General properties of thorium-based fuels in reactors

The high fission efficiency of U-233 (the ‘eta’ value) results in a swing in the fissile content and reactivity over the in-core lifetime of thorium-based fuel. However, this effect is much smaller than in uranium fuel. Thus, thorium reactor cores are more manageable than uranium cores because, over the lifetime of the core, the variations in reactivity and power distribution (power peaking) are less. Further, at low average fluxes and low burn-up, the in situ breeding is also better than for uranium fuel. This property gives significantly greater flexibility to programmes based on thorium use from a reactor operation standpoint. In general, the conversion factor in current thermal reactors is higher for Th-U/233 cycles, compared to uranium-plutonium cycles (the increase is usually between 20 to 30%).This point will be further developed at the end of this section.

It must be mentioned also that U-233 is much more flexible in thermal reactors than plutonium, because its nuclear properties (mainly its cross sections) provide greater margins for fuel management in the core. Partly, this is because the three main isotopes of plutonium have great resonances at a very low energy, which complicate the neutronic behaviour of plutonium fuels.

Other specific features of thoriumbased fuel are as follows:

Thorium and its oxide (ThO2 ) have better behaviour under irradiation than uranium and its oxide (UO2), allowing higher burn-ups. This is a consequence of the higher melting point and superior thermal conductivity of both thorium and thorium oxide when compared to uranium and UO2.[17] Furthermore, the chemical interaction of metallic thorium with water and steam is less intense than for metallic uranium.

There is a significant weaker neutron spectrum dependence of U-233 thermal cross sections, compared to those of plutonium isotopes. This is favourable for reactor safety (temperature effects) and operation (power changes), especially when switching LWR from ‘cold’ to ‘hot’ conditions (and conversely).

The yield of fission products affecting reactor poisoning during operation (such as xenon and samarium) is significantly lower for U-233, compared to U-235 and plutonium. The average cross-section values of neutron absorption by U-233 fission products is decreased by about 25 to 30%. Hence, reactivity loss is lower and the core lifetime (i. e. burn-up) increases. This also contributes to a better global neutron economy.

In terms of reactor operation and fuel performance, therefore, fuel based on Th/U-233 has many advantages over uranium-plutonium based fuel.

Conversely, one of the main drawbacks to the use of thorium fuel in reactor cores is the production of Pa-233, a neutron absorber, in rather high concentration. This is explained by the relatively long decay period of Pa-233 (27 days half-life forming U-233), compared to its equivalent in uranium fuel, Np-239 (2.3 days forming Pu-239). It results in a ‘delayed reactivity’ increase after reactor shutdown that must be carefully accounted for.

In a reactor, the rate of loss of neutrons by Pa-233 capture is proportional to the number of neutrons, thus to the neutron flux, and to the capture cross section of Pa-233, which is high for thermal neutrons. Consequently, the concentration of Pa-233 during reactor operation is particularly penalizing for high flux thermal neutron reactors. The loss of a Pa-233 nucleus by neutron capture is equivalent to the loss of a U-233 nucleus, which would otherwise have been formed by normal radioactive decay of Pa-233. This phenomenon leads to a significant reduction of the conversion ratio. Because it is most significant at high thermal neutron fluxes, studies of thorium fuel have tended to be done on reactor cores with low thermal neutron flux and, therefore, low power density. Such cores are not so attractive economically, of course, but they are a feature of ADS, which is one reason why these reactors have been considered to be particularly suitable for thorium fuel.