William J. Weber

RADIATION EFFECTS IN CRYSTALLINE CERAMICS FOR THE IMMOBILIZATION OF HIGH-LEVEL NUCLEAR WASTE AND PLUTONIUM

This review provides a comprehensive evaluation of the state-of-knowledge of radiation effects in crystalline ceramics that may be used for the immobilization of high-level nuclear waste and plutonium. The current understanding of radiation damage processes, defect generation, microstructure development, theoretical methods, and experimental methods are reviewed. Fundamental scientific and technological issues that offer opportunities for research are identified. The most important issue is the need for an understanding of the radiation-induced structural changes at the atomic, microscopic, and macroscopic levels, and the effect of these changes on the release rates of radionuclides during corrosion. {copyright} {ital 1998 Materials Research Society.}

Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and “minor” actinides

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compo...

Electrochemical properties of mixed conducting perovskites La1-xMxCo1-yFeyO3-δ (M = Sr, Ba, Ca)

Perovskite compositions in the system La 1-x M x Co 1-y Fe y O 3-δ (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia.Perovskite compositions in the system La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility and concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia. This material is an attractive candidate for several important applications, including solid oxide fuel cell cathodes.

Radiation effects in nuclear waste forms for high-level radioactive waste

High-level nuclear waste in the United States comprises large volumes (tens of millions of cubic meters), high total activities (billions of Curies) and highly diverse and complex compositions. The principal sources of nuclear waste are: (i) spent nuclear fuel from commercial and research nuclear reactors; (ii) liquid waste produced during the reprocessing of commercial spent nuclear fuel; (iii) waste generated by the nuclear weapons and naval propulsion programs. The latter category now includes over 100 metric tons of plutonium and many hundreds of tons of highly enriched uranium from the dismantling of nuclear weapons. Most of these wastes will require chemical treatment, processing and solidification into waste forms for permanent disposal. The long-term effects of radiation on waste form solids is a critical concern in the performance assessment of the long-term containment strategy. In the case of spent nuclear fuel, the radiation dose due to the in-reactor neutron irradiation is already substantial, and additional damage accumulation during disposal is not anticipated to be significant; thus, this is not a subject addressed in this review paper. In contrast, the post-disposal radiation damage to waste form glasses and crystalline ceramics is significant. The cumulative α-decay doses which are projected for nuclear waste glasses reach values of 1016 α-decays g−1 in 100 yr. Similarly, crystalline waste forms, such as Synroc will reach values of > 1018 α-decay events g−1 in 1000 yr for a 20 wt% waste loading. These doses are well within the range for which important changes in the physical and chemical properties may occur, e.g. the transition from the crystalline-to-aperiodic state in ceramics. This paper provides a comprehensive review of radiation effects (due to γ-, β- and α-decay events, as well as from actinide doping experiments and particle irradiations) on nuclear waste form glasses and crystalline ceramics, particularly Synroc phases, zircon, apatite, monazite and titanite. The paper also includes recommendations for future research needs.

Electrochemical properties of mixed conducting perovskites La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr, Ba, Ca)

Perovskite compositions in the system La 1-x M x Co 1-y Fe y O 3-δ (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia.Perovskite compositions in the system La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility and concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia. This material is an attractive candidate for several important applications, including solid oxide fuel cell cathodes.

The radiation-induced crystalline-to-amorphous transition in zircon

A comprehensive understanding of radiation effects in zircon, ZrSiO[sub 4], over a broad range of time scales (0.5 h to 570 million years) has been obtained by a study of natural zircon, Pu-doped zircon, and ion-beam irradiated zircon. Radiation damage in zircon results in the simultaneous accumulation of both point defects and amorphous regions. The amorphization process is consistent with models based on the multiple overlap of particle tracks, suggesting that amorphization occurs as a result of a critical defect concentration. The amorphization dose increases with temperature in two stages (below 300 K and above 473 K) and is nearly independent of the damage source ([alpha]-decay events or heavy-ion beams) at 300 K. Recrystallization of completely amorphous zircon occurs above 1300 K and is a two-step process that involves the initial formation of pseudo-cubic ZrO[sub 2].

