A Simple Nondestructive Measurement System for Spent-Fuel Management

A method for nondestructive assay of spent light water reactor fuel assemblies based on a combination of active and passive neutron counting is presented. After geometrical optimization, the Fuel Assembly Monitoring System (FAMOS) is a rather simple system. It allows the burnup, initial enrichment, type of fuel (uranium or mixed oxide), and criticality of the spent-fuel assembly to be determined.The results of a characterization program with emphasis on boiling water reactor (BWR) fuel assemblies are discussed. Burnup-dependent neutron emission data for spent BWR fuel are now available. The effect of steam void on plutonium and curium buildup is demonstrated. Because of this effect, the axial measurement position is of importance for an accurate assay. If the measurement is done at the upper part of the BWR fuel assembly, the error in burnup remains below ±2 GWd/tonne U, and the initial enrichment can be determined with an accuracy of ±15%. This still allows a clear distinction between the different enrichment regions used for BWR fuel assemblies.