ATOMIC ENERGY ^JfiJS L'ENERGIE ATOMIQUE OF CANADA … · .Slieriil;iii J'jrk Rcsciircli...

AECL-5704

ATOMIC ENERGY ^ J f i J S L'ENERGIE ATOMIQUEOF CANADA LIMITED t f i j DU CANADA LIMITEE

CANDU CHANNEL DECAY POWER

by

A.C. WHITTIER, D.W. BLACK and C.R. BOSS

Power Projects

Sheridan Park Research Community

Mississauga, Ontario

January 1977

CANDU CHANNEL DECAY I'OWER

by

A.C. WhiltierDAY. BlackC.R. Boss

Atomic Energy of CanadaPower 1'rojccls

.Slieriil;iii J'jrk RcsciircliMississiiuLi;i. Onknio L5K I B2

Junuary 1477

AECL-5704

LA PUISSANCE DE DECROISSANCE DANSLE CANAL DE COMBUSTIBLE CANDU

par

A.C. WhittierD.W. BlackC.K. Boss

RESUME

La decroissanee de la puissance des oinaux do combustible CANDU (Canada DeuteriumUranium) a etc calculec a I'-He du code ORIGEN pour la clialcur de decroissanee desproduHs de fission, des actinides et des elements de structure et a Taide du code ANISN pourlc transfert d;1 la puissance a I'interieur du canal de caloportage. On a suppose quo la puissancede fission to tv bait a zero uu moment zero, instantanement. L.cs calcnls ORIGliN de lapuissance due a la decroissance des produits de fission du -35 u Ont etc compares avecplusieurs experiences et la decroissance de puissance a etc eomparee avec la puissance due a ladecroissance calculee a Puide du code ANS 5.1.

L'L:ncrgie Atomiijue dti Canada, LimiteeCroupe Eilectronucleaire

Sheridan Park Research CommunityMississuugu, Ontario

Janvier 1977

AHCL-5704

CANOU CHANNEL DECAY POWER

by

A.C. WhiltierD.W. BlackC.R. Boss

ABSTRACT

The power rundown of C'ANDLI (Canada Deuterium Uraniuml fuel channels was calculatedusinu C/KKil-.N for the fission product, actinidc. ;ind structural clement decay heat, andANISN for (he power deposition wilhin the coolant channel, lission Power was assumed todrop insianleoLisly to zero al time zero. The ORIGMN calculations for --''-Hi fission productpower decay uere compared will) sc\eral experiments ami Ihe power ruiulown was comparedto the power decay calculated usin;j ANS 5.1.

Atomic Energy of Canada LimitedPower Projects

Sheridan Park Research CommunityMississauga. Ontario

January 1977

AI-CL-1704

TABLE OF CONTENTS

Puge

ABSTRACT

1. INTRODUCTION I

2. THEORY I

3. Ml 1 HOD 2

4. RESULTS 3

5. APPLICABILITY 3

<\ REFERENCES 4

APPENDIX'A' 13

APPENDIX'B' 17

APPENDIX'(•" 19

APPIiNDIX'D' ">3

LIST OF FIGURES

Page

Figure 1 CANDU Channel Power Rundown 5

Figure 2 i Second After Shutdown. 6

Variation of P/Po with Operating Time as a Function of BundleOperating Power (Fission).

10 Seconds After Shutdown. 7




105 Seconds After Shutdown. 1 1

Decay After Shutdown From ] Fission Per Second From -3->U for 15an Infinite Time. Comparison of ORIGEN with Experiment.

Decay After Shutdown From 1 Fission Per Second from -^\J 16for an Infinite Time

A Comparison of Channel Power Rundown Calculated Using: 24

(a) ORIGEN

(b) ANS S.I

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

3

4

5

6

7

A-1

A-2

D-1

CANDU CHANNEL DECAY POWER

by

A.C. Whiltier. D.W. Black and C.R. Boss

1. INTRODUCTION

There is a need for .a reference decay power curve for use in C'ANDU (Canada DeuteriumUranium) safety and other studies where power rundown is involved. Recent experimentalmeasurements have shown that the isotope generator program ORIGEN (I), using ENDF/B-1Vdata, gives quite reliable results for 235\j decay heat. Since this program accounts for Pu and238(j as well as 235u fission product buildup, it should give more accurate results for naturaluranium than can be obtained from the proposed ANS standard 5.1 which applies only to235u decay power.

