Development and Testing of an Updated Plutonium Isotopic Monitor for Nuclear Materials Inventory Control

This paper describes the development and testing of an updated plutonium isotopic ratio spectrometer. The instrument is intended to perform automatic measurements of the plutonium isotopic ratios of samples contained in sealed cans and containers. The measured data is intended for use in nuclear materials inventory control and nuclear safeguards. The instrument constitutes a comprehensive measurement and handling system to determine the plutonium isotopic ratios which can be used in association with either neutron or calorimetry measurements in order to determine the masses or activities of the individual plutonium isotopes. It may also be used for the isotopic ratio measurement of mixed oxide fuel material containing a mixture of both plutonium and uranium where the uranium ratios are also determined. The instrument employs a high resolution high purity germanium (HPGe) gamma ray spectrometer and data acquisition electronics as well as incorporating an automatically adjustable variable aperture collimator to reduce the count rate at the detector where necessary. The detector has a graded lead shield and the instrument case provides some operator shielding. The HPGe detector can employ either electromechanical or liquid nitrogen (LN) cooling. The spectroscopic isotopic ratio analysis is performed either by the PC/FRAM Pu isotopic analysis software code developed at the Los Alamos National Laboratory (LANL) or the MGA Pu isotopic analysis software code developed at the Lawrence Livermore National Laboratory (LLNL). During an isotopic ratio measurement, the sample can is rotated on a turntable and raised and lowered so that the measurement includes both rotation and vertical scanning of the sample can. Performance data is reported from measurements of Am-241 and other radionuclides with gamma ray energies similar to the gamma rays arising from plutonium.