Impact of self-irradiation damage on the aqueous durability of zircon (ZrSiO4):: implications for its suitability as a nuclear waste form

Zircon has been proposed as a nuclear waste form to safely encapsulate weapons-grade plutonium. In order to study the impact of self-irradiation damage in zircon on its aqueous durability, we performed a hydrothermal experiment (2 M CaCl2 solution, 600degreesC, 100 MPa) with several variably radiation-damaged, i.e. amorphized, zircon samples. We found an anomalous increase in the alteration rate at two critical concentrations of amorphous domains. The first dramatic increase sets in when the amorphous domains form interconnected clusters in the structure. The second increase is related to the percolation of fast diffusion pathways consisting of nano-sized regions of depleted matter that are formed during strongly overlapping a-recoil events, as seen by molecular-dynamics simulations and small-angle x-ray scattering measurements. The two percolation thresholds provide model benchmarks for the safety performance of a zircon waste form.