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Nuclear Engineering and Design 249 (2012) 419– 431

Contents lists available at SciVerse ScienceDirect

Nuclear Engineering and Design

j ourna l ho me page: www.elsev ier .com/ locate /nucengdes

evelopment of sodium leak detectors for PFBR

.I. Sylvia ∗, P. Vijayamohana Rao, B. Babu, K. Madhusoodanan, K.K. Rajanndira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India

r t i c l e i n f o

rticle history:eceived 2 November 2011ccepted 16 April 2012

a b s t r a c t

The 500 MWe Prototype Fast Breeder Reactor (PFBR) is under advanced stage of construction atKalpakkam near Chennai in India. The wide and high operating temperature, highly chemically activenature of sodium and its reaction with air make the sodium instrumentation complex over the conven-tional instrumentation. Over the years, traditional instruments such as wire type leak detectors, sparkplug type leak detectors were developed and used in different sodium systems. The redundant and diverseleak detection method calls for development of special instrumentation for sodium systems which includesodium ionization (leak) detector for detecting minute sodium leak in addition to those systems basedon mutual inductance principle. For detection of sodium leak from reactor Main Vessel (MV), diversemethods are used such as miniature, remotely locatable, Mutual Inductance type Leak Detector(MILD)and specially modified spark plug type leak detector. The design of MILD is suitably modified for detectingleak in double wall pipes and Diverse Safety Rod drive Mechanism (DSRDM). Steam/water leak in steamgenerator produces hydrogen and leads to high pressure and temperature in the system. Rupture discis used as a safety device which punctures itself due to sudden pressure rise. To detect the discharge of

sodium and its reaction products at the downstream of the rupture disc due to bursting of the rupturedisc, cross wire type leak detector has been designed, developed and tested. The selection of the leakdetection system depends on the location where leak has to be detected. This paper describes the recentdevelopments carried out in leak detector systems to meet the challenges involved in developing robustsensors for PFBR including miniaturization of leak detectors and the ability to locate or withdraw themremotely through a long guide tube with complex profile.

. Introduction

Sodium, because of its good heat transfer and nuclear prop-rties, is used as coolant in fast reactors. Liquid sodium meetslmost all the requirements of a fast-reactor coolant with its highhermal conductivity, reasonable specific heat, low neutron mod-ration and absorption, and high boiling point at near atmosphericressure. Different types of leak detectors are used in PFBR foretecting sodium leak. As sodium is good conductor of electricity,ensors based on electrical conductivity are found very attractiveor sodium leak detection system. Different types of leak detectorsre used for detecting sodium leak in single wall pipelines, dou-le wall pipelines, tanks and components in PFBR as discussed inijayakumar et al. (2004).

The primary and secondary circuits of PFBR uses large inven-

ory of sodium as coolant. Leaks in sodium systems have theotential of being exceptionally hazardous due to the reaction of

iquid sodium with oxygen and water vapor in the air. Sodium is

∗ Corresponding author. Tel.: +91 044 27480500x22352; fax: +91 044 27480086.E-mail address: sylvia@igcar.gov.in (J.I. Sylvia).

029-5493/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.nucengdes.2012.04.010

© 2012 Elsevier B.V. All rights reserved.

reactive with air and catches fire depending on the temperatureof sodium. Temperature in the plant ranges from 150 ◦C to 550 ◦C.Leaked sodium also reacts with concrete leading to damage andloss of strength. Steam/water leak into sodium in steam genera-tor produces hydrogen and leads to high pressure and temperaturein the system. Even though these systems are designed to makesodium leak most improbable, it is essential to detect the leaks atan early stage. Detectors are provided such that leak is detected andcontained before any possible damage could occur.

As per standards on performance requirements for sodium leakdetectors stipulated by ASME Boiler and Pressure Vessel Code, Sec-tion XI, Division 3; a leak rate of 100 g/h of sodium in air filledvaults must be detected in 20 h and a leak in inert vault within250 h.

2. Detectors based on electrical conductivity of sodium

Wire type, spark plug type and Extended spark plug type leakdetectors (ESPLD) work on the principle of electrical conductiv-ity of sodium. The detector wire or detector electrode makes ashort circuit during sodium leak with the pipeline which is alreadyelectrically earthed and results in grounding.

