a new method for detecting pressure tube failures in indian phwrs

Nuclear Engineering and Design 196 (2000) 337–351

A new method for detecting pressure tube failures in IndianPHWRs

V.K. Sharma *, V.K. GuptaHealth Physics Di6ision, Bhabha Atomic Research Centre, Mumbai – 400 085, India

Received 17 April 1999; received in revised form 3 November 1999; accepted 4 November 1999

Abstract

For the annulus gas system (AGS) of the standardised Indian pressurised heavy water reactor, an elaboratepressure tube (PT) crack monitoring and detection system is envisaged to ensure safety through leak-before-break.The parameters that are monitored relate to the detection of D2O moisture leaking in from the primary heat transport(PHT) system through a cracked PT. Since a slow build-up of moisture in the AGS may also occur for reasons otherthan PT failure, it is desirable that a diverse measurement technique should be available. This paper suggests such atechnique, based on the observation that a small reference concentration of fission gases is normally present in theannulus gas. This concentration would change sharply upon PT failure, when the heavy water from the leaking PHTsystem releases the dissolved fission gas content into the annulus. This paper presents a theoretical study of theparameters that influence the build-up of fission product noble gases in the AGS and shows that leakage rates as lowas 10 g h−1 from a PT crack can be detected in a few tens of minutes by this method. This is expected to substantiallyincrease the available time between the leak detection and the PT failure, thus serving as an important tool in meetingthe leak-before-break criterion of a critical component in PHWRs. © 2000 Elsevier Science S.A. All rights reserved.

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1. Introduction

In the evolution of the Indian pressurised heavywater reactor (PHWR) from Rajasthan to thestandardised design, an important change made inthe core area was the replacement of the open-ended air annulus between the pressure tube (PT)and the calandria tube (CT) by a closed gasannulus. The carbon dioxide gas envelope aroundthe pressure tube could then be monitored to

assure the integrity of the pressure tube. If anincipient crack developed in the pressure tube,penetrating the tube wall and leaking the primaryheat transport (PHT) system heavy water into theannulus, then, provided that the crack was de-tected before it reached critical length, leak-be-fore-break would be achieved.

The emphasis is obviously on the speed of leakdetection and location of failed tube. Since thecrack propagation velocities are such that the timeavailable from the start of PT leakage to itsgrowth to critical length is of the order of severalhours (Shalaby, 1988), the method of leak detec-

* Corresponding author. Tel.: +91-22-550-5050, ext. 2266;fax: +91-22-550-5151.

E-mail address: [email protected] (V.K. Sharma)

0029-5493/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.

PII: S0029 -5493 (99 )00304 -0

V.K. Sharma, V.K. Gupta / Nuclear Engineering and Design 196 (2000) 337–351338

tion and location has to be quick. The detectionmethods normally used employ the effects ofmoisture ingress from the PHT system into thedry annulus gas, such as a change of dew point,the increase of gas pressure, and the presence ofliquid water detected by electrical methods.

This paper suggests a diverse, highly sensitivemethod of PT crack detection. It is based on theobservation that a small amount of fissionproduct noble gases (FPNGs) is normally presentin the annulus gas system (AGS) purge stream tothe stack. This arises from the fission fragmentrecoils originating in the uranium impurity in thePT/CT wall surfaces and terminating in the chan-nel annulus. The resultant equilibrium fission gasconcentration should change sharply upon PTfailure, when the leaking PHT system heavy waterflashes to steam to release its dissolved FPNGsinto the annulus. The increase in FPNG concen-tration in the annulus can be detected by sensitiveon-line g-spectrometric instrumentation and agross activity monitor.

2. System description

2.1. The Indian PHWR

The standardised 220-MWe Indian PHWR es-sentially consists of a calandria vessel containing

heavy water at 343 K, with its tube sheets piercedby 306 horizontal fuel channels through which thecoolant heavy water at 8.6 MPa (87 kg cm−2 g)and 522–566 K circulates. Each fuel channel con-sists of an outer calandria tube and a co-axialpressure tube with an annular gap between thetubes. A calandria tube is typically 107.7 mminside diameter annealed Zircaloy-2 tube of wallthickness 1.25 mm, with its ends expanded andsandwich-rolled into the calandria tube sheets. Atypical pressure tube is a 82.55-mm inside diame-ter cold-worked seamless tube. The PT materialused is Zircaloy-2 in the earlier reactors, for whichthe minimum wall thickness is 4.03 mm and themaximum allowable stress is 110.4 MPa (1125 kgcm−2 g) at 573 K.

