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Abstract238Pu-substituted zirconolite (CaPuTi 2O7 ) was stored near ambient temnerature for 231 days, equivalent to an aipha decay dose of 3.1×1025 α/m3 or 3×105 years of storage time for SYNROC ceramic nuclear waste. Periodic indentation testing showed that hardness was decreased by alpha decay-induced conversion to the metamict state, while fracture toughness and resistance to cracking were increased, apparently as a consequence of the formation of a heterogeneous microstructure. These results imply improved stability of this nuclear waste phase as a result of self-irradiation damage.