Abstract

This paper is devoted to some aspects of the physical and chemical evolutions of irradiated fuels in a closed system, which implies no mass transfers with the surrounding medium. This situation is not only relevant for dry interim storage, but also for the first stages of a geological disposal scenario, prior to the re-saturation of the near field and the access of water to the spent fuel. Some potential evolutionary mechanisms are presented and discussed:

• The chemical evolution of spent fuel: natural radioactive decay leads to the formation of elements with different oxidation states and consequently, could modify the oxygen potential of the irradiated fuel. Thermo-chemistry calculations performed for decay periods of up to 300 years, show however that the change in composition of spent UOX fuel is not sufficient to significantly affect the oxidation state of the oxide matrix. The temperature dependency of oxygen potential is not modified with time, and therefore the oxygen potential will decrease with the temperature decrease in the long term.

• Helium behaviour in the fuel has been identified as a major issue since little is known on the mobility of helium in uranium oxides or mixed oxides or on the potential consequences of helium release on long term fuel rod integrity. A comparative assessment of the amount of helium generated in standard UOX and MOX fuels, at the same rod average burnup is presented for storage periods of up to 10,000 years. More helium is generated in MOX fuels than in UOX fuels due to their higher content in alpha emitters. Potential consequences for fuel storage are discussed based on two extreme hypotheses whereby helium is entirely released or retained within the oxide. A preliminary analysis of existing data on helium solubility, its diffusion in uranium dioxides and reactor feedback is presented and reveals that uncertainties make it difficult to predict with confidence the extent to which helium release could occur under storage conditions or if indeed the integrity of the pellet could be undermined in the long term.

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