EXPERIENCE WITH THE MOLTEN-SALT REACTOR EXPERIMENT

EXPERIENCE WITH THE MOLTEN-SALTREACTOR EXPERIMENTPAUL N. HAUBENREICH and J. R. ENGELoak Ridge National Laboratory, oak Ridge, Tennessee TTBS7

Received August 4, 1969Revised September 19, 1969

The x/IsRE is an 8 - MW(th) reactor in whichmolten fluoride salt at r200"F ciratlates through acore of gva0hite bars. Its purpose was to demon-strate the ?racticali.ty of the hey feafu,res ofmolten-salt pow er reactsrs .

Operation with 235 U Gg% enrichment) in the fuetsalt began in June 796s, and, by March 1968rntclear operation amounted to 9000 equiualentfull-potoer hours. The goal of demons,trating re-linbility had been attained-ouer the last Is monthsof 235t1 operation the reactor had been criticat g0%of the time. At the end of a 6-month run whichclimaxed this demonstratioyt, the reactor was shutdoutn and the 0.9 mole% uranium in the fuet ?,uas

stripped uery efficiently in an on-site ftuorinatioytfacility. uranium-27? was then added to thecarrier salt, mahi,ng the MSRE the world,s firstreactor to be fueled utith this fis si,le material.Nuclear operation uas resumed in october 196g,and, oaer 2500 equiualent full-power hours hauenow been froduced with 233ry.

The MSRE has shoutn that salt handtirry in anoperatins reactor is quite practical, the saltchemistry is well behau€d, there is fracticaily nocorrosion, the rruclear characteristics are aeryclose to fredictions, and the system is dynami-cally stable. Containment of fi.ssion products hasbeen excellent and maintenance of radioactiuecomponents has been accomplished without un-reasonable delay and with aery little radiationexpos?tre.

The successful operati.on of the x/ts&E is anachieuement that should strengthen confidence inthe fracticality of the molten-salt reactqr concept.

K EYWORDS: molten-soh re-ocfors, operotion, per{ormon ce,testing, MSRE

The paper by Rosenthal et al.l describes theorigin of the molten-salt reactor concept and howthe encouraging results of the aircraft reactorprogram led to recognition of the potential ofmolten-salt reactors for economical production ofelectricity. The role of the Motten-salt ReactorExperiment (usnn ) was to demonstrate the prac -ticality of this high-temperature fluid-fuel conceptwhich seemed so promising on the basis ofmaterials compatibility information and calculatedfuel cycle costs. when design of the MSRE wasinitiated in 1960, therefore, a primary objectivewas to make the reactor safe, reliable, andmaintainable. How well these efforts succeeded istold in this paper.

DESCRIPTION OF THE MSRE

The MSRE was designedz to use essentially thesame materials as the proposed molten-saltbreeders but, for economy and simplicity, therewas no attempt to make the reactor actuallybreed. The core is small (b4 in. in diam x 64 in.high) so that the neutron leakage is high, and thereis no blanket of fertile material. The fuel saltcontains no thorium because at the time thereactor was being designed we were thinking interms of the two-fluid breeder and we made theMSIRE salt similar to the anticipated breeder fuelsalt. The power leve1 was to be limited to 10Mw(th) or less, but we wanted to try fairly largemolten-salt pumps. As a result, the temperaturerise of the salt as it passes through the core is<50'F. The average temperature of the fuel saltwas to be 1200'F in the range proposed for thepower breeders. Even at this temperature, thevapor pressure of the salt is <0.1 mm Hg, so thepressure of the gas blanket over the salt was set

il8 NUCLEAR APPLICATIONS & TECHNOI,OGY VOL. 8 FEBRUARY 19?O

at only 5 psig. The flowsheet of the MSRE (Fig. 1)

shows the normal operating conditions at 8 MW,the murimum heat removal capability of the air-cooled secondary heat exchanger. The specialmaterials used in this reactor system are listedin Table I.

The physical arrangement of the salt systemsis shown in Fig. 2. The building housing thereactor is the one in which the Aircraft ReactorExperiment was operated in 1954. The cylindricalreactor cell was added for the Aircraft ReactorTest (which was never built) and was adapted forMSRE use.

Details of the MSRE core and reactor vesselare shown in Fig. 3. The 54-in. -diam core ismade up of graphite bars, 2 in. square and 64 in.tallr €ryosed directty to fuel which flows inpassages machined into the faces of the bars. Thegraphite was especiatly produceds to have lowperm eability, and, sinc e salt does not wet thegraphite, very high pressure would be required toforce any significant amount of salt into thegraphite. Some cracks developed in the manu-facture of the graphite, but cracked bars wereaccepted when tests showed effects attending heat-ing and salt intrusion into cracks were incon-sequential.

AII metal components in contact with moltens alt ar e m ade of H ast elloy - N (f orm erly c alledINOR-8). Metal corrosion by salt mixtures con-sists of oxidation of metal constituents to theirfluoride salts, which d o not form protectivefilms.a Attack is therefore limited only by the

Haubenreich and Engel EXPERIENCE WffH MSRE

thermodynamic potential for the oxidation re-action, and is selective, removing the least-nobleconstituent, which in the case of Hastelloy-N ischromium. However, the diffusion coefficient ofthe chromium in the metal is such that there ispractically no chromium leaching at temperaturesbelow t 500"F. f mpurities in the salt, such as

FeFz, react with Hastetloy-N, but this is a limitedeffect which goes to completion soon after thesalts are loaded. The metallurry and technologyof Hastelloy -N have been throughly developeds and

it has been approved for construction under ASMEUnfired Pressure Vessel and Nuclear VesselC odes. Hastelloy -N is stronger than austeniticstainless steel and most nickel-base alloys but,tike these metals it is subject to deterioration ofhigh-temperature ductility and stress-rupture lifeby neutron irradiation. (These effects are due toaccumulation in grain boundaries of helium pro-duced by n, ot reactions.) In the MSRE neutronspectrum the f ast neutron reactions with nickelare insignific ant compared to the slow neutronreactions with impurity boron. C areful alalysisof stresses and neutron fluxes in the MSREU led tothe conclusion that the service life of the reactorvessel would extend at least 20 000 h beyond thepoint at which the properties of the metal beganto be seriously affected by the neutron exposure.

The control rods are flexible, consisting ofhollow cylinders of Gd2Os-Al2Os ceramic, cannedin Inconel and threaded on a stainless-steel hosewhich also serves as a cooling-air conduit. Anendless-chain mechanism, driven through a

TABLE I

MSRE Materials

aAnother batch of salt of this composition is used to flush the fuel system before it is opened to minimize fissionproduct escape and again after it is resealed to pick up moisture that may have entered.

Fuel SaItComposition:

Properties at 1200'F (650"C)

DensitySpecific heatThermal conductivityViscosityVapor pressure

Liquidus temperahrre

Coolant salta

Moderator

Salt containers

Cover gas

tLir'-geFz- zrF+-lJFq (65. 0 -29. 1-5. 0 -0. 9 mol4o

t4t Lb/ff0.47 Btu/lb-oF

0.83 Btr/h-ft-"Fle \blh-ft

<0.1 mm Hg

913'F

tLiF-BeFz (66-84 mole7o)

Grade CGB graphite

Hastelloy-N (68 Ni-17 Mo-? Cr-iHeIium

2.3 q/ crr-g

2.0 x 10" /kg-ocL.43 Vm-'C

2e W/h-m<1 x 10-n bar

434"C

Fe)

NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 1970 il9

Haubenreich and Engel EXPERIENCE WITH MSRE

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120 NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 19?O

Haubenreich and Enge1 EXPERIENCE WITH MSRE

REMOTE MAINTENANCECONTROL ROOM

REACTOR CONTROLROOM

-<-a

I. REACTOR VESSEL2. HEAT EXGHANGER

3. FUEL PUMP4. FREEZE FLANGE5. THERMAL SHIELD6. COOLANT PUMP

)g

7. RADIATOR

8 COOLANT DRAIN TANK

9. FANS

IO. FUEL DRAIN TANKSI I. FLUSH TANK12. GONTAINMENT VESSELI3. FREEZE VALVE

Fig. 2. Layout of the MSRE.

clutch, raises and lowers the rods at 0"5 in"/sec.When scramm€d, the rods fall with an acceler-ation of. -12 tt/ secz .