A number of different types of channels exist in the various operating and projectedC'ANDU reactors - 8.3 cm (3.25 in.) channels containing both 7 and ' 9 element fuel, and 10.4cm (4.08 in.) channels containing 28 element and 37 element fuel. The decay power charac-teristics vary only slightly for different types of channels, so the present study is based on the10.4 cm channel fuelled with 37 element fuel. In fact, where needed, the data for the 1250MW(e)* reactor channels were used. The discrete ordimites program ANISN (2) was used tocalculate power deposition in the channel. The results of the study are plotted in Figure 1. Theapplicability of these results to other channels is briefly discussed at (he end of this report.

2. THEORY

When natural uranium is irradiated. 235u is consumed and fissile -39p u and -41 Pu areproduced. During the irradiation lifetime of CANDL1 fuel roughly half the energy output isproduced by Pu. Just before removal from the reactor, about 70?; of the fuel power isproduced by Pu. Radioactive fission products contribute about l'/i of the reactor power duringoperation: after shutdown this fission product power decreases at a rate that depends on theoperating time and cooling time.

The decay energy from a Pu fission is about 25/r less than that from a 235y fission. As aresult, the fission product decay power from natural fuel is a complicated function of operat-ing and decay time: a long irradiation time tends to increase the effect of the long-lived fissionproducts hence to increase the decay power at a given time after shutdown: however, highburnup, which is the result of long irradiation, means more Pu which tends to reduce the decaypower. Thus, for example, at 100 s after shutdown, the ratio of total decay power to total fissionpower during operation, P/Po, from a fuel bundle operated at 43 kW to 18.0 MW-h/kg U is1.04 times that for a bundle operated at 574 kW to 180.0 MW-h/kg U. A smaller effect of Puis due to the fact that a ^-^Pu fission yields 37c more energy than does a 235TJ fission: thiseffect further reduces the value of P/Po for irradiated natural uranium.

; el1, ctrical

A fuel channel will, in general, contain fuel with a large variation in burnup between oneend of the channel and the other. This burnup varies with time due to irradiation and due tofuel shifting. Since decay power depends on burnup, and since not oniy burnup but burnupdistribution varies from channel to channel, it is not possible to produce a single curve ofpower rundown that applies to all channels. It is possible, however, to calculate the highestvalue of P/Po of all the channels for various times after shutdown. The highest value of P/Pofor any given shutdown time is, of course, also a function of operating time.

In addition to the fission product decay heat, there is u substantial contribution from thedecay of -39(J and 239Np, the precursors of 239pu. While this amounts to only 0.5'.* of thepower during operation, it accounts for about 20'A of the decay power 104 seconds aftershutdown, and about 3Q'/r at 105 seconds.

Some heating is contributed by the decay of Zircaloy activities, but this amounts to lessthan l'/r of the decay power for the decay times of interest for this report.

3. METHOD

The fuel decay power was calculated using the computer program ORIGEN. The isotopicnuclear data used was based on ENDF/B-IV with recent modifications as recommended inENDF-230 (3) and in ENDF-223 (4). A comparison of calculation with measurement is givenin Appendix 'A', for decay power from 235(j fission products.

The rate of production of Pu, and the contribution of 238JJ fyst fissions to the fissionproduct and actinide (mainly 239ij und23(>Np for CANDU fuel) inventories are quite sensitiveto the neutron spectrum during irradiation. The spectrum is taken into account by three inputparameters.

THERM =

RES = r

.flux >l MeVFAST = 1.45 x

t;

thermal neutron flux'

where T, To, and r have their usual meanings for CANDU lattice physics (5). In addition, theeffect of spectrum and self shielding on the cross sections, in particular the U and Pu absorp-tion and fission cross sections, is taken into account by replacing the ORIGEN library crosssections by cross sections that are appropriate to CANDU lattices.