A different material, Zr–2.5 weight percent Nb,is used for the PTs in recent reactors, for whichthe minimum wall thickness is 3.32 mm and themaximum allowable stress is 147.2 MPa (1500 kgcm−2 g) at 573 K. The pressure tubes after trim-ming are �5380 mm long and are supported atthe ends at the rolled joints in the end fittings andat the intermediate points by four suitably locatedgarter spring spacers. The end fittings are locatedwithin a lattice tube of the end shield and areconnected through tail pipes to the PHT distribu-tion headers. The annular space between the twotubes is filled with dry carbon dioxide to insulatethe cold moderator volume from the hot coolant

Fig. 1. Assembly of pressure tube and calandria tube.

V.K. Sharma, V.K. Gupta / Nuclear Engineering and Design 196 (2000) 337–351 339

Fig. 2. AGS for the standardised Indian PHWR.

tubes. A sketch of the typical fuel channel assem-bly is shown in Fig. 1.

2.2. The annulus gas system

An open annulus gas system was conceived inthe earliest PHWR designs to insulate the coldmoderator from the hot pressure tubes. It wassubsequently modified into a closed inert gas en-velope that could be held in a slightly oxidisingcondition to preserve the integrity of the PT sur-face treatment. The closed-circuit operation of theAGS immediately led to the third function, viz.the integrity monitoring of the pressure tubes andcalandria tubes against possible leaks. Currently,the monitoring of the integrity of the pressuretubes and the PT/end-fitting rolled joints is con-sidered to be the most important function of theAGS.

A single annulus gas channel is formed byconnecting a PT/CT annulus with the gap be-

tween the inside surface of a lattice tube and thesplit sleeve of the end-fitting at each end of thereactor. A fixed number of channels (25 or 26) areindividually connected to sub-headers, which arein turn connected to the main distribution headers(Fig. 2). The gas medium used is carbon dioxidedoped with 1% (v/v) O2. In order to monitor theannulus gas, it must either be recirculated in aclosed circuit, or bled continuously past the dew-point monitor. For the standardised Indian PH-WRs, it is proposed to recirculate andcontinuously monitor the annulus gas.

2.2.1. Design objecti6es of the annulus gasmonitoring system (AGMS)

As the AGS is intended to monitor leakagesfrom the PHT system through the PT or from themoderator system through the CT, its operatingpressure must be less than that of the moderatorat the top of the calandria which is �130.7 kPa(0.3 kg cm−2 g), so the system is maintained at a


pressure of 116.0 kPa (0.15 kg cm−2 g). As thesystem has to be supplied by high pressure CO2

gas bottles, together with oxygen cylinders fordoping, it is necessary to have pressure reducingvalves on the cylinders in addition to pressurerelief valves in the system (set point 277.8 kPa (1.8kg cm−2 g)). The temperature of the annulus gasat the core exit is expected to be 461 K. TheKaiga AGS system is designed for a pressure of493.6 kPa (4.0 kg cm−2 g) and a maximumtemperature of 473 K (Kaiga Atomic Power Pro-ject, 1995).

The system should be capable of on-line detec-tion of small, incipient leakages on the basis ofsampling/monitoring of the bulk gas in a reason-able time period. The identification of the sub-header (into which 25/26 individual channels maybe connected) and the isolation of the leakingchannel should be completed in a reasonable time(Shalaby, 1988).

The initial dew point of the AGS gas whenrecharged should be low, �233 K (−40°C), sothat the system can be operated for a reasonableperiod of time without having to dump the gas onaccount of the build-up of humidity from pre-dictable chronic causes. The make-up gas is ex-pected to have a dew point of �213 K (−60°C)(Bureau of Indian Standards, 1992).

2.2.2. Monitoring schemes in the AGMS at KaigaThe annulus gas monitoring system (Kaiga

Atomic Power Project, 1995) is provided with (i)dew point sensors, (ii) differential pressure instru-mentation, (iii) moisture-sensing beetles, and (iv)cold-finger sample stations.

