The Thorium Fuel Cycle, Its Neutron Economy

WASH-1097 remains a good source of information on the thorium fuel cycle. In fact, some major recent studies of the thorium fuel cycle rely heavily on WASH-1097.

Sometimes, however, sources on thorium may draw indirectly on WASH-1097, without mentioning it in their bibliography. Although a recent IAEA report on Thorium appears to have been prepared without overt reliance on WASH-1097.

Because it is widely referenced and continues to be an important source of information, I will rely on Wash 1097 for most of the information found is this account.

One of the first things physicists discovered about chain reactions was that slowing the neutrons involved in the process down, promoted the chain reaction. Kirk Sorensen discusses slow or thermal neutrons in one of his early posts.

Under low energy neutron conditions, Th232 can be efficiently converted to U233. The conversion process works like this. Th232 absorbs a neutron and emits a beta ray. A neutron switches to being a protron and the atom is transformed into Protactinium 233. After a period average a little less than a month, Pa 233 emits a second beta ray and is transformed into U233. U233 is fissionable, and is a very good reactor fuel. When a U233 atom encounters a low energy neutron, chances are 9 out of 10 that it will fission.

Since U233 produces an average of 2.4 neutrons every time it fissions, this means that. Each neutron that strikes U233 produces a average of 2.16 new neutrons. If you carefully control those neutrons, one neutron will continue the chain reaction. That leaves an average of 1.16 neutrons to generate new fuel.

Unfortunately the fuel generation process cannot work with 100% efficiency. The left over U234 that was produced when U233 absorbed a neutron and did not fission will sometimes absorb another neutron and become U235. Xenon 135, an isotope that that is often produced when a U233 splits, is more likely to capture neutrons than U233 or Th232. This makes Xenon 135 a fission poison. Because Xenon in a reactor builds up during a chain reaction, it tends to slow a reactor down as the chain reaction continues. The presence of Xenon creates a control problem inside a reactor. Xenon also steals neutrons needed for the generation of new fuel.

In conventional reactors that use solid fuel, Xenon is trapped inside the fuel, but in a fluid fuel Xenon is easy to remove, because it is what is called a noble gas. A noble gas does not bond chemically with other substances, and can be bubbled out of fluids where it has been trapped. Getting Xenon 135 out of a reactor core, makes generating new U233 from Th232 a whole lot easier.

It is possible to bring about 1.08 neutrons into the thorium change process for every U233 atom that splits. This means that reactors that use a thorium fuel cycle, are not going to produce a large excess of U233, but if carefully designed, they can produce enough U233 that burnt U233 can be easily replaced. Thus a well designed thorium cycle reactor will generate its own fuel indefinitely.