Alpha-recoil damage in zirconolite (CaZrTi 2 O 7 )
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R. B. Greegor and F. W. Lytle The Roeing Company, P. 0. Box 39992T-05,Scuttle, Washington 98124
E. M. Foltyn and F. W. Clinard, Jr. Los Alamos National Laboratory, Los Alamos, ,YewMexico 87545
L. A. Boatner and M. M. Abraham Solid State Division, Oak Ridge Nationul Laboratory, Martin Marietta Energy Systems, Inc., Ouk Ridge, Tennessee 3 7831 (Received 5 March 1986; accepted 1 July 1986)
Radiation effects in a natural, metamict zirconolite from Sri Lanka that has received an alpha-decay dose greater than lo2' alpha decays/m' have been studied using x-ray diffraction ( X R D ) , high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance spectroscopy ( EPR), extended x-ray absorption fine structure spectroscopy (EXAFS), and x-ray absorption near edge structure spectroscopy (XANES). The same techniques were applied to the sample annealed between 1000" and 1100 "C. The heat of recrystallization was measured by differential thermal analysis ( D T A ) with values of 40 to 50 J/g. In contrast to previous work [A. E. Ringwood, Am. Scientist 70, 201 (1982); Mineralog. Mag. 49, 159 (1985)], these results demonstrate that there are fundamental differences at the atomic level between the annealed, crystalline, and the natural, fully damaged zirconolite. We suggest that the most likely structure for the fully damaged state is that of a random, three-dimensional network with no atomic periodicity extending beyond the first coordination sphere. Even within the first coordination sphere, there is a reduction in coordination number and an associated decrease in bond length for principal cations (Ca and Ti). Despite this structural modification and the great age of the specimen ( -550 million years), the natural zirconolite shows only minor signs of geochemical alteration. The metamict structure is readily annealed, initially to a disordered, flourite-type structure, and finally, at higher temperatures, to a highly twinned, monoclinic zirconolite structure.
1. INTRODUCTION Radiation effects are an important consideration in the evaluation of the long-term behavior of the individual phases that occur in proposed crystalline, high-level, nuclear waste forms. Zirconolite (ideally CaZrTi,O,) is an important phase in SYNROC, a crystalline, titanate assemblage that has been proposed as a nuclear waste form.'.' The zirconolite is the principal host for actinide elements (e.g., uranium, thorium, curium, americium, and neptunium3) and, consequently, it will be subject to a major portion of the radiation damage caused by the short-range, alpha-decay events that are characteristic of the decay scheme of many actinide isotopes. The alpha-event results in the formation of two particles: ( 1 ) a high-energy alpha particle (4-5.5 MeV) with a range of approximately 20 p m and ( 2 ) a recoil nucleus (0.1 MeV) with a range of approximately 0.02 p m . Almost all of the energy of the recoil nucleus is lost through elastic collisions with the surrounding atoms. The recoil nucleus produces several thousand atomic displac
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