Two on-line dew point sensors of metal-oxidefilm type with a range from 193 to 293 K (−80 to+20°C) are provided, one at the inlet side andthe other at the outlet side of the recirculating gasstream. The outputs from the two sensors arecompared. If the difference exceeds 10 K, or if theabsolute value of the outlet side dew point sensorexceeds 263 K (−10°C), leakage into a PT/CTannulus is indicated. Dew point measurement isthe principal method of moisture detection.

Pressure switches of the metallic diaphragmtype, with a range of 0–150.4 kPa (0–0.5 kgcm−2 g), are provided in each of the 12 outlet

sub-headers. A value of 125.8 kPa (0.25 kg cm−2

g) in a sub-header indicates leakage from acoolant channel.

Moisture-detecting probes of conduction type(also called ‘beetles’) are provided in each of theinlet and outlet sub-headers. Condensing moisturein an outlet header is indicative of heavy wateringress.

In addition, it is possible to collect cold-fingersamples at a suitable gas sampling station andcount the condensate for tritium to confirm thepresence of excessive system D2O vapour in thebulk gas. A relatively high specific activity of thecondensate indicates the source to be themoderator.

2.2.3. Method of leak identificationAs stated earlier, during the recirculation mode

of operation, if the absolute value of the bulksystem humidity reaches 263 K (−10° C) or if thedifference between the inlet and the outlet dewpoints is 10 K, leakage is indicated. The systemthen automatically changes over to the once-through mode at reduced flow rate.

The leak identification proceeds in three stages.In the first stage, the main inlet headers contain-ing the leaking channel needs to be identified (Fig.2). In the first place, both the inlet header c2and the corresponding outlet header c2 are iso-lated, and a reduced flow of 10 m3 h−1 is directedthrough inlet header c1 and the coupled outletheader c1. If indications meet the leak criteria,the leaking channel belongs to one of the channelsconnected to inlet header c1, otherwise the othergroup of channels connected to inlet header c2is investigated.

In the second stage, the sub-header containingthe ruptured tube is identified. For this purpose,all save one of the six sub-headers connected to amain header are isolated, and a one-pass flow of10 m3 h−1 is passed through each sub-headersequentially until the leaking sub-header isidentified.

In the third stage, the leaking channel in theaffected sub-header is identified. This is done byblanking each channel in sequence and examiningflow for moisture. It is expected that the sub-header containing the leaking channel may be


identified in �12 h, while the particular channelcan be pin-pointed in another 2 h.

2.3. Leak-before-break for pressure tubes

The safety of the coolant tube in the PHWR isensured by an application of the leak-before-break criterion. If a pressure tube develops athrough-wall crack shorter than the critical cracklength, it will leak into the annulus gap, providingenough opportunity for the plant operator to usethe sensitive annulus gas monitoring system todetect a failure, identify the leaking tube, and thenreplace or plug the pressure tube. This implies, ofcourse, that (Moan, 1990):� the length of the crack at tube wall penetration

is less than the critical crack length (CCL) forunstable growth;

� the leak is detected; and� the failed tube can be identified and replaced

before the crack length exceeds the CCL.There are, thus, two time intervals of prime

importance:1. the time T1 required for the crack to grow

from the length at start of leak to the CCL;2. the time T2 required to detect the leak, identify

the tube and shut-down the reactor.If T2BT1, leak-before-break is demonstrated.

2.3.1. Determination of time inter6al T1

Not considering the relatively slow PT failuremechanisms (such as fatigue cracking), the mainfailure mechanism for Zr–2.5Nb PTs underPHWR operating conditions is delayed hydridecracking (DHC). All known failures in reactors ofthis type have been of this type (Shalaby, 1988;Moan, 1990). DHC assumes the presence of aminor surface defect, the presence of hydrides,and a circumferential tensile stress. If the avail-able hoop stress exceeds the threshold for DHCfor the considered defect in the surface of the PT,then the presence of defect can lead to the initia-tion of a crack, which under a suitable environ-ment can then propagate to failure.

