A multi layer core catcher concept for future sodium cooled fast reactors

Abstract

Sodium cooled fast reactors (SFRs) have an in-vessel core catcher to collect, contain and cool the core debris settling upon it following a severe accident. Liquid sodium with its excellent heat transfer characteristics is able to cool the core debris by natural circulation, rejecting heat to the safety grade decay heat removal system. The core catchers for reactors like Superphenix-1 in France and Prototype Fast Breeder Reactor (PFBR) in India are designed to handle the core debris resulting from seven subassemblies melting. But for future reactors it would be desirable if the core catcher is able to accommodate and cool debris resulting from a whole core melt down. Such a design would enhance the safety of the reactor by maintaining the main vessel integrity even for the worst accident scenario. Several design options of core catchers for fast breeders are discussed in the present work with particular emphasis on a multi layer core catcher. A multi layer core catcher comprising of a sacrificial layer of Molybdenum, delay bed of thoria or magnesia and the bottom most base layer made of SS316LN is proposed and its adequacy is substantiated by heat transfer analysis. The heat transfer analysis which is aimed at optimizing the thickness of the delay bed layer reveals that, 4cm thick thoria layer or 5cm thick magnesia layer can serve as delay bed in order to restrict the temperature at the core catcher bottom within design safety limits for core debris spread evenly on the core catcher plate.