As the isotopic composition of spent fuel is dependent on the burnup, higher burnups will result in somewhat different characteristics of spent fuel. Higher burnup will result in increased levels of fission products, degraded composition of uranium isotopes and increased levels of transuranics, Mainly Pu.3 By degraded composition of uranium it is meant that concentrations of the U-232 and U-236 will increase and, due to higher initial enrichment, the concentration of residual U-235 will also be higher until a burnup above 70 GWd/tU when it starts to decrease. This effect (shown in Fig. 15.4) occurs because of the current limits on initial enrichment of fuel to 5%. The isotopic composition will

affect later spent fuel management, which may be either reprocessing and recycling or disposal. Higher concentrations of transuranics are important for the safety analysis of further spent fuel management steps as well as the usability of spent fuel for recycling (i. e. U-232 contributes to radioactivity since it is a precursor to the high-energy gamma emitter Tl-208 and the non-fission long-lived neutron absorber U-236).

The higher enrichment of fresh fuel will also raise concerns about criticality safety in spent fuel management where, as is sometimes the case, the (simplifying and conservative) assumption is made that this is unchanged by irradiation. As enrichment increases, therefore, the benefit to be gained from ‘burnup credit’ also increases. Increased enrichment also typically results in higher contents of fissile material in the spent fuel with burnup in the vicinity of 70 GWd/t. Applying burnup credit for such nuclear fuel would show more realistic safety margins for storage and disposal and may also have an impact on the cost of storage, transportation and disposal system designs.

Figure 15.5 shows the change in the concentration of the various plutonium isotopes with burnup (the higher burnups above 70 GWd/tU were test burnups). Figure 15.6 shows the transuranics inventory of spent fuel in relation to burnup. Pu-238 inventory is an interesting case: in-reactor it is generated from Am-238 and has a relatively short half-life (87.7 years) and high heat generation because almost all its decay is through alpha emission. High concentrations of Pu-238 in spent fuel may render the extracted plutonium unattractive for use as a nuclear weapon because this high heat generation makes it unmanageable.

15.5 Concentration of plutonium isotopes in spent fuel as a function of burnup.3

The specific activity of the fission products in spent fuel is almost directly proportional to the discharge burnup. So, for example, the activities of Sr-90 and Cs-137 would double for double the burnup.

Other physical-chemical characteristics of spent fuel are mostly the result of the isotopic composition in addition to the radiation exposure of, for example, the cladding and other non-fuel materials. The key characteristics are:

• decay heat

• radioactivity of spent fuel

• gas and volatile radionuclide build-up in the fuel pellet and properties of the fuel pellets (formation of pellet rim with high porosity)

• cladding properties (rod growth, clad hardening hydrogen build-up)

All these characteristics may lead under adverse circumstances to increased incidences of rod failure at higher burnup either in the reactor or subsequent storage.

15.3.2 Decay heat

Decay heat also increases linearly with fuel bumup. Most decay heat during fuel storage (first 20 years) is from fission product beta particles with less from actinides. Later, this contribution is shifted towards predominantly alpha-emitting actinides. Figure 15.7 shows decay heat dependence on spent fuel burnup showing the contributions of fission product to the total decay heat. It shows also decay heat for different cooling periods (5-200 years).