420 J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431

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Fig. 1. Wire

.1. Wire type leak detection system

This is the simplest method of leak detection system employedn the experimental sodium systems, single walled pipelines, com-onents and tanks of secondary system of PFBR. It consists of aetallic Nickel wire insulated with ceramic bead insulators. The

ross shaped ceramic insulator is stung over the wire and this strings laid on the surface of the pipe all along its length as shown in Fig. 1.he exposed portion of the wire is in close proximity to the pipeurface. Sodium leaks from the pipe make contact with the wirend bridges the small air gap between the wire and the pipe sur-ace results in earthing. The electronics system detects the earthingnd gives sodium leak alarm.

In order to make the leak system more effective, the ceramicnsulated wire is laid underneath the horizontal pipelines of smalleriameter. For larger pipes of sizes 250 mm and above (not in verticalun) the wire type leak detectors are provided at 90◦, 180◦ and70◦ taking top as 0◦. These three leak detectors along the length of

m helps in reducing the distance between the leak point and theetectors. For vertical run of the pipe wire type leak detectors areound helically with the pitch of 0.5 m so that the maximum length

etween a leak point and a detector is less than 0.5 m. They arerouped in series to minimize the number of electronic channels.he series grouping ensures healthiness of system as any fault inhe circuit is also indicated as a leak alarm.

.2. Spark plug type leak detection systems

Spark plug type leak detectors are used for detecting sodiumeak in enclosed space like bellow sealed valves and thermowell. It

s mounted in a small trough/pocket into which the leaked sodiumollects and causes electrical short between its insulated centralonductor and the body. It is typically used to detect sodium leakrom the double wall pipelines and vessels of fast reactor system

Fig. 2. Sensor porti

ak detector.

with the inter-space containing nitrogen gas. The leak detector ismounted in a small trough at the bottom most level of the envelopesurrounding the capacities such as reactor vessel. Same electronicsis used for detection of leak.

2.2.1. Extended spark plug type leak detector (ESPLD)For detecting leak from Main vessel, two diverse methods are

used. One of which is based on electrical conductivity and other isbased on Mutual Inductance principle. Two numbers of ESPLDs areused which works on electrical conductivity principle. Sensing endof the leak detector is located at bottom most point inside safetyvessel where sodium is getting collected. The active sensor portionis as shown in Fig. 2. It consists of a centre capillary syringe insulatedfrom the outer Stainless steel (SS) tube by alumina. The ceramicto metal brazing was done in vacuum. The overall dimension ofthe active sensor portion is 4 mm dia. and 49.5 mm long which islimited by the complex guide tube profile in the Safety Vessel (SV).25 m long MI cable was brought through a 4 mm dia thin walledextension tube to protect the cable from damage and also for easeof insertion/withdrawal. SS sheath of MI cable was removed fora length of 30 mm and the SS conductor was passed through theceramic insulators to the detector sensor portion and welded tothe end using laser beam welding. The active sensor portion (1 mmSS tube) is protected from damage by welding an SS protective capduring insertion and withdrawal. Once the leaked sodium bridgesthe SS protective cap and the centre capillary syringe leak will bedetected. The sensor in assembled condition is shown in Fig. 3.

As per ASME code, a leak rate of 100 g/h of sodium should bedetected in 250 h in the inert atmosphere. After leaking from MV,

sodium is collected at the bottom of the SV. 25 kg of liquid sodiumtakes a level of 20 mm from the bottom of the MV. As the activeportion of the sensor is 4 mm dia, 4 mm height of leaked sodiumcan be detected using this sensor which is much earlier than 250 h.

on of ESPLD.

J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431 421

assembled condition.

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Table 1Set values of ESPLD.

