Comparison of life of two types of decay heat exchangers used in prototype fast breeder reactor

Abstract

The decay heat generated by radioactive decay of fission products in the core after reactor is shut down is to be removed for limiting of temperatures of various components. In prototype fast breeder reactor (PFBR), a dedicated safety grade decay heat removal (SGDHR) system consisting of four independent loops of each 8 MWt heat removal capacity is used for removing decay heat. Each loop consists of a sodium to sodium heat exchanger (DHX) and sodium to air heat exchanger (AHX). While DHX transfers heat from the radioactive primary sodium to intermediate sodium, AHX dissipates the heat to atmospheric air. Diversity in the design is adopted from reliability consideration. This paper compares the design life of two types of DHX (Type-A & B). Both DHX are shell and tube type with primary sodium on shell side and intermediate sodium on tube side. While Type-A has the tubesheet design similar to IHX, Type-B is a U ‘ tube two-pass heat exchanger. In this tubes are supported at the top by two separate tube sheets (split tubesheets). The tubesheets along with attached shells are analysed using CAST 3M, issued by CEA, France. The creep-fatigue damage is evaluated as per RCC-MR (2002). For the specified design life of 40 years, the fatigue damage is small and the creep damage evaluated is 0.26 in the type-A and 0.32 in the type-B, thus meeting design code limits. Two types of DHX are found to have 121y and 154y as allowable life after considering appropriate factor of safety given in design code RCC-MR.