Year
1990
Abstract
Reactor facilities worldwide are beginning to use fuel assemblies made with reprocessed plutonium. The fresh mixed-oxide (MOX) assemblies, which are generally stored underwater at reactor facilities in preparation for core reloading, can contain several kilograms of plutonium. Safeguarding these assemblies in a timely fashion is very important because of the quantity and the accessibility of the plutonium. An underwater neutron coincidence counting system has been developed for measuring the plutonium mass in MOX assemblies. Neutron signals, which come primarily from the even plutonium isotopes, are measured with 3He tubes in a fork-type detector. By using standard techniques to correct for neutron multiplication effects on the coincidence counter response, the system is capable of measuring the effective 240Pu mass accurately. Fuel pin removal and replacement patterns for 6% of the pins have been investigated and assayed correctly. Independent of facility operator data, the system determines the boron concentration in the water and adjusts for boron poisoning effects.