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Thorium Fuel for Nuclear Energy

An unconventional tactic might one day ease concerns that spent fuel could be used to make a bomb

Mujid Kazimi

Now You're Cooking with Thorium

The use of thorium in power reactors has been considered since the birth of nuclear energy in the 1950s, in large part because thorium is considerably more abundant than uranium in the Earth's crust. Roughly speaking, there is about three times more thorium than uranium. Unfortunately, thorium atoms cannot themselves be easily induced to split—the basic requirement of a fission reactor. But when a quantity of thorium-232 (the common isotope of that element) is placed within a nuclear reactor, it readily absorbs neutrons and transforms into uranium-233, which, like the uranium-235 typically used for generating nuclear power, supports fission chain reactions.

Figure 2. Thorium-based fuels areClick to Enlarge Image

Thorium is thus said to be "fertile" rather than fissile. In this respect it is similar to uranium-238, which makes up more than 95 percent of most nuclear fuels. A conventional reactor breeds various isotopes of plutonium from uranium-238, and some of that plutonium in turn undergoes fission in the reactor, adding to the power the uranium-235 provides.

The hitch with using thorium as a fuel is that breeding must occur before any power can be extracted from it—and that requires neutrons. Some engineers have proposed using particle accelerators to generate the needed neutrons, but this process is costly, and the only practical scheme at the moment is to combine the thorium with conventional nuclear fuels (made up of either plutonium or enriched uranium or both), the fissioning of which provides the neutrons to start things off.

The breeding of uranium-233 from thorium is more efficient than the breeding of plutonium from uranium-238, because less of various nonfissile isotopes is created along the way. Designers can take advantage of this efficiency to decrease the amount of spent fuel per unit of energy generated, which reduces the amount of waste to be disposed of. There are some other pluses as well. For example, thorium dioxide, the form of thorium used for nuclear power, is a highly stable compound—more so than the uranium dioxide typically employed in today's fuel. So there is less concern that the fuel pellets could react chemically with the metal cladding around them or with the cooling water should there be a breach in the protective cladding. Also, the thermal conductivity of thorium dioxide is 10 to 15 percent higher than that of uranium dioxide, making it easier for heat to flow out of the slender fuel rods used inside a reactor. What is more, the melting point of thorium dioxide is about 500 degrees Celsius higher than that of uranium dioxide, and this difference provides an added margin of safety in the event of a temporary power surge or loss of coolant.

Knowledge of such advantages has repeatedly spurred nuclear engineers to conduct experiments, and some groups have even gained experience running commercial power reactors on thorium-based fuels. For example, a gas-cooled, graphite-moderated reactor called Peach Bottom Unit One, located in southeastern Pennsylvania, used a combination of thorium and highly enriched uranium in the mid-1960s. Another gas-cooled reactor at Fort St. Vrain in Colorado was run on a similar thorium-based fuel between 1976 and 1989. Tests with relatively simple mixtures of thorium oxide and highly enriched uranium oxide also began with water-cooled reactors during the 1960s, at the "BORAX" (Idaho) and Elk River (Minnesota) facilities and at the Indian Point (New York) power plant. And between 1977 and 1982, more complicated combinations of thorium and either uranium-235 or uranium-233 were also employed in a water-cooled reactor at Shippingport, Pennsylvania, in an experimental program seeking to develop a fuel that produces more fissile material than it consumes. Interestingly, Shippingport, which began operation in 1957, was the very first nuclear power plant built in the United States for the commercial generation of electricity.

Figure 3.  Conventional nuclear fuelClick to Enlarge Image

Work with thorium-based nuclear fuels has by no means been restricted to the United States. German engineers, for example, have used combinations of thorium and highly enriched uranium, or thorium and plutonium, in both gas- and water-cooled power reactors. Thorium-based fuels have also been tried in the United Kingdom, France, Japan, Russia, Canada and Brazil. But despite these considerable early efforts, most nations long ago abandoned the notion of using thorium to power their nuclear generating stations. One country that has maintained interest is India, which began fueling some of its power reactors in the mid-1990s with bundles containing thorium. Although one of the reasons for employing thorium was simply to even out the distribution of power within the cores of these reactors, Indian engineers also took the opportunity to test how well thorium could function as a fuel source. The positive results they obtained motivated their current plans to use thorium-based fuels in more advanced reactors now under construction.

India's attraction to thorium-based fuels stems, in part, from its large indigenous supply. (With estimated thorium reserves of some 290,000 tons, it ranks second only to Australia.) But that nation's pursuit of thorium, which helps bring it independence from overseas uranium sources, came about for a reason that has nothing to do with its balance of trade: India uses some of its reactors to make plutonium for atomic bombs. Thus India refuses to be constrained by the provisions that commercial uranium suppliers in countries such as Canada require: They demand that purchasers of their ore allow enough oversight to ensure that the fuel (or the plutonium spawned from it) is not used for nuclear weapons.

Previous work on thorium elsewhere in the world did not lead to its adoption, largely because its performance in water reactors, such as the first core at the Indian Point power station, did not live up to expectations. Given this history, it may come as something of a surprise that thorium-based nuclear fuels are once again being considered, this time as the means to stem the potential proliferation of nuclear weapons. Using thorium to prevent the buildup of plutonium requires that the fuel be configured differently than in most of the experiments of years past. Those trials incorporated highly enriched uranium (something that is currently discouraged because of worries over proliferation) and presupposed that the spent fuel would be reprocessed for the extraction of its fissile contents. Neither practice is now envisaged. The thorium-based fuel assemblies currently being designed are different from past examples in other ways too. For example, they can withstand greater exposure to the heat and radiation experienced inside the core of a reactor, which allows more of the fertile thorium-232 to be converted into fissile uranium-233. So what's being talked about now is definitely not your father's thorium-based nuclear fuel.




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