Tritium and uranium inventory measurements with the JET AGHS precision calorimeter

The principle of temperature stabilization by inertial feedback control was further developed to provide a stable base for a thermoelectric calorimeter. Noise and long-term drift of the base temperature have been reduced to ±3×10^-10 Ks^-1. With this temperature stability, samples with large heat capacities, such as Amersham MKIV uranium getter beds for tritium transport and storage (total heat capacity of calorimeter chamber, uranium bed secondary containment and getter bed itself is 1370 JK^-1), can be measured with a reproducibility of ±2 μW. During a 14-day campaign, all Amersham getter beds presently held by JET were measured. The bed with the highest tritium content (22,40Ci) was repeatedly measured in 2-day intervals, and the results were found to follow the trend of the tritium decay well within the error band of ±0.4%, (±0.3% due to tritium decay heat uncertainty, and ±0.1% due to calorimeter calibration accuracy). Tritium-free getter beds showed a power output of 7.5±2 μW, due to the decay heat of the total sum of all isotopes contained in their depleted uranium inventory of 320 g. This compares reasonably well with subsequent measurements on a 4 kg depleted uranium test sample of certified composition, procured from British Nuclear Fuels Limited