Today some 103 nuclear power plants in the United States produce about one million kilowatts each of electrical power, supplying some 20% of US electrical needs. They do this by the use of the neutron chain reaction in uranium-oxide ceramic pellets, sustained by the regeneration of neutrons through the fission process.
Each fission in the light isotope of uranium—U-235 that constitutes 0.7% of natural uranium and is enriched to about 5% concentration in the 25 tons of fuel loaded into the reactor each year, where it produces heat for about 85% of its 4-year sojourn—liberates about 2.5 neutrons on the average, and 30 billion fissions contribute about 1 joule of heat.
| Of the 25 tons of fuel–heavy metal– loaded each year into the reactor as essentially non- radioactive fuel rods and fuel elements, about one ton is fissioned during its 4-yr stay in the reactor— that is, the U-235 is split into a light and a heavy fission product largely retained in the solid fuel pellets in their tubular metal sheaths. The accompanying heat is transferred to water in the high pressure reactor vessel, and the water boils to steam in the upper portion of the vessel (for a boiling water reactor—BWR) or after a heat exchanger in the case of a pressurized water reactor—PWR. Because these reactors use ordinary water both to transfer heat from the reactor fuel to the steam turbine, they are called light-water reactors—LWRs. The plentiful U-238 does not fission to a significant extent in LWR, but it does have an appetite for the slow neutrons; instead of fission U-238 undergoes capture of a neutron to form U-239, which in short order decays in the reactor to Np-239 and then to plutonium—Pu-239. Pu-239 is even more readily fissile than is U-235 and is quite suitable for making nuclear explosives, as is highly enriched U-235 in the range of 80% U-235 or more. The spent fuel elements removed from the reactor in there fueling operation are highly radioactive. Even after 100 years they are regarded as self protecting in that a single fuel element would irradiate a person at one meter distance with more than a dose of 1 sievert (1 Sv) in 1 hour. Delivered in an instant, a lethal dose of 4Sv would raise the body temperature only about 0.001ºC. Within the operating reactor, each kg of fuel generates about 30kW of heat. A week after reactor shutdown, fuel elements transferred to the spent-fuel pond still generate about 100W/kg, from the decay of the radioactive fission products. If the water were lost, the spent fuel would heat within hours to the melting temperature of the fuel-rod sheath; the zirconium alloy would burn in air. After 10 years, spent fuel still creates 2W/kg, little enough that the fuel can be stored in massive casks to protect people from the gamma radiation of the fission products; the casks are cooled by natural air convection. All US power reactors are fueled with low-enriched uranium—LEU—ceramic fuel, and almost all spent fuel thus far has been held in at-reactor water pools that provide cooling of the fuel elements and shielding of plant and public personnel against nuclear radiation. It has long been planned that after 10 years or so of pool storage and cooling, fuel elements would be transferred to long-term storage casks that would then be shipped to the Yucca Mountain, Nevada, mined geologic repository; a recent National Academies study provides an independent assessment of the safety of such shipment. Following the long-delayed opening of YM, fuel elements in storage casks would be loaded into the underground horizontal tunnels—drifts—with about 1.1 metric tons of initial heavy metal per meter length of drift—MTIHM/m. The US industry in this way has been practicing the open fuel cycle or the once-through or direct disposal fuel cycle—at least up to final disposal in a mined geologic repository. In contrast, for decades France has been reprocessing spent fuel from its 58 LWRs, using the PUREX process to separate about 16 tons per year of plutonium from about 1600 tons of spent fuel. Much of the spent fuel was of German or Japanese origin, and the separated Pu and vitrified fission products were by law and contract to be returned to the country of origin. France has used its own Pu to fabricate mixed-oxide—MOX—ceramic fuel pellets that displace LEU fuel elements—UOX— and thus reduce the uranium demand by about 20%. PUREX was used by the US and other states to separate plutonium for nuclear weapons from lightly irradiated fuel from Pu-production reactors; less than one ten millionth of the radioactive fission products remains with the separated Pu. The civil plutonium is stored and shipped in small welded stainless-steel cans containing 2 kg of plutonium oxide. In contrast to the fierce gamma radiation of the spent fuel, so little radiation emerges from the pure plutonium oxide that the cans can be carried without harm in one’s bare hands, and the MOX fuel elements can be fabricated without the use of heavy shielding. However, plutonium is an intense emitter of alpha particles and must therefore be handled in a glove box to prevent ingestion or inhalation. Per gram, weapon plutonium emits about 60,000 times less alpha radiation as does the polonium-210 that killed Alexander Litvinenko in 2006; this is a consequence of the 24,000-yr half life of Pu-239 compared with the 140-day half life of Po-210. The French approach to the closed fuel cycle has been demonstrated by French government analyses to be more costly than the open fuel cycle. Despite persistent claims that this approach to plutonium recycle has substantial benefits in reducing the burden on the repository, there has been recent awareness that the capacity of the repository is not limited by the bulk of the spent fuel but by the continuing heat evolution from the fission products and the transuranics—that is, plutonium, americium, neptunium, curium. The US Global Nuclear Energy Partnership—GNEP— stresses that major gains in repository capacity can be achieved only with a suite of fast-neutron reactors that can actually fission the transuranics—the minor actinides. GNEP was announced by President George W. Bush in February, 2006. Testimony by the Department of Energy at the April 6, 2006 session of the Energy Subcommittee of the House of Science Committee highlighted the fact that of the proposed first-year GNEP budget of $250 M, some $155 M was toward the building of a demonstration reprocessing plant, dubbed UREX+. The intent was to demonstrate at perhaps 10% full-scale the reprocessing of all the fuel emerging from the 103 operating US LWRs, in order to begin to provide fuel for a generation of fast-neutron Advanced Burner Reactors—ABRs. A key element of GNEP was to have a reprocessing approach more “proliferation resistant” supposedly by leaving enough fission products with penetrating gamma radiation—lanthanides—especially europium-154 with a half-life of 8.8 years. Part of the GNEP program is to offer foreign reactor operators a secure fuel cycle at advantageous rates— leasing of fresh fuel and take-back of the spent fuel—and also cartridge reactors that would be delivered loaded with fresh fuel and could operate for 20-30 years without refueling. The cartridge reactor would then be replaced by a fresh one and taken back for de-fueling. National and international regulations and customs need to be changed in order to permit spent fuel to be transferred from one country to another for ultimate disposition, either by direct entombment in a mined geologic repository or by reprocessing followed by entombment in a repository. The secure fuel cycle makes good sense economically from the point of view of the using country, and for the world from the point of view of limiting facilities capable of providing weaponusable materials: enrichment plants and reprocessing plants that, respectively, produce enriched uranium (and could produce highly enriched uranium), and the reprocessing plant that produces plutonium, even if it is mixed with 50% uranium in some of the recent proposals. The proposal to lease and take back reactor fuel was published long ago by Harold M. Agnew, then Director of the Los Alamos Scientific Laboratory, in the Bulletin of the Atomic Scientists (May 1976, page 23), as “Atoms for lease: An alternative to assured nuclear proliferation.” States that express concern about the reliability of future fuel supply under potentially tense international conditions could well buy a stockpile of LEU fuel for 10 years of operation of their reactors; fortunately, LEU fuel is safe and cheap to store and cheap to buy, in comparison with fossil fuels. |
As for terrorist acquisition of nuclear weapons, to acquire plutonium from spent fuel elements is a daunting task because of their intense radioactivity and the fact that to obtain the 10kg of reactor-grade Pu for a nuclear weapon a terrorist would need to steal and reprocess a ton of intensely radioactive spent fuel.