Methodology for estimation of conditional probability of stagnation channel break for PHWRs

A methodology to estimate the conditional probability of a break in inlet feeder being the stagnation channel break in Pressurized Heavy Water Reactors is presented in this paper. According to AERB Safety Guide AERB/SG/D-5, rupture at any location of PHT system up to size of double ended largest feeder pipe belongs to category-2 events which are events of moderate frequency. For a particular range of break sizes in inlet feeder, the flow in the corresponding reactor channel reduces almost immediately to stagnant/very low flow conditions and remains sustained due to high flow inertia of PHWR pipe structure. As it is a single channel event out of hundreds of channels, the signal for reactor trip is generated very late. Consequently the fuel temperature rises sharply and fuel failure may occur. The probability of break in a feeder leading to flow stagnation conditions in the corresponding channel is low but needs to be estimated. In this paper the conditional probability of stagnation channel break for feeder pipe is estimated deterministically for a reactor similar to Indian PHWR. In this work, the stagnation channel break is identified if minimum critical heat flux ratio (MCHFR) is sustained lower than one anywhere in the corresponding reactor channel. It leads to sudden rise in fuel clad temperature due to sudden significant reduction in heat removal. There is a range of break sizes for which the clad surface temperature rises above fuel failure limits. Depending on this range and break locations in the feeder, the conditional probability is estimated. It is assumed that conditional probability depends on the range of break sizes leading to stagnation. It is observed from the calculations that the conditional probability of stagnation channel break is significant. The analysis has been carried out using thermal hydraulic code RELAP5/Mod 3.4.

► System modeled using RELAP5/Mod 3.4.
► Stagnation channel break is identified for which MCHFR less than one is sustained in the core.
► Stagnation break range increases as the break location shifts towards core.
► Conditional probability of stagnation channel break is 0.2458.