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 cycleA nuclear reactor consumes certain specific fissile isotopes to produce energy. As of the 2010s, the most common types of nuclear reactor fuel were:
* Uranium-235, purified (i.e. " enriched") by reducing the amount of uranium-238 in natural mined uranium. Most nuclear power has been generated using low-enriched uranium (LEU), whereas high-enriched uranium (HEU) is necessary for weapons.
* Plutonium-239, transmuted from uranium-238 obtained from natural mined uranium.—including the much greater abundance of thorium found on Earth, superior physical and nuclear fuel properties, and reduced nuclear waste production. One advantage of thorium fuel is its low weaponization potential. 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 (LWBR) 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(e), produced power from 1977 through 1982 (producing over 2.1 billion kilowatt hours of electricity) 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.Moir, Ralph W. and Teller, Edward. "Thorium-fuelled Reactor Using Molten Salt Technology", ''Journal of Nuclear Technology'', September 2005 Vol 151
PDF file available
. This article was Teller's last, published after his death in 2003.Barton, Charles.

, Nuclear Green Revolution, 1 March 2008
Between 1999 and 2022, the number of operational 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.Thorcon design document: (2010
Powering up our world with cheap, reliable, CO2-free electric power, now.
World Nuclear New
(26 Jan 2022) Empresarios Agrupados contracted for first ThorCon reactor
Use Molten salts— Flibe both as fuel and as coolant transfer fluid
(2020) Molten-Salt Reactor Choices - Kirk Sorensen of Flibe Energy
. Keep operational temperatures below 700 °C, use prismatic graphite as moderator, pump the molten salts from one reactor vessel in cooldown stage to the active, operating reactor vessel. Mitigate tritium using the CO2 cycle in the supercritical CO2 power conversion system; capture the tritium with the oxygen in the supercritical CO2 as mitigated water. This approach keeps the materials in chemical equilibrium during the process, while reducing the volume of waste materials such as CO2, with shorter radioactive half-lives than the uranium series' half-life.

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." However, development of thorium power has significant start-up costs. Development of breeder reactors in general (including thorium reactors, which are breeders by nature) will increase proliferation concerns.

History

The use of thorium to breed uranium-233 (²³³U) 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.

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 (at a power level somewhat under 8 MWth). 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 (LMFBR) 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 (LWBR) 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 (THTR-300), 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

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