This invention relates to a process for releasing volatile fission products from irradiated fuel pellets with either NO.sub.2 or a mixture of NO.sub.2 with its dissociation products, O.sub.2 and NO.
One of the problems encountered in the reprocessing of irradiated nuclear fuel is the extreme difficulty in separating the volatile fission products from the reaction solutions of the reprocessing process. This is especially important with regard to removing tritium in that the tritium isotope is extremely difficult to separate from aqueous solutions. The hydrogen isotope tritium, has a half life of about 12.26 years and is a ternary fission product produced in small mass quantities, approximately 1/1000 of an oz. in each ton of fuel. However, because of the volume of irradiated fuel anticipated to be produced in order to satisfy the world's electrical needs for the year 2000, there would be significant volumes of tritium produced. Recovery of tritium in nuclear fuel reprocessing plants of conventional designs is not practical because of the very small amounts of tritium in each ton of fuel. Tritium, in the form of tritiated water, becomes intimately mixed with the thousands of gallons of process water in the plant, Isotopic separation of the tritium from this large volume of water would be required before the water could be released as liquid or vapor. Since the isotopic separation of tritium in such volumes of water is not practical, it is necessary to develop a technique for the removal of volatile fission products from irradiated fuel prior to aqueous reprocessing.
The problem of removing volatile fission products from irradiated fuel is encountered when reprocessing irradiated uranium dioxide fuel and/or irradiated mixed oxides, i.e., UO.sub.2 and PuO.sub.2. The problem of removing the volatile fission products is increased by the fact that volatile fission products are trapped within the crystal lattice of the irradiated fuel and thus cannot be removed by grinding or pulverizing the fuel.
A process for the removal of volatile fission products from irradiated fuel has been developed by the Oak Ridge National Laboratories and is referred to as voloxidation and is described in the Oak Ridge National Laboratories Report ORNL-TM-3723. Voloxidation is a process for oxidizing irradiated fuel in the presence of oxygen at a controlled temperature in order to form a very fine powder of U.sub.3 O.sub.8. The voloxidation process is sensitive to temperature. It has been estimated that in a large commercial plant the reaction temperature would have to be maintained at a narrow range probably at about 480.degree. C. plus or minus 10.degree. C. Because of the narrow temperature range, the difficulty in operating large scale reprocessing plants would be increased. Therefore a method for releasing the volatile fission product from irradiated fuel in a commercially reasonable time and under commercially reasonable processing conditions would be desirable and could result in major process economies in reduction of costs for the reprocessing of nuclear fuels. The present invention is such a method.