The debate has begun again over the disposition of nuclear fuel and waste.
The release of radioactivity into the atmosphere from the Fukushima Daiichi accident has been estimated by JapanÕs government as about one-tenth of that from the Chernobyl accident. The area in Japan contaminated with cesium-137Ñat the same levels that caused evacuation around ChernobylÑis also about one-tenth as large. The estimated number of resulting cancer deaths in the Fukushima area from contamination due to more than 1 curie per square kilometer is likely to scale correspondinglyÑon the order of 1,000. On March 16, 2011, the Nuclear Regulatory Commission advised Americans in the region to evacuate out to 50 miles (NRC 2011 a). If the Japanese government had made the same recommendation to its citizens, it would have resulted in the early evacuation of about two million people instead of 130,000. Because contaminated milk was interdicted in Japan, the number of (mostly non-fatal) thyroid cancer cases will probably be less than 1 percent of similar cases in Chernobyl. Unless properly dealt with, however, fear of ionizing radiation could have longterm psychological effects on a large portion of the population in the contaminated areas.
The factors influencing the level of U-232 contamination in U-233 are examined for heavy-water-moderated, light-water-moderated and liquid-metal cooled fast breeder reactors fueled with natural or low-enriched uranium and containing thorium mixed with the uranium or in separate target channels. U-232 decays with a 69-year half-life through 1.9-year half-life Th-228 to T1-208, which emits a 2.6 MeV gamma ray upon decay.We find that pressurized light-water-reactors fueled with LEU-thorium fuel at high burnup (70 MWd/kg) produce U-233 with U-232 contamination levels of about 0.4 percent. At this contamination level, a 5 kg sphere of U-233 would produce a gammaray dose rate of 13 and 38 rem/hr at 1 meter one and ten years after chemical purification respectively. The associated plutonium contains 7.5 percent of the undesirable heat-generating 88-year half-life isotope Pu-238.However, just as it is possible to produce weapon-grade plutonium in low-burnup fuel, it is also practical to use heavy-water reactors to produce U-233 containing only a few ppm of U-232 if the thorium is segregated in "target" channels and discharged a few times more frequently than the natural-uranium "driver" fuel. The dose rate from a 5-kg solid sphere of U-233 containing 5 ppm U-232 could be reduced by a further factor of 30, to about 2 mrem/hr, with a close-fitting lead sphere weighing about 100 kg.Thus the proliferation resistance of thorium fuel cycles depends very much upon how they are implemented.
Because of the unavailability of off-site storage for spent power-reactor fuel, the NRC has allowed high-density storage of spent fuel in pools originally designed to hold much smaller inventories. As a result, virtually all U.S. spent-fuel pools have been re-racked to hold spent-fuel assemblies at densities that approach those in reactor cores. In order to prevent the spent fuel from going critical, the fuel assemblies are partitioned off from each other in metal boxes whose walls contain neutron-absorbing boron. It has been known for more than two decades that, in case of a loss of water in the pool, convective air cooling would be relatively ineffective in such a "dense-packed" pool. Spent fuel recently discharged from a reactor could heat up relatively rapidly to temperatures at which the zircaloy fuel cladding could catch fire and the fuel's volatile fission products, 2 Alvarez et al.including 30-year half-life 137 Cs, would be released. The fire could well spread to older spent fuel. The long-term land-contamination consequences of such an event could be significantly worse than those from Chernobyl.No such event has occurred thus far. However, the consequences would affect such a large area that alternatives to dense-pack storage must be examined-especially in the context of concerns that terrorists might find nuclear facilities attractive targets. To reduce both the consequences and probability of a spent-fuel-pool fire, it is proposed that all spent fuel be transferred from wet to dry storage within five years of discharge. The cost of on-site dry-cask storage for an additional 35,000 tons of older spent fuel is estimated at $3.5-7 billion dollars or 0.03-0.06 cents per kilowatt-hour generated from that fuel. Later cost savings could offset some of this cost when the fuel is shipped off site. The transfer to dry storage could be accomplished within a decade. The removal of the older fuel would reduce the average inventory of 137 Cs in the pools by about a factor of four, bringing it down to about twice that in a reactor core. It would also make possible a return to open-rack storage for the remaining more recently discharged fuel. If accompanied by the installation of large emergency doors or blowers to provide largescale airflow through the buildings housing the pools, natural convection air cooling of this spent fuel should be possible if airflow has not been blocked by collapse of the building or other cause. Other possible risk-reduction measures are also discussed.Our purpose in writing this article is to make this problem accessible to a broader audience than has been considering it, with the goal of encouraging further public discussion and analysis. More detailed technical discussions of scenarios that could result in loss-of-coolant from spent-fuel pools and of the likelihood of spent-fuel fires resulting are available in published reports prepared for the NRC over the past two decades. Although it may be necessary to keep some specific vulnerabilities confidential, we believe that a generic dis...
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