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Thorium: The Preferred Nuclear Fuel of the Future

Nuclear engineer Ramtanu Maitra explains how the development of thorium fuel cycles will enhance the efficiency and economy of nuclear power plants. This article is excerpted from the Fall 2005 issue of 21st Century Science & Technology magazine. Australia has the world’s largest extractable reserves of thorium, so it would be crazy for us not to be world leaders in developing thorium-fuelled reactors.

Thorium is an abundant element in nature with multiple advantages as a nuclear fuel for future reactors of all types. Thorium ore, or monazite, exists in vast amounts in the dark beach sand of India, Australia, and Brazil. It is also found in large amounts in Norway, the United States, Canada, and South Africa. Thorium-based fuel cycles have been studied for about 30 years, but on a much smaller scale than uranium or uranium/plutonium cycles. Germany, India, Japan, Russia, the United Kingdom, and the United States have conducted research and development, including irradiating thorium fuel in test reactors to high burn-ups. Several reactors have used thorium-based fuel.

India is by far the nation most committed to study and use of thorium fuel; no other country has done as much neutron physics work on thorium as have Indian nuclear scientists. The positive results obtained in this neutron physics work have motivated the Indian nuclear engineers to use thorium- based fuels in their current plans for the more advanced reactors that are now under construction.

India decided on a three-stage nuclear program back in the 1950s, when its nuclear power generation program was set up. In the first stage, natural uranium (U-238) was used in pressurized heavy water reactors (PHWRs), of which there are now 12. In the second stage, the plutonium extracted from the spent fuel of the PHWRs was scheduled to be used to run fast breeder reactors. The fast breeders would burn a 70-percent mixed oxide (MOX) fuel to breed fissile uranium- 233 (U-233) in a thorium-232 (Th-232) blanket around the core. In the final stage, the fast breeders would use Th-232 and produce U- 233 for use in new reactors. One main advantage of using a combination of thorium and uranium is related to the proliferation question: There is a significant reduction in the plutonium content of the spent fuel, compared with what comes out of a conventional uranium- 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, and 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, and the mix determines the plutonium production.

India has completed the first phase of its program, and moved into the second phase with a small experimental fast breeder reactor, and, in 2004, began the construction of a 300 MW Advanced Heavy Water Reactor (AWHR), as a prototype for the third phase. The innovative design of the AHWR is characterized by extensive passive safety features, making it very safe.

Abundance of Thorium

The thorium fuel cycle has many attractive features. To begin with, thorium is much more abundant in nature than uranium. Soil commonly contains an average of around six parts per million (ppm) of thorium, three times as much as uranium. Thorium occurs in several minerals, the most common being the rare earth thorium-phosphate mineral, monazite, which usually contains from 3 to 9 percent, and sometimes up to 12 percent thorium oxide. In India, the monazite is found in its southern beach sands.

Th-232 decays very slowly (its half-life is about three times the age of the Earth). Most other thorium isotopes are short-lived and thus much more radioactive than Th-232, but of negligible quantity.

In addition to thorium’s abundance, all of the mined thorium is potentially usable in a reactor, compared with only 0.7 percent of natural uranium. In other words, thorium has some 40 times the amount of energy per unit mass that could be made available, compared with uranium.

From the technological angle, one reason that thorium is preferred over enriched uranium is that the breeding of U-233 from thorium is more efficient than the breeding of plutonium from U-238. This is so because the thorium fuel creates fewer non-fissile isotopes. Fuel-cycle 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 benefits. For example, thorium oxide, the form of thorium used for nuclear power, is a highly stable compound— more so than the uranium dioxide that is usually employed in today’s conventional nuclear fuel. Also, the thermal conductivity of thorium oxide is 10 to 15 percent higher than that of uranium dioxide, making it easier for heat to flow out of the fuel rods used inside a reactor.

In addition, the melting point of thorium oxide is about 500 degrees Celsius higher than that of uranium dioxide, which gives the reactor an additional safety margin, if there is a temporary loss of coolant.

The one challenge in using thorium as a fuel is that it requires neutrons to start off its fission process. These neutrons can be provided by the conventional fissioning of uranium or plutonium fuel mixed into the thorium, or by a particle accelerator. Most of the past thorium research has involved combining thorium with conventional nuclear fuels to provide the neutrons to trigger the fission process.

The approach undergoing the most investigation now is a combination that keeps a uranium-rich “seed” in the core, 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, when he was chief scientist of the U.S. Naval Reactors Program. Radkowsky, who died in 2002 at age 86, headed up the design team that built the first U.S. civilian nuclear reactor at Shippingport, Pennsylvania, and made significant contributions to the commercial nuclear industry during the 1960s and 1970s.

