The Growing Problem of Spent Nuclear Fuel
October 31, 2012
Nuclear reactors apply advanced engineering and radioactive materials for an exceedingly simple task: to boil water. The heat generated by radioactive fuel heats water to boiling; the resulting steam drives turbines that turn electric generators. Two types of radioactive fuel are employed in this process. Fresh uranium dioxide nuclear fuel is approximately four percent fissile uranium-235 and 96 percent non-fissile uranium-238. It has relatively low radioactivity, can be fused into ceramic pellets, packed into thin metal tubes, and can be easily handled. However, when arranged in assemblies of multiple rods in close proximity, the neutrons from the fission of the U-235 becomes dense enough to sustain a chain reaction releasing tremendous heat and producing radioactive waste products. It is also possible to use mixed uranium-plutonium oxide (MOX) fuel, as was the case of the #3 reactor at Fukushima. This fuel is inherently more dangerous and difficult to handle at all stages of its lifecycle. Fresh MOX fuel contains plutonium-239; when spent, it has more plutonium and other transuranics than normal uranium-based nuclear fuel and much higher thermal activity.[i]
Partially and fully spent nuclear fuel is extremely hot and radioactive. According to a General Accounting Office report,
One of the most hazardous materials made by man is spent nuclear fuel—the used fuel periodically removed from reactors in nuclear power plants. Without protective shielding, the fuel’s intense radioactivity can kill a person exposed directly to it within minutes or cause cancer in those who receive smaller doses. As the fuel ages, it begins to cool and becomes less radiologically dangerous—some of the radioactive particles decay quickly, within days or weeks, while others exist for many thousands of years.[ii]
Safe management of spent fuel rods is a major challenge. Because the U.S. has no permanent repository for high level nuclear waste, the 103 active nuclear power reactors in the U.S. store their spent fuel rod assemblies in on-site cooling ponds, 40 feet deep, steel-lined and with concrete walls up to six feet thick. The spent rods must remain in cooling pools for five to ten years under least 20 feet of actively circulating water. Loss of cooling water to a cooling pool is extremely dangerous, as it can lead to an explosion and catastrophic release or radiation, as was demonstrated in Fukushima.
A U.S. Nuclear Regulatory fact sheet states that after 10 years in a cooling pool, the surface radioactivity of a spent fuel assembly is still about 10,000 rem/hour.[iii] To understand the danger that poses to health, consider that a 500-rem dose delivered to a whole person in a single exposure is fatal. Close proximity to a single 10-year-old spent fuel assembly would deliver a fatal whole-body radiation dose in about three minutes.
After at least five years in cooling pools, fuel rod assemblies can be packed into massive sealed dry casks for safer storage. A typical dry cask is nine feet in diameter and 20 feet tall, with walls of steel and concrete for radiation shielding. Each dry cask contains 20 to 50 spent fuel assemblies.[iv] The sealed casks are pressurized with an inert gas that circulates passively through cooling fins to dissipate the heat generated by the contained spent fuel.[v] Casks weigh upward of 120 tons and are not designed for transport to off-site locations. Should a deep geological repository for spent fuel, or any other long-term storage solution, ever be developed, the radioactive assemblies would have to be unloaded from dry casks underwater and transferred to smaller hardened transfer casks for safe transport. In any foreseeable scenario, transportation to a permanent repository will be a daunting undertaking.
Today there are 103 active nuclear power reactors in the U.S. They generate 2,000 metric tons of spent nuclear waste per year and to date have accumulated 71,862 tons of spent fuel, according to industry data.[vi] Of that total, 54,696 tons are stored in cooling pools and only 17,166 tons in the relatively safer dry cask storage.
The toxic “lifespan” of spent nuclear fuel is about one million years.[vii] Dry casks are designed for about 100 years of spent fuel storage. Because a permanent solution has not been found in half a century of trying, owners of nuclear power plants are essentially required to manage this most hazardous of all man-made wastes forever. If new nuclear power plants are built, accumulation of this million-year waste will accelerate. Not only do the costs of storage become effectively unlimited; in addition, the risk of a devastating cooling pool accident becomes steadily more likely.
[i] John Deutch and Ernest J. Moniz, et al., Massachusetts Institute of Technology Report, The Future of Nuclear Power: An Interdisciplinary MIT Study, 2003, 180 pages, accessed online April 16, 2011.
[ii] U.S. General Accounting Office Report, Spent Nuclear Fuel: Options Exist to Further Enhance Security, July 2003, 50 pages, accessed online March 22, 2011.
[iii] U.S. Nuclear Regulatory Commission Backgrounder on Radioactive Waste, 5 pages, accessed online April 15, 2011.
[iv] U.S. General Accounting Office Report, Nuclear Regulatory Commission: NRC Needs to Do More to Ensure that Power Plants Are Effectively Controlling Spent Nuclear Fuel, April 2005, 42 pages, accessed online March 19, 2011.
[v] John Deutch and Ernest J. Moniz, et al., Massachusetts Institute of Technology Report, The Future of Nuclear Power: An Interdisciplinary MIT Study, 2003, 180 pages, accessed online April 16, 2011.
[vi] Associated Press, Spent nuclear fuel throughout US stored by state, Forbes, March 22, 2011, accessed online April 16,2011.