Thorium Fuel for Nuclear Energy
An unconventional tactic might one day ease concerns that spent fuel could be used to make a bomb
As I mentioned, the lack of uranium-233 in nature necessitates using a different fissile material, such as uranium-235 (or perhaps plutonium-239), to prime a reactor running on thorium. Given the present-day proscription against commercial fuels that are too highly enriched in uranium-235, a considerable amount of (nonfissionable) uranium-238 would clearly need to be included in the primer; current standards require at least 80 percent, and more is typical. As is the case with conventional reactors, this would make it impossible to use the fresh fuel for a bomb without first having to go through the technically difficult step of isotopically enriching the uranium.
The main advantage of using a combination of thorium and uranium is the significant reduction in plutonium content of the spent fuel compared with what comes out of a conventionally fueled reactor. Just how much less plutonium is made? The answer depends on exactly how the uranium and thorium are combined. For example, uranium and thorium can be mixed homogeneously within each fuel rod. In this case the amount of plutonium produced is roughly halved. But mixing them uniformly is not the only way to combine the two elements.
Indeed, the approach undergoing the most investigation now is a combination that keeps a uranium-rich "seed" separate from a thorium-rich "blanket." The chief proponent of this concept was the late Alvin Radkowsky, a nuclear pioneer who, under the direction of Admiral Hyman Rickover, helped to launch America's nuclear Navy during the 1950s as chief scientist of the U.S. Naval Reactors Program. Radkowsky went on to make significant contributions to the commercial nuclear industry during the 1960s and '70s. Then, at the urging of Edward Teller (one of his former teachers) to find a way to reduce the threat of nuclear weapons getting into the wrong hands, Radkowsky turned his attention to the use of thorium-based fuels, which he had already recognized as a means of lessening the amount of nuclear waste created. In 1992 he helped to found a private company, Thorium Power, Inc., to commercialize this technique. Sadly, Radkowsky would not live to see his vision materialize: He died last year, at the age of 86.
Radkowsky's idea was to construct special fuel assemblies that could be used in typical water-cooled reactors with very little modification. These units are made up of a central seed region containing fuel rods filled with reactor-grade uranium (that is, having no more than 20 percent uranium-235). Surrounding the seed is a blanket region with fuel rods containing thorium and a small amount of uranium. Having uranium-238 in the blanket prevents anyone from withdrawing these rods and using only chemical means to separate out the fissionable uranium-233 that is created over time.
With support from the U.S. Department of Energy and technical assistance from Brookhaven National Laboratory, Thorium Power is now working with the Kurchatov Institute in Moscow to investigate this strategy more fully. Their concept calls for using a metallic alloy as the seed fuel and for keeping the seed units in a Russian reactor for three years before replacing them but leaving the blanket rods in the reactor for 10 years. Their results are not going to be directly applicable to the nuclear power stations in most other parts of the world, however, because the fuel material is not in the form of an oxide (as preferred in the West) and because the Russian reactors involved in these tests use a hexagonal array of rods for each fuel assembly, whereas most facilities operating in Western countries use a square array.
Radkowsky and his colleagues had calculated that their scheme would reduce the amount of plutonium produced by 80 percent compared with what goes on in a conventionally fueled reactor of the same energy output. What is more, they found that the mix of plutonium isotopes generated, mostly in the seed fuel, would not be particularly desirable for military use, because a bomb made from it would be extremely unlikely to give much explosive yield—in the slang of weapons designers, it would probably "fizzle." Also, the plutonium has such a high content of the 238Pu isotope that its decay heat may be sufficient to melt or damage the other materials used in constructing a weapon.
Even if a terrorist group wanted to use the blanket plutonium for making a terrifying (if not terribly powerful) bomb, extracting it from Radkowsky's thorium fuel—indeed from any thorium fuel used in a reactor—would be more difficult than removing it from today's spent fuel. The spent blanket fuel contains uranium-232, which in the course of a few months decays into isotopes that emit high-energy gamma rays. Thus pulling out the plutonium would require significantly beefed-up radiation shielding and a more widespread use of remotely operated equipment within the reprocessing facility, further complicating an already challenging task. And the abundance of uranium-232 and its highly radioactive products in the spent fuel would probably thwart any effort to separate uranium-233 (which, being fissionable, could also be used for a bomb) from uranium-238.
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