The growth by DHC occurs in both radial andaxial directions, generally at different rates. Leak-age starts when the defect in the radial directionpenetrates the tube wall and will keep increasingas the crack propagates in the axial direction to

reach a length at which it becomes unstable. Thetime available for the crack to grow to the criticalcrack length (CCL) is determined on the basis ofa simple model (Shalaby, 1988; Moan, 1990):

T1=(CCL−L0)

2V(1)

where T1 is time for crack to grow from its lengthat wall penetration (s), CCL is critical cracklength (cm), L0 is crack length at time of wallpenetration (cm), and V is velocity of crack prop-agation on both sides of the initial crack (cms−1).

The crack length at wall penetration, L0, hasbeen found in experiments to lie between 4 and7w, where w is the wall thickness of the PT(Shalaby, 1988). Thus, for a 3.32-mm thick PT, L0

lies between 13.3 and 23.2 mm.The critical crack length has been statistically

estimated from experiments on tube segments,both irradiated and unirradiated, and at varioustemperatures to have a value of �50 mm at 95%lower confidence level (Moan, 1990). The DHCcrack propagation velocity V depends on the stateof irradiation of the material and may be consid-ered to be independent of the stress intensityfactor during stable crack growth. It may betaken at a value of �2.7×10−7 m s−1 (Shalaby,1988) for irradiated material under PHWR oper-ating conditions.

For the Indian PHWRs, the wall thickness ofthe Zr–2.5Nb tubes is 3.32 mm, and the availabletime T1 is seen to lie in the range 14–19 h.

2.3.2. Determination of time inter6al T2

The main factors determining the time requiredto detect the leak are:1. the rate of leakage from the crack,2. the dryness of the annulus gas and the rate of

moisture build-up under normal circumstances(for methods based on moisture detection),and

3. the monitoring system design, the minimumdetection level, and the monitoring scheme.

The leak rate from a crack depends on theoperating pressure: at low pressures, the leak ratedecreases with time primarily on account of clog-


ging by oxide and debris; at the normal operatingpressures, the leak rate steadily increases, and hasbeen observed to show a rapid increase beyond acertain threshold crack length which varies fromtube to tube. This threshold value has been foundto be well above 1 kg h−1 at crack lengths wellbelow the CCL (Moan, 1990)

Since moisture sensing is the conventionalmethod of leak detection, it is necessary that thenormal humidity levels in the AGS should be heldat a low value. Even if there is no leakage fromthe adjoining bodies, the moisture content of theAGS is expected to rise slowly on account of thediffusion of D2 from the PHT side through thestainless steel end fitting which can then combinewith the system CO2 or with available O2 to formheavy water. The moisture build up in the AGS isheld at a low level by passing the inlet gasthrough a drier vessel filled with molecular sieve.Where necessary, the AGS may be purged beforethe moisture level becomes too high. Purging alsohelps to keep the deuterium gas concentration inthe AGS at a reasonably low value.

It is well known that beetles are relatively slug-gish in the detection of small leaks since nosignificant condensation of moisture may be tak-ing place in the headers in the initial stages ofcrack growth. Again, the measurement of tritiumfrom cold finger samples has its limitations onaccount of the long sampling times required andsince the annulus gas may have a background ofdiffused tritium; moreover, results may not beavailable quickly.

It is worth noting that the decision regarding apossible leak has to be made based on measure-ments on the bulk gas. On account of the consid-erable volume of the gas, the rate of build-up ofthe dew point may be slow. For the recirculatingAGS of the standard PHWR, it is expected thatthe detection of the leak and identification of thefailed PT can be completed in �14 h (KaigaAtomic Power Project, 1995). This time period T2

is almost equal to the lower limit of the timeavailable T1, and needs to be reduced. A keyelement in this effort is an improvement in theminimum detection level of the bulk gas monitor.

3. Method

Theoretical studies on detection of leak fromPT, based on the build-up of fission product noblegases in the AGS, are now developed.

3.1. Source of fission product noble gases in theAGS

Right from the early days of operation of theNarora reactor, FPNGs, along with 41Ar andtritium, have been detected in the CO2 annulusgas purge stream (Joshi, M.L., Health PhysicsDivision, Bhabha Atomic Research Centre, per-sonal communication, 1997). The fission gases aregenerally masked by the relatively intense 41Aractivity and have not been precisely measureduntil recently. The presence of these gases may betraced primarily to the uranium impurity in thezirconium structural members, which may havean upper limit concentration of 3.5 ppm (ASTM,1985). Fission product noble gas radio-nuclidessuch as 88Kr, 87Kr, 85mKr, 135Xe and 133Xe havebeen detected in the annulus gas after a steadyoperation of the reactors at Narora andKakrapar.

