A systematic program to study the long term generation of gas which results when tritiated water is stored on molecular sieve is ongoing at the Tritium Systems Test Assembly (TSTA). The gas that is generated, as the tritium decays, may result in safety concerns because the pressure generated in the disposal container may lead to a failure of the container.

Tritiated water is generated from processing tritiated effluent waste gases from TSTA process systems and experiments. Tritium contaminated waste gases are generated primarily from glovebox purges and process system evacuation. The various tritium compounds in the waste gas are converted to the oxide form and absorbed on molecular sieve. The tritiated water is collected on a fixed molecular sieve bed and when saturated, the water is regenerated into a 60 liter moisture (MC) filled with molecular sieve (approximately 45 kg of either type 4A or 13X molecular sieves). When the waste container is removed from the waste treatment system the container is evacuated to approximately 200 torr. The 60 liter container contains approximately 11 kg of water with amounts of tritium varying from 200 Ci to 30,000 Ci per container. The tritium content is determined by ion chamber measurement. These containers are eventually packaged and buried in retrievable shafts at the LANL waste facility located on site. Because of difficulty in the disposal of tritiated waste, TSTA currently has 20 waste containers on site that are part of this study. Periodically the gas in the waste container is sampled. The gas composition is measured with a mass spectrometer and an ion chamber. Properties measured are; pressure, gas composition (hydrogen, oxygen, nitrogen, helium-3, …) and tritium content. The waste containers have been stored at TSTA beginning in 1990. Measurements began in 1992.

Gas is generated in the container from the decay of tritium to helium-3 and from the effects of radiolysis. For every mole of tritium that decays two moles of helium-3 are generated. In addition, as the beta particle from the tritium decay loses energy it can cause the decomposition of the water absorbed on the sieve. Previous experiments predict that for every mole of tritium that decays, up to 26 moles of hydrogen and 16 moles of oxygen can result. Measurements to date indicate that significant hydrogen is generated, however it is generated at approximately one half the amount predicted from the previous experiments. Little oxygen has been found. The amount of helium-3 generated does not correlate with the expected amount from the estimated from the tritium loaded on the MC.