Thermo-mechanical behaviour of primary system components of future FBR during crash Cooling: A numerical simulation

In a pool-type fast breeder reactor, the shutdown by SCRAM involves crash cooling of primary sodium at the rate of 480 K/h. This high cooling rate can develop high thermal stresses at the thicker junctions that are immersed in sodium, due to a lower Fourier number and a higher Biot number, which results in higher Fitz number. Thermo-mechanical analysis of various critical locations within sodium with the transient loading, and parametric studies with varying cooling rates and thicknesses has been carried out. Due to the drop in sodium temperature during crash cooling, there is a fall in the sodium level, resulting from the volumetric contraction of sodium. Thus the locations of the hot pool components that are at the free level interface of sodium and argon experience an abrupt shift in the ambience, i.e., from sodium to argon cover gas. A numerical model has been developed to simulate the complex loading condition using CAST3M code. The model has been validated with theoretical equations and the overall heat balance equations. Transient analysis of all the hot pool components has been performed to estimate the structural damage due to thermo-mechanical behavior during crash cooling. Based on this study, it is concluded that for the future design of fast reactors with 60 years of design life, controlled cooling is not essential from the structural damage point of view. The recommendation simplifies the design of decay heat removal system.