Radiation Effects in SYNROC-D

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Abstract
SYNROC-D is a ceramic material proposed as a waste form for defense high-level nuclear waste. During the first million years of storage, it would be subjected to ∼8 × 1024 alpha decay/m3 of SYNROC-D and a total ionization dose of ∼1 × 1011 rad. There are several methods of simulating the resulting radiation effects, including external bombardment using gamma rays, electrons, light ions, heavy ions, or neutrons, and internal bombardment using short half-life actinide doping to bring about internal alpha decay, or doping with uranium, boron, or lithium, coupled with neutron irradiation, to induce internal fissions or (n, α) reactions. Previous work by others using several of these methods as well as data from natural minerals has been compared on a displacements per atom basis. The results show that dose rate effects are not important in determining the swelling and metamictization of the perovskite and zirconolite phases over a wide range of dose rate for low temperatures and doses of 2 to 3 × 1025 alpha/m3 of each phase, corresponding to expected million year doses in SYNROC-D. Based on this observation and a consideration of the basic processes involved, we argue that the million-year radiation damage expected in SYNROC-D can be adequately simulated in a few months by doping samples with 238Pu, and simultaneously carrying out external gamma-ray bombardment. The 238Pu will undergo alpha decay, producing the same type of damage in the same phases as would long-term actinide decay in actual waste. The gamma irradiation will simulate the ionization dose, which would result primarily from fission product decay in actual waste. SYNROC-D samples have been fabricated and characterized using cerium and uranium, respectively, as stand-ins for plutonium. These samples show good properties, and 239Pu doping experiments are expected to take place soon to determine if plutonium will dissolve properly in SYNROC-D.