The Tritium Systems Test Assembly self-assaying portable uranium bed

Abstract

Tritium storage at the Tritium Systems Test Assembly (TSTA), and at proposed tritium-burning fusion reactors, is accomplished with uranium beds. Inventory determination at TSTA is performed by off-loading the beds into a known volume and using PVT measurement combined with mass spectroscopic analysis to compute the tritium quantity. The accuracy of this approach is approximately 3 per cent. Transferring tritium at TSTA to experiments not part of the main flow loop is done with pressurized, secondarily contained gaseous cylinders. Both inventory determination and tritium transfer involve more tritium handling than would be necessary if the uranium storage beds had a self-assay feature and if the beds were portable with a means of connecting to glovebox experimental piping. A uranium bed having a self-assaying feature, portable and with means to make transfer connections to glovebox-contained piping, has been designed and is in operation at TSTA. The bed is insulated within a highly evacuated secondary enclosure which provides containment in the event of a tritium leak from the uranium bed. The high thermal resistance between the bed and the secondary enclosure surfaces, which are at room temperature, causes any heat generation within the uranium bed to result in a large, easily measurable, and repeatable temperature rise. The decay heat of tritium is approximately 0.309 watt per gram. In the TSTA uranium bed, this causes a temperature rise of 15 degrees Celsius which is sensed by three platinum resistance thermometers sandwiched in the wall of the bed. Because of the very small quantities of heat transferred and because of the high thermal resistance from the bed to the environment, the bed is essentially isothermal. The temperature rise of the bed (compared to the temperature of the vacuum jacket) can be sensed to less than 0.1 degree Celsius, which results in an ability to detect tritium down to 100 curies. Stabilization time is minimized by sizing the bed thermal mass so that the temperature rise resulting from hydriding is approximately the same as the steady state temperature rise caused by decay heating. Several tritium loadings have been performed with this bed and transfers between gloveboxes accomplished. This paper describes the design of the uranium bed and the results of the self-assay measurement.