Condition Voltage (V) Output value indigital form

Normal 4.2 0100Leak 1.02 0010Wire open 11.96 1000Wire short .020 0001

Fig. 3. ESPLD in

.3. Electronics for leak detectors

The wire type and spark plug type leak detectors use the similarlectronics for processing the leak signal. Wire type leak detectorsre directly connected to Field Terminal Boxes (FTB) where twoesistors, one is 100 ohm and other is 470 ohms are connecteds shown in Fig. 4. The control card located in the control bin ishe heart of LD system where all the signals are processed, storednd manipulated. The Control Bin consists of RT20 CPU Card, VMEthernet Controller Card for communicating the data through Eth-rnet, memory card to store the database and configuration, I/Onterface board to convert VME bus to I/O bus where leak detectorards (analog input) and Relay output cards are located.

Each Leak detector card consists of 32 analogue input chan-els in which four channels are used for diagnostics purpose. Thisard, after receiving field signals, converts analog signals to dig-tal signals. The set values are shown in Table 1. Digital signals

Fig. 4. Schematic o

Undefined Other than above values 0000

which represent field leak conditions are compared with set valuesstored in Erasable Programmable Logic Device (EPLD). This sys-tem consists of one Relay output module and depending on the

group configuration, if any one of the leak detector shows leak orany unhealthy condition, the relay will be energized to alert theoperator through Alarm Annunciator window. This electronics was

f electronics.

422 J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431

Table 2Sensor specification.

S. no. Leak detector Sensor specification

Sensor Pocket Winding details/features

Material Size Material Size

1. Main vessel leak detector(miniature & remotelylocatable)

SS410 (magnetic materialimproves the sensitivity)

Dia. – 5.0 mmActive length – 20 mmTotal length – 25 mm

SS316L OD – 7.5 mmID – 5.5 mm

0.5 mm dia. M.I. cable in two layersin bifilar fashion.Primary: 10 turnSecondary: 10 turnRemotely locatable22 m long free leads inside theextention tube of 4 mm OD and3.6mmID

2. DSRDM leak detector(miniature)

SS410(magnetic materialimproves the sensitivity)

Dia. – 5.0 mmActive length – 20 mmTotal length – 22 mm

SS316L OD – 7.5 mmID – 5.5 mm

0.5 mm dia. M.I. cable in two layersin bifilar fashion.Primary: 10 turnSecondary: 10 turn15 m long free lead installed insidethe EM

3. DWP leak detector SS316L Dia. – 32 mmActive length – 22 mm

SS316L OD – 42.16 mmID – 36.62 mm

1 mm dia. M.I. cablePrimary: 1 layer

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eveloped in-house and tested in the sodium rigs available in Fasteactor Technology group (FRTG).

It was experimentally proved in FRTG that the highest detec-ion time observed was about 6 h for a sodium leak rate of 222 g/hs discussed in Vijayakumar et al. (2011). A relationship betweeneak rate and detection time was established from the experimen-al results and found that sodium leak rate of 100 g/h is likely to beetected in 11 h which far better than the stipulated value of 20 h

n ASME code.

. Leak detector based on mutual inductance (MI) principle

Mutual Inductance type Leak Detector (MILD) works on the prin-iple of decrease in mutual inductance between two coils whenodium surrounds it. The primary coil is excited with AC constanturrent of 100 mA and an emf will be induced in the secondary coilf the sensor. The secondary coil emf will be reduced when sodiumurrounds the pocket. The reduction in secondary emf indicatesodium leak.

MI type sensors have several advantages such as (i) non contactperation with sodium as it is located inside a thimble; (ii) facili-ates maintenance, as the sensor can be removed from its pocket fornspection and/or replacement; (iii) the sensor needs to be installednto the thimble only prior to sodium-filling of the vessel and noturing the integrity (pressure/vacuum) testing, normally carriedut on the vessel after fabrication at the manufacturer’s premises;nd (iv) immune to sodium wetting. All the above factors weighedn favour of developing a leak detector based on the MI principle.his leak detector forms a redundant and diverse method of leaketection. As remote handling and miniaturization is possible withhis type of detectors it is used for detecting sodium leak in mainessel, double walled pipes, electromagnet of diverse safety rodrive mechanism etc. by suitably modifying the design of the sensors indicated in Table 2.