Fission of the uranium impurity in theZircaloy-2 calandria tube and in the moderatorheavy water was earlier postulated to successfullyexplain the low level noble gas and other fissionproduct (FP) activities in the moderator/cover gassystems of several Indian PHWRS (Sharma andManganvi, 1997). The annulus gas system, whichis in contact with the pressure tube as well as thecalandria tube surface, should contain copiousamounts of fission gases.

3.2. Method for estimating background fission gasconcentrations in the AGS

There are four potential pathways for the build-up of FPNGs in the annulus:1. by recoil of the fission fragments originating in

the surface layer of the zirconium alloy used inthe PT/CT;

2. by activation of the Kr or Xe impurity in theannulus gas;


3. by diffusion of the fission product gases pro-duced in the zirconium alloy PT to the an-nulus; and

4. by diffusion of the fission gases from thePHT system through the intact pressure tubewall to the annulus.

Since industrial grade CO2 is used in theAGS, there is a likelihood of trace quantities ofstable noble gas isotopes being present in theannulus, which on being activated may give riseto radio-nuclides of Xe and Kr. Radio-nuclideslike 41Ar and 125Xe have been found to bepresent at Narora (Lal Chand, 1998), which canbe produced by activation only. Calculationsshow that the activation of the inert gas ele-ments Xe and Kr could be but a minor contrib-utor to the overall activity.

It has been demonstrated (Sharma and Man-ganvi, 1997) that for the relatively low operatingtemperature of the PT (�573 K), the diffusioncoefficients for the migration of the fission gasesin zirconium are rather low, and the mechanismc) above would lead to elimination by decay ofall but the long-lived nuclides among them, no-tably 85Kr. The mechanism d) is even less likelyas it would require an additional liquid-side filmresistance to be overcome. The considerablethickness of the PT will certainly reduce thethrough-wall diffusion. This rules out mecha-nisms c) and d) as processes for the build-up ofshort-lived fission gases in the annulus. Thus,the build-up by recoil is the principal mecha-nism for the accumulation of the fission gases inthe AGS.

3.2.1. FPNG build-up due to recoilThe range of fission fragments in Zircaloy is�9.1 mM (McLain and Martens, 1964). For thefission fragments originating at a depth less than9.1 mm, there is a likelihood of the fission frag-ments crossing the annulus and getting embed-ded in the wall across the gap. Thus, anintegrated treatment of the efficiency of the gapfor the capture of the fission fragments is re-quired. The release-to-birth ratio for the recoilrelease of a particular nuclide is given as follows(Lewis, 1987):

(R/B)recoil=o(Sg/V)mf (2)

where mf is average range of fission fragments inZircaloy (cm), Sg/V is ratio of geometric surfacearea to volume of the emitting solid (cm−1),and o is release efficiency (defined as the fractionof those fission fragments born within a depthmf of the surface, which stop in the gas-filledannulus.)

Rewriting Eq. (2)

(Ri)recoil=oSgmfF: Yi (3)

where (Ri)recoil is release rate by recoil of nuclidei (atoms s−1), F

.is fission rate, (fissions cc-s−1),

and Yi is cumulative yield of nuclide i.The release efficiency o for the PT/CT annulus

can be derived following the parallel plate as-sumption (Lewis, 1987). The calculated valuesare as follows:

o=0.1909, for Zr-2 PTs and=0.2007, for Zr–2.5Nb PTs

The fission rate F.

and the surface area Sg canbe shown to be:

F: =5.336Z×106 fis cc-s−1

with Z ppm as the U impurity in zirconiumalloy.

Sg=3.164×104 cm2, for the Zr-2 PTs and=3.142×104 cm2, for the Zr–2.5Nb PTs

The recoil rate of any specific species i beinggiven as:

Ri=omfF: SgYi,

=2.933×107YiZ atoms s−1,

for Zr-2 pressure tubes, and

=3.062×107YiZ atoms s−1, for Zr

–2.5Nb pressure tubes.