The bowl of the fuel pump is the surge spacefor the circulating loop. Dry, deoxygenated he-Iium at 5 psig blankets the salt in the pump bowl.About 50 of the I20 gal/min discharged by thepump is sprayed into the gas space to prov_ide

contact between salt and cover gas to allow tsxe

to escape from the salt. (The solubility of xenonand krypton in the salt is very low,) A flow of

4liters/min STP of helium caruies the xenon andkrypton out of the pump bowl, through a holdupvolume providing -40-min delay, a filter station,and a pressure-control valve to charcoal beds.The charcoal beds consist of pipes filled withcharcoal, submerged in a water-filled pit at

-90oF. The beds are operated on a continuous-flow basis and delay xenon for -90 days andkrypton for '-''7 days. Thus, only stable or long-Iived gaseous nuclides are present in the heliumwhich is discharged through a stack after passingthrough the beds.

All salt piping and vessels are electricallyheated to prepare for salt filting and to keep_thesalt molten when there is no nuclear power.t Inthe reactor and drain-tank ce1ls, where radiationlevels make remote maintenance necessary,heater elements and reflective metal insulationare combined in removable units. Thermocouplesunder each heater monitor temperatures to avoidoverheating the empty pipe. The radiator isequipped with doors that drop to block the air ductand seal the radiator enclosure if the coolant salt

NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 1970 121


GRAPHITE SAMPLE ACCESS POR

circulation stops and there is danger of freezingsalt in the tubes.

Around the reactor-vessel furnace is a shield,16 in. thick consisting of a tank of stainless steelfilled with steel balls and circulating water. Theshield absorbs most of the energy of neutrons andgammas escaping the reactor vessel (zo kflIvIW ofreactor power). It also cuts down on neutronactivation of components in the reactor cell,facilitating maintenance. The cooling-water sup-ply for the shield is deaerated to remove radio-lytic gas.

Neutron chambers are located in tubes in a36-in. -diam water -filled shaft that slopes downthrough the reactor cell to the inner surface of thethermal shield. Included are 3 uncompensated ionchambers driving safety channels , 2 eompensatedchambers, and 2 servo-driven fission chambers

122 NUcLEAR

FLEXIBLE CONDUIT TOCONTROL ROD DRIVES

COOLING AIR LINES

ACCESS PORT COOLING JACKETS

FUEL OUTLET EACTOR ACCESS PORT

CONTROL ROD THIMBLES

CORECENTER ING GR I

OUTLET STRAINER

FLOW DISTR IBUTOR

cRAPH ITE-iTfODERATORSTRINGE

FUEL INLET

REACTOR CORE CA

REACTOR VESSEL

ANTr-swrRL vANEgWVESSEL DRAIN LIN

TODE RATORSUPPORT GR ID

Fig. 3. Details of the MSRE core and reactor vessel.

that provide a l0-decade power indieation. Thecompensated chambers are connected to multiple-range ammeters and a flux- or power-servosystem. Any one of the three rods can beconnected as a regulating rod to the servo system.The fuel salt, which is a mixture containing bothalpha emitters and beryllium, is itself a strongneutron source, but there is also an Am-Cm-Bestart-up source in a thimble in the thermal shield.

There are no mechanical valves in the saltpiping. fnstead, flow is blocked by plugs of saltf.rozen in flattened sections of the lines. Temper-atures in the "freeze valves" in the fuel andeoolant drain lines are controlled so they willthaw in 10 to 15 min when a drain is requested. Apower failure of longer duration also results in adrain because the cooling air required to keep thevalves ftozen is interrupted.

APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 19?O

The drain tanks are -4 ft in diameter, but themolten fuel is saf ely subcritical bec ause it isundermoderated away f rom the core graphite.Water-cooled bayonet tubes e xt e n d down intothimbles in the drain tanks to remove up to 100kW of heat if necessary. Steam produced in the

tubes is condensed and returned by gravity toprovide reliability.

The reactor cell and drain-tank cell are con-nected by a large duct so they form a singlecontainment vessel. The tops of the two cellsconsist of two tayers of concrete blocks, with a

weld-sealed stainless-steel sheet between the lay-ers and the top layer fastened down to containinternal pressure. The drain cell and the top ofthe reactor cell were designed for 40 psig and

were tested at 48 psig. The sides and bottom ofthe reactor cell, built to house the AircraftReactor Test, were tested at 300 psig before thetop was modif ied for the MSRE. The cell atmo-sphere is kept at 130"F by water-cooled, forced-air space coolers.

The cooling "airtt for the f.reeze valves and thefuel-pump bowl is actually reactor-cell atmo-sphef€, compressed and cooled. Some of theblower output is discharged past a radiationmonitor and up the ventilation stack to keep thereactor cell and drain-tank cell at -2 psig duringoperation. A small bleed of nitrogen into the cellkeeps the oxygen content at 3Vo to preclude fire iffuel-pump lubricating oil should spill on hotsurfaces. The cell inleakage is calculated frompurge flow-in, discharge rate, and differentialpressure between the cell and a temperature-compensated, sealed volume in the cell.

All the components in the reactor and drain-tank cells are designed and laid out so they can be

removed by the use of long-handled tools fromabove. When maintenance is to be done, the fuelis secured in a drain tank and salt plugs f.tozen inthe connecting lines. The upper layer of blocks isremoved, a hole is cut in the stainless membraneand one or two lower blocks are removed over theitem to be worked on. A large duct from thereactor cell to the upstream side of the ventilationfilters is opened to draw air down through theshield openings. Tools, lights, and viewing de-vices are inserted through fitted openings in asteel work shield. Items removed from the cellsare bagged in plastic and removed for storage inanother cell in the reactor building, inspection, ofdisposal.

In the same buildihgr adjacent to the drain-tankcell, there is a simple facility for processing thefuel f or flush salt.8 The purpose of twofold: toremove oxide contamination from the salt if thisshould be necessary and to recover the uraniumfrom the salt at the end of the experiment. One


whole batch of salt (-?5 fts) is transferred into a

tank where it is sparged with gas-either hydrogenfluoride to remove HzO and convert the metaloxides to fluorides, of fluorine gas to convert UF*to the volatile UFe which leaves the salt to be

trapped on a bed of sodium fluoride pellets.Ancillary equipment includes gas-handling equip-ment and a large filter to remove solids from thesalt after processing. n

CHRONOLOGY OF THE MSRE A

Design of the MSRE began in the summer of1960 and 18 months later fabrication of theHastelloy-N primary-system c o m p o n e n t s wasstarted. Production of the newly develoPed, low-permeability graphite proved to be the criticalpath but by the end of 19 63 it was ready f orassembly in the reactor vessel. Installation of thesalt systems was completed in the summer of1964. Activities during the start-up period thatfollowed are outlined in Fig. 4.

Prenuclear tests showed that all systems oper-ated well and there were no unanticipated prob-lems in handling the molten salt. The fuel carriersalt that was circulated in the final prenucleartest contained depteted uranium (150 kg). Thereactor was made critical by adding 69 kg of

highly enriched 235U in the form of a UF+-Lif'eutectic salt containing 6IVo uranium by weight.Most of the uranium was added to the salt in thedrain tanks, with the finat amounts added,85 g at atime, through the sampler-enricher to the circu-Iating fuel salt. After the initial criticality on

June !,1965, more uranium was added while thecontrol rods were calibrated and the reactivitycoefficients that would be needed in later analysisof the system were measured.

At the end of the zero-power nuclear e>!peri-ments, the reactor was shut down while finalpreparations for power operation were made.

This consisted mainly of finishing construction;the only repair or modification dictated by theprevious testing was replacement of the heat-warped radiator doors with doors of an improveddesign.

After tests in the kilowatt range showed thatthe dynamics of the system were as expect€d, theapproach to the full power was started in January1966. Trouble was e nc ou nt e r e d immediately-within a f ew hours, plugs developed at severalpoints in the fuel off-gas system and the reactor

tActivities in the Molten-Salt Reactor Program, includ-ing the MSRE , are described in a series of semiannualprogress reports issued by Oak Ridge National Lab-oratory.