.1. MI type leak detector (MILD) for main vessel (MV) of PFBR

This is one of the diverse methods for detection of leak, from

he Main Vessel (MV) of PFBR for the safe operation of reactor. Inase of sodium leak from the Main Vessel (MV), the leaked sodiumollects in the Safety Vessel (SV). One of the detectors developed forhis purpose is miniature, remotely locatable Mutual Inductance

22 turnsSecondary: 2 layer22 turns/layer

type Leak Detector. MILD will be positioned at the bottom mostelevation of the inner surface of SV through a guide tube from thetop of the reactor vault. Since unhindered space for the in-vesselinspection vehicle has to be provided, the size of the guide tubeis small and this puts a limitation on the size of the probe bobbin.The challenges involved in developing this sensor for PFBR includesminiaturization of leak detector and the ability to locate/withdrawthem remotely through a long complex guide tube profile.

3.1.1. Design of MILDThe guide tube with three different segmented profiles suitable

to the reactor is shown in Fig. 5. The sharp bending radius of theguide tube is of the order of 350 mm. For easy insertion and with-drawal of the sensor a cylindrical bobbin of 2.8 mm diameter and20 mm long was selected.

The insertion and withdrawal of the probe inside the guide tubeassembled with three different segmented profiles suitable to thereactor has been tested and confirmed. However, the miniatureprobe has a limitation on sensitivity of the probe. In order to eval-uate the sensitivity, sensor with different bobbin material namelySS316L and SS410 (magnetic) were fabricated. The probe coils werewound on a cylindrical bobbin of 2.8 mm diameter and 20 mmlong made up of either SS316 or SS410 using 0.5 mm diameter SSsheathed, MgO insulated, copper conductor (MI) cable over two lay-ers in bifilar fashion. In order to evaluate the sensitivity of the probe,two probes are fabricated with short free leads of identical dimen-sions but of different bobbin material namely SS316 & SS410. Thepreliminary experiment was carried out in brick sodium with twoprobes. The sensitivity obtained was around 7% for the probe madeof SS316 material whereas the sensitivity of sensor made of SS410material was around 30% which is sufficient to provide thresholdsetting in the electronics. Therefore SS410 is selected as materialfor MILD.

This was again validated by simulation using Finite ElementModeling and the flux pattern at 5.25 kHz for SS410 bobbin withand without sodium is shown in Fig. 6. The sensitivity obtained is50% for SS410 bobbin but for SS316L sensitivity is only 9%. Hence

miniature MI probes are designed with SS410 bobbin. The SS410gives more sensitivity compared with SS316. This is because SS410is having better permeability compared with SS316. Hence moreflux is generated in SS410 which results in increase in sensitivity.

J.I. Sylvia et al. / Nuclear Engineering a

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Fig. 5. Location of MILD probe in Safety Vessel (SV) of PFBR.

At one end of bobbin, a collar is provided for attaching to ahin-walled tube of 4 mm OD and 3.6 mm ID. MI cable leads fromhe probe are brought through the capillary tube. Fig. 7 shows thehotograph of the sensor.

.1.2. Experimental procedureThe detailed experiments were carried out in pure sodium simu-

ating the various reactor conditions. Fig. 8 shows the experimentalet up and Detail-A shows the MI type leak detector probe. Thisxperiment was carried out to find optimum frequency at whichhe sensitivity is maximum. The guide tube is closed at the bottomnd welded with one SS block which simulates the PFBR safety

essel thickness. The guide tube carrying the MILD is held by glandnd nut arrangement mounted on the top of the flange, which alsorovides the leak tightness for the argon cover gas. L1, L2, L3 areodium level indicators. Level of sodium increases slowly up to the

nd Design 249 (2012) 419– 431 423

operating level (L2). The loop is initially filled with sodium at 150 ◦Cand the reading is taken from the probe (secondary output is mea-sured in mV). Then the sodium level is quickly drained below theguide tube end and the reading is again taken. The difference inreadings is the reduction in secondary output of MI probe. Thisoperation is for one temperature and frequency. Similarly the oper-ations have to be repeated for temperature upto 550 ◦C in stepsof 50 ◦C and the frequency was varied from 2 to 8 kHz in steps of0.25 kHz.

