‘Burning’ Nuclear Waste in Nuclear Reactors

I was recently asked for my opinion on the DUPIC (Direct Use of spent PWR fuel In CANDU) reactor. Although I do not have any comments about this reactor compared to other reactors which ‘burn nuclear waste’ I thought I would say a few words about the concept in general. Before we go onto that I just want to make few general points.

Spent Fuel

First of all it is important to get past the nuclear industries misleading wording on what is actually happening. They often say that ‘nuclear is the only power source which can actually burn its own waste’. What is actually happening is that modern fission power plants are very inefficient utilising between 4% and 6% of the energy available1. So most of the energy that is available in the fuel is usually thrown away in the ‘spent fuel’. Originally it was envisenged that the current type of nuclear power stations would be replaced by better designs which would utilise much more of the energy. However, despite many attempts this has not happened. There are several reasons for this including the more difficult technology, the fact that nuclear power did not take off as expected and there was not the expected shortage of natural uranium and that other technologies are more expensive.

Transuranic Elements and Fission Products

There are two types of radioactive elements present in spent nuclear fuel – fission products and transuranic elements produced by neutron capture (see Composition of Spent Fuel). Fission products tend to be much shorter lived than the transuranic elements. For example two of the major fission products Cs-137 and Sr-90 have half-lives of 30 and 28 years respectively. However, there are several fission products which have much longer half-lives, in particular Tc-99 (211,000 years).

On the whole transuranic elements have longer half-lives than fission products. For example Plutonium-239 has a half life of 24,000 years and Neptunium-237 has a half life of 2 million years. Unlike fission products the transuranic elements are fissionable and/or fertile i.e. they can either undergo fission or can undergo neutron capture and be transformed into something that can undergo fission.

‘Burning’ Nuclear Waste

Since the transuranic elements can still be used to produce power and their long half-lives creates more problems for long term disposal it is theoretically possible to ‘burn’ them to dispose of them and possibly create more power from the spent fuel. Several technologies have been proposed for this including fast reactors, PRISM and DUPIC.

When the transuranics are ‘burnt’ in the reactor they end up as fission products which tend to be much more radioactive but have shorter half-lives than the transuranics. This is a good thing since we do not know what to do with long lifetime waste (tens of thousands of years). However, it is not so good because you end up with much more radioactivity and we do not know what to do with the highly radioactive medium lifetime waste (hundreds of years). Even if all the transuranics were to be removed there are also several long lived fission products which would make the waste dangerous for tens of thousands of years. Even ‘natural uranium’ is not safe (see Is Natural Background Radiation is Safe?).

‘Burning’ Money

Any technology that reuses spent fuel will take a lot of money and time to develop. It decreases but does not eliminate the problem of disposing of long lived waste but also drastically increases the amount of highly radioactive medium lived waste (hundreds of years).

Nuclear power is already finding it difficult to compete with renewables. It is time to admit that enormous resources put into the development of nuclear fission (see UK R&D Expenditure on Energy)has been a waste of money and we should stop throwing good money after bad.

1 This is reasonably easy to estimate. If you assume that the fissioned atom splits 40%/60% it is easy to work our a rough idea of the mass change from looking at the atomic masses of the fission products, then you can calculate the expected energy using E = mc2. This gives a figure of about 1000GWd/TU. This figure is slightly too high since it ignores radiation losses including neutrinos. Alternatively you could just use the figure of 193.7MeV per fission and use this to calculate the maximum burnup which gives you 907GWd/TU.

Current burnup is about 40GWd/TU but it is hoped to get this to 60GWd/TU for new reactors.


One Response to “‘Burning’ Nuclear Waste in Nuclear Reactors”

  • PCAH says:

    The proposed Hinkley C would burn MOX fuel from Sellafield. Thank you for pointing out that this would increase spent fuel waste stored on site in Somerset for thousands of years.

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