Although thorium is not fissile like U-235, Th-232 absorbs slow neutrons to produce U-233, which is fissile. In other words, Th-232 is fertile, like U-238. The Th-232 absorbs a neutron to become Th- 233, which decays to protactinium- 233 (Pa-233) and then to fissionable U-233. When the irradiated fuel is unloaded from the reactor, the U-233 can be separated from the thorium, and then used as fuel in another nuclear reactor. Uranium- 233 is superior to the conventional nuclear fuels, U-235 and Pu-239, because it has a higher neutron yield per neutron absorbed. This means that once it is activated by neutrons from fissile U-235 or Pu- 239, thorium’s breeding cycle is more efficient than that using U- 238 and plutonium.

The Russian-U.S. Program

Since the early 1990s, Russia has had a program based at Moscow’s Kurchatov Institute to develop a thorium-uranium fuel. The Russian program involves the U.S. company Thorium Power, Inc. (founded by Radkowsky), which has U.S. government and private funding to design fuel for the conventional Russian VVER-1000 reactors. Unlike the usual nuclear fuel, which uses enriched uranium oxide, the new fuel assembly design has the plutonium in the center as the “seed,” in a demountable arrangement, with the thorium and uranium around it as a “blanket.” ...

One study concludes:

“Thorium fuel offers a promising means to dispose of excess weapons-grade plutonium in Russian VVER-1000 reactors. Using the thorium fuel technology, plutonium can be disposed of up to three times as fast as MOX at a significantly lower cost. Spent thorium fuel would be more proliferation- resistant than spent MOX (mixed oxide) fuel. ... [The thorium fuel technology] will not require significant and costly reactor modifications. Thorium fuel also offers additional benefits in terms of reduced weight and volume of spent fuel and therefore lower disposal costs.”

Thorium Fuel Operating Experience

There have been four decades of research and development on thorium fuel cycles, including in the ultra-safe pebble-bed modular reactor (PBMR) now being built in South Africa and China.

Between 1967 and 1988, the AVR (Arbeitsgemeinschaft Versuchsreaktor – “working group test reactor”) experimental pebble bed reactor at Jülich, Germany (the basis for the South African and Chinese PBMRs), operated for more than 750 weeks at 15 megawatts-electric, about 95 percent of the time with thorium-based fuel. The fuel used consisted of about 100,000 billiard ball-size fuel elements. Overall, a total of 1,360 kilograms of thorium was used, mixed with highly enriched uranium (HEU). Maximum burn-ups of 150,000 megawatt-days were achieved. Thorium fuel elements with a 10:1 ratio of thorium to highly enriched uranium were irradiated in the 20-megawatts-thermal (MWt) Dragon reactor at Winfrith, United Kingdom, for 741 full-power days. Dragon was run as a cooperative project of the Organization of Economic Cooperation and Development and Euratom, involving Austria, Denmark, Sweden, Norway, and Switzerland, in addition to the United Kingdom, from 1964 to 1973. The thorium-uranium fuel was used to “breed and feed,’’ so that the U-233 that was formed, replaced the U-235 at about the same rate, and fuel could be left in the reactor for about six years. The General Atomics Peach Bottom high-temperature, graphite- moderated, helium-cooled reactor (HTGR) in the United States operated between 1967 and 1974 at 110-MWt, using highly enriched uranium with thorium.

In India, the Kamini 30-kWt experimental neutron-source research reactor started up in 1996 near Kalpakkam, using U-233 which was recovered from thorium-dioxide fuel that had been irradiated in another reactor. The Kamini reactor is adjacent to the 40-MWt Fast Breeder Test Reactor, in which the thorium-dioxide is irradiated.

In the Netherlands, an aqueous homogenous suspension reactor has operated at 1 megawatt-thermal for three years. The highly enriched uranium/thorium fuel is circulated in solution, and reprocessing occurs continuously to remove fission products, resulting in a high conversion rate to U-233.

Thorium in Power Reactors

The 300-MWe Thorium High- Temperature Reactor (THTR) in Germany was developed from the Jülich, Germany AVR noted above, and operated between 1983 and 1989 with 674,000 pebbles, over half of them containing thorium/ highly enriched uranium fuel (the rest of the pebbles were graphite moderator and some neutron absorbers). These pebbles were continuously recycled on load, and on average the fuel passed six times through the core. Fuel fabrication was on an industrial scale.