3.1.3. Experimental resultsThe experiment was conducted up to 8 kHz for the temperature

from 150 ◦C to 550 ◦C. It was observed from Fig. 9 that at 550 ◦Cthe sensitivity is maximum of 49.5% at a frequency of 5.25 kHz.Sodium tests of the probe have revealed a sensitivity ranging from40 to 50% over the sodium temperature range of 150–550 ◦C andthe electronics is provided with alarm threshold corresponding tothe probe output at 550 ◦C (probe fully covered with sodium).

Skin depth corresponding to the frequency of 5.25 kHz is3.8 mm. The thickness of sodium equal to twice the skin depth cor-responding to the operating frequency is virtually equivalent to aninfinite sea of sodium as discussed in Mcgonigal et al. (1971). Theheight of the guide tube (8 mm) and the height of the leaked sodiumabove the guide tube (8 mm) contributes to a total height of 16 mmwhich can be detected using this sensor. Hence the detection time isearlier than the 20 mm height thereby meeting the recommendedASME standard.

3.2. MI type leak detector for DSRDM

In PFBR there are two shutdown systems, one of the systems isDiverse Safety Rod Drive Mechanism (DSRDM). In bottom portion ofthe DSRDM there is a dissimilar weld joint, between Electro Magnet(EM) made of iron and stainless steel. The failure of weld can resultin entry of sodium inside the electro magnet, which can increasethe response time of electromagnet, thereby increasing ShutdownControl Rod Accelerated Drive Mechanism (SCRAM) time. So it isnecessary to detect the entry of sodium inside the EM. Consider-ing the space, Miniature MILD was required for detecting the leak.As remote handling is not required as in MILD for MV, extensiontube over the free leads are not provided. But 15 m long MI freeleads are required to mount the sensor within the electromagnethousing.

Miniature MI type sodium leak detector to detect the sodiumleak in electromagnet housing in DSRDM is indigenously devel-oped and fabricated. The sensor construction is same as that ofMILD for MV except the mounting arrangements. Special jig wasfabricated for winding the M.I. cable on the bobbin without twist-ing and damaging the MI cable sheath. Jig was fabricated in sucha way that during winding, the free lead (15 m) also rotate alongwith the bobbin and the MI cable sheath should not touch any metalpart, otherwise the sheath will get damaged (sheath thickness is 50microns). Special jig facilitates to get the repeatability of windingwithout damaging the free leads of the M.I. cable.

The detailed assembly drawing is shown in Fig. 10. After wind-ing the bobbin the free leads are inserted through bobbin supporttube and sensor flange which is having 4 holes of 0.8 mm diameteron 2 mm PCD. The bobbin was welded with the support tube byLaser beam welding. The primary and secondary leads are twistedto cancel the noise. Then the sensor assembly will be inserted intothe pocket and tack welded between pocket and sensor flange toavoid rotation. Insert the cap over the sensor flange and fix with 3

screws. The screws and the cap are tack welded to avoid unscrew-ing during vibrations in the DSRDM. The cold ends are prepared andsealed after improving the insulation value. The insulation value ismeasured at every stages of assembly.

424 J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431

& air at 200 ◦C for SS410 bobbin.

Fig. 6. Flux pattern in sodium

Fig. 7. MILD Sensor for Main Vessel (MV). Fig. 9. Frequency vs sensitivity of the probe at 550 ◦C.

Fig. 8. Experimental setup & probe.

J.I. Sylvia et al. / Nuclear Engineering a

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Fig. 10. Assembly drawing of DSRDM leak detector.

The above assembly is qualified in three stages. Before fabri-ation, chemical composition of raw materials and dimensionalhecks are carried out, welding procedures are qualified andpproved. During fabrication dimensional check of fabricated parts,urface treatments and welding qualification are carried out. Afterabrication, visual inspection and Liquid penetrant inspection ofeld joints, dimensional inspection of final assembly, and perfor-ance checking of sensor were carried out. Three numbers of leak

etectors are successfully fabricated with qualification proceduress shown in Fig. 11. The sensor was not only indigenously developednd fabricated but also l it was finally installed inside the DSRDMor deployment in PFBR.