For instance, given a U impurity level of 1ppm (Z=1.0) in the Zr–2.5Nb alloy, the rateof formation of Kr-88 (Yi=3.584%) in the an-nulus gas would be 1.097×106 atoms s−1.


3.2.2. Calculations for the noble gas acti6ity inAGS

3.2.2.1. Acti6ity build-up in a closed AGS. Theactivity build-up in a closed AGS is calculated asfollows:

Ai=Xi{1−exp(−li t)} (4)

where Ai is gas concentration for the isotope i(mCi l−1), Xi is formation rate, for the isotope i(mCi l−1), li is decay constant for isotope i (s−1)and t is duration of irradiation (s).

3.2.2.2. Acti6ity build-up in a continuously purgedAGS. The activity build-up of the nuclide i in thecontinuously-bled AGS is calculated as follows:

Ai= (Bi/GiK)l iexp(−0.5�li tr) (5)

where Ai is gas concentration of the isotope i, atcore outlet (mCi l−1), Bi is addition rate perchannel (atoms s−1), Gi is gas bleed rate perchannel (l s−1), li is decay constant for nuclide i(s−1), tr is residence time in a channel (s−1) and Kis 3.7×104 dps mCi−1.

3.2.2.3. Acti6ity build-up in a recirculating AGS.The activity build-up of the nuclide i in a recircu-lating AGS is calculated iteratively as follows.

If AIN (I) refers to the gas activity at the inletheader in the Ith cycle, and BOUT (I) that at theoutlet, then

AIN (I+1)=BOUT (I) exp (−liTex) (6)

BOUT (I+1)

=AIN (I+1) exp ( −liTr)+ADDi (7)

where li is decay constant of nuclide i (s−1), Tex isout-of-core cycle time (s−1), Tr is in-core resi-dence time (s−1), and ADDi=activity additionrate (mCi l−1), of nuclide i, =Ai, with no coolantchannel failure (see section above), =Ai+(CONCi�LRATE), with coolant channel failure,

where CONCi is concentration of nuclide i (mCil−1), in the leaked PHT system water, andLRATE is leak rate (l s−1) of PHT system water.

When a failed PT is leaking moisture and theassociated radioactivity into the annulus, the con-centration in the failed channel will be consider-ably different from that in the sub-header or thatin the bulk gas. The above equations then corre-spond to the activity in the leaking channel, andthe activities in the sub-header and bulk gas aregiven as below:

(ACT)Bulk= (305�Ai+ (ACT)max)/306 (8)

(ACT)Sub-header= [(N−1)�Ai+ (ACT)max]/N (9)

where (ACT)max is activity of the failed channel atits outlet, N is number of channels feeding eachsub-header (25/26), and 306 is total number offuel channels in the core.

A computer program RECIRC was developedon a 80486 microprocessor-based desktop com-puter to carry out the above calculations.

4. Results

The computed background activities of five typ-ical noble gas nuclides, viz. 85mKr, 87Kr, 88Kr,133Xe and 135Xe in the recirculating AGS whichare shown in Fig. 3. The main assumptions madefor the calculations are given in Table 1. Thefigure also shows the aggregate for all the fivenuclides which is termed the ‘gross’ activity. Itmay be noted that the same five NG activities arealso routinely measured in the PHT system, sothat in the assessment of the leakage of radioac-tivity from the PHT to the AGS, any of theindividual nuclides or the gross activity may bestudied.

Figs. 4–6 give the time-dependent variation ofthe Kr-88, Xe-133 and the gross activity, respec-tively, in the AGS with one PT leaking at the rateof 0.1 g s−1 (0.36 kg h−1). In the first part ofeach of the curves, the background activities inthe AGS build up to an equilibrium level, follow-ing which a PT is assumed to develop a leak. Thechanges in the concentrations of the affected an-nulus, the corresponding sub-header and the aver-age bulk CO2 activities are shown.


Fig. 3. Recirculating AGS, background activity (standard case).

Figs. 7 and 8 give the gross NG activity build-upin the AGS for two leak rates: 1 g s−1 (3.6 kg h−1)and 10 g h−1, respectively, where in addition to thenormalised activity of the bulk CO2, the rate ofchange of the activity is also plotted.