NUCLEAR APPLICATIONS & TECHNOLOGY VOL. B FEBRUARY 19?O 123

Haubenreich and Engel EXPERIENCE wmH MSRE

IN SPECTION AN DPRELIM INARY TESTING

PRE N UCLEAROF COMPLETE

TESTSSYSTEM

FINAL PREPARATIONSFOR POWER OPERATION

FIN ISHINSTALLATION

OF SALT SYSTEMS

TEST AUX. SYSTEMS

-

OPERATOR TRAINING

-17,

LEAKT EST,PURGE A HEATSALT SYSTEMS

J-

LOAD SALTINTO DRAIN TANKSCOOLANT FLUSH

SALT SALTr74 vv4

INSTALLCONTROL RODS

SAMPLER-ENRICHER

-

INSTALL CORE SAM PLESINSPECT FUEL PUMP

HEAT-TREAT CORE VESSEL777.77-7-l

TEST TRANSFER, OPERATORFIL!_8, lRAIN OPS. TRAtNtNG1- r+

FINISH VAPOR-COND. SYSTEMMODIFY CELL PENETRATIONSREPLACE RADIATOR DOORS

-

LOAD 235sIN ZERO-POWER

N UCLEAREXPERIMENTS

-

TEST SECONDARYCONTAINMENTw,-l

ADJUST A MODIFYRADIATOR ENCLOSURE

LOW- POWER(o-50 kwt

E X PTS.l-l

-F-rD

LOAD 8CIRCULATE CIRCULATEq8 FL SALTS CARRTER

was shut down. Investigation showed that some-thing like a varnish mist had plugged the smallpassages and a small filter in the off-gas system.There had evidently been some oil in the off-gasholdup pipe in the reactor cell and this had beenvaporized and polym erized by the heat and radia-tion from the fission products in the fuel off-gas.only a few gram s of material had caused thetrouble and the installation of a more efficient,larger filter ("particle trap,,) downstream of thehold pipe brought the problem under control.

Almost three months were spent in investi -gating and remedying the off-gas problem beforeoperation was resumed. As shown in Fig. b, inthe approach to furl power, the reactor wasoperated at several intermediate power levels toobserve dynamics, xenon behavior, and fuel chem-istry. The only significant delay in this periodwas to repair an electrical short in the fuelsampler-enricher drive. In the final stages of thepower escalation it was discovered that the heatremoval capability of the air -cooled secondaryheat exchanger was substantially less than ex-pected. As a result the murimum steady-statepower of the MSRE was restricted to g Mw. (Atthe time, heat balances indicated the mu<imumpower was -7.2 Mw. Later the coolant saltspecific heat was measured and found to be llTohigher than the original value, which had beenobtained by extrapold"tion of data on other salts. )shortly after full power was reach€d, theindicated air inleakage into the reactor cell roseand, as shown in Fig. b, the reactor was shut downto investigate the difficulty. No excessive leakagepath to the atmosphere was found; the inleakaiewas from pressurized thermocouple headers and

Fig. 4. MSRE activities, July Lg64-December 196b.

was corrected. Full-power operation was thenr esum ed. In the next five weeks the only inter -ruption of more than a few hours was a four-daydelay when an electricar short in the power lead toa component cooling blower caused a salt drain.

High-power operation was abrupily halted inJuly when the hub and brades in one of the mainblowers in the heat removal system broke up.The cast aluminum hubs in the other blower and inthe spare unit were also found to have somecracks, so new units had to be procured. Throughthe vigorous efforts of the manufacturer, the huband blade castings were redesigned and a new unitwas built, tested, and installed at the MSRE within11 weeks.

while the reactor was down, the array ofgraphite and metal specimens was removed fromthe core and a new array was installed. Duringthe flushing operations before the specimen re-moval, the fuel-pump bowl was accidentatly over-filled and some flush salt f.roze in the attached gaslines. Temporary heaters were installed remote-ly to clear the lines.

operation with one blower resumed in octob€r,but it was soon found that the fuel off- gas line hadnot been completety cleared. A restriction nearthe pump bowl became worse and the reactor wasshut down. when temporary heaters were put onthe off -gas line and pressure was appli€d, the plugpartially blew out, and the pressure drop returnedto near normal. while the reactor was down, thesecond new main blower was instalted and whenthe reactor was started up again it was to go tofull power. Again it was found that the o11-gasline was not clear. once more the reactor wasshut down to take positive steps to clean out the

APPLICATIONS & TECHNOIOGY VOL. 8 FEBRUARY 19?O124 NUCLEAR

SALT INFUEL LOOP

Haubenreich and Engel

SALT IN

FUEL LOOP POWER

EXPERIENCE WITH MSRE

XENON STRIPPING

EXPERIMENTS

I TNSPECTION AND

I MAINTENANcEIt REPLACE coRE SAMPLES

TEST AND ]I/ODIFYFLI.JORINE CISPOSAL

SYSTEM

PROCESS FLUSH SALT

PROCESS FUEL SALT

LOAD URANIUM - 233REMOVE LOADING DEVICE

233u zERo - PowER

PHYSICS EXPERIMENTS

INVESTIGATE FUELSALT BEHAVIOR

CLEAR OFF.GAS LINES

REPAIR SAMPLER ANDCONTROL ROO DRIVE

233u DYNAMtcs rESTS

INVESTIGATE GASIN FUEL LOOP

HGH-POWER OPERATION

To MEASunE 233u os/o6

l

l

)

)

DYNAMICS TESTS

INVESTIGATEOFF.- GAS PLUGGING

REPLACE VALVESAND FILTERS

RAISE POWER

REPAIR SAMPLER

ATTAIN FULL POWER

CHECK CONTAINIVIENT

FULL - POWER RUN

MAIN BLOWER FAILURE

REPLACE MAIN BLOWER

MELT SALT FROM GAS LINES

REPLACE CORE SAMPLESTEST CONTAINMENT

Rt,N WITH ONE BLOWER

INSTALL SECOND BLOWER

ROD OUT OFF- GAS LINECHECK CONTAINMENT

3O-doy RUN

AT FULL POWER

REPLACE AIR LINEDISCONINECTS

SUSTAINED OPERATIONAT HIGH POWER

)

)

II REPLAcE coRE SAMPLES

I rESr coNrAtNMENr

} REPAIR SAMPLER

II nepuacE coRE sAMPLEs

l REPATR Roo DRIvES

I cuean oFF-GAs LINEs

o ? 4 6 8loFUEL N Po,lrER (Mw)

FLUsH f-l

Fig.5.

NUCLEAR APPLICATIONS & TECHNOI,OGY VOL.

oFUEL NFLUSH

MSRB activities, 1966-1969.

8 FEBRUARY 1970

2 4 6 8loPOI'I'ER (M,I'V)

r25


off-gas line and also to investigate an apparenthigh inleakage of air.

when the off-gas line was opened, crusts offrozen salt were found in the franges where theheaters had not been eff ective. These depositswere rodded out. The cell inleakage proved to befrom leaking service-air-rine disconnects. sincethese did not represent a leakage path to theoutside, provisions were simply made to measuretheir input so the cell leak rate could be calculated.

The next run began in mid-December andcontinued for 30 days at full power. A shutdowtr,which had been scheduled for the 1000-h inspec -tion of the new blower, was extended to permitreplacement of all the air-line disconnects in thereactor cell and installation of an improved par-ticle trap in the fuel off -gas line.

It turned out that the bothersome problems thathad interfered with sustained operation during theautumn had been finally overcome. Nuclear oper-ation resumed on January 28 and continued for LOzconsecutive days until a shutdown that was sched-uled to permit removal of specimens from thecore. As shown in Fig. s, during this period thereactor was at full power nearly the futt time (%Voof the time from February 1 through May g). Thelongest interruption was four days at low powerwhile a zero-xenon reactivity measurement wasbeing obtained and a rough bearing on a mainblower wag_replaced. During this run a total of?61 g of t*u was added through the sampler-enrich€r, marking the first time fuel had beenadded with the reactor at full power.

The reactor was down for 39 days while thecore specimens were replac€d, annual tests ofcontainment and instrumentation were conducted,and miscellaneous maintenance jobs were done.A_gain full-power operation was resumed. More235u was added during the run (1g capsules con-taining 1527 g of "5u in seven days) in preparationfor six months of power operation with no furtheruranium additions to obtain a measure of thecapture-to-fission ratio of "uu. However, after42 days of futl-power operation, the run wascut short by a malfunction in the fuel sampler-enriche r.

The drive cable in the sampler had becometangled and, in attempts to untangle it, the cablewas sever€d, dropping a sample capsule and thecable latch into the pump bowr. In the next fiveweeks the latch was retrieved and the drive unitwas replaced. The capsule was left in the pumpbowl.

Full-power operation was barely resumed whenone of the component cooling pumps went downwith a lubricating-oil leak. Rather than start whatwe hoped to be a 6-month run without a standby,component cooling pump, w€ shut down and fixed

it. A new start was made on september 20. Thistime everything went weII and it was 1gg daysbefore the fuel was again drained.

The long run was devoted primarily to sfudy offission-product behavior. The reactor was oper-ated for about two of the six months at a reducedpower, 5.6 Mw, to permit the fuel temperature tobe varied in a study of the effect of variousoperating conditions on ttuxe stripping. The onlyinterruption of any significance was a week inNovember when the powe" was lowered (sub-critical for two days) while an electrical short inthe fuel sampler-enricher was being repaired.