Fig. 11. Photograph of DS

nd Design 249 (2012) 419– 431 425

3.3. MI type leak detector for double walled pipeline

In the double wall piping the leaking sodium spreads andreaches the pocket since the piping is with little inclination. Sincethe guard pipe is provided with pockets for every compartment,the maximum distance sodium is to travel is the length of thecompartment before reaching the pocket. MI type leak detectorsare provided where the leaked sodium gets collected in a pocketdesigned for the purpose. The main function of the MI type leakdetector is to detect the leak of sodium from the main pipe lineinto the annulus space filled with Nitrogen envelope in the pipe.

The primary and secondary coils of the detector are wound ona cylindrical bobbin of length 22 mm and diameter of 32 mm usingSS sheathed 1 mm dia. MI cable, with copper conductor and mag-nesium oxide insulation. The sensitive portion of the detector isof hollow cylindrical cross section made of SS316. This detector asshown in Fig. 12 is a probe type, which can be kept inserted into athimble from the pocket bottom.

The electronics as shown in Fig. 13 is made simple as tempera-ture compensation over an operating range from 150 ◦C to 550 ◦C isnot required as the length of the probe is much smaller. The sodiumleak of the order of 100gm/h can be detected in a few hours withthis detector.

4. Sodium ionization detector (SID)

SID is used as global leak detection system in PFBR secondary cir-cuit area. Details of developmental works related to SID is given inElumalai et al. (1984). Sodium cooled fast reactors have numerousrequirements for instrumentation capable of detecting the leakageof sodium from component or piping faults into the surroundinggaseous atmosphere. Detection of these leaks by a sensitive earlywarning system is highly desirable to limit piping deterioration,component destruction, fire damage or corrosive effects from thesodium. The amount of oxygen and water impurities guarantee thatany leaking sodium will be partially reacted to oxide or hydroxidesand that sodium aerosol will be generated in addition to sodium

vapour. Therefore the leak detector should be capable of respond-ing to sodium compounds as well as to elemental sodium. One ofthe major advantages of SID is its operating simplicity. Detection ofleak is obtained under ambient atmospheric conditions.

RDM leak detector.

426 J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431

Fig. 12. Leak detector for double walled pipeline.

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are then either thermally ionized to become free sodium ions, or areevaporated as neutral sodium atoms. Only the free ions contributeto the SID response. Temperature of the filament contributes tothe ionization efficiency. The work function of the filament metal

Fig. 13. Elec

.1. Theory of operation

The SID works on the principle of preferential ionization oferosols of sodium and its compounds (oxides and hydroxides) inarrier gases such as argon, nitrogen and air. The schematic of SIDs as shown in Fig. 14. The detector carries a heated filament on

hich the sodium aerosols in the carrier gas fall and get ionized.hese ions are then collected on an adjacent electrode which isaintained at a negative potential relative to the filament, leading

o readily detectable current. In the absence of an easily ionisableubstance like sodium near the filament, the SID produces only

small background current, arising primarily from alkali metalmpurities in the filament material as discussed in Ref. Bauerle et al.1976).

The most critical and also the most complex aspect of SID oper-tion involve the detailed processes by which the sodium particlesre converted to free sodium ions. This can be thought of as occur-ing in the following steps:

1] The collision of sodium particles with the hot filament surfaceand their subsequent Melting. The rapid surface diffusion of themelted sodium over the filament to form a layer of adsorbedsodium atoms

2] The transfer of valence electrons from some of the adsorbedsodium atoms to the filament, converting them to adsorbedsodium ions

3] The desorption of these sodium ions from the surface to becomefree ions; also the desorption of neutral sodium atoms.

s for MILD.

The first and second steps produce a monolayer of sodium atomsin the filament surface. In the third and fourth steps, these atoms

Fig. 14. Schematic of SID.

J.I. Sylvia et al. / Nuclear Engineering a

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The SID system consists of SID sensor and the sampling sys-

Fig. 15. Photograph of SID.

urface should be greater or comparable to the ionization efficiencyf the sodium atoms.

