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Bhabha Atomic Research Centre,

Mumbai, India


The Indian Pressurised Heavy Water Reactors (PHWRs) of 220

MWe capacity have 306 fuel channels in the primary heat transport

(PHT) system. Each channel consists of 12 fuel bundles contained

in a pressure tube which is surrounded by a calandria tube. The calandria tube is surrounded by cold moderator. The primary coolant flows inside the pressure tube containing the 19 rod fuel bundles. There are several postulated accident scenarios where a quick, large voiding of certain sections of the channel is ex- pected. The fuel temperatures in these sections rise initially because of the stored heat in the fuel and the decay heat of the fuel. Later, at higher temperatures, the metal water reaction between steam and zircaloy cXad also produces heat. The modes of heat removal are convective heat transfer to the steam present in the channel and heat transfer to the moderator through the combi- nation of conduction, convection and radiation heat transfer processes. The metal water reaction has a significant effect on the peak cladding temperatures reached by the fuel. The reaction rate varies exponentially with clad temperature. In an accident involving total voiding of the channel, clad temperatures are significantly influenced by the steam flow rate. A t higher rate it leads to more reaction and hence more heat generation. On the other hand it leads to better convective heat removal.

Analysis of the above scenario has been carried out using the computer code H T / M O D 4 and presented in this paper. The hot- test channel of an Indian PHWR h a s been analysed for its hottest bundle. The stored heat and decay heat generation in different fuel rods have been accounted for. T h e metal-water/steam reaction is simulated in all zircaloy components. The analysis considers conduction, convection and radiation heat transfer in different components of the channel. Suitable assumptions have been made.

The results obtained have been discussed from the point of view of safety

1.0 INTRODUCTION The Primary Heat Transport System (PHTS) of a Pressurised Heavy Water Reactor (PHWR) of Indian design consists of inlet headers from which coolant is fed to the channels in the reactor core through feeder pipes. A s the coolant flows through the core it picks up heat generated in the fuel rods. The coolant then flows through feeder pipes to the outlet header. The hot coolant from the outlet header passes through the steam generator, on the secondary side of which steam is generated. The cold heavy water comes to the other inlet header through the pumps. The coolant path in the PHTS is in the form of a figure of eight, with the coolant flowing in opposite directions in adjacent channels. The largest diameter pipes in the PHTS are the inlet and outlet headers.

331 Unlike other water cooled reactors, the Indian PHWR has ahorizontal reactor core [1]. The pressure boundary in the core consists of 306 pressure tubes. Each pressure tube houses 12 short fuel bundles. Each fuel bundle consists of 19 fuel pins.

The pressure tube is surrounded by a thin calandria tube.

During an accident in a water cooled reactor the heat removal capability may get impaired, thusresulting in fuel damage and release of radioactivity. For e.g. in PHWRs a LOCA coupled with unavailablity of emergency core cooling system may have a potential for large release of radioactivity to the containment [2].

The effects of such accidents* are predicted using suitable analytical models. The models chosen depend on the type of analysis required.

The fuel elements of the channel are strongly coupled thermally to the coolant through convective heat transfer under normal operating conditions and are very weakly coupled to each other. But following an accident leading to loss of coolant, convective heat transfer reduces and fuel temperatures rise even when the reactor is shut down. A totally voided channel experiences significant heat exchange through radiation. The thermal coupling among fuel elements becomes strong. The pressure tube temperature also rises. However, the temperature of the calandria tube is not affected significantly since it is convectively cooled by the moderator. A rise in the temperature of pressure tube may lead to deterioration in its mechanical properties.

This may lead to deformation (balloning/sagging) of pressure tube. The mode of deformation and the temperature at which deformation occurs would depend on how the coolant tube's axial motion is restricted. The hot pressure tube may deform and touch the calandria tube leading to a temporary temperature spike in the calandria tube.

In a partially voided channel, the vapour and liquid phases may separate out. The temperatures Qf the fuel elements exposed to steam rise while the elements submerged in coolant remain at lower temperatures. This leads to large circumferential temperature gradients in the pressure tube and fuel elements.

1.1 Total Voiding of The Channel A channel may void totally for accidents such as LOCA coincident with total failure of the emergency core cooling system.

In such a case, the fuel rod temperatures rise and start radiating heat to the pressure tube. The pressure tube temperature rises. The pressure tube transfers part of the heat to the calandria tube by (a) conduction through the gas in the annulus between the two tubes, and (b) radiation to the calandria tube.

Finally the calandria tube transfers the heat to the moderator by convection.

A rise in temperature leads to a rapid deterioration in the mechanical properties of the sircaloy pressure tube. Thus, at higher temperatures the pressure tube deforms. There are two possible modes of deformation. One mode of deformation is balloning. This occurs when the pressure tube reaches high temperatures when internal pressure of the tube is still high. On ballooning it touches the calandria tube. The mode of heat transfer changes from radiation to conduction heat transfer, thus enhancing the heat transfer. The fuel temperatures continue to rise. At a certain temperature the fuel clad fails and the fuel bundle slumps on to the pressure tube. Additional contacts are established between fuel rods and -pressure tube leading to further enhancement of heat transfer to the ultimate heat sink (moderator).

The other mode of deformation is sagging. This occurs when the channel internal pressure of the tube is already low by the time it gets heated to higher temperatures. Subsequently the fuel bundle also slumps as the fuel clad fails. This again leads to further enhancement of heat transfer to the moderator, the ultimate heat sink.



