Thorium as a nuclear fuel

Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233)a, which is an excellent fissile fuel materialb. In this regard it is similar to uranium-238 (which transmutes to plutonium-239). All thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the necessary neutron dosing to produce protactinium-233. The Pa-233 that is produced can either be chemically separated from the parent thorium fuel and the decay product U-233 then recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form, especially in molten salt reactors (MSRs).

Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain reaction (and thus supply of surplus neutrons) can be maintained. The only fissile driver options are U-233, U-235 or Pu-239. (None of these is easy to supply)

It is possible – but quite difficult – to design thorium fuels that produce more U-233 in thermal reactors than the fissile material they consume (this is referred to as having a fissile conversion ratio of more than 1.0 and is also called breeding). Thermal breeding with thorium requires that the neutron economy in the reactor has to be very good (ie, there must be low neutron loss through escape or parasitic absorption). The possibility to breed fissile material in slow neutron systems is a unique feature for thorium-based fuels and is not possible with uranium fuels.

Another distinct option for using thorium is as a ‘fertile matrix’ for fuels containing plutonium that serves as the fissile driver while being consumed (and even other transuranic elements like americium). Mixed thorium-plutonium oxide (Th-Pu MOX) fuel is an analog of current uranium-MOX fuel, but no new plutonium is produced from the thorium component, unlike for uranium fuels in U-Pu MOX fuel, and so the level of net consumption of plutonium is high. Production of all actinides is lower than with conventional fuel, and negative reactivity coefficient is enhanced compared with U-Pu MOX fuel.

In fresh thorium fuel, all of the fissions (thus power and neutrons) derive from the driver component. As the fuel operates the U-233 content gradually increases and it contributes more and more to the power output of the fuel. The ultimate energy output from U-233 (and hence indirectly thorium) depends on numerous fuel design parameters, including: fuel burn-up attained, fuel arrangement, neutron energy spectrum and neutron flux (affecting the intermediate product protactinium-233, which is a neutron absorber). The fission of a U-233 nucleus releases about the same amount of energy (200 MeV) as that of U-235.

An important principle in the design of thorium fuel systems is that of heterogeneous fuel arrangement in which a high fissile (and therefore higher power) fuel zone called the seed region is physically separated from the fertile (low or zero power) thorium part of the fuel – often called the blanket. Such an arrangement is far better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U-233, in fact all thermal breeding fuel designs are heterogeneous. This principle applies to all the thorium-capable reactor systems.

Th-232 is fissionable with fast neutrons of over 1 MeV energy. It could therefore be used in fast molten salt and other Gen IV reactors with uranium or plutonium fuel to initiate fission. However, Th-232 fast fissions only one tenth as well as U-238, so there is no particular reason for using thorium in fast reactors, given the huge amount of depleted uranium awaiting use.

In Norway, Thor Energy is developing and testing two thorium-bearing fuels for use in existing nuclear power plants. Fuel rods containing thorium additive (Th-Add) and also thorium MOX (with Pu) fuel rods have been in a five-year irradiation trial since April 2013 at the Halden test reactor. The company is working towards obtaining regulatory approval for the commercial production and use of Th-Add fuel by 2017-18, and to market the fuel soon thereafter. In mid-2015 a second batch of Th-MOX fuel pellets will commence testing. Thor Energy and several utilities from North America and Europe are initiating feasibility studies to investigate the use of Th-Add fuel in commercial reactors. This fuel is promoted as a means to improve power profiles within commercial reactors.

Source World Nuclear

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