Models and mechanisms of irradiation-induced amorphization in ceramics

Abstract A number of models have been developed to describe the various amorphization processes and the effects of temperature on the kinetics of amorphization. These models are reviewed and in some cases further developed. In general, these models contain a number of parameters relating to irradiation-assisted and thermal recovery processes, which make their application to existing data sets challenging. Nonetheless, general aspects of the models yield insights into the rate-limiting processes controlling the kinetics of amorphization within a given temperature regime. Several examples are used to illustrate features of the models and to highlight differences in behavior.

Radiation stability of gadolinium zirconate: A waste form for plutonium disposition

Zirconate and titanate pyrochlores were subjected to 1 MeV of Kr{sup +} irradiation. Pyrochlores in the Gd{sub 2}(Zr{sub x}Ti{sub 1-x}){sub 2}O{sub 7} system (x=0,0.25,0.5,0.75,1) showed a systematic change in the susceptibility to radiation-induced amorphization with increasing Zr content. Gd{sub 2}Ti{sub 2}O{sub 7} amorphized at relatively low dose (0.2 displacement per atom at room temperature), and the critical temperature for amorphization was 1100 K. With increasing zirconium content, the pyrochlores became increasingly radiation resistant, as demonstrated by the increasing dose and decreasing critical temperature for amorphization. Pyrochlores highly-enriched in Zr (Gd{sub 2}Zr{sub 2}O{sub 7}, Gd{sub 2}Zr{sub 1.8}Mg{sub 0.2}O{sub 6.8}, Gd{sub 1.9}Sr{sub 0.1}Zr{sub 1.9}Mg{sub 0.1}O{sub 6.85}, and Gd{sub 1.9}Sr{sub 0.1}Zr{sub 1.8}Mg{sub 0.2}O{sub 6.75}) could not be amorphized, even at temperature as low as 25 K. (c) 1999 Materials Research Society.

Radiation effects in glasses used for immobilization of high-level waste and plutonium disposition

This paper is a comprehensive review of the state-of-knowledge in the field of radiation effects in glasses that are to be used for the immobilization of high-level nuclear waste and plutonium disposition. The current status and issues in the area of radiation damage processes, defect generation, microstructure development, theoretical methods and experimental methods are reviewed. Questions of fundamental and technological interest that offer opportunities for research are identified.

Radiation damage in zircon and monazite

Monazite and zircon respond differently to ion irradiation and to thermal and irradiation-enhanced annealing. Monazite cannot be amorphized by 800 keV Kr+ ions at temperatures greater than 175°C; whereas, zircon can be amorphized at temperatures up to 740°C. The damage process (i.e., elastic interactions leading to amorphization) in radioactive minerals (metamictization) is basically the same as for the ion-beam-irradiated samples with the exception of the dose rate which is much lower in the case of natural samples. The crystalline-to-metamict transition in natural samples with different degrees of damage, from almost fully crystalline to completely metamict, is compared to the sequence of microstructures observed for ion-beam-irradiated monazite and zircon. The damage accumulation process, representing the competing effects of radiation-induced structural disorder and subsequent annealing mechanisms (irradiation-enhanced and thermal) occurs at much higher temperatures for zircon than for monazite. The amorphization dose, expressed as displacements per atom, is considerably higher in the natural samples, and the atomic-scale process leading to metamictization appears to develop differently. Ion-beam-induced amorphization data were used to calculate the α-decay-event dose required for amorphization in terms of a critical radionuclide concentration, i.e., the concentration above which a sample of a given age will become metamict at a specific temperature. This equation was applied to estimate the reliability of U-Pb ages, to provide a qualitative estimate of the thermal history of high-U natural zircons, and to predict whether actinide-bearing zircon or monazite nuclear waste forms will become amorphous (metamict) over long timescales.