In the present study the U and Pu cross sections were calculated by LATREP (5) as afunction of irradiation, and effective values over an irradiation of 180 MW-h/kg U were used inORIGEN. LATREP used the 37 element 1250 MW(e) reactor channel us a model. Appendix'B' shows a comparison of the isotopic content as calculated by ORIGEN and as calculated byLATREP. The discrepancies will have a negligible effect in the decay power values.

ORIGEN calculates the beta decay power and a multi-group gamma decay power. Thespectral grouping is given in Appendix 'C\ In calculating the channel decay power, it wasassumed that all of the beta power was thermalized within the channel, and further, that all ofthe gamma photons with energies less than 0.2 MeV were similarly thermalized. These assump-tions introduce a negligible error and are conservative.

A series of calculations taking one gamma group at a time was run with ANISN, to get thespatial energy deposition in the lattice cell for each group. The cell was modelled as a series ofhomogenized fuel, cladding and coolant rings at the centre, surrounded by unnuli of coolant,pressure tube, void, calandria tube, and moderator. Reflecting conditions were used at thecylindrical cell boundary: in this way, account was taken for energy deposition from surround-ing cells. The gamma source data from ORIGEN were combined with the normalized energydeposition per gamma group from ANISN, to give the total decay power in the channel due tothermalization of all gammas of energy greater than 0.2 MeV. A summary of the ANISNcalculations is given in Appendix 'C .

4. RESULTS

Since the decay power of fission products for relatively short times after irradiation issensitive (o operating time, several calculations were run for various operating times at variousoperating powers. From these runs maps of P/Po vs operating time were plotted for shutdowntimes of 1 s, 10 s, 10- s, 10^ s, 10^ s, and 105 s, and are shown in Figures 2 - 7 . Using thesemaps, the decay powers for a high-powered and for a low-powered channel were calculated forvarious operating times and the six shutdown times. It was found, not unexpectedly, that theshort operating times give the highest P/Po for short shutdown times, and that long operatingtimes gave the greatest P/Po for times in excess of 100 seconds after shutdown. At 10 secondsafter shutdown P/Po for a fully irradiated high powered channel was about 4'/> less than achannel irradiated for only 25 days; at one day after shutdown the fully irradiated channelP/Po is 27'/; greater than the 25-day channel. From this analysis a set of highest values of P/Powas calculated (by "highest" is meant the highest value of channel P/Po in the reactor for eachtime after shutdown).

As discussed above, and in Appendix ' C , the variation of the ratio of channel thermalpower after shutdown to channel thermal power during operation, (P/Po), is obtained byadjusting P/Po for gamma escape from the channel both during operation and after shutdown.This was done, and the results plotted in Figure 1.

The main uncertainty is due to the uncertainty in experimental values and the dis-crepancy between calculation and measurement as discussed in Appendix 'A'. The dottedcurve in Figure 1 shows the upper limit to the expected highest power ratios.

It is of interest to compare the power decay curve of Figure 1 with one obtained usingthe proposed ANS Standard 5.1. This is done in Appendix 'D\

5. APPLICABILITY

Fuel geometry in a channel affects the pkitonium production. However, a comparison ofthe decay heat in a 37 element 1250 MW(e) reactor channel with that in a 28 elementPickering channel showed differences of only a few tenths of a percent.

Of greater impact is the channel diameter. But again the effect is small. (P/Po) values forthe 8.3 cm Douglas Point channel are about 37' lower than for a 10.4 cm 1 250 M W(e) reactorchannel.

Therefore, the curve in Figure 1 e;in be used for all present CANDU channels fuelled withnatural uranium. Since Pu decay heat is less than for 235u, the curve is conservative forPu-enriched CANDU channels. Coolants other than DiO. namely organic and ll^O. will nothave a significant effect on (P/P^).

If channel diameters are increased in the future. (P/Po) will increase: tin increase from10.4 cm (4 in.) to 12.7 cm (5 in.) will increase (P/Po) by about 3'i.

The calculations for this report assumed that cooLnt was present in the channel. Roughly\'.'< of the nuclear radiation is thermalized in the coolant. II'(here is no coolant present some ofthis will be absorbed in the fuel, cladding and coolant tubes, and some will escape to themoderator. Therefore, the energy deposition in the channel with no coolant is slightly lowerthan with coolant present.

