Design and modification of steam generator safety system of FBR MONJU

Matsuura, Makoto; Hatori, Masakazu; Ikeda, Makinori

MONJU is a prototype Fast Breeder Reactor (FBR) in Japan. The sodium-water reaction in the steam generator (SG) is one of the important safety assessment items for a sodium cooled reactor like MONJU. A latest evaluation method on the design margin against the overheating tube rupture showed that it had not enough margins on the present SG system in the worst case. For improving that margin, it needs to shorten the time of the sodium-water reaction by earlier water leak detection in the SG and sooner water ejection from the SG tubes. Therefore MONJU is now carrying out the following modification works (1) addition of steam relief valves, (2) addition of a gas pressure gauge with changing the interlock logic and lowering the trigger level, (3) reducing the opening of the valves on the SG gas flow line to the dump tank because of earlier detection for the pressure rise. After this modification, the design margin of the SG system will be sufficiently improved. For the tube materials, the value of the cumulative damage for the creep failure will be much smaller than 1, and the equivalent stress for the ductile failure will be much smaller than the tensile strength.