Isothermal Calorimeters Applied to the Measurement of Plutonium Residues for Plant Post Operational Clean-Out

The Non-Destructive Assay (NDA) of residues is required for material accountancy and criticality control during the post operational clean out decommissioning phase of plutonium pyrochemical recovery facilities. Residues can be problematic for NDA methods using gamma ray and neutron measurement. Errors can be caused by multiplication in lumps and high (α, n) production rates (neutron measurements), and self shielding and matrix heterogeneity in the case of gamma ray methods. The paper describes the application of ANTECH Isothermal Calorimeters to the measurement of the plutonium content of residues from pyrochemical recovery processes. Almost all of the energy produced in the plutonium decay process is released with the α-particles, the energy released by fission is a very minor contributor due to the much longer half life compared with that for α-decay. The α-particles are slowed down over a very short distance within the material so that almost all the thermal power generated by the decay is deposited within the sample. Thus if the thermal power is measured, and the relationship between power generated and plutonium mass is known, then the plutonium content of the material can be determined. The ANTECH Isothermal Calorimeter consists of three concentric cylinders, each maintained at a different temperature (innermost highest, outermost lowest) by an automatic control system, so that heat flows from the inner cylinder (measurement chamber) to the outer cylinder where heat is drawn away by a controlled stream of air. The basis of the measurement involves trading precisely measured electrical power for the thermal power produced by the radioactive decay of the sample. A ‘power difference’ mode is used in which the measurement chamber (the inner cylinder) is maintained at a constant temperature by electrical heating. When a heat producing sample is inserted into the measurement chamber, the control system reduces the applied electrical power, as it is replaced by the thermal power produced by the sample. At equilibrium, the difference in applied electrical power gives the sample power. The plutonium mass of the sample is determined from this measured power and a knowledge of the sample effective specific power derived from gamma ray isotopic measurement. Calorimetry is potentially the most accurate NDA method for measuring plutonium. The thermal power is related directly to simple physical quantities (voltage and current) and is immune to the matrix interferences that affect neutron and gamma-ray techniques. Moreover, corrections are not required for sample inhomogeneity or chemical form. The main disadvantage of calorimetry is the time to achieve thermal equilibrium. This can be alleviated by pre-heating of the sample to approximately the internal temperature of the thermal element or by application of an equilibrium power prediction algorithm. ANTECH have been successfully producing calorimeters for 12 years during which in excess of 20 ANTECH calorimeters have been in service in various USA DoE and UK MoD facilities for the measurement of plutonium residues, achieving a total of almost 250 ‘calorimeter operational years’. In all approximately 10,000 individual measurements have been carried out making a significant contribution to decommissioning programs.