chapter  15
26 Pages

Severe Accident Analysis

The assessment of various energy generating options by worldwide experts concludes that the sodium-cooled fast reactors (SFRs) have high potential for providing affordable energy in 10-20  years in the countries mastering this technology. The conclusion has been derived after rigorous discussions involving comprehensively all the issues such as economy, safety, and environment. In the domain of safety, the concerns are expressed since the 1950s particularly referring to the risk of an uncontrolled power excursion in case of large-sized fast reactor systems and positive sodium void effects in case of SFRs. In the context of advanced reactor concepts, for example, Gen IV systems, treatment of severe accidents in the design is one of the key issues of R&D plans [15.1]. This requires complete understanding of various scenarios and the associated phenomena in the allied domains of science, engineering, and technology that can be hypothesized for robust safety demonstration. Accident progression in the fast reactor is considerably different from that in the thermal reactor. The thermal reactor core is optimized with respect to the fuel to moderator ratio for just optimum moderation, due to which any motion of the fuel material will lead to negative reactivity under loss of moderation. In the fast reactor, the core is not in optimum reactivity configuration. This means that any motion of fuel, depending upon core compaction and core expansion or dispersion, could introduce either positive or negative reactivity, respectively. If there is fuel melting, there will be core compaction due to the downward motion of molten fuel or fuel slumping, which will lead to large positive reactivity addition. This in turn will result in a superprompt critical excursion and release of a lot of thermal energy followed by mechanical consequences. The severe accident scenario in the fast reactor is defined based on this physics.