Posted by: Kash Farooq | June 4, 2011

Nuclear energy: fast neutrons, chain reactions and fuel enrichment

I’m currently enrolled on the short Open University course: “Inside nuclear energy” (ST174) and I am blogging as I go. See all my nuclear energy posts here.

Nuclear Energy can be produced in various types of nuclear power plant. In “Fast Breeder Reactors” the energy is produced when fast neutrons are absorbed by uranium-235 (i.e. an isotope of uranium with 92 protons and 143 neutrons). A fast neutron is a free neutron with a speed of 14 million metres per second. A uranium-235 isotope may absorb one of these neutrons to form the highly unstable uranium-236 isotope.

The uranium-236 isotope then undergoes nuclear fission to produce fission products, neutrons and γ-rays. There is a mass defect (i.e. mass is missing); this missing mass has been converted into energy and is used in electricity production.

The neutrons released by this fission reaction are also fast neutrons, with energy of 1 MeV and moving at 14 millions metres per second. They could go on to be captured by other uranium-235 isotopes resulting in a chain reaction:

A uranium-235 fission chain reaction

A uranium-235 fission chain reaction. A fast neutron is captured by a U-235 isotope. This forms the highly unstable U-236 isotope, which quickly decays into fission products and more neutrons. These neutrons can go on to be absorbed by other U-235 isotopes and so on – we have a chain reaction. Also note that some of the neutrons produced do not take part in any more reactions; they may encounter a U-238 isotope, or they may simply not hit anything.

The problem is that in a sample of natural uranium, the isotope uranium-238 makes up 99.3% and only 0.7 % is uranium-235. If a fast neutron is whizzing around a sample of natural uranium, the odds are against it meeting a uranium-235 isotope and being absorbed by it. When all possible outcomes are considered, the most likely outcome is that the neutron will just scatter off a uranium-238 isotope.

So, how do we adjust the odds to make uranium-235 fast neutron absorptions events more probable? We enrich the nuclear fuel. For fast breeder reactors, nuclear fuel is enriched to a level where 20-30% of the fuel can absorb fast neutrons.

This is the level of enrichment required for nuclear energy production with fast breeder reactors; so that there are enough isotopes to get a chain reaction going. Unfortunately, enriched uranium can also be used for nuclear weapons, and hence there are always concerns whenever a country announces that they want to start a uranium enrichment programme.

There are other types of reactors that do not need fuel enrichment. One reactor type (the CANDU) can, in fact, use natural uranium. But this type of reactor requires heavy water (D2O – 2 hydrogen atoms with proton and a neutron, combined with oxygen). Heavy water is extremely expensive  – costing £100s per kilogram. So, for economic reasons, these sorts of reactors aren’t widely used.


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