Composition of spent nuclear fuel

Spent nuclear fuel composition depends on the reactor type, on the initial enrichment of ²³⁵U, and on the burn-up level. The burn up is defined as the amount of produced energy per ton of nuclear fuel and is related to the fraction of uranium that underwent fission. The fuel for light water reactors, the most widespread reactor type, usually contains 3.5 – 5.0 % of ²³⁵U, the rest is ²³⁸U.
After being used in the nuclear reactor, the initial fuel composition is changed, and contains more than 40 elements. Assuming that a light water reactor has been running for 18 months or more and the burn-up level is 33 GWd⁽¹⁾ per each ton of uranium, then the composition of the fuel is:

• 40-50 kg/ton of fission products;
• 10 kg/ton of plutonium isotopes;
• 1 kg/ton of minor actinoids.

The typical composition of spent nuclear fuel from a PWR reactor is shown in Figure 1. Although the minor actinoids form only approximately 0.1 % of the spent nuclear fuel, they are together with plutonium responsible for long-term radiotoxicity.

Figure 1 - An example of composition of spent nuclear fuel from a PWR reactor.

Uranium and plutonium
The 94% of the initial uranium is still present in the spent fuel.
Plutonium was created in the fuel by reactions of neutrons with uranium, followed by beta decays.

The most common plutonium isotope – ²³⁹Pu – is formed as follows:

The remaining Uranium and the produced Plutonium can be converted into MOX fuel (Mixed OXide) and used in the reactor again. First, they have to be separated from the spent fuel by means of the PUREX process. Then the nitrate solutions are converted to plutonium and uranium dioxide. This process consists of two steps – oxalic precipitation and air calcination.

Minor actinoids
The main isotopes causing high radioactivity and radiotoxicity of the spent nuclear fuel after the initial cooling period are ¹³⁷Cs and ⁹⁰Sr, then plutonium and the minor actinoids (Am, Cm, and Np).
The contribution of ²⁴¹Am increases during time because this isotope is generated by the decay of ²⁴¹Pu and ²⁴¹Am also forms ²³⁷Np as a decay product. ²⁴¹Am with a half-life of 432 years is the most important minor actinoid in the spent nuclear fuel. ²⁴¹Am has been widely used industrially, e.g. in-home smoke detectors or for oil logging.
The scale of nuclear reactions leading to minor actinoids formation is large. As other examples, neptunium isotopes ²³⁷Np (half-life - 2.1 million years) and ²³⁹Np (half-life - 2.4 days) are generated in nuclear reactors by the reactions:

${}^{238}U(n,2n)^{237}U\xrightarrow{\beta^{-},\ 6.8\ days}{}^{237}Np$

${}^{238}U(n,\gamma)^{239}U\xrightarrow{\beta^{-},\ 23\ min}{}^{239}Np$

²³⁷Np is produced as high purity neptunium in kilogram quantities in nuclear reactors. However, it does not have a large industrial application.
The most important radioisotopes of curium are ²⁴²Cm (half-life is 163 days) and ²⁴⁴Cm (half-life is 18.1 years). ²⁴⁴Cm can be used in radioisotope

thermoelectric generators. Radioisotope thermoelectric generators have found wide use in different satellites and space probes in recent decades. For example, the Curiosity Mars Rover (Figure 2) uses plutonium isotope ²³⁸Pu to produce the electricity for both locomotion and instrument operation.

Figure 2 - Curiosity Mars Rover. Credit: Courtesy NASA/JPL-Caltech/MSSS

Fission products
A significant part of the fission products belongs to the group of lanthanoid elements; their build-up worsens the neutron multiplication ability of the fuel due to their high neutron absorption cross-sections. Therefore, they represent neutron poisoning elements.
The main isotopes causing high radioactivity of the irradiated nuclear fuel after the initial cooling period are other fission products – ¹³⁷Cs and ⁹⁰Sr – both with a half-life of around 30 years.

Conclusion
Spent nuclear fuel consists mainly of uranium – about 94 %. The second most significant part is formed by the fission products, these are responsible for higher radioactivity in the first few decades (or hundreds of years). About 1 % of spent nuclear fuel is plutonium that can be re-used in MOX fuels together with uranium. The last part is represented by minor actinoids. There is only a small quantity of them, but they are responsible for most of the long-term radioactivity and radiotoxicity.