Thorium fuel: following the neutron captures

Thorium-232 can absorb a neutron and eventually become uranium-233, a fissile isotope with strong neutron economy in a thermal reactor. That conversion gives thorium its role in reactor fuel cycles. The details depend on the reactor, the fuel form and how the used fuel is managed.

What is thorium?

Thorium is a slightly radioactive metal, element 90 in the periodic table, named after Thor, the Norse god of thunder. Almost all natural thorium is the isotope thorium-232, which has a half-life of about 14 billion years, close to the age of the universe. That long half-life makes it only weakly radioactive.

Thorium is roughly three to four times more abundant than uranium in the Earth's crust. It occurs mainly in monazite, a phosphate mineral found in some beach and river sands alongside rare-earth elements.

What does "fertile" mean?

Reactor physicists sort heavy isotopes into two groups according to how they respond to neutrons.

• Fissile isotopes split readily when they absorb a slow, thermal neutron. The important fissile isotopes are uranium-235, uranium-233 and plutonium-239.
• Fertile isotopes absorb a neutron and then transform, through radioactive decay, into a fissile isotope. Uranium-238 and thorium-232 are the two fertile isotopes found in nature.

Thorium-232 belongs to the fertile group. On its own, it supplies raw material for fuel. A reactor using thorium therefore also contains a fissile driver, such as uranium-235, uranium-233 or plutonium-239, which provides the neutrons that start the chain reaction and keep it running while new fissile material builds up.

How does thorium-232 become uranium-233?

The conversion takes three steps: one neutron capture followed by two beta decays.

In the first step, a thorium-232 nucleus absorbs a neutron and becomes thorium-233, releasing the extra binding energy as a gamma ray. Physicists write this reaction as ²³²Th(n,γ)²³³Th. Thorium-232 has a thermal neutron capture cross-section of about 7.4 barns, roughly 2.7 times that of uranium-238, so it captures thermal neutrons comparatively well.

Thorium-233 is unstable. With a half-life of about 22.3 minutes, it undergoes beta decay: a neutron in the nucleus converts into a proton, emitting an electron and an antineutrino. The atomic number rises from 90 to 91, and the nucleus becomes protactinium-233.

Protactinium-233 then undergoes a second beta decay, with a half-life of about 27 days, and becomes uranium-233. Uranium-233 is fissile, and it remains in the fuel ready to absorb a neutron and split.

Why is uranium-233 such a good fuel?

The quality of a fissile isotope for a thermal reactor depends on a quantity called eta (η): the average number of fission neutrons produced for each neutron the isotope absorbs. Some absorbed neutrons cause fission and some are simply captured, so eta captures both effects in one number.

In a thermal neutron spectrum, uranium-233 has an eta of about 2.29, compared with about 2.07 for uranium-235 and 2.11 for plutonium-239. A self-sustaining breeding cycle needs at least two neutrons per absorption: one to continue the chain reaction and one to convert a new fertile nucleus, with any surplus covering losses to leakage and parasitic absorption. Uranium-233's higher eta leaves a usable margin above two in a thermal reactor, which makes thermal breeding with thorium possible in principle. Uranium-235 and plutonium-239 leave a much thinner margin in the same conditions.

Why does the protactinium stage matter?

The 27-day half-life of protactinium-233 has important consequences. During that time the protactinium sits in the reactor core, exposed to the same neutron flux as the rest of the fuel. Protactinium-233 has a significant thermal capture cross-section, about 40 barns. A protactinium nucleus that captures a neutron becomes protactinium-234, which decays to uranium-234, a non-fissile isotope in a thermal spectrum.

Each such capture costs the reactor twice: it removes a neutron from the chain reaction and it removes a nucleus that would have become fissile uranium-233. The fraction lost depends on the neutron flux. A higher flux increases the capture rate relative to the decay rate, so high-power cores lose a larger share of their protactinium.

