NAGRA Opalinus Clay (…002 - Version 1
FEP 5.2.11.1 Volume expansion​

Conversion of spent fuel to higher oxides of uranium results in a volume expansion that will partially fill voids in the canisters. Formation of higher oxides is expected to be limited because of reducing conditions. Even if some are formed, the void volume inside the canisters is sufficiently large that complete filling of the voids and the buildup of stresses inside the canister are not considered likely. The process is eliminated from further consideration in the safety assessment.