I will probably write about the Thorium cycle a bit more at some later point. There seems to be a growing number of people who say that we can use Thorium as a nuclear fuel. One of the reasons they state is that you cannot make a bomb from Uranium-233 and therefore the thorium cycle is proliferation resistant.

What is the Thorium Cycle

Thorium 232 is more abundant than Uranium and although cannot be used as a nuclear fuel directly, it can undergo neutron capture and eventually be turned into Uranium 233. The Uranium 233 can be used as a nuclear fuel.

Proliferation Resistance

Many people claim that the Uranium 233 cannot be used to produce a nuclear weapon. However, this is incorrect. In fact, a Uranium – 233 bomb was tested by the USA in 1955 (see Wikipedia).

Uranium-233 is in many ways a much better bomb material than Uranium-235. For example, its critical mass is about the same as Plutonium-239 which is much lower than Uranium-235 (Minimum Critical Mass Analytical Studies, Los Alamos, 1993).

Let us look at why people make these claims.

As well as Uranium-233 another isotope (Uranium-232) is also produced. The daughter products of this are highly radioactive and it is claimed that this high level or radiation makes it ‘impossible’ to make a bomb – ignoring the fact that this has already been done.

The high radioactivity of the decay products of Uranium-232 produces two problems. First it is difficult to handle when building the bomb and second the radioactivity can damage the triggering device for the bomb.

Technology has moved a long way since 1955 and handling such materials can be done remotely. Although the electronics have to be well shielded the pneumatics and electric motors are less affected. This does not create any problem that cannot be overcome.

The first and second difficulty can be overcome in the short term by chemical separation. Although it is difficult to separate Uranium 232 from Uranium 233 it is the decay products of Uranium 232 that causes the difficulties. Uranium-232 has a halflife of 68.9 years and the first decay product is Thorium-228 which has a halflife of 1.9 years. Therefore it is possible to isolate the Uranium from the decay products and reduce the amount of radioactivity to a reasonable level for a limited period.

This would not be good if you were thinking of building a huge arsenal of nuclear weapons since the weapons would have a limited lifespan. However, this would not be a problem if just want one or two weapons.

Spontaneous Fission

I have seen it mentioned that the spontaneous fission rate of Uranium-233 is too high. This would lead to a nuclear bomb detonating prematurely as the parts were brought together to the critical mass. However, this is not true since the spontaneous fission rate is >2.7×10¹⁷ years for ²³³U (Ground-state spontaneous-fission half-lives of uranium isotopes, H. R. von Gunten*, A. Grütter, H. W. Reist, and M. Baggenstos, Phys. Rev. C 23, 1110–1112). The problem is alpha neutron reactions resulting from the decay of the daughter products of Uranium-233. This again can be overcome by chemically separating the Uranium from the daughter products.