The Fort St. Vrain reactor in Colorado was the only commercial thorium-fueled nuclear plant in the United States. Also developed from the AVR in Germany, it operated from 1976 to 1989. It was a high-temperature (700EC), graphite- moderated, helium-cooled reactor with a thorium/highly enriched uranium fuel, which was designed to operate at 842 megawatts- thermal (330 MWe). The fuel was contained in microspheres of thorium carbide and Th/U-235 carbide, coated with silicon oxide and pyrolytic carbon to retain fission products.

Unlike the pebble bed design, the fuel was arranged in hexagonal columns (“prisms”) in an annular configuration. Almost 25 tons of thorium were used in the reactor fuel, achieving a 170,000- megawatt-days burn-up.

Thorium-based fuel for Pressurized Water Reactors (PWRs) was investigated at the Shippingport reactor in the United States (the first U.S. commercial reactor, started up in 1957), using both U-235 and plutonium as the initial fissile material. The light water breeder reactor (LWBR) concept was also successfully tested at Shippingport, from 1977 to 1982, with thorium and U-233 fuel clad with zircaloy, using the “seed/blanket” concept.

Another reactor type, the 60- MWe Lingen Boiling Water Reactor (BWR) in Germany also utilized fuel test elements that were thorium-plutonium based.

Proliferation Issues

In the early days of the civilian nuclear program, the Acheson-Lilienthal Report in 1946 warned of the connection between civilian nuclear power and nuclear weapons, and concluded that the world could not rely on safeguards alone “to protect complying states against the hazards of violations and evasions”—illicit nuclear weapons. Acheson-Lilienthal proposed international controls over nuclear power, but also considered possible technical innovations that would make it harder to divert nuclear materials into bomb-making. The thorium fuel cycle is one such technical innovation—as yet untapped.

A 1998 paper by Radkowsky and Galparin describes the most advanced work in developing a practical nuclear power system that could be made more “proliferation resistant” than conventional reactors and fuel cycles. Based on a thorium fuel cycle, it has the potential to reduce the amount of plutonium generated per gigawatt-year by a factor of five, compared to conventional uranium-fueled reactors. It would also make the generated plutonium and uranium- 233 much more difficult to use for producing bomb material.

Heightened current concerns about preventing the spread of bomb-making materials, have led to an increase in interest in developing thorium-based fuels. The U.S. Department of Energy has funded Radkowsky’s company (Thorium Power) and its partners in their tests with Russian reactors, as well as three other efforts (two national laboratories, two fuel fabrication companies, and a consortium of three universities). This research is geared to designing a thorium fuel system that will fit with conventional reactors. There is also a new company, Novastar Resources, that is buying up thorium mines in anticipation of thorium-fueled reactors in the future.

The proliferation potential of the light water reactor fuel cycle may be significantly reduced by using thorium as a fertile component of the nuclear fuel, as noted above. The main challenge of thorium utilization is to design a core and a fuel cycle that would be proliferation- resistant and economically feasible. This challenge is met by the Radkowsky Thorium Reactor concept. So far, the concept has been applied to a Russian design of a 1,000-MW pressurized water reactor VVER, designated as VVERT.

The main results of the preliminary reference design are as follows: The amount of plutonium contained in the Radkowsky Thorium Reactor spent fuel stockpile is reduced by 80 percent, in comparison with a VVER of conventional design. The isotopic composition of the reactor’s plutonium greatly increases the probability of pre-initiation and yield degradation of a nuclear explosion. An extremely large Pu-238 content causes correspondingly large heat emission, which would complicate the design of an explosive device based on plutonium from this reactor.

The economic incentive to reprocess and reuse the fissile component of the Radkowsky Thorium Reactor spent fuel is also decreased. The once-through cycle is economically optimal for its core and cycle.

To reiterate the proliferation difficulties: the replacement of a standard (uranium-based) fuel for nuclear reactors of current generation by the Radkowsky Thorium Reactor fuel will provide a strong barrier for nuclear weapon proliferation. This barrier, in combination with existing safeguard measures and procedures, is adequate to unambiguously disassociate civilian nuclear power from military nuclear power.

Other scientists point out that even if a terrorist group wanted to use the blanket plutonium for making a bomb, the process of extracting it from thorium fuel would be more difficult than removing it from conventional spent fuel. This is because the spent blanket fuel from a thorium fuel cycle would contain uranium-232, which over time decays into isotopes that emit high-energy gamma rays. To extract the plutonium from this spent fuel would require significantly more radiation shielding plus additional remotely operated equipment in order to reprocess it for weapons use, making a daunting task even more difficult. It would also be more complicated to separate the fissionable U-233 from uranium- 238, because of the highly radioactive products present.

Overall, the development of thorium fuel cycles makes sense for the future, for advancing the efficiency and economy of nuclear power plants, ease of recycling, and making it more difficult to divert radioactive materials for weapons.

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