Thorium-based nuclear power
Thorium-based nuclear power generation is fueled primarily by the nuclear fission of the isotope uranium-233 produced from the fertile element thorium. A thorium fuel cycle can offer several potential advantages over a uranium fuel cycle—including the much greater abundance of thorium found on Earth, superior physical and nuclear fuel properties, and reduced nuclear waste production. Thorium fuel also has a lower weaponization potential because it is difficult to weaponize the uranium-233 that is bred in the reactor. Plutonium-239 is produced at much lower levels and can be consumed in thorium reactors.
The feasibility of using thorium was demonstrated at a large scale, at the scale of a commercial power plant, through the design, construction and successful operation of the thorium-based Light Water Breeder Reactor core installed at the Shippingport Atomic Power Station. The reactor of this power plant was designed to accommodate different cores. The thorium core was rated at 60 MW, produced power from 1977 through 1982 and converted enough thorium-232 into uranium-233 to achieve a 1.014 breeding ratio.
After studying the feasibility of using thorium, nuclear scientists Ralph W. Moir and Edward Teller suggested that thorium nuclear research should be restarted after a three-decade shutdown and that a small prototype plant should be built.
Between 1999 and 2022, the number of operational non molten-salt based thorium reactors in the world has risen from zero to a handful of research reactors, to commercial plans for producing full-scale thorium-based reactors for use as power plants on a national scale.
Advocates believe thorium is key to developing a new generation of cleaner, safer nuclear power. In 2011, a group of scientists at the Georgia Institute of Technology assessed thorium-based power as "a 1000+ year solution or a quality [|low-carbon bridge] to truly sustainable energy sources solving a huge portion of mankind's negative environmental impact."
History
The use of thorium to breed uranium-233 was first discovered in 1940 by Glenn Seaborg, from the neutron bombardment of thorium in a cyclotron. During the Manhattan Project, after the construction of the X-10 Graphite Reactor, Seaborg quickly realized the potential of uranium-233 as a fissile material. Research continued throughout the Manhattan Project; however, it was largely sidelined in the weapons program in favor of plutonium, which had been discovered by Seaborg in February 1941.With the formation of the Atomic Energy Commission, uranium-based nuclear reactors were built to produce electricity. The first to do so was the experimental uranium breeder EBR-I. In the United States, many of these reactors were light-water reactors starting with the Shippingport Atomic Power Station. These were similar to the reactor designs that produced the propulsion for propelling nuclear submarines. Several other types of reactors were constructed, such as the Liquid metal cooled reactors like EBR-I, or gas-cooled reactors such as Peach Bottom Unit 1 and Fort St. Vrain.
During this period, thorium was investigated by the AEC for use in nuclear weapons, as well as for power generation. Several tons of uranium-233 were bred from thorium in AEC reactors, some of which was processed at the Rocky Flats Plant. Uranium-233 was used in the MET shot of the Operation Teapot series of nuclear tests.
File:Thorium reactor ORNL.jpg|thumb|Early thorium-based nuclear reactor at Oak Ridge National Laboratory in the 1960s
Several commercial power-generating reactors were also fueled with thorium-uranium mixed oxides, including Indian Point, Peach Bottom, and Fort St. Vrain. Around the same time, the government of the United States built the Molten-Salt Reactor Experiment, a prototype molten salt reactor, using uranium-233 fuel. The MSRE reactor, built at Oak Ridge National Laboratory, operated critical for roughly 15,000 hours from 1965 to 1969. In 1968, Glenn Seaborg, the chairman of the AEC, publicly announced that a 233U-based reactor had been successfully developed and tested. For its final year of operation, the reactor was briefly fueled with plutonium fluoride. The project's leaders also proposed a test run using plutonium fuel, however this was never carried out due to the project's cancellation.
Despite the project's apparent success, the MSRE was shut down in December 1969 due to pressure from Milton Shaw, the director of the AEC's Reactor Development and Testing Division. At the time, the US government was heavily interested in breeder reactors to meet the projected need for nuclear fuel. Shaw pressured the MSRE team, led by Oak Ridge director Alvin Weinberg, to end the project due to his preference for the other competing breeder design at the time, the liquid metal fast breeder reactor.
By 1973, due to Shaw's influence, the US government had essentially settled on uranium technology and largely discontinued thorium-related nuclear research. The reasons were that uranium-fueled reactors were considered more efficient, the research into uranium was proven and thorium's breeding ratio was thought insufficient to produce enough fuel to support development of a commercial nuclear industry. As Moir and Teller later wrote, "The competition came down to a liquid metal fast breeder reactor on the uranium-plutonium cycle and a thermal reactor on the thorium-233U cycle, the molten salt breeder reactor. The LMFBR had a larger breeding rate ... and won the competition." In their opinion, the decision to stop development of thorium reactors, at least as a backup option, "was an excusable mistake". The government pushed ahead with the LMFBR design with the Clinch River Breeder Reactor Project starting in 1970, however the project encountered substantial political opposition and was finally cancelled in 1983.
In 2009, science writer Richard Martin stated that Weinberg, who was director at Oak Ridge and primarily responsible for the MSRE, lost his job as director because he championed development of thorium reactors. Weinberg himself recalls this period:
At the time, Martin claimed that Weinberg's unwillingness to sacrifice potentially safe nuclear power for the benefit of military uses forced him to retire. However, in his 2012 book SuperFuel, Martin refuted the idea that thorium was rejected by the AEC because of the desire for weapons production:
Even after the cancellation of the MSRE, thorium research continued. Admiral Hyman Rickover, the developer of naval nuclear propulsion and head of the U.S. Naval Reactors office, had pushed for a thorium-fueled breeder project since 1963 and organized the construction of a light-water breeder reactor project in 1976. First reaching criticality on August 26, 1977, this project successfully turned the Shippingport Atomic Power Station, the first peacetime nuclear power plant, into a demonstration 232Th−233U breeder reactor. After successful operation until 1982, the reactor was found to have a breeding ratio of 1.4%.
