Retention of gaseous isotopes

Retention of gaseous fission products during fuel reprocessing has, in the past, been limited to a modest retention of 131I when processing fuels decayed less than about 180 days. The projected rapid growth of the nuclear power industry along with a desire to minimize environmental effects is leading to reassessment of requirements for retention of gaseous fission products, including 131I, 129I, 85Kr, 3H and 14C. Starting in the late 1960s, a significant part of the LMFBR reprocessing development programme has been devoted to understanding the behaviour of gaseous fission products in plant process and effluent streams, and the development of advanced systems for their removal. The general environmental effects of gaseous fission products are discussed and the existing and proposed regulations in the USA regarding their release are summarized. The paper also describes specific gaseous fission-product removal systems. Systems for iodine control include methods for evolving up to 99% of the iodine from dissolver solutions to minimize its introduction and distribution throughout downstream equipment. An aqueous scrubbing system (Iodox), using 20M HNO3 as the scrubbing media, effectively removes all significant iodine forms from off-gas streams while handling the kg quantities of iodine present in head-end and dissolver off-gas streams. Silver zeolite is very effective for removing iodine forms at low concentration from the larger-volume plant off-gas streams. Removal of iodine from plant liquid effluents by solid sorbents either prior to or following final vaporization appears feasible. Krypton is effectively released during dissolution and can be removed from the relatively small volume head-end and dissolver off-gas stream. Two methods appear applicable for removal and concentration of krypton: (1) selective absorption in fluorocarbons, and (2) cryogenic absorption in liquid nitrogen. Retention of tritium in a reprocessing plant is difficult as the tritium combines and distributes with the water inventory throughout the plant. To avoid this, a high-temperature (5500C) volatilization step, termed voloxidation, is being developed at Oak Ridge Natl. Lab. which drives most (up to 99%) of the tritium out of the fuel and traps it prior to aqueous processing. Other processes for tritium control under study include total retention of water and isotopic separation of tritium from plant aqueous effluents. Carbon-14 is produced in oxide fuels primarily by n-p reactions with 14N contaminant in the fuel. Little is known about the actual behaviour of 14C in a reprocessing plant, and most of the effort is directed toward identifying the chemical forms and process sources of the 14C effluents. (author)