6. REFERENCES

(1) M.J. Bell, ORIGHN - the ORNL Isotope Generation and Depletion Code. Oak RidgeNational Lab. report ORNL-4628. May 1073.

(2) W.W. Engle, Jr., ANISN - A One Dimensional Discrete Ordinates Transport Code withAnisolropic Scattering. Union Carbide Corporation report K-1693. March ll)o7.

(3) ENDF-230 Vol. I, Benchmark Testing of L-NDI/B-IV, Brookhaven National Laboratoryreport, BNL-NCS-211 18, March 1976.

(4) T.R. England and R.E. Schenter. ENDF/B-IV Fission Product Files: Summary of MajorNuelide Data, Los Alamos Scientific Lab. report LA-ollMvlS (ENDF-223). October1975.

(5) G.J. Phillips and J. Griffiths, LATRF.P User's Manual. Atomic Energy of Canada Limitedreport AECL-3857, September 1971.

(6) K. Shure, Fission Product Power Decay Energy. Westinghouse report WAPD-BT-24. ll>dl.

(7) ANS-5.1, Proposed American Nuclear Society Standard - Decay Energy Release RatesFollowing Shutdown of Uranium - Fuelled Thermal Reactors. October ll>7 I.

(8) A.M. Perry et al, Fission-Product Afterheat - A Review of Experiments Pertinent to theThermal-Neutron Fission of 235u, Oak Ridge National Lab. report ORNL-TM-4197.October 1973.

(9) P.M. Lolt et al. Mesures ealorimetriques de la puissance emis par les produits de fissiondu -35(j pour des temps de refroidissement eompris entre 70 secondes et 7- IO(> secoiules.Bulletin d'informations scientifiques et techniques. 181. May 1973.

CANDU CHANNEL POWER RUNDOWN

Fission power assumed 0 at t =0.

Plotted is the ratio of shutdownthermal power to operating thermalpower.

z

u. -057

1 SECOND AFTER SHUTDOWNVARIATION OF P/Po WITH OPERATING

TIME AS A FUNCTION OF BUNDLEOPERATING POWER (FISSION).

The fission power is assumed to fall tozero immediately after shutdown.

Burnup (MW-h/kg U) is shown in brackets

• Note Suppressed Zero

.058 g

280 360

OPERATING TIME, DAYS

10 SECONDS AFTER SHUTDOWNVARIATION OF R/Po WITH OPERATING



Burnup (MW'h/kg U) is shown in brackets.

100 SECONDS AFTER SHUTDOWNVARIATION OF P/Po WITH OPERATING



Burnup (MW-h/kgU) is shown in brackets.

rj i i i i i i f r i i i i i i i i i i i i i i n i i 11 H T :

F 1000 SECONDS AFTER SHUTDOWN '-_ \E VARIATION OF P/Po WITH OPERATING ; yC TIME AS A FUNCTION OF BUNDLE ± ;F OPERATING POWER (FISSION). H

r The fission power is assumed to fall to ; 4 ;\ zero immediately after shutdown. ; -

". Burnup (MW-h/kgU) is shown in brackets. - -

1—I • 1 1 1 1 1 1 1 - • «• A 1 • 1 I * I - 1 1—1 • i. I • • • • • • • * • • | • | |




Burnup (MW-h/kgU) is shown in brackets.




Burnup (MW-h/kgUj is shown in brackets.

13

APPENDIX A'

Comparison of ORICiEN with Measurements

The major source of uncertainty in the decay power calculations is in (he fission productdecay heat calculations of ORIGEN.

After analysing ineasurements available, K. Shure constructed a decay heat curve lor235|J fission products in 1%1 (6). This was later used as a proposed ANS Standard (7). Itcovered the range 10"' to 10'-* seconds after shutdown from infinite operation. The un-certainty limits given for the curve were quite large, as follows:

Cooling Time, seconds Uncertainty

t < io 3 +:o',;. 40',

i o 3 < t < io 7 +io';;. :o',

t > i o 7 +25',;. soy

In I''73 A.M. Perry carried out a statistical analysis of a number of beta and gammadecay measurements (8), and proposed a -35 LI decay curve on more recent data than wereavailable to Shure. Perry's values were a few percent above Shure's for all times exceptbetween 10 and 100 seconds after shutdown. Perry's curve covered the range 1 to 10° secondsafter shutdown following irradiation for an infinite time. His analysis gave one standard devia-tion (1 a) limits of about Tit over most of the time range and about 10'-; at the upper end. IO(l

seconds.