In Germany, the AVR pebble-bed reactor utilized mixed-oxide 235U−232Th TRISO fuel, and operated from 1967 until 1988. Based on the AVR design, the Thorium High-Temperature
Reactor, a 300MWe commercial reactor was constructed and commenced operation in 1985. However, both reactors were plagued with design issues. THTR-300 was shut down in 1989, after only four years of operation.
Despite the documented history of thorium nuclear power, and successful demonstration of thorium-based breeding by the operation of the LWBR core at Shippingport, by 2009 many nuclear experts were nonetheless unaware of it. According to Chemical & Engineering News, "most people—including scientists—have hardly heard of the heavy-metal element and know little about it", noting a comment by a conference attendee that "it's possible to have a Ph.D. in nuclear reactor technology and not know about thorium energy." Nuclear physicist Victor J. Stenger, for one, first learned of it in 2012:
Others, including former NASA scientist and prominent thorium advocate Kirk Sorensen, agree that "thorium was the alternate path". According to Sorensen, during a documentary interview, he states that if the US had not discontinued its research in 1974 it could have "probably achieved energy independence by around 2000".
On 18 May 2022, US Senator Tommy Tuberville introduced US Senate bill S.4242 – "A bill to provide for the preservation and storage of uranium-233 to foster development of thorium molten-salt reactors", the 'Thorium Energy Security Act', a measure which Sorensen had urged since 2006. However, it was not adopted by Congress.
Benefits
- Abundance. Thorium is three times as abundant as uranium and nearly as abundant as lead and gallium in the Earth's crust. The Thorium Energy Alliance estimates "there is enough thorium in the United States alone to power the country at its current energy level for over 1,000 years." "America has buried tons as a by-product of rare earth metals mining", notes Evans-Pritchard. Almost all thorium is fertile Th-232, compared to uranium that is composed of 99.3% fertile U-238 and 0.7% more valuable fissile U-235.
- Less suitable for bombs. It is difficult to make a practical nuclear bomb from a thorium reactor's by-products, allowing governments to potentially pursue further nuclear power without worsening nuclear arms proliferation. Thorium is not fissile like uranium, so packed thorium nuclei will not begin to split apart and explode. However the uranium-233 used in the cycle is fissile and hence can be used to create a nuclear weapon though plutonium production is reduced. According to Alvin Radkowsky, designer of the world's first full-scale atomic electric power plant, "a thorium reactor's plutonium production rate would be less than 2 percent of that of a standard reactor, and the plutonium's isotopic content would make it unsuitable for a nuclear detonation." Several uranium-233 bombs have been tested, but the presence of uranium-232 tended to "poison" the uranium-233 in two ways: intense radiation from the uranium-232 made the material difficult to handle, and the uranium-232 led to possible pre-detonation. Separating the uranium-232 from the uranium-233 proved very difficult, although newer laser isotope separation techniques could facilitate that process. In the United States, the AEC and DOE processed several kilograms of uranium-233 at Rocky Flats, and successfully used multiple chemical isolation steps to isolate uranium-232 decay products.
- Less nuclear waste. There is less high-level nuclear waste when thorium is used as a fuel in a liquid fluoride thorium reactor—up to two orders of magnitude less, state Moir and Teller, eliminating the need for large-scale or long-term storage; "Chinese scientists claim that hazardous waste will be a thousand times less than with uranium." The radioactivity of the resulting waste also drops down to safe levels after just one or a few hundred years, compared to tens of thousands of years needed for current nuclear waste to cool off. However, the production of activation products and fission products is broadly similar between thorium and uranium based fuel cycles.
- Fewer reaction startup ingredients. According to Moir and Teller, "once started up needs no other fuel except thorium because makes most or all of its own fuel." Breeding reactors produce at least as much fissile material as they consume. Non-breeding reactors, on the other hand, require additional fissile material, such as uranium-235 or plutonium to sustain the reaction.
- Harvesting weapons-grade plutonium. The thorium fuel cycle is a potential way to produce long term nuclear energy with low radio-toxicity waste. In addition, the transition to thorium could be done through the incineration of weapons grade plutonium or civilian plutonium.
- No enrichment necessary. Since all natural thorium can be used as fuel, no expensive fuel enrichment is needed. However the same is true for U-238, as fertile fuel in the uranium-plutonium cycle.
- Efficiency. Comparing the amount of thorium needed with coal, Nobel laureate Carlo Rubbia of CERN, estimates that one ton of thorium can produce as much energy as 200 tons of uranium, or 3,500,000 tons of coal.
- Failsafe measures. Liquid fluoride thorium reactors are designed to be meltdown proof. A fusible plug at the bottom of the reactor melts in the event of a power failure or if temperatures exceed a set limit, draining the fuel into an underground tank for safe storage.
- Mining. Mining thorium is safer and more efficient than mining uranium. Thorium's ore, monazite, generally contains higher concentrations of thorium than the percentage of uranium found in its respective ore. This makes thorium a more cost efficient and less environmentally damaging fuel source. Thorium mining is also easier and less dangerous than uranium mining, as the mine is an open pit—which requires no ventilation, unlike underground uranium mines, where radon levels can be potentially harmful.