Investigation into Cause of Increasing Count Rate on PIMS at RRP (III)

The Plutonium Inventory Measurement System (PIMS) is permanently installed on glove boxes (GB) in U-Pu co-denitration process at Rokkasho Reprocessing Plant (RRP) and collects data of total neutron count rates (Singles) all the time. Singles measured by PIMS have been increasing continuously even under the condition that the amount of nuclear material have not been changed since the completion of commissioning test in 2009. As a result of various investigations, the increased 19F(a,n) reaction resulted from the higher fluorine contamination in residual MOX powder in the process was considered to be the main cause of the PIMS issue. However, it was not clear how the fluorine was generated and absorbed into MOX powder. Therefore JNFL planned a test to study the mechanism of the fluorine generation and absorption by using MOX and PTFE (polytetrafluoroethylene) in cooperation with Japan Atomic Energy Agency (JAEA). The result of the test showed that Singles continuously increased when MOX and PTFE existed together in the same environment. Then, the differences in forms of PTFE (plates or powder) and in contact conditions between MOX and PTFE (contacted or separated) led to the different trends toward an increase in Singles. It was found that the Singles increase in PIMS can be roughly extrapolated from an estimated fluorine absorption rate obtained from the test results. This paper describes the consideration on the mechanism of the Singles increase in PIMS, which is caused by fluorine materials used in the process equipment.