Thorium fuel cycle

The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, ²³²
Th, as the fertile material. In the reactor, ²³²
Th is transmuted into the fissile artificial uranium isotope ²³³
U which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as ²³¹
Th), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, ²³²
Th absorbs neutrons to produce ²³³
U. This parallels the process in uranium breeder reactors whereby fertile ²³⁸
U absorbs neutrons to form fissile ²³⁹
Pu. Depending on the design of the reactor and fuel cycle, the generated ²³³
U either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

The thorium fuel cycle has several potential advantages over a uranium fuel cycle, including thorium's greater abundance, superior physical and nuclear properties, reduced plutonium and actinide production, and better resistance to nuclear weapons proliferation when used in a traditional light water reactor though not in a molten salt reactor.

Concerns about the limits of worldwide uranium resources motivated initial interest in the thorium fuel cycle. It was envisioned that as uranium reserves were depleted, thorium would supplement uranium as a fertile material. However, for most countries uranium was relatively abundant and research in thorium fuel cycles waned. A notable exception was India's three-stage nuclear power programme. In the twenty-first century thorium's claimed potential for improving proliferation resistance and waste characteristics led to renewed interest in the thorium fuel cycle. While thorium is more abundant in the continental crust than uranium and easily extracted from monazite as a side product of rare earth element mining, it is much less abundant in seawater than uranium.

At Oak Ridge National Laboratory in the 1960s, the Molten-Salt Reactor Experiment used ²³³
U as the fissile fuel in an experiment to demonstrate a part of the Molten Salt Breeder Reactor that was designed to operate on the thorium fuel cycle. Molten salt reactor (MSR) experiments assessed thorium's feasibility, using thorium(IV) fluoride dissolved in a molten salt fluid that eliminated the need to fabricate fuel elements. The MSR program was defunded in 1976 after its patron Alvin Weinberg was fired.

In 1993, Carlo Rubbia proposed the concept of an energy amplifier or "accelerator driven system" (ADS), which he saw as a novel and safe way to produce nuclear energy that exploited existing accelerator technologies. Rubbia's proposal offered the potential to incinerate high-activity nuclear waste and produce energy from natural thorium and depleted uranium.

Kirk Sorensen, former NASA scientist and Chief Technologist at Flibe Energy, has been a long-time promoter of thorium fuel cycle and particularly liquid fluoride thorium reactors (LFTRs). He first researched thorium reactors while working at NASA, while evaluating power plant designs suitable for lunar colonies. In 2006 Sorensen started "energyfromthorium.com" to promote and make information available about this technology.

A 2011 MIT study concluded that although there is little in the way of barriers to a thorium fuel cycle, with current or near term light-water reactor designs there is also little incentive for any significant market penetration to occur. As such they conclude there is little chance of thorium cycles replacing conventional uranium cycles in the current nuclear power market, despite the potential benefits.

In the thorium cycle, fuel is formed when ²³²
Th captures a neutron (whether in a fast reactor or thermal reactor) to become ²³³
Th. This normally emits an electron and an anti-neutrino (ν) by β⁻
decay to become ²³³
Pa. This then emits another electron and anti-neutrino by a second β⁻
decay to become ²³³
U, the fuel: