Testing of an Iodine and Tritium Removal System for Advanced Tritium Pretreatment Off-Gas

The reprocessing of used nuclear fuel would release volatile radionuclides into the off-gas of a processing plant, including 3H, 14C, 85Kr, and 129I. For a reprocessing plant sited within the United States and governed by US regulations, the abatement of some or all of these radionuclides from the plant off-gas would be required before discharge of that gas to the environment. A potential simplification of the off-gas management system can be achieved through an efficient advanced tritium pretreatment (ATPT) step in which the UO2 fuel is oxidized by NO2 prior to dissolution. Oxidation of the UO2 fuel matrix results in the release of tritium contained within. Up-front removal of tritium from the fuel in a pretreatment step, followed by the abatement of tritium from the advanced tritium pretreatment off-gas (ATPTOG) stream, can minimize or eliminate the distribution of tritium throughout the plant, also decreasing or eliminating the need for tritium capture on multiple off-gas streams. The use of NO2 as the oxidant allows tritium pretreatment to be performed at lower temperatures and may result in the quantitative release of iodine from the fuel. A total of five tests examined AgA as a potential iodine sorbent at an operating temperature of 150°C and both 3A MS and silica gel as potential tritium sorbents with an operating temperature of 40°C. 3A MS and silica gel are intended for use as regenerative sorbents, in which the adsorbed tritium (as tritiated water or tritiated nitric acid) is loaded onto the sorbent at operating temperature and then subsequently desorbed at high temperature into a small-volume gas stream for recovery and conversion to a waste form. This testing leaves many fundamental questions regarding the ATPTOG iodine and tritium abatement systems unanswered. These include questions regarding sorbent capacity and sorbate desorption. Overall, this testing indicates that tritium and iodine abatement from an ATPTOG stream by solid sorbents is currently at a low stage of technological development and that deployment of such an abatement system would require substantial additional research and development.