The shutdown on March 26,1969 was the end ofnuclear operation with 235U. For several months,preparations had been underway to strip theuranium from the fuel salt and replace it with "ttr.The uranium was to be removed on-site by thefluoride volatility process. For the zs'u aoaitiotr,equipment had been set up in the nearby Thorium-uranium Recycle Facility (TURF) at oRNLro toproduce half a cubic foot of uF+ -LiF eutecticcontaining 35 kg of "ttr. (This uranium contained-220 ppm zszrJ, which made it so radioactive as topractically prohibit any other use. )

In the first month of the shutdown the corespecimens were replac€d, and the fuel piping andvessels were surveyed using remote gamma-rayspectroscopy to determine the distribution offission products. At the same time necessarymaintenance was completed. This included repairof two heaters from the primary heat exchatrge",rodding out the fuel off-gas line at the pump bowland fishing (unsuccessfully) for another samplecapsule that had been accidentatly dropped into thepump bowl. Attention then focused on the saltprocessing system where the system for disposalof excess fluorine was as yet untried.

In the preparations for uranium recovery,difficulties were encountered in obtaining therequired efficiency in disposal of fluorine byreaction with SOz gas, and eventually the processwas changed to reaction in a caustic solution.P roc essing of the flush -salt charge started onAugust l; eleven days later the 6 kg of uranium inthe salt had been fluorinated out as uFu andcollected on sodium fluoride pellets, corrosion-product fluorides had been reduced to filterablemetals, and the salt had been filtered on its returnto the reactor. Fluorination of the fuel salt torecover the 218 kg of uranium present took only47 h of fluorine sparging over a 6-day period.corrosion products were reduced and filtered inanother 10 days. Two cubic feet of the strippedcarrier salt, still loaded with fission products,was left in the processing vessel for a future testof a vacuum distillation process. The remainderwas returned to the reactor.

126 NUCLEAR APPLICATIONS & TECHNOI-..OGY VOL. 8 FEBRUARY 19?O

Loading of 233U began immediatety. The bulk of

the uranium required for criticality was loaded

into the carrier salt through equipment attachedtemporarily to one of the drain tanks. C ans of

frozen eutectic, containing up to 7 kg 233U each,

were lowered into the hot tank where the saltmelted and poured into the carrier salt. Neutronmultiplication measurements were made with the

salt in the core after the addition of 2t, 28, and

33 kg LI. After the last of these, the addition

equipment was removed and the cell was closedand leak-tested before the addition of the re-maining 400 g required for criticality. Thisamount was added in capsules through the sam-pler -enricher and on October 2 the MSRE was

iirst critical with 233u fuel. capsule additions

were continued and on October 8, U.S. AtomicEnerry commission chairman seaborg took the

reactor power to 100 kw for the first timeafter ceremonies marking the world's firstpower operation with 233U fuel. Over the next

month another 1. ? -kg U was added while the

control rods were catibrated and the temperatureand concentration coefficients of reactivity were

measured.Earty in the zero-power physics experiments

the amount of gas entrained in the circulating fuelwas observed to increase from <0.1 to -0.5 vol%o.

The run was therefore extended two weeks forexperiments aimed at determining the cause of theincrease. The initial increase was coincident withthe addition of beryllium reductant to the fuel saltand further additions confirmed that beryllium had

an effect on bubble behavior. Magnets dipped intothe salt showed that small amounts of finelydivided, reduced iron were floating on the surfacein the pump bowl. This evidence was later fittedwith other observations to conclude that minorchanges in surface tension occurred when beryl-lium was exposed in the salt containing some

unreduced corrosion productS. These minorchanges aff ected the behavior of the bubbleschurned into the fuel in the pump tank by the xenon

stripper sprays just enough to affect the entrain-ment of gas into the circulating stream.

In preparation f or prggision assays of the

uranium to measure the 233U capture-to-fissionratio during subsequent power operation, the re-actor was shut down to mix the uranium in the

loop and in the drain tanks. while it was down the

fuel off-gas line, which had become restricted near

the pump bowl, was rodded out. The reactor was

started up, but before power operation was begun'

it was shut down again to repair a gear in the

sampler drive mechanism. This work extended

into JanuarY 1969.As shown in Fig. 5, the reactor power was

stepped up to !, 5, and 8 MW over a two-week


period in Januafy, with dynamics tests and obser-vations of reactivity and radlation heating at each

leve1. As expect€d, the system was quite stable

over the entire range of power, with dynamic

response characteristics very close topredictions.

During the approach to futl power, there wereobserved for the first time sporadic small in-creases (-5 to I}Vo) in nuclear power for a few

seconds, occurring with a varying frequency

somewhere around ly/h. The perturbations in-volved brief reactivity increases of 0.01 to 0-02Vo

6k/h, and temperature and pressure changes too

small to be of consequence. The characteristicsof the transients pointed to changes in gas volume

in the fuel loop and it appeared that they weremost likely caused by occasional release of some

gas that collected in the core. This hypothesiswas supported when, late in February, a variable-frequency generator was used to operate the fuelpump at reduced speed. The gas entrainment inthe circulating fuel decreased sharply (from 0'? to<0.1 vol Vo) and the perturbations ceased entirely.Upon resumption of full-speed circulation theperturbations appeared again but gradually be-came smaller and less frequent, until by the end

of March they were indistinguishable from the

continuous noise in the neutron level (* ZVo of

power). The disappearance was tentatively attri -buted to gradual, slight changes in salt surfacetension and circulating gas fraction'

In May, f €strictions began to show up again inthe fuel off- gas system. partial restrictions ap-

peared at several points including two places inthe main fuel off- gas line where they had occurredpreviously as weti as two other locations that had

not plugged bef ore. Although the restrictionswere Somewhat more severe than in the past, they

did not prevent operation of the reactor at high

power .rtrtit the scheduled end of the run on June 1.

The power operation in this run burned up enough233U io provide a good measure of the ratio of

neutron captures to fissions in this fissile ma-

terial. (Samples taken f or this purpose wereprepared for high-precision mass-spectrographicanalyses at the time of this writing')

The primary purpose of the shutdown was

to permit removal and replacement of surveil-lance specimens in the core and to investi-gate the distribution of fission products in the

primary loop by gamma scanning. Preparationswere also made for more extensive gamma scan-

ning during and after the next period of power

operation. In additioor maintenance operations

were performed on the reactor control rods

and drives and to relieve the off- gas restrictions.The return to power operation was scheduled forAugust.

NUCLEAR APPLICATIONS & TECHNOI'OGY VOL' 8 FEBRUARY 1970 127


SUi,IMARY OF EIPERIENCE

operation of the MSRE has served to demon-strate and emphasize the basic soundness of themolten-salt reactor concept. It has also providedsome valuable new information. The following isa discussion of the experience in each of severalimportant areas.

Fuel Chemistry

Fuel chemistry is an area of vital importanceto a fluid-fuel reactor comparable to fuel struc-ture, cladding integrity, and coolant stability in asolid-fuel reactor.

In assessing the safety of fluid-fuel reactorsperhaps the first question that comes to mind isthe possibility of uranium separating from thefluid. This has occurred in aqueous homogeneousreactors, where severar mechanisms exist forsuch separation and operating conditions may beclose to a region of chemical instability. such isnot the c ase in molten-salt reactors, however.The MSRE fuel is subject to only two mechanismsfor uranium separation: the contamination of thesalt with moisture or oxygen to the point thaturanium oxide precipitates, and the reduction ofso much uFn to uFs that the uF, begins todisproportionate into uF+ and insoluble uranium.Neither condition is approached during operation.n

oxide formation in the MSRE salt is controuedby providing a blanket gas (helium ) that has hadoxygen and moisture reduced to <10 ppm bypassage through a 1200"F titanium sponge. whenthe core access is opened for specimen removal,moist air intrusion is minimized by first fillingthe system with denser argon and working througira standpipe filled with dry nitrogen. Then beforethe fuel salt is put back in the core, the ftush saltis first circulated to react with any moisture thatmay have entered. Evidenc e of the eff ectivenessof these measures is the extremely low oxidelevel in the fuel after four years in the reactor:analyses consistently show only -60 ppm oxide.This small amount is in solution: the chemistryof the fuel salt is such that the first oxide to formis zroz and its solubility is -?00 ppm. The zrFawas put in the fuel as a cushion; without it, theuo, that would have been formed would still havebeen far below its solubility limit of 1000 ppm.The success of the MSRE of keeping oiioecontamination down argues that zrEa need not beincluded in the fuel of future molten-salt reactors.

