SIDEBAR 1: Economic Perspective for Uranium Enrichment he future demand for enriched are the customary measure of the effort uranium to fuel nuclear power required to produce, from a feed material plants is uncertain, Estimates of with a fried concentration of the desired T this demand depend on assump- isotope, a specified amount of product en- tions concerning projections of total electric riched to a specified concentration and tails, power demand, financial considerations, and or wastes, depleted to a specified concentra- government policies. The U.S. Department tion. For example, from feed material with a of Energy recently estimated that between uranium-235 concentration of 0.7 per cent now and the end of this century the gener- (the naturally occurring concentration), ation of nuclear power, and hence the need production of a kilogram of uranium enrich- for enriched uranium, will increase by a ed to about 3 per cent (the concentration factor of 2 to 3 both here and abroad. Sale of suitable for light-water reactor fuel) with tails enriched uranium to satisfy this increased depleted to 0.2 per cent requires about 4.3 demand can represent an important source SWU. of revenue for the United States, Through Gaseous diffusion is based on the greater fiscal year 1980 our cumulative revenues rate of diffusion through a porous barrier of from such sales amounted to over 7 billion the lighter component of a compressed dollars, and until recently foreign sales ac- gaseous mixture. For uranium enrichment counted for a major portion of this revenue. the gaseous mixture consists of uranium The sole source of enriched uranium until hexafluoride molecules containing 23$ 1974, the United States now supplies only uranium-235 ( UF6) or uranium-238 238 about 30 per cent of foreign demand, New ( UF6). The enrichment attainable per dif- enrichment capacity planned in this country fusion unit is quite low, being limited theo- should include means of reducing enrichment retically to less than the square root of the 238 235 costs to permit capturing a larger share of UF6 to UF6 mass ratio, or about 1.004. the foreign trade in this commodity. Laser Therefore, the slightly enriched product from isotope separation shows promise of ac- one diffusion unit, consisting of a com- complishing this goal. pressor and a diffusion chamber, is passed Currently, gaseous diffusion is the process through a second unit whose product in turn by which uranium is enriched at the is passed through a third unit, and so on. large-scale production facilities in the United (The theory of separative work and optimal States. Three such facilities exist (at Oak arrangement of separation units was Ridge, Tennessee; Paducah, Kentucky; and pioneered by R. E. Peierls and P. A. M. Portsmouth, Ohio) whose total enrichment Dirac.) To enrich uranium from 0.7 per cent capacity will soon reach 27.3 million separa- uranium-235 to about 3 per cent requires tive work units per year. approximately 1250 units, Gaseous diffusion Separative work units (abbreviated SWU) plants are therefore very large and expensive. 6 LOS ALAMOS SCIENCE Separating Isotopes With Lasers They have, however, proved very reliable. smaller, including only about ten separation The main disadvantage of the process is the units. In addition, the process would con- great amount of electric power required to sume an equal or lesser amount of energy operate the many compressors. A standard per SWU than the gas centrifuge. These gaseous diffusion plant operating at full advantages lead directly to reduced capital capacity demands about 3000 megawatts and operating costs. electric. For comparison, a typical large For a laser isotope separation process 235UF electric power plant produces 1000 mega- involving selective excitation of 6 watts electric. As the cost of electric power molecules with infrared lasers and their dis- increases, its consumption becomes an in- sociation with an ultraviolet laser, a facility creasingly important factor in the cost of with the standard capacity of 8.75 millions enriched uranium. (In fiscal year 1980 about SWU per year is estimated to cost about 1 75 per cent of the production costs at billion dollars. (Laser casts account for ap- gaseous diffusion plants was for electricity.) proximately half of the direct capital costs.) With the expectation of reducing power This is considerably lower than the estimated consumption, attention is now focused on cost of a new gaseous diffusion plant (about the gas centrifuge, another method for en- 5 billion dollars) or that of a gas centrifuge riching uranium. The Gas Centrifuge Enrich- plant (about 6 billion dollars). The annual ment Plant now being constructed at operating cost for a laser isotope separation Portsmouth, Ohio will contribute 8.75 mil- facility is estimated to be about 100 million lion SWU per year to the national enrich- dollars, in contrast to about 500 million for a ment capacity by 1994. In a gas centrifuge gaseous diffusion plant and 100 to 200 235 238 UF 6 and UF 6 are separated by the million for a gas centrifuge plant. Our esti- centrifugal force imposed on UF6 by a mates of capital and operating costs for a rapidly rotating container. For this process a laser isotope separation facility indicate a considerably smaller number of centrifuge cost per SWU of about $30; the current units (less than 10) are required to reach the commercial cost for enriched uranium is desired enrichment. However, the through- $110 per SWU. put per centrifuge unit is very small com- The considerably lower cost per SWU for pared to that of a diffusion unit-so small, in laser isotope separation opens the possibility fact, that it is not compensated by the higher of turning the large stockpile of wastes from enrichment per unit. To produce the same gaseous diffusion plants into a valuable amount of reactor-grade fuel requires a con- uranium resource. These wastes contain siderably larger number (approximately 23S about a third of the UF6 originally present 50,000 to 500,000] of centrifuge units than in the feed material and are estimated to diffusion units. This disadvantage, however, amount to more than 500,000 metric tons by is outweighed by the considerably lower (by the end of 1990—an amount containing a factor of 20) energy consumption per enough uranium-235 for 1800 reactor-years SWU for the gas centrifuge. of operation. Compared to diffusion and the centrifuge, These advantages imply that laser isotope laser isotope separation offers the potential separation should be thoroughly investigated for much greater enrichment and throughput as a potentially economical process for per separation unit. Therefore, a laser large-scale production of enriched isotope separation facility would be much uranium. ■ LOS ALAMOS SCIENCE 7.