Implications of High Burnup Fuel – Basic Notes

An important issue around nuclear new build is the amount and what happens to the spent fuel. The proposed EPR reactors at Sizewell and Hinkley are designed to run oh ‘high burnup’ fuel. This post has a basic look at what the implications are.

This is a rough wordy explanation and I hope to be able to a more detailed analysis in a later post.

You should have a basic understanding of some of the issues about spent nuclear fuel – see Spent Fuel – Basic Notes.

In the recent consultation document published by EDF it states:

2.2.16 The UK EPR design generates less spent fuel than other nuclear reactors in the UK per unit of electricity generated. It optimises fuel use which, when coupled with fuel design and manufacture, ensures that less radioactive spent fuel is created.1

We are going to have a look what this really means in terms of radioactivity produced and spent waste storage.

When they say that they will generate less spent fuel per unit of electricity generated what they mean is that they will be using ‘high burnup’ fuel.

First of all let us go back to some basic nuclear physics. Most of the energy produced in a nuclear reactor is through the fission process – a neutron hits a fissionable atom (U-235 or Pu- 239) and that atom splits producing two new atoms (fissile products). The energy released during the fissile process is very large (about 200MeV – 3×10-11Joules per fission). The fissile products are often radioactive and this produces some energy (less than 1MeV per decay on average). After removal from the reactor the fission process stops (since there are no longer enough neutrons to maintain the chain reaction). However the fuel is very radioactive due to the fissile products and also continues to produce heat – called ‘decay heat’.

The only way to increase the amount of energy from the same amount of fuel is to increase the number of fission events in that amount of fuel. This is mainly achieved by increasing the amount of enrichment of the fuel (from about 3.6% to 5%).

Radioactivity and Decay Heat

However by increasing the number of fissions for a given amount of fuel you also proportionally increase the number of radioactive fissile products and hence decay heat. i.e. if you double the amount of energy you get from a given amount of fuel then you double its radioactivity and its decay heat.

Amount of Uranium Used

As said above the main way that the ‘high burnup’ is achieved is by increasing the enrichment of the fresh fuel. However this does not decrease the amount of natural uranium which has to mined to generate the same about of energy. If you increase the enrichment then you produce less enriched uranium from a given amount of natural uranium and consequently more depleted uranium (see Uranium Enrichment Formula).

 


 

1 Initial Proposals and Options Consultation Document, EDF 2012 (http://sizewell.edfenergyconsultation.info/wp-content/uploads/Consultationdocument.pdf)

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