CHARACTERISTICS AND PERFORMANCE OF A LARGE VOLUME TWIN CELL HEAT-FLOW CALORIMETER FOR PLUTONIUM AND TRITIUM MEASUREMENT

In this paper the characteristics and performance of a large volume twin cell heat-flow calorimeter is described and reported. The transportable calorimeter incorporates a removable electrical calibration heater and an internal calibrated power supply for both calibration and performance checks. Provision is made for calibration using a calibrated external precision power supply or radioactive heat standards. The calorimeter is able to achieve a high degree of sensitivity and thermal stability through the use of high output voltage thermopile differential temperature sensors and enhanced thermal insulation. Using twin cell heat-flow measurement technology, a significant sensitivity improvement has been achieved for large volume measurement cells, each with a volume in excess of 50 litres. The present instrument achieves improved performance with an external heat sink made of a laminate of dry solid insulation combined with high thermal conductivity panels, which represents an improvement in technology over water bath calorimeters. The elimination of water as a heat sink medium reduces any potential criticality hazard and the possibility of tritium contamination. Measured precision and accuracy for different sample powers will be presented and minimum levels of detection are determined using the electrical calibration sample and zero power measurements. The stability of the calorimeter and its response to variations of ambient temperature has been studied. Measurements of both electrical samples and radioactive samples with varying thermal powers will be considered. Results will be compared for measurements at different thermal powers using both equilibrium end point power prediction and end point equilibrium power determination. The paper includes results of measurements of the spatial variation of sensitivity within a measurement chamber cell. Sensitive large sample volume calorimeters of this type have application to the measurement of a wide range of radioactive materials in variety of containers.