Evaluation of neutron fluence on JOYO core structure components

Ishida, Koichi; Maeda, Shigetaka   ; Saikawa, Takuya*; Masui, Tomohiko*

It is essential to evaluate the radiation damage of core structure materials used for core support plate and reactor vessel to maintain the safe operation of nuclear reactor plant. Therefore, surveillance tests for the irradiated specimen have been conducted in the experimental fast reactor JOYO to assure the integrity and to evaluate the life time. Neutron fluence and related spectral information are key palameters in evaluation of irradiation effects on the mechanical properties. They are usually predicted based on the calculation using the DORT two-dimensional transport code. In order to evaluate the calculation accuracy, the surveillance irradiation rigs (SVIRs) with dosimeter sets and gradient-monitor to monitor neutron fluences and temperatures were loaded several positions of the JOYO MK-II core. They were irradiated between 34 and 35 cycle. Based on the verification, the JOYO neutron field was precisely characterized and the calculated neutron flux at the positions of irradiated specimen and those of the core structure components need to be evaluated were corrected based on the experiments. As a result of this study, the following items are concluded: (1)The maximum fast neutron fluence (E 0.1Mev) on surveillance test specimen is determined as 2.0710n/cm at 9th row of the core. (2)The neutron fluences at the positions of surveillance test specimen were higher than those of the core structure components. (3)For the core support plate which seems to be most critical for JOYO life time, the fast neutron fluence at present is 9.3810n/cm and will reach 2.3110 n/cm at the end of life. The fast neutron fluence of reactor vessel is 3.1210n/cm at present and will reach 4.8310n/cm at the end of life.