Fig. 9 is drawn for the PHT system D2O contam-ination level that may be termed ‘slight’ (Table 2).The figure gives the effect of the leakage rate on thepeak values of the curves for the normalised rateof change of activity. The parametric leak ratesused are 10 g h−1, 360 g h−1, and 3.6 kg h−1.

Fig. 10 is similar to Fig. 9, having been drawnfor the PHT contamination level corresponding to‘fresh core’ (Table 2). The leak rates used are 0.1,3.6, 7.2, 10.8 and 14.4 kg h−1.

5. Discussion

5.1. Background acti6ities in the recirculatingAGS

The background concentrations of the variousnoble gas nuclides are calculated to lie in the range

0.25–1.5 mCi l−1 (Fig. 3). These values are obvi-ously proportional to the assumed impurity level ofU in the in-core zirconium materials. In order tocheck the basic assumptions of the calculations,background activity measurements have been madein the closed AGS systems at Narora (Lal Chand,1998) and the continuously-purged AGS of Kakra-par (Joshi, M.L., Health Physics Division, BhabhaAtomic Research Centre, personal communication,1997), where 220-MWe PHWRs are operating.Reasonable agreements have been found for theshort-lived gaseous fission products.

Table 1Standard inputs

Impurity content of zirconium alloys (3.51. 1.0 ppmppm max.)Rate of recirculation of AGS (where ap-2. 30 m3 h−1

plicable)3. 25/26Number of channels feeding each sub-

headerModerate4. Degree of contamination of PHT system

(moderate/slight)


Fig. 4. Recirculating AGS (standard case): Kr-88.

Fig. 5. Recirculating AGS (standard case): Xe-133.


Fig. 6. Recirculating AGS (standard case): gross activity.

Fig. 7. Recirculating AGS (standard case): gross activity of bulk CO2.


Fig. 8. Recirculating AGS (standard case): gross activity of bulk CO2.

Fig. 9. Recirculating AGS (slight PHT contamination): gross activity.


5.1.1. AGS acti6ity build-up as function of leakrate

Figs. 4–6, give the time-dependent concentra-tion in the AGS for Kr-88, Xe-133 and the grossNG activity, respectively, for a small leak rate of0.1 g s−1 (360 g h−1) when the PHT contamina-tion level is ‘moderate’ (Table 2). Each figureincludes curves for the bulk gas, the sub-header,and for the ruptured channel. The initial failuredetection has to be made on the basis of the bulkactivity signal, but it is found that the bulk activ-

ity does not change substantially over a reason-ably long period of time.

This situation improves considerably if the rate-of-change of activity is plotted as a function oftime for the bulk activity, as has been done inFigs. 7 and 8 for the bulk system gross activity,using leak rates of 1 g s−1 (3.6 kg h−1) and 10 gh−1. A leak rate of 10 g h−1 implies only incipi-ent leakage. It is observed that immediately fol-lowing a break, the leak rate of 3.6 kg h−1 showsa peak rate change as high as 3000, while the leak

Table 2Activities in RAPS PHT system

Fresh core activitiesa (mCi kg−1) Moderate contaminationb (mCi kg−1)Slight contaminationb (mCi kg−1)Nuclide

3.860.0086 11.561. I-1310.45 14.592. Kr-88 41.480.47 9.813. Kr-87 19.95

27.968.634. Kr-85m 0.120.031 262.065. Xe-133 1542.80

43.440.35 173.696. Xe-135

a Calculated values.b Measured averaged values.

Fig. 10. Recirculating AGS (effect of fresh core): gross activity.


rate of 10 g h−1 gives 10–50. Since the bulk ofthis peak is constituted in less than an hour,this parameter offers a highly sensitive methodof leak detection.

5.1.2. AGS acti6ity as a function of PHTcontamination le6el

Two levels of fission product contamination inthe PHT system are envisaged, one termed slightand other moderate, the former correspondingto an 131I concentration of 3–5 mCi kg−1 andthe latter to 10–15 mCi kg−1. The typical long-term averaged fission product activities for thetwo contamination levels are taken from the Ra-jasthan Atomic Power Station (RAPS) Unit 2PHT system activity records (Bhatt, H.R., Sta-tion Chemist, Rajasthan Atomic Power Station,Rawatbhata, personal communication, 1990). Inaddition, a hypothetical case is considered inwhich the PHT system contains no leaking fuelelements and the activity therein is derived fromthe fission of 1 ppm U impurity in the PT andcladding alloys, and 1 ppb tramp U concentra-tion in the PHT system heavy water. The corre-sponding calculated activities are indicated asfresh core activities in Table 2.