The second possibility, precipitation of re -duced uranium, was the reason for specifying thatthe uranium to be used in the original powe"operation by only - 33Vo 235u instead of ttigtrtyenriched. At that time we did not know precisely

what the average total valence of the products ofone fission would be in the reactor environment.we expected it to be close to four, but if it hadbeen more it would, in effect, tie up more than thefour fluorine atoms made availabte by the de -struction of the uranium atom. The ingrowth offission products would then gradually reduce someof the uFn to uFr. with more moles of UFn in thefuel charge, more fissions could be sustainedbef ore the UFr/UFn ratio reached the point ofdisproportionation. Thus the composition of thefuel was specified to contain O.gTo total uraniumeven though <0.3 moLeVo of highly enriched ura-nium would have been sufficient to make the MSREcritical. operation of the MSRE has shown thatin fact the average totar valence of the products ofa fission is slightly less than four. This meansthat the tendency is actually away from reductionof uF+ to uFr, thus eliminating the possibility ofuranium p recipitation.

Although the chemical effect of the fissionprocess in the MSRE is in the direction of excessfluoritr€, as long as there is any uF, present, theresult is simply the gradual conversion of UF, touFo. Therefore, a small fraction of the uraniumis maintained as uF, to guarantee a reducingrnoncorrosive environment. This is aeeomplishedsimply by occasionally exposing a rod of Be metalto the salt in the fuel-pump bowl. In a 10-hexposure, -10 g of Be forms BeF2, reducing twomoles of uFn to uF, and counteracting the effectof the fissions over more than 10 000 Mwh ofoperation. After the fuel processing in september1968, some unreduced corrosion-product fluoridesfrom the fluorination process were apparenily leftin the salt, and the first three additions ofberyllium were consumed in reducing these fluo-rides to the metals. Further additions then pro-duced the desired UFr.

Corrosion products in the salt in the reactorhave never amounted to more than -200 ppm and,with the exception of the possible interferencewith the uF+-uFr reduction just mentioned, theyhave had no significant chemical effects.

Fission product behavior in the molten-saltreactor environment was a question on which theMSRE operation has shed new light.a The noblegas€sr xenon and krypton, are stripped into theoff- gas even more efficienily than had been ex-pected. Only small fractions of their radioactiveisotopes decay in the salt or in the pores of thegraphite. As expect€d, most of the other fissionproduct species remain entirely within the fuelsalt. The only exception is the *noble-metal,group-Mo, Ru, T€, and Nb, which do not formstable fluorides in the normal MSRE environment.Analyses of salt samples typically show only onepercent or less of the noble metals circulating


with the salt. (An exception was during the 233U

start- up when niobium showed up in the salt untilberyllium additions incneased the reducing powerof the salt. ) Specim ens from the core arrayshowed that around half of the noble-metat fissionproducts were plated out on graphite and metalsurfaces exposed to the salt. The remainingsubstantial fraction of the noble metals goes intothe cover gas above the fuel salt as a smoke oraerosol of submicron particles. The depositionand "smoking" of the noble metals create no

problems in the MSRE, but forewarn of afterheatproblems to be dealt with in the breeder design.

Also of interest for future reactors is thebehavior of plutonium in molten salt. The stableform is PuFs, which is sufficiently soluble in thesalt to be of interest as a potential fuel formolten-salt reactors. The MSRE operation withpartially enriched uranium produced '-'600 g ofplutonium and analyses of samples show that allhas stayed with the salt.

Materials Compatibi I ity

Anatyses of several hundred samples of fuelsalt taken over a period of three years and

examination of specimens exposed in the core formany thousands of hours have demonstrated thecompatibitity of the salt, the moderator, and thecontainer material.

Chromium in the fuel salt is the best indicatorof corrosion of the Hastelloy-N, since corrosionselectively attacks the chromium in the alloy and

the product , C rFr, i s quite soluble in the salt.The MSRE fuel is sampled and analyzed forchromium at least once a week. Results showed

an increase in the three years between May 1965

and March 1968 from 38 to 85 ppm. This corre-sponds to the amount of chromium in a 0.2-millayer of Hastelloy-N over the entire metal surfacein the circulating system. However, the data

suggest that much of the chromium appeared inin the salt white it was in the drain tank between

runs so that the chromium was leached from even<0.2 mil over the circulating loop surfaces inthree years. This extremely low rate in the loop

is less than was predicted from thermodynamicdata and diffusion in the metal. One hypothesis isthat the corrosion is so low because the surfacesare protected by the film of deposited noble-metalfission products, which

"ry"""!ed -10 A in thick-

ness by March 1968.

The chromium indication of very low corrosionin the fuel loop during the '*U operation was

substantiated by the condition of Hastelloy-N sur-veillance specimens exposed in the core. Therewas no sign of localized attack, and metallo-graphic examination showed no appreciable corro-


sion. Carburization of metal specimens in contactwith graphite extended to a depth of only 1 mil.

During the first 3 months of fuel circulation inthe 233U start-up, the chromium concentrationcame up from 35 to 70 ppm and then virtuallyleveled out. This behavior was attributed to thepresence in the salt of some FeFz from thefluorination process that reacted with the chro-mium in the walls until it was reduced by addi-tions of beryllium metal.

Graphite specimens exposed in the fuel salt foras long as 22 500 h (2.5 years) showed no attackby the salt. There was no change in the surfacefinish and no cracks other than those presentbefore e>iposure. Only extremely small quantitiesof salt were found to have penetrated the graphiteeither through pores or cracks. (essuming thespecimens are typical, there is <2 g of uranium inthe 3?00 kg of graphite in the entire core- )

The physical properties of the Hastelloy -Nwere affected by irradiation in the MSRE as

e>rpected on the basis of prior irradiation experi-ments.tt There was littte change in ultimatestrength, yield strength, and secondary creep rate.Rupture ductility and creep rupture life in speci-mens of the heats of Hastelloy -N used in thefabrication of the MSRE were reduced as had beenanticipated in the design. Specimens of improvedHastetloy -N (containing small amounts of tita-nium) irradiated in the MSRE showed very good

ductility and lif e at 1200'F, even surpassingunirradiated standard Hastelloy -N.

Ghemical Processing

Fluorination to recover the original charge ofuranium was aceomplished in the simple equip-ment depicted in Fig. 6. Mixtures of fluorine and

helium were bubbted at rates up to 33 std liters/min of fluorine through a 1-in. dip tube immersed64 in. in the tank of salt. At the beginning of theprocessing there was a high utilization of fluorineas the UFa was rasied to UFs. Then volatile UFsbegan to be formed and the fluorine utilizationdecreased to -30V0. The effluent gas streampassed through a bed of sodium fluoride pellets at

?50'F to remove volatile impurities and then intoa series of cannisters filled with NaF pellets at

200 to 300'F where the uFe was absorbed.The uranium concentration in the flush salt was

reduced to ? ppm in ? h of fluorination; in the fuelsalt processing , 47 h of fluorination removed 218

kg of uranium from the 4730 kg of salt, leaving a

concentration of 26 ppm IJ. The UFo has not yetbeen desorbed from the NaF for an accuratematerial balance, but the weight gain of the

absorbers checks the expected UFe recoverywithin the accuracy of the measurement.

NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 1970 129


GAS ADDITIONSTATION

tn,ffi

SAM PLERSALT IS SAMPLEDTO MEASURE UF+

UFC ABSORBERS

uF6 ls ABSORBED ONNoF PELLETS AT 25O"F

SOMECONTA M I NANT

FLUO R I DESABSORB ON

NoF PELLETSAT 75O"F

FUEL STORAGE TANKFLUORINE REACTS WITH

DrssoLVED UF+ TO FORMvoLATtLE UFe

Decontamination of uranium from the fissionproducts was very effective; the processing tankread over 2000 R/h, while cannisters containing12 kg u read <0.002 R/h, The measured decon-tamination factors for gross beta and grossgamma activities were l.z x 10n and 8.6 x 10t,respectively. This permitted direct handling ofthe absorbed cannisters containing the recovereduranium.

C orrosion of the tank during fluorination wasless than had been observed in fluoride volatilityprocessing in other equipment. This was probablybecause the processing at the MSRE was done at alower temperature (850"F, just above the saltliquidus) and also because the MSRE tank isrelatively large so that fluorine concentrations atthe wall tended to be lower. The depth of corro-sion for the whole campaignr averaged over thesurface exposed to the salt, was 11 mil. At thisrate the tank would last for processing manybatches, although the dip tube would likely have tobe replaced after each few batches.

At the end of fluorination the fuel salt contained850 ppm Ni, 410 ppm Fe, and 435 ppm Cr asfluorides. The fluorides were reduced by treat-ment with hydrogen and finely divided zireonium.

CAUSTTC SCRUBBER TRACES OF

FLUoRTNE REAcrs wrrH FLUORINE

KoH. roDrNE rs ABSoRBED o#?lt"%VEt

Fig. 6. MSRE salt fluorination process.

