Decay heat removal in sodium cooled fast Reactors-An overview

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Shutdown systems and decay heat removal systems form the backbone of the nuclear plant protection system. While the former ensures safe shutdown of the fission reaction, the latter is essential to remove the heat from the decay of the fission products during earlier fissions. Thus, heat continues to be generated even after shutdown. Residual or decay heat removal (DHR) systems are needed to ensure that fuel clad temperatures do not rise beyond limits after the reactor shutdown. In fast reactors which have higher power density in the core, decay heat removal becomes very important to keep the core temperatures and structures within acceptable limits as otherwise fuel failure can result with its attendant consequences of adding radioactivity to the primary coolant. Also, in situations of loss of off-site and on-site power, this heat needs to be removed. Toward this, one has to ensure that the design is amenable to natural circulation cooling in different situations. The primary coolant system in Sodium cooled fast Reactors (SFR) can easily be configured to provide natural circulation shutdown heat removal. Different reactors designed/built/operated have resorted to different ways of decay heat removal. This paper traces the evolution of different decay heat removal options used by different fast reactor designs in different countries. Also presented is a brief review of some of the studies carried out on the efficacy of the different DHR systems besides parametric investigations carried out in different countries, backed up by experiments. The paper makes important observations on the type of DHR system to be adopted based on the above studies.