A feasibility study of PIMS for the Dounreay evaporator cell

A calculation-based study has been carried out to determine the feasibility of monitoring the plutonium content of the Dounreay Evaporator cell by PIMS (Plutonium Inventory Monitoring System) using total neutron counting. The objective of the study was to optimize the disposition of neutron detectors so as to give counting rates proportional to the amount of plutonium in the cell (independent of its position), while keeping the number of detectors required to a minimum. An approximate model of the cell was developed using the Monte Carlo neutron transport code MCNP. Neutron fluxes (weighted by the 3He neutron cross-section) were calculated at various positions about the cell, for both the ''working'' and ''shut-down'' plutonium source configurations. A location within the cell was identified where the number of detector counts per source neutron showed little sensitivity to the source distribution. Consideration of the absolute count-rate appears to show that the technique is feasible, although the effect of sources of background on the reliable measurement of small amounts of plutonium has yet to be determined quantitatively. (author)