Year
2015
Abstract
We have made non-destructive measurements as part of the investigation into the cause of the increasing count rate on the Plutonium Inventory Measurement System (PIMS) at the Rokkasho Reprocessing Plant in Japan. PIMS measures the total (uncorrelated) neutron counting rate from MOX material in gloveboxes. From 2009 to 2010 the counting rate steadily increased by about 9% without any change in the plutonium content of the gloveboxes. We have made neutron and gamma measurements on samples of material removed from plant gloveboxes. Neutron multiplicity analysis was performed to determine the ratio of (a,n) to spontaneous fission neutrons from the material. This showed that the ratio was higher than that expected from pure MOX material. The same detector showed a correlation between increasing (a,n) emission and lower average neutron energy. These results are consistent with increased (a,n) emission being caused by the 19F(a,n) reaction. These results are not consistent with increased emission caused by moisture alone. An increase in the counting rate of 9% would require about 7% by weight of water, which would have to be accumulating at a constant rate. The same increase could be caused by 200 ppm of fluorine. Gamma measurements, with a HPGe detector, show clear lines from 19F(a,n) and 19F(a,p) reactions in samples of the removed hold-up material that are not present in clean MOX samples. This again is clear evidence that fluorine is present in the material in the glovebox. Gamma measurements of PuO2 and PuF4 were carried out at Los Alamos National Laboratory for comparison purposes.