Calorimetry as a Safeguards Tool: The Routine Use of the Argonne Bulk Calorimeter for Plutonium Assay in the United Kingdom, and an Investigation of the Potential for Improvement of Measurement of Material Likely to be Met in the Field

During 1985 the UKAEA have made extensive use of an Argonne Bulk Calorimeter on loan from the IAEA. Over 100 plutonium-bearing samples of varying forms and geometrical arrangements have been measured, and the thermal power converted to plutonium mass using both decay-corrected historic mass-spectrometric and recent gamma-spectrometric determinations of isotopic ratios. Agreement between mass determined by calorimetry, by neutron coincidence counting, and declared masses corrected for decay has been very satisfactory when account is taken of all sources of error. This exercise gives great confidence in NDA by a combination of gamma spectrometry and either calorimetry or neutron coincidence counting, but a major current disadvantage of calorimetry as a safeguards technique is the length of time to make a measurement. Using some routine measurements and in addition some specially prepared samples, an investigation of the effect of sample geometry and thermal capacity on the time of measurement has indicated the need for improvements in both the preheating strategy and the control and predictive technology of the calorimeter. It is concluded that such improvements are technically feasible and may shorten measurement times to under one hour in many samples likely to be encountered in the field.