Summary of requirements on SASS for a large FBR core

In order to improve the reliability of safety system of FBR employing passive safety functions, requirements on a self-actuated shutdown system (SASS) have been summarized, which employs Curie-point magnets in its safety rods, for a large homogeneous reactor core (1500 MWe), which was designed by JNC in JFY1999 as one of candidates in the Feasibility Studies on Commercialized FBR System. The requirements were based on the sensitivity analyses, conducted by using the safety analysis code SAS4A, of uncertainty factors that affect the maximum coolant temperature in unprotected loss-of-flow events. The study has given the following requirements: 1) The detachment temperature of the SASS magnet is to be set above 638degC (911K) to avoid the possibility of unintended rod drop under normal operations, and to be set below 666degC (939K) to prevent coolant boiling under ULOF conditions conservatively assuming a boiling temperature of 960degC in the plant. 2) The parameter that has the largest sensitivity on the maximum coolant temperature under ULOF conditions is the exit coolant temperatures of the adjacent fuel assemblies to the SASS assembly, with a sensitivity coefficient of 20.5degC/σ (1σ=6.7degC for the exit coolant temperature). It has been concluded based on the parametric analyses that a SASS design even with a nominal detachment temperature of 666degC, the SASS has a safety margin of about 3.4σ for the exit coolant temperature of the most sensitive adjacent fuel assembly in preventing coolant boiling under ULOF conditions, if the incoherency of SASS detachments are appropriately taken into account together with a more realistic criterion on the coolant boiling temperature (1018degC) under the ULOF conditions of this plant. (author)