The kinetics of alpha-decay-induced amorphization in zircon and apatite containing weapons-grade plutonium or other actinides
Introduction
A challenge facing the world community is the stabilization and immobilization of high-level radioactive waste (HLW) and special high-actinide waste streams into solid waste forms that are destined for geologic repositories. These wastes include the HLW stored in tanks at US. Department of Energy sites, an estimated 100 metric tons of weapons-grade plutonium recovered from the dismantling of several thousand nuclear weapons under the first and second Strategic Arms Reduction Treaties, several tens of tons of plutonium residues/scraps, and other high- actinide waste streams. Potential waste forms include glasses, glass-ceramics and crystalline ceramics, and in the case of glass-ceramic or crystalline-ceramic waste forms, the actinides, including plutonium, and many fission products can be stabilized in crystalline ceramic phases. Two such phases of interest are zircon 1, 2and apatite 2, 3, 4, 5, 6, which readily accommodate actinides and some fission products in their structures and are well-known as extremely durable mineral phases. In addition to being primary, actinide-bearing waste form phases, zircon and apatite (or closely related structure types) have been identified among the alteration (i.e. corrosion) products of HLW glasses 7, 8, 9and the UO2 of spent nuclear fuel [10].
Long-term durability is an important consideration in HLW and actinide disposition, but it is a particularly important consideration in Pu disposition because the fissile (half-life of 2.41×104 years [11]) and its fissile daughter, (half-life of 7.04×108 years [11]), present a risk of nuclear criticality (however small) for exceedingly long time periods; durability also ensures that neutron absorbers (e.g. Hf or Gd), which are also readily incorporated in the zircon and apatite structures, stay with the fissile nuclides. High Pu incorporation (5 to 20 wt%) into the waste form is desirable to minimize the total volume necessary for disposition; however, lower Pu loading (1 wt%) may be dictated by criticality concerns. In a near-surface geologic repository, temperatures are expected to decrease from initial values that could range as high as 250°C at emplacement to values less than 100°C after several hundred years [12].
Zircon (ZrSiO4) forms as one of several crystalline phases in glass ceramic waste forms 13, 14, 15, 16, 17and has been proposed as a durable ceramic for the immobilization and disposal of both excess weapons-grade Pu in the United States 1, 2and high-actinide wastes in Russia 18, 19. Zircon is also a prominent actinide-bearing phase formed by crystallization from the core melt at the Chernobyl Nuclear Power Plant 18, 20and has been observed as a corrosion product on HLW glass [7]. Natural zircon is an extremely durable mineral and individual grains undergo many cycles of weathering and erosion followed by transport and deposition with limited physical abrasion or dissolution. Zircons have been dated at 4.1 to 4.3 billion years 21, 22, 23, the oldest terrestrial minerals yet dated. The widespread distribution of zircon in the continental crust, its tendency to concentrate a wide variety of impurities (such as hafnium, lanthanides and actinides), the very low diffusion and loss rates of these impurities and decay products (e.g. Pb) and its resistance to chemical and physical degradation have made zircon one of the most useful accessory minerals in geologic studies. High Pu (10 wt%) 24, 25, 26, 27and U (>10 wt%) [18]incorporation in zircon have been demonstrated and extensive substitution of Pu for Zr is suggested by the synthesis of isostructural PuSiO4 [28].
Natural apatites, Ca10(PO4)6(OH, Cl, F)2, are the most abundant of the phosphate minerals [29], attesting to the durability of this structure. The durability of natural apatites is also well demonstrated by apatites from the Oklo natural reactor site in the Republic of Gabon, Africa; these apatites have retained a significant enrichment [4]and high fission-product contributions in their structure 4, 30despite exposure to the geologic environment for nearly 2 billion years. Apatite phases, analogous to natural apatites, have been observed in glass-ceramic waste forms [31]and as recrystallized alteration products on the surfaces of HLW glasses as a result of aqueous corrosion 8, 9. The latter data indicate the inherent stability of apatite relative to HLW glasses [31]. Rare-earth silicates with the apatite structure also have been observed as actinide host phases in a devitrified borosilicate HLW glass [32], a multiphase ceramic waste form [33], a glass- ceramic waste form [34]and a cement waste form [35]. These apatites are generally of the composition Ca4−xRE6+x(SiO4)6−y(PO4)y(F, OH, O)2 (where RE=La, Ce, Pr, Nd, Pm, Sm, Eu and Gd) and are isostructural with natural apatite. More recently, the apatite structure has been proposed as a host phase for the disposition of Pu and high-actinide waste 2, 3, 4, 5, 6.
Section snippets
Radiation effects
As actinide host phases, zircon and apatite will experience self-radiation damage from α-decay. The α-decay of U/Th impurities in natural zircon minerals 27, 36, 37, 38, 39and of Th impurities in several natural apatite minerals, containing appreciable rare earths and silica 40, 41, leads to a radiation-induced, crystalline-to-amorphous (metamict) transformation. The α-decay of incorporated into the apatite phases that formed in a devitrified HLW glass and in a multiphase ceramic waste
Proposed models
As discussed, α-decay in apatite and zircon generally leads to a radiation-induced, crystalline-to-amorphous transformation. In the absence of any significant recovery due to temperature or geologic time periods, the dependence of amorphous fraction, fa, on cumulative α-decay dose, D, in silicate apatite at approximately 30°C has been previously shown [26], as illustrated in Fig. 4, to follow the direct impact model [58]for amorphization (i.e. amorphization occurring directly in a displacement
Model calculations
The predicted amorphization dose as a function of temperature is shown in Fig. 8 for rare-earth silicate apatite and zircon phases containing 1 and 10 wt% weapons . The effect of damage rate or α-activity (i.e., actinide content) on the temperature dependence is clearly evident and is consistent with the calculated values of Tc, based on Eq. (6), which for silicate apatite containing 1 and 10 wt% are 15 and 29°C, respectively, and for zircon are 196 and 218°C, respectively, for 1 and
Conclusions
The models presented here demonstrate that apatite incorporating will remain crystalline, despite self-radiation effects, because of the high thermal recovery rates. The incorporation of into zircon, however, will lead to a crystalline-to-amorphous transformation at conditions expected in a near surface repository, such as Yucca Mountain. The initial elevated temperatures in a geologic repository may retard amorphization in zircon, but eventually zircons containing Pu will undergo a
Acknowledgements
The experimental studies and original model development were supported by the Division of Materials Sciences, Office of Basic Energy Sciences, US. Department of Energy under Contract DE-AC06-76RLO 1830 (PNNL) and Grant DE-FG03-93ER45498 (UNM). The extension and application of the models to nuclear waste materials was supported by the Environmental Management Science Program, Office of Environmental Management, US. Department of Energy under Contract DE-AC06-76RLO 1830 (PNNL) and Grant
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