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nuclear properties of plutonium vs uranium

Some time ago I saw a comment that the nuclear properties of plutonium make a dried out reactor core more dangerous.

I think that this is not true.

While plutonium does have different nuclear properties to uranium it is not impossible to run a power reactor on MOX or even pure plutonium fuel in a safe manner.

One of the key differences is the ratio of prompt to delayed neutrons from plutonium fission is different. So a reactor which is to run on plutonium must have a control rod system which is faster to cope with this difference in the two fuels.

If someone was to put a MOX fuel into a reactor which was designed for UOX (uranium dioxide) without checking that the control system of the reactor was able to respond at the right speed something might go wrong, but if the reactor is able to control itself OK on MOX then in that system then MOX is not an evil menace. 

A light water reactor needs water to make the nuclear reaction work. The slower the neutrons the more able the neutrons are to interact with the nucleus of atoms. The fission (splitting) of a plutonium or uranium atom is caused by the absorption of a neutron by the nucleus to form an excited state.

1n + 235U –> 236*U –> fission products + 1n + 1n + 1n

As the new neutrons on the right hand side of the equation are very fast moving they are not likely to react with a nucleus of an atom, they need slowing down to make them more able to cause fission.

The moderator works by allowing the neutrons to bounce around, each time the neutron hits a nucleus of a moderator (typically 1H, 2H, Be or 12C) the neutron slows down. When the neutrons are at a low speed then they are more able to cause further reactions. The fast neutrons are likely to escape from the core, by the time they hit something which they interact with they are likely to be too far outside the core to be able to randomly bounce back into the core. As a result they are lost, when the rate at which neutrons are lost is greater than the rate at which they are created the reactor becomes subcritical and then the reaction dies out.

So as a result by boiling off the water or draining it from the reactor the reactor becomes less able to become critical. This is true both for a reactor with uranium dioxide inside it and for one with MOX inside it.

In addition with the Japanese reactors when the earthquake was detected the control rods were inserted into the reactor. The control rods are rods of metal which contain a substance (typically cadmium, silver, boron carbide or hafnium) which absorbs the neutrons. When these control rods absorb neutrons no fission reaction occurs these absorbed neutrons are lost for ever from the reaction.

In addition when a reactor is suddenly shut down one of the fission products has a strong calming effect on the reactor. It is the xenon, one of the xenon isotopes has a very large ability to capture neutrons. I will write more about xenon soon.

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