Investigation result and correction proposal for neutron count rate increase of Plutonium Inventory Measurement System at RRP (2)

It is observed that the measured value of PIMS (Plutonium Inventory Measurement System), installed in U-Pu co-denitration building at RRP (Rokkasho Reprocessing Plant) has been monotonously increasing although there is no inventory change after the end of test operation in 2009. Since PIMS measured value increasing leads to negative MUF systematically in the case of small inventory and suspension condition though increasing value is negligibly small compared with throughput, several investigations were performed and it was found that (α, n) reaction by fluorine extricated from process equipment is the main cause. Because it is quite difficult to grasp both the quantity of fluorine separated from equipment material in glove boxes and the distances between nuclide emitting alpha particles and fluorine under the actual condition, fluorine behavior was hypothesized to meet with each operation condition and correction function for each glove box was established empirically from the measured values. The validity of this correction method was also confirmed. We are planning to apply the correction to material accountancy in future. (author)