DEVELOPMENT OF A URANIUM OXIDE CANISTER COUNTER FOR THE ROKKASHO REPROCESSING PLANT*

We report on the design of a uranium oxide canister counter that will be used for measurement of the uranium output stream. An initial design was proposed that followed earlier canister counter designs. The counter consists of a hollow cylinder into which the uranium oxide canister is lowered from the top. There are no doors or other moving parts. A graphite plug at the counter bottom is used to maintain detection efficiency at the lower detector end. Extensive Monte Carlo neutron and photon simulations were conducted to evaluate the counter performance. Two problems were discovered, a slight multiplication at high-mass loadings and a nonuniform axial response due to the presence of j unction boxes, an open top, and other interferences. A novel design that exploited plastics other than the common polyethylene was used to flatten the axial response. The multiplication effect was comected using the “known-alpha” method of neutron coincidence counting.