Betafite, a radioactive mineral, is the Ti-rich member of the pyrochlore group, A1–2B2X6Y0–1, (Fd3m, Z = 8), where A = Ca, Na, U, Th, REE (rare earth elements), Y, Ba, Sr, Bi, Pb; B = Nb, Ti, Ta, Zr, Sn, Fe: X = oxygen; Y = 0, OH and F. The pyrochlore structure (or structural derivatives, e.g. zirconolite in SYNROC) is a common constituent of polyphase, crystalline radioactive waste forms. Naturally occurring minerals with this structure often occur in the metamict state. Therefore, a principal concern is the effect of alpha-recoil damage on the structure and dissolution behavior of synthetic, analogue, waste-form phases which contain actinides.
We describe the dissolution behavior of 238U, 234U, 232Th, 230Th and 228Th in betafite during leaching in a bicarbonate-carbonate solution. Results indicate enhanced leaching of recently produced alpha-recoil damage as evidenced by the more rapid dissolution rate of 228Th relative to 232Th and 230Th, and only slightly enhanced leaching of older damage as evidenced by a small increase in the dissolution rate of 234U relative to 238U. These experiments demonstrate that individual alpha-recoil tracks are preserved for some time as disordered regions of higher chemical reactivity in already fully-damaged, aperiodic structures. We estimate an annealing time for the alpha-recoil tracks of 2,000 ± 1,300 yr in the metastable, aperiodic structure. Similar alpha-recoil tracks should be formed and annealed in other aperiodic nuclear waste forms, such as borosilicate glass.