F.W. Clinard

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.

Neutron irradiation damage in MgO, Al2O3 and MgAl2O4 ceramics☆

The consequences of point defect aggregation in single-crystal and polycrystalline Al2O3 and MgAl2O4 and polycrystalline MgO, irradiated to fast-neutron (>0.1 MeV) doses ⩽2–3 × 1026nm−2, have now been investigated over a range of temperatures using macroscopic density measurements of swelling and transmission electron microscopy of defect aggregates. Al2O3 irradiated between 925 and 1100 K exhibits 2–4 vol% swelling, dense dislocation networks and aligned voids, while single-crystal MgAl2O4 spinel irradiated under similar conditions, does not swell and forms only isolated interstitial dislocation loops which do not unfault. Planar arrays of voids form either side of grain boundaries in polycrystalline MgAl2O4 spinel, however. MgO irradiated at 430 K swells 2.8% and contains a high density of small elongated loops and exhibits a tendency towards dislocation network formation despite the low irradiation temperature. Al2O3 and MgAl2O4 irradiated at 430 K exhibits dense arrays of small loops. The nature and origin of the observed aggregate defect structures are described and correlated with swelling behaviour and with previous studies which are reviewed.

Ceramics for applications in fusion systems

Six critical applications for ceramics in fusion systems are reviewed, and structural and electrical problem areas discussed. Fusion neutron radiation effects in ceramics are considered in relation to fission neutron studies. A number of candidate materials are proposed for further evaluation.

Self-irradiation effects in 238Pu-substituted zirconolite: I. Temperature dependence of damage

238Pu-substituted cubic zirconolite (CaPuTi2O7) was stored near ambient temperature, at 575 K, and at 875 K until alpha decay doses as high as 6.1 × 1025 α/m3 were attained. Self-damage at the lowest temperature proceeded by accumulation of alpha-recoil tracks, but this was partly or wholly suppressed at the higher temperatures. Full conversion to the metamict condition near ambient temperature was accompanied by bulk swelling of 5.4 vol%. Self-damage at 575 K and 875 K was characterized by lesser amounts of swelling (4.3 vol% and 0.4 vol%); however, subsequent reduction of the storage temperature to ambient resulted in further swelling. Details of this swelling response imply structural differences between metamict states formed at 575 K and ambient temperature. Implications for self-irradiation effects in nuclear waste forms under variable temperature conditions are discussed.

Structural performance of ceramics in a high-fluence fusion environment

The ceramics MgAl2O4, A12O3 (single crystal), Si3N4, and a SiC/graphite laminate were irradiated to ∼2 × 1026 n/m2 (E > 0.1 MeV) at 680 and 815K. Spinel exhibited near-zero dimensional change, while Al2O3 and Si3N4 swelled ∼3 vol% and 1 vol% respectively. Strength of MgAl2O4 was increased, while strength of Al2O3 and Si3N4 were not greatly altered. The SiC/graphite composite, tested only at 680K, suffered almost complete delamination as a result of swelling of the SiC and densification of the graphite. These results are discussed in terms of microstructural alterations and related to various fusion applications.

Alpha decay self-irradiation damage in 238Pu-substituted zirconolite☆

Abstract Samples of 238Pu-substituted cubic zirconolite (CaPuTi2O7) were cold-pressed and sintered, then stored at ambient temperature for ~200 days. Alpha decay of the plutonium isotope resulted in an accumulated radiation dose of 2.7× 1025 α/m3. Macroscopic swelling reached a saturation value of 4.7 vol% at this dose, while X-ray dilation saturated at 7× 1024 α/m3 and an apparent volume increase of 2.2%. The material became X-ray amorphous at 1.3× 1025 α/m3. A minor amount of microcracking accompanied swelling, but general breakup did not occur. Transmission electron microscopic observations of heavily damaged material indicated a metamict state principally characterized by a highly disordered, electron diffractionamorphous structure. Fracture surfaces showed a reduction in number of areas exhibiting distinct cleavage at high damage levels, consistent with the disordered structure.

Structural damage in a self-irradiated zirconolite-based Ceramic☆

The zirconolite phase of SYNROC nuclear waste was fabricated with 5 mol% 238PuO2 substituted for a like amount of ZrO2, in order to induce self-irradiation damage. The resulting product exhibited a matrix of monoclinic zirconolite containing ∼~ 3.8 mol% PuO2 along with roughly 20 vol.% of the cubic polymorph with approximately twice the PuO2 content of the matrix. After a dose of 2.1 × 1025α decays/m3 at room temperature (800 d storage) swelling reached 5.5 vol.% and neared saturation. The monoclinic phase became X-ray metamict at ∼~1.0 × 1025α/m3 after slight atomic rearrangement within the crystalline material. Periodic TEM examination revealed a gradual evolution from the crystalline state to an amorphous condition with residual crystallites, consistent with a model involving conversion by alpha recoil tracks. Optical metallography showed extensive microcracking, attributed to differences in swelling rates of the two zirconolite polymorphs.

Structural properties of MgO AND MgAl2O4, after fission neutron irradiation near room temperature

Polycrystalline MgO and MgAl/sub 2/O/sub 4/ samples were irradiated at 430 +- 5 K in HFIR to a fast neutron fluence of 2.1 x 10/sup 26/ n/m/sup 2/, E > 0.2 MeV, and 4.6 x 10/sup 26/ thermal n/m/sup 2/. Following irradiation, swelling, microstructure and mechanical strength were evaluated relative to control samples. Both materials swelled substantially, 2.6-3.0% in the case of MgO, and 0.8% in the case of MgAl/sub 2/O/sub 4/. The substructure of the MgO was found to contain a dense array of dislocation loops while the spinel showed heavy but unresolved damage. Results of mechanical strength evaluation by diametral compression testing showed significant strengthening for both materials. This result, which has important implications for use of cubic technological ceramics, is discussed in terms of the observed fracture modes and microstructural damage.

Ceramic and organic insulators for fusion applications

Abstract Fusion insulator requirements, operating conditions, and anticipated performance of candidate materials are reviewed, with emphasis on recent developments. Critical problem areas are highlighted, and research and development activities needed to meet the demands of advanced fusion devices are identified.

The effect of elevated-temperature neutron irradiation on fracture toughness of ceramics☆

Single-crystal forms of MgAl/sub 2/O/sub 4/, Y/sub 3//Al/sub 5/O/sub 12/, and Al/sub 2/O/sub 3/ were irradiated in the EBR-II fast fission reactor to a fluence of 0.3 x 10/sup 26/ n/m/sup 2/ at 1015/sup 0/K, and to /sup -/1 to 2 x 10/sup 26/ n/m/sup 2/ at 925 and 1100/sup 0/K. Fracture toughness was subsequently measured at room temperature by an indentation technique, and radiation-induced defect aggregates were characterized by transmission electron microscopy. A slight increase in toughness of MgAl/sub 2/O/sub 4/ was observed, and attributed to interaction of cracks with strain fields around dislocation loops. No significant change was noted for Y/sub 3/Al/sub 5/O/sub 12/, despite the presence of a high concentration of unresolved defect clusters. Fracture toughness of Al/sub 2/O/sub 3/ was markedly increased, with the enhancement apparently attributable in large part to impedance of crack propagation by interaction with the irradiation-induced void lattice.