Transmutation of radioactive nuclear waste

Abstract

Lack of a safe disposal method for radioactive nuclear waste (RNW) is a problem of staggering proportion and impact. A typical LWR fission reactor will produce the following RNW in one year: minor actinides (i.e. {sup 237}Np, {sup 242-243}Am, {sup 243-245}Cm) {approx}40 kg, long-lived fission products (i.e, {sup 99}Tc, {sup 93}Zr, {sup 129}I, {sup 135}Cs) {approx}80 kg, short lived fission products (e.g. {sup 137}Cs, {sup 90}Sr) {approx}50kg and plutonium {approx}280 kg. The total RNW produced by France and Canada amounts to hundreds of metric tonnes per year. Obtaining a uniform policy dealing with RNW has been blocked by the desire on one hand to harvest the energy stored in plutonium to benefit society and on the other hand the need to assure that the stockpile of plutonium will not be channeled into future nuclear weapons. In the meantime, the quantity and handling of these materials represents a potential health hazard to the world's population and particularly to people in the vicinity of temporary storage facilities. In the U.S., societal awareness of the hazards associated with RNW has effectively delayed development of U.S. nuclear fission reactors during the past decade. As a result the U.S. does not benefit from the large investment of resources in this industry. Reluctance to employ nuclear energy has compelled our society to rely increasingly on non-reusable alternative energy sources; coal, oil, and natural gas. That decision has compounded other unresolved global problems such as air pollution, acid rain, and global warming. Relying on these energy sources to meet our increasing energy demands has led the U.S. to increase its reliance on foreign oil; a policy that is disadvantageous to our economy and our national security. RNW can be simplistically thought of as being composed of two principal components: (1) actinides with half lives up to 10{sup 6} years and (2) the broad class of fission fragments with typical half lives of a few hundred years. One approach to the RNW storage problem has been to transmute the radioactive elements into other radioactive isotopes with much shorter half-lives. Transmutation of both RNW components using neutrons has been discussed and studied over the past four decades. Most transmutation studies have examined the feasibility of using neutron-induced reactions where the neutrons would be provided by accelerator-based spallation neutron sources, tokamak fusion reactors, sub-critical fission reactors and other novel concepts. Studies have shown that all proposed transmutation processes to treat RNW using neutron reactions are deficient or marginal at best from the point of view of energy consumption and/or cost. We suggest an alternative approach that has not been considered to date: the transmutation of RNW elements using high-energy photons or gamma rays. The photo-disintegration of RNW may provide an effective way to treat reprocessed waste; waste that has been chemically separated or the residual waste left over after neutron processing. Photo-disintegration is attractive in that any isotope can be transmuted. This approach is now potentially practical because of the development of micropole undulators (MPUs) that allow us to use small storage rings to economically generate photons with gamma-ray energies and to tune these ''gamma rays'' to the peak of the cross-section resonance for various RNW elements. Because the cross sections for all RNW nuclei have a broad peak with the maximum in the 12-18 MeV range, a single MPU could be used to treat both actinide and fission fragment components of RNW. The goal of this study is to make estimates of the reaction rates and energy efficiency of the transmutation of typical RNW elements using gamma rays to establish whether or not gamma-ray transmutation should be examined as a viable alternative solution to RNW warranting further study.