.2. Sensor design

The optimum sensor design used in PFBR is to achieve maximumensitivity with minimum background sodium ion current. The SID

s shown in Fig. 15 scores over other detectors in the sense that itan detect a quantity of as low as one nanogram of sodium presentn a cubic centimeter of carrier gas.

Fig. 16. SID as area

nd Design 249 (2012) 419– 431 427

Filament used : platinum filament of 50 mm long, 25micron thick and 1 mm width

Collector plate material : perforated cylinder made of StainlessSteel (12 mm OD 10 mm ID, 35 mmlong) 1.8 mm dia perforated hole

Filament to collector spacing : 5 mmCollector voltage : 200 VFilament temperature : 950–1000◦ CCarrier gas flow : 1 l/minMaximum background current : 0.08 nASensitivity : varies from 0.1 to 1 nanoampere per

nanogram of sodium per cubiccentimeter of gas

Minimum detectable sodiumconcentration

: better than 0.3 nanogram of sodiumper cubic centimetre of gas

4.3. Electronics for the SID

The electronics unit for the SID consists of

[1] an adjustable current source for heating the filament[2] a medium voltage power supply to provide the electric field for

collecting the sodium ions[3] a wide range electrometer (10−12 to 10−6 ampere) for measur-

ing sodium ion current[4] Thermal cycling circuitry to maintain the sensitivity of the fila-

ment

The electronics chassis was qualified for immunity level to EMInoise. To maintain the sensitivity of the filament, thermal cyclingcircuit has also been incorporated in the SID electronics.

4.4. Sampling system

tem with fluted pipelines as shown in Fig. 16. Hence a singledetector can cater to many locations through sampling of thecarrier gas. The inherent response time of the SID to a single

monitoring.

428 J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431

ID with long sampling lines.

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Fig. 17. Sensitivity of S

erosol particle should be comparable to the ion drift time betweenhe filament and the collector, typically less than a millisecond.n a practical detection system employing aerosol sampling line,owever the time response is controlled by the much slowerrocesses of gas flow to the sensor chamber and mixing in thehamber as discussed in Swaminathan and Elumalai (1984). Theensitivity of SID system using long sampling lines, has beenemonstrated experimentally as shown in Fig. 17. The SID work-

ng on sampling method has been used in experimental area andecondary circuit area of PFBR. The installation of SID is shownn Fig. 18. The response of SID to burning 5 g of sodium in thexperimental area is shown in Fig. 19. Every time the test is car-ied out, the alarm threshold set at 1 nA is exceeded by the SIDesponse, confirming the sensitivity and effectiveness of the SIDystem.

During the operation of SID system it was observed thathere was an increase in background value crossing the thresh-ld limit. After cleaning the sensor, sampling lines and blower

ith water the background started coming down. As part of the

urveillance periodic cleaning of sensor and sampling system isecommended.

Fig. 18. Installation of SID.

Fig. 19. Sodium fire test.

5. Cross wire type leak detector

The main function of the Cross Wire type Leak Detector (CWLD)is to detect the discharge of sodium and its reaction products at thedownstream of the rupture disc due to bursting of the rupture disc.This provides the information on sodium water reaction caused dueto water/steam leak in the Steam Generator (SG) and the developedhydrogen gas pressure bursts the rupture disc. Literature surveyshows that this type of sensor is deployed in Monju reactor workson bursting technique as in Takahashi et al. (1983). These S. S. foilsare located at the downstream of the rupture disc in the verticalpipe as shown in Fig. 20. When sodium/sodium reaction productsflow past the S. S. foils break them and the loss of electrical conti-nuity of the foils is detected and alarm signal is given. The CWLDwas designed with following specification and the functionalitywas checked in experimental water test setup.

5.1. Sizing of SS foil

Based on calculations the width and thickness of the SS foilwere fixed to 15 mm & 50 microns respectively. Experiments werecarried out to optimize the design of the foil. Based on these exper-

iments, the depth and number of V-notches were determined. Theeffect of thermal stress in the foil due to the temperature differencebetween pipe and foil was studied by numerical simulation. It wasfound that, in order to avoid yielding of the foil due to thermal stress

J.I. Sylvia et al. / Nuclear Engineering and Design 249 (2012) 419– 431 429

Fig. 20. Location of CWLD.