The agglomerated reduced metals were then re-moved by a 9-ft2 fibrous-metal filter as the saltwas returned to the reactor. Samples from thereactor showed only 60 ppm Ni, 110 ppm Fe, and35 ppm Cr, indicating the effectiveness of thispart of the processing. As discussed before,howev€r, it appears that at least some of the ironthat got through was in the form of the fluoride.

Reactivity

The MSRE has gone through two series ofnuclear start-up experiments: oncett with'*Uand again with "tU. In each case the criticalconcentration of fi ssile material was carefullydetermined and the control rod calibrations andreactivity coefficients necessary for analysis ofthe nuclear operation were measured. Table IIsummariz es the most important results. C om -parison of predicted and observed values showsthat the available data and procedures were quiteadequate for calculating the nuclear character -istics of the MSRE at start-up.

During routine reactor operation, the systemreactivity is monitored by a calculation executedevery five minutes by an on-line digitat computer

APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 19?O130 NUCLEAR

Haubenreich and Engel EXPERIENCE WmH MSRE

TABLE II

summary of MSRE Nuclear Parameters with 23sU and 233U Fuels

Parameter Units

235u Fuel233u Fuel

Calculated Measured Calculated Measured

Initial critical concentration in salt g tJ /LitetReactivity loss due to circulation

of delayed-neutron precursors Vo 6k/ h

Control-rod worth at initial critical loadingd To 6k/k

1 Rod

3 Rods, banked

Temperahrre coefficient of reactivity ry {x 105)at operating loading oF '

Total

Fuel

concentration coefficient of reactivit To6fu/!YM

0

33 06.l

222

-8.1

-4.L

0.234

2.LL

5.46

32.85 + 0.25^

0.2L2 + 0.004

2.26

5.59

-7 .3 + 0.2

-4.9 + 2.3

0.223

15.30b

0.0 93

2.7 5

7.01

-8.8

-5.7

0. 389

15.15 + 0.1b

c

2.58

6.9

-8.5

e

0.369

""uu oolo.bu"Jo}'ot the isotopic composition of the material added during the critical experiment (gf%'su).tMeasurement obscured by effect of circulating voids.dNormal full travel of rod(s).eNot separately evaluated.

at the reactor site.la The computer is program -med to take signals of control rod positiotrrneutron flux, and fuel temperature and compute areactivity balance based on present conditions andthe power history. The basic eQuation solved bythe computer is

- KRES - KTEMP + KPOW + IA(E + I(SAM+ KFP + KU235 + KROD,

where

KRES - the deviation of the observed re-activity from the expected behavior

I(TEMP = the reactivity variation associatedwith variations of the system tem-perature f rom a reference value(r zoo "r)

KPOW = the reactivity difference caused bythe calculated temperature distri-bution within the core at power rel-ative to the isothermal core

IO(E = the reactivity effect of 13$"

IiSAM = the reactivity effect of l4eSm and tuts*

KFP = the reactivity effect due to buildup ofnonsaturating fission products andother long-term isotoPic changes(e.g., Pu buildup, U isotopic chang€sr

%i burnout in fuel, toB burnout ingraphite)

KU23 5 - the reactivity eff ect of chatses in theconcentration of 235U and 233U

KROD = the reactivity effect of the controlrods relative to a baseline condition.

If the deviation, KRES, exceeds preset limits(usualty t 0.25V0 6h /h) tfre computer automaticallyannunciates the fact. In any event the completebalance is printed out hourly for the guidance ofthe reactor operators. The results are used towatch for unexpected short-term changes in re-activity and also to study the long-term behaviorof the reactor. The precision of the results hasbeen excellent, with the scatter being generally<0. 02Vo 6h /k No significant deviations from thee>ipected short-term behavior have been observed.

A major term in the on-Iine reactivity balanceat power is the description of ttsxe poisoning.Xenon and other noble gases are only slightlysoluble in the molten salt and therefore tend totransfer into any gas phase in contact with thefuel. Contact with the helium cover gas is pro-vided by the 50- gaI/ min spray in the fuel-pumpbowl, which not only produces salt spray in thegas space but also carries bubbles of helium intothe salt, some of which enter the circulating loop.Atthough most of the t35xe is swept out into theoff-gAs system, some diffuses into the pores of the

r3rNUCLEAR APPLICATIONS & TECHNOLOGY VOL. B FEBRUARY 1970


core graphite and some remains in the salt until itdecays or captures a neutron. The various com -peting mechanisms operate to gtve a xenon poi-soning in the MSRE around 0.3V0 6k/h, only + to* of the value that would exist if all the xenonremained in the fuel salt. The reactivity measureof xenon poisoning has been confirmed by deter-minations of the ttuK"/ttnxe ratio in off -gas sam -ples which show the effect of neutron captures intsxe.

The steady-state xenon poisoning in the MSREvaries somewhat with system temperature andpressure and, to some extent, with the level ofsalt in the fuel-pump tank. These variations areaeeompanied by small changes in the volumefraction of circulating helium bubbles in the fuelsalt, and it appears that they are caused bychanges in gas-stripping efficiency in the pumptank. The correlation between stripping efficiencyand the various operating parameters has not beenfully described. Instead, the on-Iine calculation ofxenon poisoning uses average values for strippingefficiencies and transfer coefficients obtained em -pirically from analysis of many xenon poisoningtransients.

Reactivity balance calculations are done onlywhen the reactor is critical with the fuel circu-lating, so the reactivity effects of circulation arepart of the baseline condition. In the operationwith 235U, circulation reduced reactivity byO.zl%o 6h/k due to the loss of delayed neutrons andonly 0.02Vo \h/h or less due to the effect of gasbubbles entrained in the fuel. Since the saltprocessing, howeverr g&s bubbles are carrieddeeper into the salt in the pump bowl by the xenonstripper jets and more gas gets into the loop. Asa result, the void effect attending circulation inthe recent operation has amounted to around 0.25V06h/h. An exception has been during operation atreduced pump speeds when the circulating bubblesare eliminated by a 15% reduction in speed. Thereactivity decrease due to loss of delayed neu-tron amounts to -0.09V0 6k/k with the 233U fuel.

The long-term reactivity behavior is mostaccurately defined by measurements made in theabsence of t35xe poisoning. During the '*U op-eration, such measurements showed the reactivitydecreasing very slightly more than predieted"(The deviation was -0.05V0 6h/k at the end of g006equivalent full-power hours. ) After the end of the'*U operation, a careful reexamination of thecalculation revealed small errors in some of theIong-term effects. At about the same time a morereliable value for the heat capacity of the coolantsalt was obtained which shifted the heat-balancepower of the reactor from 7.2 to 8 VIW, thusincreasing the calculated burnup and fission-product inventories. When these errors were

corrected the observed-calculated reactivity de-viation showed a gradual increase to + 0.2V0 \h/kin 9006 equivalent fulI-power hours of operationover a period of more than 2 years. Although ithad been anticipated that distortion of the coregraphite by neutron irradiation might have apositive reactivity effect, tro attempt had beenmade to predict the effect quantitatively becauseof the limited amount of data originally availableon MSRE graphite. After the '*U shutdown, withthe benefit of more data on graphite distortionr w€evaulated this effect and found that at most itcould aceount for only a small part of the calcu-lated reactivity drift. The drift must, therefore,be assigned to inaccuracies in calculating otherIong-term effects on reactivity.

In more than 2000 equivalent full-power hoursof operation with "tU, there has been no distin-guishable long-term trend in the deviation betweenobserved and predicted reactivity.

Reactor Dynamics

The MSRE is stable and self-regulating withregard to changes in heat load, with a responsethat becomes quicker and more strongly dampedas the power level is increased. Responsible inlarge part for this behavior are the strong nega-tive temperature coefficients of reactivity associ -ated with both the fuel salt and the graphitemoderator. When the reactor is operated at lowpower, because of the very large temperaturedifference between the salt and air, only a verysmall area of the secondary heat exchanger needbe e4posed. This introduces a long time constantin the system thermal equations and produces arather sluggish system at low power.

Loss of delayed neutrons by precursor decayoutside the core significantly reduces the effectivedelayed-neutron fraction in the MSRE. In fact,with the salt circulating, the effective fractionsare 0.0045 and 0.001? for the '*U and 233U fuels.Despite these low fractions, the system responseto perturbations is quite acceptable with eitherfuel.

The dynamic behavior of the MSRE was ex-tensively examineci, by theoretical techniquestubefore the reactor was operated and by e>rperi -mentstG during the operation. Calculations hadindicated that the reactor would be inherentlystable at all power levels and that the degree ofstability would increase with increasing power,and experimental measurements of the reactordynamic response agreed very closely with thepredictions. In addition, m e a s u r e m e nt s madethroughout the operation with 235U fuel showed thatthere was no change in dynamic behavior withtime.


