Protected plutonium production at fast breeder reactor blanket – Chemical analysis of uranium-238 samples irradiated in the experimental fast reactor Joyo

For improvement of proliferation resistance, the protected plutonium production (PPP) concept has been proposed. The concept is to produce 238Pu which has high decay heat in fuels with appropriate 238Pu/239Pu ratio for higher proliferation resistance. Estimation of Pu production with high accuracy under various irradiation conditions should be needed for evaluation of fuel specification and its proliferation resistivity. In previous studies, 238Pu production from 237Np and 241Am has been experimentally investigated in our group (Osaka et al., 2005, Koyama and Mitsugashira, 2008; Koyama et al., 2010). However, experimental data of 239Pu production from 238U is not enough opened. In this study, 239Pu production from 238U is investigated on the basis of experimental data. Nine small 238U samples were irradiated in the experimental fast reactor Joyo under various irradiation conditions. Isotopic composition of U, Pu and Nd was measured by chemical analysis with accuracy. Plutonium-239 production ratio (a ratio of 239Pu atoms to initial 238U atoms) and burnup were also determined. Behavior of 239Pu production depended on sample loading position (e.g. neutron spectrum), and the tendency is different between fuel stack and reflector region. In order to evaluate this spectrum dependence quantitatively, the 239Pu production rate is defined as the ratio of 239Pu production per 1026 n/m2 of neutron fluence. The 239Pu production rate was decreased linearly with increasing fast flux ratio (a ratio of fast flux with energy of more than 0.1 MeV to the total flux). Therefore, the 239Pu production from 238U by neutron capture reaction could be evaluated under wide irradiation area and/or position based on experimental results.