Posted by: Kash Farooq | July 15, 2011

Nuclear energy: thermal neutrons, thermal reactors and heavy water

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.

In a previous post I described how fast breeder reactors require substantially enriched fuel to increase the probability that a fast neutron will be captured by a U-235 isotope and hence produce a fission reaction. Unfortunately, enriched uranium can also be used for nuclear weapons.

So, what is the alternative?

It turns out that the probability of a fission reaction is also greatly increased if fast neutrons are slowed down so that they become thermal neutrons. These neutrons have energy of less than 1 eV and a speed of 2.2 kilometres per second.

In thermal nuclear reactors, fast neutrons produced by previous fission reactions are are slowed down by a moderator. The moderator contains low-mass nuclei that, essentially, absorb kinetic energy from the fast neutrons to turn them into thermal neutrons. They can then go on to produce further fission reactions.

The reaction is the same as I described in my previous post about fast breeder reactors:

A uranium-235 fission chain reaction

A uranium-235 fission chain reaction. A neutron is captured by a U-235 isotope. This forms the highly unstable U-236 isotope, which quickly decays into fission products and fast neutrons. These neutrons are slowed down by a moderator surrounding the fuel. They become thermal neutrons, which can go on to be absorbed by other U-235 isotopes and so on – we have a chain reaction.

There are a few major advantages of thermal reactors. The fuel does not need to be enriched. This makes the fuel cheaper, and takes away the nuclear weapons worry. Also, the CANDU reactor, the main thermal reactor design used around the world, requires 30-40% less mined uranium than light-water reactors.

So, why doesn’t everyone just use thermal reactors?

It comes down to economics.

The moderator needs to be something that has light nuclei. This is so that the nuclei absorb kinetic energy from the neutrons;  neutrons would just bounce of heavy nuclei and not lose speed. The moderator also has to be neutron rich itself so that it doesn’t just absorb the neutrons rather than remove their kinetic energy.

Taking these requirements into consideration, the best choice is heavy water (i.e. water made with deuterium rather than hydrogen).  Deuterium is “heavy hydrogen” – a hydrogen atom with a neutron in the nucleus alongside the proton. A normal hydrogen nucleus only contains a proton.

Heavy water is expensive.

The total cost to construct the Darlington Nuclear Generating Station in Ontario, Canada was $5,117 million. This cost included the heavy water cost: $1,528 million – i.e. 23% of the total cost. The amount of heavy water required as a moderator also requires a larger construction – for example, it increases the cost of the containment building required.

The only other moderator that is used in nuclear energy production (that can use non-enriched fuel) is graphite. There is only reactor design in current use:  the Russian designed RBMK. This is the reactor that was involved in the Chernobyl disaster. There are no plans to build more of these reactors and there is international pressure to close the 10+ RBMK reactors that are still operating.


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