Figs. 9 and 10 are drawn to assess the sensi-tivity of detection for relatively low PHT con-tamination levels. For a slight contamination ofthe PHT system, the detection sensitivity re-mains high even for rather low leak rates. Thus,the normalised rate-of-change peaks to a valueof �3 for a leak rate as low as 10 g h−1.However, for fresh core activity levels, the limitof detection is reached at �100 g h−1, when arate-of-change gives a peaking factor of 1.5.

5.2. Implementation of the measurement technique

The results presented here indicate that theFPNG activity measurement in the AGS repre-sents a powerful technique for detecting a failedPT. However, on-line activity measurements arebeset with problems, primarily on account ofthe interference by the background activationproducts, 16N and 41Ar. While the former has ashort half-life, the latter with a half-life of 110min and a gamma energy of 1.29 MeV offers

formidable problems in the on-line measurementof a large majority of the FPNG nuclides. AtNarora, the 41Ar activity comprises over 99% ofthe total activity in the purge stream (LalChand, 1998). Currently, exploratory work is inprogress for developing an instrument based ongas chromatograph column coupled to a multi-channel analyser.

6. Conclusions

The measurement of the fission product noblegas activity in the annulus gas system of a mod-ern PHWR offers a unique and diverse tech-nique for the detection and location of acracked pressure tube at an early stage of leakmanifestation. The method is both prompt andsensitive. It becomes particularly effective whenthe normalised rate of change of activity is stud-ied as the characteristic leak parameter, and hasthe capability of detecting PT cracks givingleakages as low as 10 g h−1. This is expected tosubstantially increase the available time betweenthe leak detection and the PT failure, thus serv-ing as an important tool in meeting the leak-be-fore-break criterion of a critical component inPHWRs. The method suggested in this paper is,however, efficient only during the steady stateoperation of the reactor. The technique is con-ceived as an add-on feature to supplement theexisting leak detection instrumentation.

References

ASTM, 1985. Standard Specification for Wrought Zr and ZrAlloy Seamless and Welded Tubes for Nuclear Service.ASTM, Philadelphia, PA B353-83.

Bureau of Indian Standards, 1992. Carbon Dioxide. Bureau ofIndian Standards, New Delhi Second Revision, IS-307:1966, Reaffirmed 1991.

Kaiga Atomic Power Project, 1995. Annulus Gas MonitoringSystem-Process and Instrumentation, DM-34710, Kaiga 1,2, NPCIL, Mumbai, India.

Lal Chand, 1998. Radiological Monitoring of Annulus Gas atNAPS. Health Physics Unit, Narora Atomic Power Sta-tion, Narora, Rajasthan (internal report).

Lewis, B.J., 1987. Fission product release from nuclear fuel byrecoil and knockout. J. Nucl. Mater. 148, 28–42.


McLain, S., Martens, J.H. (Eds.), 1964. Reactor Handbook,vol. 4, second ed., Table 5.13, p. 174, Interscience Pub-lishers, New York.

Moan, G.D., Coleman, C.E., Price, E.G., Rodgers,D.K., Sagat, S., 1990. Leak-before-break in the pressuretubes of CANDU reactors. Int. J. Press. Ves. Piping 43,1–21.

Shalaby, B.A., Price, E.G., Moan, G.D., Coleman, C.E.,

1988. Leak-before-break and leak detection systems ofCANDU fuel channels. In: CNS 9th Annual Conference,13–15 June, Winnipeg, Canada, pp. 401–407.

Sharma, V.K., Manganvi, S.S., 1997. Sourcing of FissionProduct Activity in the Moderator/Cover gas System ofRAPS-2/MAPS-1/NAPS-2: An Analytical Approach.Bhabha Atomic Research Centre, Mumbai, India BARC/1997/I/006.

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a new method for detecting pressure tube failures in indian phwrs

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