Radioactive corrosion products in experimental fast reactor 'Joyo'

The liquid metal-cooled fast breeder experimental reactor ''Joyo'' attained the criticality in April, 1977, and the operation of 12,968 hours with the Mk-1 core was completed in December, 1981. Thereafter, the change to the Mk-2 core was carried out for 11 months, and now it is operated as an irradiation reactor. During the Mk-1 operation, the break of fuel never occurred, and the contamination with fission products was not found at all. However, with the increase of the cumulative operation time, the rise of surface dose rate in the piping and equipment of primary cooling system due to the accumulation of radioactive corrosion products was observed. The corrosion products in primary sodium are formed by the neutron activating reaction of the components of SUS 316 stainless steel and impurity metals, and released into coolant by sodium corrosion. Accordingly, the purity control of primary sodium and especially the control of oxygen contributing to corrosion are important. The source of formation, release, transfer, deposition and transfer to the outside of primary system of corrosion products in a LMFBR are explained. The distribution of corrosion products in Joyo and the countermeasures to the problems of corrosion products are reported. (Kako, I.)