Fig. 21. Dimensions of CWLD.

4 ring and Design 249 (2012) 419– 431

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he foil temperature (which will be nearly equal to the temperaturef argon gas surrounding the foil) should be limited to 15 ◦C only. Ashis is difficult to achieve, slight extra length (2–3 mm) of foil will berovided. This will prevent any thermal stress getting developed inhe foil and thus prevent the breakage of the foil due to differentialemperature.

.2. Specification of SS Foil

aterial : SS304ength of foil : 338 mm

idth of foil : 15 mmhickness : 0.05 mmumber of ‘V’ notches : 7 NOS.epth of ‘V’ notches : 6 mm

V’ notches are kept at a distance of 40 mm as shown in Fig. 21.

.3. Experimental setup

In order to validate the design, experiment was carried out. Thexperimental test set up for testing the S.S. foils along with the

etector electronics is shown in Fig. 22. The tests were carried out

n a water loop in Engineering Hall-II by suddenly allowing watert some pressure to flow over the foils. To measure the pressure inhe pipeline at the time of opening of the valve, a dynamic pressure

Fig. 23. SS foils before a

Fig. 24. Graph showing specim

Fig. 22. Experimental test setup of CWLD.

transmitter is installed. In this set-up, all the 3 foils are installed in a

pipe at uniform gap of 10 mm and 60 degree apart without makingcontact with each other. To electrically insulate the foils from themetallic pipeline, a spark plug is installed in pipe with the help of

nd after breaking.

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J.I. Sylvia et al. / Nuclear Enginee

leeves. A triangular piece welded to the central electrode of thepark plug is used to hold the foil firmly. Electronic connectionsre given to the terminals of the spark plug. Fig. 23 shows the SSoils before and after breaking. The breaking of these SS foils areetected electronically using two out of three logic.

It was found from the hydraulic experiments repeated over2 trials that all the SS foils break at an average pressure of.15–0.25 MPa which corresponds to a velocity of 2 m/s as shown

n Fig. 24. The expected minimum pressure and velocity in PFBRs 0.37 MPa and 2 m/s respectively. It is found that the results areonservative and the snapping/breaking technique of the CWLD haseen demonstrated successfully.

. Conclusion

As the commercially available conventional sensors cannot besed for sodium leak detection, the sensors and instrumentationre specially designed using the state-of-art technology to suit thisostile environment. Wire type and spark plug type leak detectorsorking on ‘earthing’ principle is used on pipelines and different

apacities such as bellow sealed valves and leak collection traysespectively with improved electronics in PFBRs. Miniature Mutualnductance type leak detectors with remote insertion/withdrawalacility was developed for MV leak detector of PFBR. The selec-ion of SS410 bobbin for improving the sensitivity of this sensoras experimentally proved. ESPLD’s are used as diverse method

or detecting leak in Main vessel. The sensor was assembled andested for its functionality and easy insertion and withdrawal. TheSRDM leak detectors with longer length of free leads (15 m) haseen fabricated, qualified and installed in the DSRDM electromag-

et successfully. For double walled pipe line where pockets arerovided at the lowermost elevation, MI type leak detectors of

arger size was designed and used in multiple numbers. This typef leak detector is non-intrusive and easily replaceable and hence

nd Design 249 (2012) 419– 431 431

it gains importance for usage in the reactor systems. Sodium Ion-isation leak detectors with improved electronics have been usedin secondary system as area monitoring in PFBR. As this sys-tem has been employed in sodium facilities, lot of expertise hasbeen gained in maintaining the same. The Cross wire type leakdetectors are designed and experimentally tested at required pres-sure of PFBR. The different types of sodium leak detectors weredesigned, developed and tested in various sodium facilities whichgave us enough confidence for deploying them in PFBR and futureFBRS.

Acknowledgements

Authors acknowledge the encouragement, guidance and sup-port received from K. Swaminathan, Former Head, Component &Instrumentation Development Division, FRTG, Shri S.C. Chethal,Director, Indira Gandhi Centre for Atomic Research (IGCAR) andall other agencies who supported during the course of thiswork.

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