The breeding calculation therefore depends on many interacting factors: fuel composition, neutron spectrum, flux level, core geometry and the irradiation history of each region of fuel. Some liquid-fuel concepts propose removing protactinium from the core and letting it decay outside the neutron field to avoid these losses, which adds chemical processing steps to the plant.

What happens to uranium-233 in the reactor?

Once formed, uranium-233 participates in the chain reaction alongside the original fissile driver. Over time, the share of power coming from bred uranium-233 grows. Each uranium-233 fission releases about 200 MeV of energy and a new set of neutrons, some of which convert more thorium. A thorium-fuelled core thus evolves continuously: thorium is consumed slowly, uranium-233 builds up toward an equilibrium level, and the original driver is gradually used up.

Engineers measure the effectiveness of this process with the conversion ratio, the rate of new fissile production divided by the rate of fissile consumption. A ratio above one indicates breeding, where the reactor produces more fissile material than it burns.

How does thorium oxide compare with uranium oxide?

Thorium is usually used as thorium dioxide, ThO₂. This ceramic has several favourable properties for reactor fuel:

• It melts at about 3,350 °C, around 500 °C higher than uranium dioxide.
• It conducts heat better than uranium dioxide, which lowers temperatures at the centre of fuel pellets.
• Thorium has a single stable oxidation state in the oxide, so the fuel resists oxidation and chemical change.

The same chemical stability makes thorium oxide difficult to dissolve during reprocessing. The established process for separating thorium and uranium, called THOREX, uses concentrated nitric acid with a small amount of hydrofluoric acid to break down the oxide.

What role does uranium-232 play?

Alongside uranium-233, thorium fuel produces small amounts of uranium-232 through side reactions involving fast neutrons. Uranium-232 has a half-life of about 69 years, and its decay chain includes thallium-208, which emits a highly penetrating gamma ray of 2.6 MeV. As a result, recycled uranium-233 becomes more radioactive over time, and fuel fabrication with recovered uranium-233 requires heavily shielded, remotely operated facilities. This intense radiation also serves as a strong deterrent to diversion, a property that features in discussions of proliferation resistance.

Solid or liquid fuel?

Thorium fuel cycles divide broadly into two families according to the physical form of the fuel.

Solid fuels place thorium oxide in pellets, rods or coated particles, just as uranium fuels do. The fuel is irradiated, cooled and, in a closed cycle, reprocessed to recover uranium-233 for new fuel. Solid thorium fuel has been tested in light-water reactors, heavy-water reactors and high-temperature gas-cooled reactors. The Shippingport light-water breeder reactor in the United States operated from 1977 to 1982 with a thorium and uranium-233 core and demonstrated breeding in a light-water reactor.

Liquid fuels dissolve thorium and uranium as fluoride salts in a molten salt that serves as both fuel and coolant. The Molten Salt Reactor Experiment at Oak Ridge National Laboratory operated in the 1960s and ran on uranium-233 fuel from 1968, demonstrating the use of this isotope in a liquid-fuelled reactor. Liquid fuel allows chemical processing during operation, including protactinium removal, at the cost of handling a highly radioactive circulating fuel.

Each family involves different materials, processing systems, safeguards arrangements and waste streams. A complete fuel-cycle assessment covers fabrication, irradiation, cooling, reprocessing where applicable, refabrication and long-term management of used fuel.

How should a thorium fuel cycle be evaluated?

The IAEA's review of thorium fuel options describes design-specific benefits and constraints, including the fissile material needed to start a thorium core and the effect of protactinium. A sound evaluation follows the isotopes through every stage:

• the fissile driver needed at start-up and during the transition to equilibrium;
• the conversion ratio achieved in the chosen spectrum and flux;
• protactinium losses at the planned power density;
• uranium-232 build-up and its effect on handling;
• the reprocessing and refabrication facilities the cycle requires;
• the characteristics of the used fuel and waste.

Thorium offers a valuable nuclear property, the high neutron yield of uranium-233, linked to a substantial engineering chain. Its potential is realised when the reactor physics and the fuel-cycle facilities are designed together.