In 1973 Lott published the results of a calorimetric measurement of -35(j decay lieal fordecay times of 70 to 7-10̂ > seconds after a single fission (°). Me claimed an uncertainty of lessthan +5'// over the whole time range. Since these measurements were carried out to a relativelylong decay time, they can, with a negligible increase in uncertainty, be converted to the decayheat after infinite irradiation and hence, compared to the ANS Standard and to Perry's analysis.

In Figure A-l are plotted the values obtained by Shine, Perry and Lott. In order to showthe relationships and uncertainties with good resolution over the time range of I to l()7seconds, the decay heat values (MeV per fission) have been multiplied by T..0.23. (Figure A-2shows the unmultiplied values versus time. To get P/Po divide by 200 MeV per fission.) Alsoplotted is the curve calculated by ORIGEN.

Uncertainty flags have been put on Lott's measurements only where these measurementswere made, and on Perry's values at shorter times. The very wide limits on Shlire's points, andthe range on Perry's, result in substantial overlapping of all these results over the entire rangeof decay times.

14

While the ORIGEN curve is within the experimental limits except for ti?nes less thanabout 100 seconds where it falls a bit below Perry's 1 o range, it is a fact that it is generallybelow the experimental values, especially at short decay times. Tnis is due to lack of data forsome short-lived fission products in the ENDF/B-IV file.

In the assignment of uncertainty in the present analysis, it was assumed that a reasonableupper limit to the 235|j fission product decay power is given by adding S7< to Lott's measuredvalues for ts >103 seconds. In the vicinity of 10- seconds the upper limit was set at the misvalue of the upper limits of Shure (+10%), Perry (+6.9%) and Lott (+5'/i). For shorter timesthe upper limit was taken to be Perry's derived values plus la (ref (8) p41).

DECAY AFTER SHUTDOWN FROM1 FISSION PER SECOND FROM235u FOR AN INFINITE TIME

Comparison of ORIGEN withExperiment

41 Legend -

4 ORIGEN ;

1 A Lott (9) '•

; O Perry (8): • ANSSTD5.1 (6) (7)

17

APPENDIX 'B'

Heavy Element Composi'on ot mel

The fission product decay power depends, to some extent, on the ratio of plutoniuni touranium in the fuel. The following table compares the isotopic content calculated by LATRHPand by ORIGfcN for a 37 element C'ANDU fuel bundle in a 4 x 1250 MW(e) reactor channel,irradiated to 1 SO MW-h/kg U at a power of 574 kW (fission).

TABLEB- 1

Isotopic Composition g initial Kg V

LATREP ORIGEN % discrepancy

-•11

*">Pu

-40Pu

241pu

2.260

2.469

1.054

0.202

2.19X

2.415

1.078

0.225

2..S

+ 2.3

+ 11.4

19

APPENDIX C

ANISN Calculations

The fraction of the fission product gamma energy deposited in the fuel channel wascalculated using the discrete ordinates transport code ANISN (2). The energy grouping wasbased on the OR1GHN system with a slight modification. The photon source energies weredivided into 1 2 groups, the scattered energies into 19 groups. The source groups correspond tothe I 2 groups used in OR1GKN for the fission products; the lowest energy group of the twelveis divided into seven subgroups corresponding to the ORIGEN grouping for the actinides.Finally, there is a lowest energy group added as a "sink group" to minimize a peculiarity ofANISN that allows some loss of energy from the lowest group without being accounted for.

The grouping used is shown in Table C-l.

The photon source distribution within the fuel regions of the coolant channel was takenfrom a WIMS calculation.

The energy deposition in various components of the lattice cell was calculated for each ofthe twelve source groups, normalized to one photon per group.

Reflecting conditions were used at the cell boundary, so that radiation from neighbouringcells was accounted for.

The results for a series of P3 S16 ANISN calculations are shown in Table C-2.

The gamma decay power in each group is multiplied by the fractions given in the columnentitled "Total Within Coolant Tube" in Table C-2 to give the gamma decay thermal power ina channel. A sample calculation for a single bundle in a channel is shown in Table C-3. Here theratio of gamma decay power from the bundle to the total fission power from the bundle ismultiplied by the Table C-2 fractions to give the ratios of gamma thermal power after shut-down to total fission power during operation; these ratios are given in the last column of TableC-3. To the sum of the gamma thermal power ratios are added the ratio of beta decay power tooperating fission power, the ratio of actinide decay power to operating fission power andfinally the small contribution from structural elements. This sum, 0.02943, is the ratio ofthermal power generated in the channel by the bundle at 100 seconds after shutdown to thetotal fission power during operation. To get the ratio of thermal power after shutdown tothermal power generated within the channel during operation the sum must be multiplied bythe ratio of total fission power during operation to thermal power generated in the channelduring operation, 1.058:

thermal power in channel after shutdown; :—; rr— :— = 1.058 x.02943 = 0.0311

thermal power in channel during operation

20

TABLE C - 1

Gamma Energy Groups Used in ORIGEN

Group Lower Energy Bound MeV Mean Energy MeV

Source Groups

1 5.0 5.25

2 4.5 4.70

3 4.0 4.22

4 3.5 3.7

5 3.0 3.2

6 2.6 2.75

7 2.2 2.38

K l.X ].<><>

" 1.35 1.55

10 0.9 1.10

11 0.4 0.63

12 0.2 0.3

Low Energy Scattered Gamma Groups

1 0.25 0.3

2 0.175 0.2

3 0.125 0.15

4 0.075 0.1

5 0.050 0.06

6 0.035 0.04

V 0.020 0.03

Sink Group

0.010

TABLEC-2

Fractional Gamma Heat Distribution

TABLEC-3

Ratio of Power after Shutdown to Operating Power

GammaSourceGroup

1

3

4

5

6

7

8

<)

10

11

Operating

Operating

Burnup

Cooling T

Decay 7 Power

Operating Fission Power

0.2X

0.60

1.76

0.58

8 J 4

ZJ3

9.48

6.52

21.1

32.9

52.1

11.4

Time = l!5cl

Power = 574 kW (fission»

84.00MW-h/kg(J

ime = 100 s

Fraction of 7Power Absorbedin Channel

0.796

0.790

0.7X2

0.776

0.769

0.76.5

0.761

0.765

0.777

0.K14

0.8KS

0.970

Decay 7 Power Absorbed (

\ 1 uOperating Fission Power

0.22

0.47

1.3K

0.45

6.41

2.54

7.21

4.99

16.4

26.N

46. ;>

1 1.1

14K.4 1 24.2

Add- Fission Product Mela decay 125..1

Aclinide ik'cay 44.1

.Structtirai I'lcmont decay 0.7

294..:1

23

APPENDIX D'

In the USA the Nuclear Regulatory Commission regulations 10 CRFR 50.46 requirepower rundown to be calculated using the proposed ANS Standard 5.1 values multiplied by1.2. This is more appropriate for light water reactors than tor CANDU's since virtually all ofthe fission energy of the former is carried by the coolant and also since in LWR's a muchgreater fraction of the fission products come from -35u than is the case in CANDU's.

The ANS Standard 5.1 does not separate beta and gamma decay energies: however, thework on which it is based (6), does. Using (6) a power rundown curve was constructed and isshown in Figure D-l, along with its 20'/? upper limit. This curve includes contributions fromthe aetinides and structural decay.

Also shown, for comparison, is the Figure 1 curve.

qOI 01

Tfrl

iOZ + l " S SNV

l ' S SNV

pue'sapiuuoe sapnpui - L'9 SNV (M) i

sionpojdsapnpui • N39IHO (e)

:ONISnb3MOcn3NNVH3 JOIMOSiaVdlAlODV ,--

«Olfflf^ 4O

The International Standard Serial Number

ISSN 0067-0367

has been assigned to this series of reports.

To identify individual documents in the series

we have assigned an AECL-number.

Please refer to the AECL-number whenrequesting additional copies of this document

from

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