Rodney C. Ewing

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...

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.

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].

The corrosion of uraninite under oxidizing conditions

Abstract Uraninite is the most common uranium mineral, which, because of chemical and structural similarities to synthetic UO 2 , is a useful natural analogue for UO 2 in spent fuel. Uraninite, however, contains “impurities” such as Pb, Ca, Si, U 6+ , Th, Zr, and lanthanides. These affect the thermodynamic properties of uraninite, the rate of uraninite alteration, and the composition of the corrosion products. Uraninite can contain a significant amount of radiogenic Pb, and the Pb-uranyl oxide hydrates (Pb-UOH) are the most common corrosion products formed by the oxidative alteration of Pb-bearing uraninites. Incongruent alteration of the Pb-UOHs in natural waters produces increasingly Pb-enriched uranyl phases, effectively reducing the amount of U lost from the corrosion rind. This is not true of other uranyl oxide hydrates, such as schoepite, UO 3 · H 2 O, or becquerelite, CaU 6 O 19 · 11H 2 O, which can dissolve completely under similar geochemical conditions. The most common end product of Pb-UOH alteration is curite. Curite may provide surface nucleation sites for certain uranyl phosphates, thereby enhancing their formation. Uranyl phosphates are generally less soluble than other uranyl phases. In the absence of Pb, schoepite and becquerelite are the common initial corrosion products. The reaction path for the alteration of Pb-free uraninite results in the formation of uranyl silicates, which are generally more soluble than the uranyl phosphates. Thus, the long-term oxidation behavior for ancient, Pb-bearing uraninite is different from young, Pb-free uraninite. Because the presence of Pb effectively reduces the mobility of uranium in oxidizing waters, the concentration of uranium in ground waters associated with oxidized uranium ore deposits will depend in part on the age of the primary uraninite.

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 effects in ceramics

Abstract Ceramics represent a large class of solids with a wide spectrum of applicability, whose structures range from simple to complex, whose bonding runs from highly ionic to almost entirely covalent and, in some cases, partially metallic, and whose band structures yield wide-gap insulators, narrow-gap semiconductors or even superconductors. These solids exhibit responses to irradiation which are more complex than those for metals. In ceramic materials, atomic displacements can be produced by direct momentum transfer to often more than one distinguishable sublattice, and in some cases radiolytically by electronic excitations, and result in point defects which are in general not simple. Radiation-induced defect interaction, accumulation and aggregation modes differ significantly from those found in metals. Amorphization is a frequent option in response to high-density defect perturbation and is strongly related to structural topology. These fundamental responses to irradiation result in significant changes to important applicable properties, such as strength, toughness, electrical and thermal conductivities, dielectric response and optical behavior. The understanding of such phenomena is less well-understood than the simple responses of metals but is being increasingly driven by critical applications in fusion energy production, nuclear waste disposal and optical communications.

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.

Zircon: A host-phase for the disposal of weapons plutonium

Zircon, ZrSiO 4 , is a well-characterized, naturally occurring phase that is extremely durable. Zircon has been synthesized with Pu-concentrations up to 10 wt. % and radiation-damage effects studied to saturation doses of nearly 0.8 displacements per atom. We propose that zircon be used as a waste form for the disposal of the more than 100 metric tons of plutonium that will result from the dismantling of nuclear weapons. There are already several demonstrated processing technologies, of which hot pressing offers the most potential. This highly durable material, even under hydrothermal conditions, with its high waste loading and smaller volume allows deep, permanent disposal of the weapons plutonium in geologic environments in which the borosilicate waste-form glass would not be stable.