Modification of steam generator system to prevent overheating tube rapture accidents at MONJU

MONJU is a prototype fast breeder reactor (FBR) of Japan. Sodium-water reaction in the SGs is one of important safety assessment items for sodium cooled reactors. A latest evaluation method on the overheating tube rupture was introduced to evaluate the steam generator (SG) tube failure of MONJU. The results revealed that the design margin against the overheating tube rupture, which would be caused by a creep failure or a ductile failure, was not enough on the present SG system in the worst case. For increasing the margin, it is required to shorten the time of the sodium-water reaction by an earlier water leak detection of the SG and a sooner water blow from the SG tubes. Therefore the following modifications are to be taken; [1] provision of supplementary steam relief valves, [2] addition of another cover-gas pressure gauge and improvement of the interlock logic, [3] alteration of the trigger level of the cover-gas pressure, [4] regulation of valve opening in the cover-gas line. After these modifications, the design margin would be increased: the value of the cumulative damage for the creep failure would be sufficiently smaller than 1 and the equivalent stress for the ductile failure would be sufficiently smaller than the tensile strength, respectively. (authors)