The fuel in modern nuclear power plants is enriched to 3-5% U-235 (which is the isotope that undergoes fission). Therefore we might expect that spent fuel contains roughly 3-5% fission products. It is a bit more complicated than this which I will go into in this blog. Despite the small percentage of fission products in the spent fuel they create a massive increase in the radioactivity with spent fuel being over a million times more radioactive than fresh fuel¹.

Direct Fission Products

Every fission event products two fission products (occasionally three – I may go into that in a future post). For the moment let us just concentrate on the fission of U-235 – the data I have used is from http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0659980 and I have put it into an Excel spreadsheet available here.

The split is on average roughly a 40:60 split in terms of atomic mass (see figure 1). Note that this graph is often shown with a logarithmic y-scale (as shown in Very Heavy Atoms – Fission). The different atomic masses contain various isotopes of different elements.

Figure 2 shows the yield of different element. Note that nuclei with an even number of protons are slightly more stable than those with an odd number and hence the alternating difference in yield.

Overall 776 different  isotopes are shown in this data.

So the fission process produces a bit of a mess with different isotopes of various elements – but it gets worse.

Decay of Fission Products

Many of the fission products are not stable and undergo radioactive decay (see Decay Chains). The products of the radioactive decay may not be stable and they themselves undergo radioactive decay. Therefore an even wider range of elements and isotopes are formed.

Neutron Capture

Some of the fuel can undergo neutron capture. For example U-238 can undergo neutron capture to produce U-239 which decays to form Np-239 which then decays to form Pu-239.

Pu-239 – plutonium-239 is fissile and can undergo fission in a similar fashion to U-235. So some of the non-fissile U-238 is turned into fissile -Pu239. (e^– is a beta particle – an electron and is a neutrino – don’t worry about these since they interact very weakly with matter).

So some of the energy produced in a standard nuclear reactor is from this plutonium. The ‘breeding’ of plutonium from U-238 can be enhanced in a ‘fast breeder reactor’.

However, some of the fission products can also undergo neutron capture. Such fission products ‘poison’ the nuclear reactor since they remove neutrons that are needed for fission without producing any fissile isotopes.

An important example is Xe-135 which is produced by the radioactive decay of the fission product I-135:

The build up of such ‘poisons’ in the fuel is the reason why the fuel is removed from the reactor before all the U-235 (and Pu-239) is used up.

So overall you can see that spent fuel is a complete mess which is one of the reasons why it presents such a problem for disposal.

---

1. Spent fuel has an activity of about 1×10¹⁷ Bq/TU (see http://dspace.mit.edu/handle/1721.1/16603) while U-238 has an activity of about 1.24×10¹¹Bq (see Radioactivity of U-238)