Similar theoretical and experimental evalu-ations were made of the dynamic behavior with233U fuel. The calculations indicated that, despitethe lower delayed-neutron fractiotr, the reactorstability would be greater with 233u-due primarilyto the larger negative temperature coefficient ofreactivity of the fuel salt. Drperimental mea-surements of system transfer functions and tran-sient response are in good agreement withpredictions.

Because of the good self -regUlating character-istics of the MSRE, the system is quite simple toc ontrol. In more than 1 5 000 h of c ritic aI op -eration, not once have the nuclear pow€rr period,or fuel temperature gone out of limits so as tocause a control-rod scram.

Equipment Performance

Some of the MSRE equipment items have un-usual f eatures but none differ radically fromcomponents tested and proved before the MSRE.In addition to conservative design, great care wasexercised in quality assurance for every part ofthe primary systems. These factors aecount inlarge measure for the high degree of reliabilityattained by the MSRE.

Salt htmps. The fuel and coolant salts are circu-lated by centrifugal pumps of conventional hydrau-Iic design. The vertical shafts have oil-lubricatedbearings above the salt surface. Oil leakage pastthe lower seal is drained out of the pump housingand a split purge of helium into the shaft annulusprevents oil vapors from going down into the pumpbowl or gaseous fission products from coming upthe shaft. The two original pumps are still per-forming satisfactorily. The fuel pump has circu-lated salt for more than 19 400 h; the coolantpump, more than 23 500 h.

The only problem with the salt pumps has beenwith oil leakage into the pump bowl. There isevidence that in the fuel pump a small amount of

oil (on the order of 1 cmt/a^y) Ieaks from the sealdrainage passage, past a gasketed joint, and intothe pump bowl. The oil does not react with thesalt or affect it in any way, but the oil is ther-mally decomposed and its products are drawn intothe off-gas line where they have caused problemsas discussed below. Spare pump rotary elementshave a seal weld that prevents oil leakage, but theoff- gas problems have been adequately handledotherwise, obviating the need for pump replace-ment.

Off-Gas System" The primary function of the fueloff- gas system is to discharge cover gas fromwhich radioactive materials have been filtered or


allowed to decay. Tests have shown that the timesrequired for krypton and xenon to work their waythrough the zA}-ft-long charcoal beds are at least7 and 90 days, respectively. As a" result, therehas been no significant activity in the effluentother than l0-year 85Kr.

The problem with the oil decomposition prod-ucts first appeared when the power was raised to1 MW. A light fraction that had condensed in theholdup pipe was vaporized by the heat from thefresh fission-product gases and the intense betaradiation transformed it into a cross-linked poly-mer that ptugged a small sintered-metal filter and

a throttling valve and partially plugged the steelwool in the entrances of the charcoal beds. AIarge fitter, consisting of coarse stainless-steelmesh, sintered fibrous-metal filter media, andFiberfrur, was designed and proved quite effec -tive. The first unit developed excessive pressuredrop when the trapped fission products raisedinternal parts of the filter above 1200'F, causingthe inlet pipe to expand against the filter medium.No more troubles of this sort were experiencedafter a revised unit was instalted in January 1967.

SaIt mist is produced in the pump bowl by thexenon stripper spray and some drifts into theoff- gas line where it freez es into tiny beads (1 tollt diam). The rate is very low (probably a fewgrams per month) but the frozen mist doesgradually buitd up in the off- gas line at the firstcool point just outside the heated enclosure aroundthe pump bowl. Accumulations there were roddedout in 1966, ir April 1968, and in December 1968.

Intermittent heating at the point of accumulationproved effective in a similar situation in a salt-pump test loop and a small heater unit that can be

installed remotety in the MSRE has been de-signed and tested.

Heat Exclmngers. MSRE operation has shown thatconventional design calculations adequately predictheat transfer in molten-salt heat exchangers andthere is no change in heat transfer over an ex-tended period of operation. In the primary heatexchanger, which is of conventional shell-and-tubedesign, the predicted overall heat transfer coeffi-cient is 600 ntu/h-ftz-oF and that observed is656 r 15 gtu/h-ft2-oF. Values measured overmore than 15 000 h of salt circulation since thebeginning of power operation show no detectablechange in the coefficient. In the air-cooled secon-dary heat exchanger the overall coefficient, whichis very close to the air-side coefficient, proved tobe lower than the design value by '-'21Vo- Part ofthe reason for the disparity wa^s traced toan errorin the choice of air properties used in the designcomputation. Even after correction of this error,the design coefficient of 51.5 Btu/h-ftz-"F is still

NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 19?O r33


higher than the observed value of 4 z"B Btu/h -ftz -oF. This discrepancy is most likely attributableto inapplicability of the basic heat transfer corre-lation used in the design to the specific geometryof the air flow through the MSRE heat exchanger.

The tubes in the primary heat exchanger arewelded and back-brazed into the tube sheet. Inthe secondary heat exchanger, the tubes arewelded to stubs forged in the headers. There hasnever been any leakage in either exchanger.

The doors on the secondary -heat-exchangerenclosure warped during prenuclear testing at1200'F so that they could not be opened. Reviseddoors with the structural members protected fromhigh temperatures did not warp. The gasketedclosure was not perfect at first and air inleakagecaused salt to freeze in several of the lZ0 tubeson two occasions when the coolant-salt circulationstopped and the doors were shut. The tubes werethawed without damage, using the heaters builtinto the enclosure (the coolant-salt densitychanges very little on freezing and thawing). Asegmented metal strip pressing against an asbes-tos-filled braided Inconel gasket gave a tight doorseal and no more freezing has occurred.

Salt Samplers. The samplers on the fuel pump,the coolant pump, and the processing tank are a1lsimilar. A motor-driven windlass in a" shieldedeontainment enclosure is used to lower and r€-trieve capsules latched to the end of a drive cable.Isolation valves close off the sampler tube beforea sample is withdrawn from the containment €o-closure. The samplers have operated very wellexcept for two occasions when a capsule beinglowered in the fuel sampler hung up, causing thedrive cable to tangle. Both times the capsule wasaccidentally dropped down into a cage in the pumpbowl. Efforts at retrieval were unsuccessful Twoother brief delays in operation were occasioned byelectrical shorts in the wiring penetrating thecontainment enclosure to the drive motor. Onanother occasion, loose setscrews in a drive gearrendered the sampler inoperative until repairswere made. Altogether the three salt samplershave been. used for over b80 sampling operationsand the addition of l4l uranium enriching cap-sules. Never has there been any significantrelease of activity into the operating area.

other EEtiQment" The unique flexible control rodsand drives developed for the MSRE proved satis-faetory. In four years after the beginning of nu-clear operation, no control rod ever failed toscram when requested. One rod hung one time onJune L, 1969 and the trouble was traced to gallingon an air tube which slides inside the hollow rod.

The "fteeze valvesr" which take the place of me-chanical valves in the salt piping, operated asintended after the cooling-air controls were prop-erly adjusted during the prenuclear start- up of thereactor. The valves in the transfer lines are slowto use, however, because a gas-tight seal requiresmolten salt on the high-pressure side of thef roz en plug and sometimes this takes severalhours to set up. The fuel and coolant drain valvesoperate reliably, with thaw times between 10 and15 min. Removable heaters on the salt piping andvessels in the reactor cell have had a lowincidence of failure: only 4 units of ,-,92 have everbeen out of service. Atthough not a part of theprimary systems, the positive-displacement blow-ers used to circulate cell atmosphere for coolingthe control rods and fteeze valves are essential toreactor operation. Failure due to bett slippag€rlubricating oil leakage, and a short in the powerwiring have shut the reactor down at varioustimes. These problems have been corrected andthe blowers are now considered to be quitereliable.

Containment

The MSRE design aimed at zero leakage fromthe system of piping and vessels that is theprimary containment for the fission products. rnadditioD, a secondary containment system wasprovided to limit the release of fission products tothe environs in the event of a failure in theprimary containment. Stringent leakage criteriawere set for the secondary containment by theassumption that the fuel salt system and thewater-cooled thermal shield could rupture simul-taneously, mixing all the salt with the amount ofwater that would grve the maximum steam pres-sure in the cell. This was calculated to be 39 psigand the reactor cell and drain-tank cell arerequired to leak at a rate {l%o per day at thispressure.