The prefered mode of deformation of coolant channel should lead to maximum heat transfer contact area between coolant tube and calandria tube. The two independent modes of deformation of coolant tube are sagging and ballooning. The sagging deformation of a coolant tube is essentially due to bending deflection caused by fuel bundle weight. A contact with the calandria tube due to this mode of deformation can initiate either at a point between the two garter springs or at a point between a garter spring and rolled joint. The ballooning mode of deformation is essentially due to circumferential membrane stretching of the coolant tube caused by remaining PBT pressure. Due to its localised nature of deformation, the entire length of the coolant tube is susceptible to ballooning mode of deformation except the zone near to garter springs. The conusion which can be drawn through this discussion on the deformation pattern, is that the ballooning mode of deformation would generally lead to much higher contact heat transfer area than the sagging mode of deformation. Hence a ballooning mode of deformation is preferable over sagging mode. Such scenario may be achieved by delaying sagging mode during the heatup time of the coolant tube under accidental conditions, provided there is sufficient internal PHT pressure following LOCA. One of the important considerations during ballooning mode of deformation is the possibility of rupture of coolant tube. This possibility further enhances in case there is an initial defect in the coolant tube inner surface. In the following section, we discussed this issue of possibility of pressure tube rupture during ballooning under the presence of a flaw. The subsequent section of the present paper deals with the sagging behaviour of a coolant tube. The effect of coolant tube axial restraint on the sagging temperatrure is also highlighted.



As discussed above, ballooning mode of deformation of the coolant tube is a prefered mode of deformation. This mode of deformation is likely to provide more heat transfer area than the sagging mode of deformation.* However, there exists always a possibility of coolant tube rupture during ballooning. The probability of coolant tube rupture is high in the locations having defects. The defect may be caused due to unfavourable sliding of fuel bundles on the coolant tube inner surface during refuelling operation or following earthquake. A double ended rupture in coolant tube may lead to propagating damage. This may cause axial flying out of coolant tube into fuelling machine vault in case of axially free tube. This may

–  –  –

1.2.2 Analysis details Geometrical modelling- In this analysis, a cross section of the tube is modelled as a two dimensional case. Finite element technique is employed for this purpose. There are 180 isoparametric elements with 560 nodes. The mesh is modified to accomodate a particular size of the flaw to have large number of elements near the crack tip.

Location of flaws- Coolant tubes in a PHWR may have part through flaws. These flaws may be generated during manufacturing or during the movement of the fuel bundles while refuelling. A severe earthquake may lead to sliding of the fuel bundles inside the channel and may cause part through flaws.

These flaws act as stress risers during ballooning of the tube and may initiate rupture. The chances of having a flaw at the lower portion of the tube is more due to fuel bundle weights. On the other hand the maximum temperature along the circumference of the pressure tube during severe accident is likely to occur at the top surface of the tube due to channel stratification. However, in the present analysis, the location of the flaw is conservatively assumed at the maximum temperature point.

Temperature profile- The temperature profile following severe accident in a coolant tube can be known through a detailed thermal-hydraulic calculations. These profiles may change case by case depending upon the severity of the accident.

In the present parametric analysis, we have assumed a closed form expression for circumferential temperature profile as

T{6) = Tmin + AT[ 1-

Here At is the circumferential temperature drop and n is an exponent. Different computed temperature profile using thermal hydraulic codes can be simulated using this expression by assigning different values to Tmin, DT and n. It may be noted here that the lower value of n localises the high temperature zone near to ft =0. Thus uniform temperature is obtained aaymtotically by using a large value of n. In the present parametric study, we have varied DT from 100 Deg.C to 300 Deg.C. The exponent n has been varied to obtain linear, quadratic, cubic, quartic and uniform temperature profiles. The temperature is assumed to increase at a rate 5 Deg.C/sec to determine the creep properties of zircalloy.

334 Pressure loading- The temperature at which ballooning occurs in a coolant tube, depends on the internal pressure during ballooning. In a typical PHWR, the normal primary operating pressure is lOMPa. With the occurance of loss of coolant accident, the pressure decreases with a rate depending on the size and the location of the break. In the present analysis on ballooning, different coolant channel pressure ranging from lOMPa to 0.5MPa have been considered.

Computer code THESIS- The present analysis has been performed by using inhouse code THESIS (THErmal elastic-plastic AnalySIS), developed by the present authors. This is a 2-D/axisymmetric finite element code with the capability of considering geometric and material nonlinearities. The material nonlinearities include elastoplasticity and thermoplasticity.

The capability of the code has further been enhanced to analyse viscoplastic behaviour of materials. Four different general forms of creep laws, such as, power, exponential, strain hardening and time hardening, have been implemented in the code.

High temperature creep model- The progress of ballooning in a coolant tube is essentially due to flow of material at high temperature dominantly under internal pressure. This phenomenon can be modelled by using a high temperature creep model of Zr-2.5 wt% Nb. In the present study, we have used the creep model developed by Shewfelt [1984]. This model has been developed essentiallly through ballooning experiment on Zr-2.5 wt% Nb tubes at high temperature. The creep equations are given as follows.

E = exp(-36600/T) F = exp(-29200/T) G = exp(-19600/T) H = Cr-1105)3-72 Creep rate equation for 450 Deg.C to 850 Deg.C

–  –  –

Where tl is the time when T=700 Oeg.C and t2 is the time when T=850 DegC.

Rupture criteria- With the progress of ballooning, the local strain at the tip of the flaw increases. It is conservatively assumed that the rupture occurs when the local effective strain at the tip of the flaw reaches a value 100%. II 335 1.2.3 Analysis Results The ballooning analysis has been carried out for various assumed temperature profiles characterised by different combinations of DT and n. Three values of DT have been assumed.

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