The MSRE has met the criterion for primarycontainment. By the use of welded constructionwith a minimum of gasketed joints, and thosepressure-buffered, zero leakage has been attainedduring all periods of operation" Annual tests androutine measurements of air leakage into thereactor cell during normal operation at -2 psigindicate that the reactor has always operatedwithin a satisfactory seeondary containment. ontwo occasions in 1966, the reactor was shut downwhen a high air leakage rate into the reactor cellwas measured. The sources proved to be leaks inpressurized thermocouple penetrations and valqeoperator air line disconnects. In neither case didthe leaks constitute an open path for outleakagehad the cell been pressurized.

APPLICATIONS & TECHNOLOGY VOL. I FEBRUARY 19?0134 NUCLEAR

Maintenance of Radioactive Systems

For the molten-salt reactor to be accepted as apractical possibility for commercial power, itmust be maintainable at a reasonable cost. De-velopment and demonstration of safe and efficientmaintenance is therefore a prime objective of theMSRE. The reactor equipment was designed and

taid out with an eye to maintenancer and philos-ophy, methods, and tools were developed in ad-vance for all conceivable jobs of radioactivemaintenance. Many of the most important jobs(fuel-pump rotary element removal, for example)were practiced before the system became highlyradioactive.

Although there has been tittle trouble with themajor components of the reactor, enough jobs

have involved working in the reactor cell tothoroughly test the maintenance Scheme. Fourtimes the specimen array in the core has been

replaced. The fuel off-gas line has been rodded out

and a removable section replaced. Off- gas systemvalves and filters have been replaced. Eighteendisconnects in service air lines in the reactor cellwere replaced because of air leakage. Two largeheater units surrounding the primary heat ex-changer were remov€d, repair€d, and replaced.AII of these jobs were done with long-handledtools working through penetrations in the mainte-nance shield. Such work generally takes severaltimes as long as similar tasks done directly, but

with experience and with the help of detailedprocedures, it has proved possible to estimatetime requirements for remote maintenance as

for conventional jobs.Through careful planning and use of temporary

containment enclosures, radioactive contaminationhas been successfully controlled and cleanup dur-ing and after radioactive jobs has required onlyvacuuming and mopping or wiping. Exposure ofpersonnel to radiation has been held well belowpermissible limits: the maximum exposure of any

individual in any quarter has been <0.5 rem.

coNcLUsloNs

The MSRE operated long enough with 235U to be

a good test of the practicality of molten-saltreactors. Some of the highlight dates and statis-tics from this period are listed in Table III. Inretrospect, the 30-day run that started in De-cember 1966 was the real beginning of the sus-tained power operation phase of the test program.Up to that time sundry problems related to start-uphad prevented any long run. After that pointr overthe next 15 months until the shutdown, the reactorwas critical 80Vo of the time. Counting four weeks


that were spent on core specimen replacement,the reactor was available f or planned experi -mental work f or 87Vo of that period. This isobviously a good record, but when measuredagainst the yardstick of other reactors in a

comparable stage of development, it is seen to be

indeed remarkable.From the months of operation and experiments,

a very favorable picture emerged. In properlydesigned equipment, handling the high-melting saltproved to be easy. Maintenance of the radioactivesystems was not easy, but there were no unfore-seen difficulties, and control of contaminationwas, if anythingr less difficult than expected. FueIchemistry and materials compatibility lived up toe>rpectations, showing no adverse effects due tothe reactor environment. The noble gases, xenonand krypton, were stripped efficiently. It wasfound that the noble-metal fission products, whosebehavior had hitherto been uncertain, partiallyplated out and partially came off as a smoke, thusproviding important information for the design offuture reactors. Although the operation of theMSRE showed that the design of some equipmentand systems could be improved, key componentsperformed well.

The on-site removal of the original uraniumfrom the fuel and the loading of the 233U into thestripped carrier salt extended the usefulness ofthe MSRE and the simplicity and efficiency ofthese steps illustrated one of the virtues of thefluid-fuel, molten-salt system.

TABLE III

Important Dates and Statistics in Operation of the MSRE

Dates

Satt first loaded into tanks October 24, Lg64

Salt first circulated through core January 12, 1965

First criticality June 1' 1965

First operation in megawatt range January 24, t966Reach full power MaY 23, 1966

Complete 30-day run January 14' 1967

Complete 3-month run APril 28, 196?

Complete 6-month run March 20, 1968

End nuclear operation operation with '*U March 26, 1968

Strip U from fuel carrier salt August 23-291 1968

First critical with 233U October 2' 1968

First operation at significant powerwith 233U October 8' 1968

Reach full power with 233U January 28' 1969

Statistics

Critical hoursIntegrated power, MW(th) hEquivalent full-Power hoursFuel pump circulating salt, hCoolant pump circulating saltr h

23su a zssg b Totalb

11 51572 44LI 006

L5 04216 906

3 91020 3632 5494 3636 660

t5 42492 80511 55519 40523 566

asalt circulation times include prenuclear testing.bThrough Jrure 1, 1969.

NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 1970 r35


The nuclear start-up experiments and operationat power confirmed the adequacy of the data andprocedures used to predict the nuclear behavior.The system _was quite stable and easy to eontroleven during 233U operation with a delayed neutronfraction lower than in any other reactor. Finally,burnup over an extended period at high powershould yield very accurate information on 233u

cross-section ratios in a neutron enerry spectrumtypical of molten-salt reactors.

A net result of the MSRE operation is enhancedconfidence in the practicality and performance offuture molten-salt reactors.

ACKNOWLEDGiIENTS

This research was sponsored by the u. s. AtomicEnergy Agency commission under contract with theUnion Carbide Corporation.

REFERENCES

1. M. W. ROSENTHAL, P. R. KASTEN, and R. B.BRIGGS, "Molten- Salt Reactors-History, Status, andPotential," Nucl. Appl. Tech., 8, 102 (1970).

2. R. c. ROBERTSON, "MSRE Design and operationsReport, Part I-Description of Reactor Design,t' ORNL-TM-278, oak Ridge National Laboratory (January 196b).

3. w. H. cooK, "MSRE program semiannual pro-gress Report, July 31, 1964,r, ORNL-320g, pp. BZB-Bgg,Oak Ridge National Laboratory.

4. 'w. R. GRIMES, "Molten-salt Reactor chemistry,t'Nucl. Appl. Tech., 8, 137 (1970).

5. A. TABOADA, "MSRE Program semiannual pro-gress Report, July 31, LgG ,r' ORNL-3?08, pp. gB0-972,Oak Ridge National Laboratory.

6. R. B. BRIGGS, "Effects of Irradiation on the Ser-vice Life of the MSRE,t' Trans. Am. Nucl. Soc., 10, 166(1965).

7. T. L. HUDSON, "Design and Operation of the 1200"FHeating System for the MSRE," Trans. Am. NucI. Soc.,8, L47 (1965).

8. R. B. LINDAUER, "MSRE Design and OperationsReport, Part vII-Fuel Handling and Processing plant,ttORNL-TM-907 Rev., Oak Ridge National Laboratory(December L967).

9. R. B. LINDAUER and C. K. McGLOTHLAN, ,,De-sign, Construction, Development, and Testing of aLarger Molten Salt Filter,r' ORNL-TM-2478, Oak RidgeNational Laboratory (February 1969).

10. J. M. CHANDLER and S. E. BOLT, "Preparationof Enriching salt ?LiF-233 UFe for Refueling the MoltenSalt Reactot," ORNL-437L, Oak Ridge National Labora-tory (March 1969).

11. H. E. McCOY, '3MSRE Program Semiannual Pro-gress Report, August 31, 1968,t' ORNL-4344, pp. 2LG-223, Oak Ridge National Laboratory.

L2. R. B. LINDAUER, " Processing of the MSRE Ftushand Fuel Salts," ORNL-TM-2578, Oak Ridge NationalLaboratory (July 1969).

13. B. E. PRINCE, S. J. BALL, J. R. ENGEL, P. N.HAUBENREICH, and T. W. KERLIN, "Zero-PowerPhysics Experiments on the Molten-Salt Reactor Ex-periment," ORNL-4233, Oak Ridge Nationat Laboratory(February 1968).

L4. J. R. ENGEL and B. E, PRINCE, "The ReactivityBalance in the MSRE," ORNL-TM -1796, oak RidgeNational Laboratory (March 10, L}GT).

15. s. J. BALL and r. w. KERLIN, "stability Analysison the Molten-Salt Reactor Experiment,t, ORNL-TM-1070, oak Ridge National Laboratory (December 1g6b).

16. T. 'W. KERLIN and S. J. BALL , ,,ExperimentalDynamic Analysis of the Molten-salt Reactor Experi-ment," ORNL-TM-L647, Oak Ridge National Laboratory(October 1966).

f36 NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 19?O

EXPERIENCE WITH THE MOLTEN-SALT REACTOR EXPERIMENT

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