[en] The macroscopic swelling from alpha-decay events in Pu-doped ZrSiO₄, natural ZrSiO₄ minerals, and Cm-doped Ca₂Nd₈(SiO₄)₆O₂ is interpreted in terms of both unit-cell expansion from defect accumulation and the volume change that occurs as the local crystal structure and long-range order collapse during amorphization. The derived amorphous fraction in ZrSiO₄ as a function of cumulative alpha-decay events is consistent with models based on the multiple overlap of displacement cascades, suggesting that amorphization in ZrSiO₄ occurs as a results of the local accumulation of high-defect concentrations. In Ca₂Nd₈(SiO₄)₆O₂, on the other hand, the rate of amorphization is higher and the derived amorphous fraction as a function of cumulative alpha-decay events is consistent with amorphization occurring directly within the displacement cascade. 23 refs., 4 figs., 1 tab

[en] Radiation effects on the stability of high-level nuclear waste (HLW) forms are an important consideration in the development of technology to immobilize high-level radioactive waste because such effects may significantly affect the containment of the radioactive waste. Since the required containment times are long (10³ to 10⁶ years), an understanding of the long-term cumulative effects of radiation damage on the waste forms is essential. Radiation damage of nuclear waste forms can result in changes in volume, leach rate, stored energy, structure/microstructure, and mechanical properties. Any one or combination of these changes might significantly affect the long-term stability of the nuclear waste forms. This report defines the general radiation damage problem in nuclear waste forms, describes the simulation techniques currently available for accelerated testing of nuclear waste forms, and reviews the available data on radiation effects in both glass and ceramic (primarily crystalline) waste forms. 76 references

[en] The effects of radiation from alpha and beta decay on the performance and properties of borosilicate nuclear waste glasses are reviewed and discussed. Alpha decay results in volume changes of up to +-1.2%. Stored energy, released over a wide temperature range, does not generally exceed 150 J/g. Changes in leach rates are not as well defined, but increases of at least a factor of 3 can be expected. Ionization from alpha, beta, and gamma radiation results in volume changes, radiolytic decomposition, significant bubble formation, decreases in hardness, and increases in fracture toughness. 39 refs., 9 figs., 3 tabs

[en] Barium hollandite and nickel-iron spinel have been irradiated with alpha particles emitted from a PuO₂ source in order to simulate the effects from alpha particles emitted in adjacent actinide-containing phases of the SYNROC assemblage. The unit cell of barium hollandite undergoes an apparent volume expansion of several percent and a weak transformation from a tetragonal to a monoclinic structure when irradiated with alpha particles to a fluence of 3 x 10²⁰ alphas/m² (approx. 0.05 dpa). The spinel structure, however, is very stable with respect to alpha irradiation and the irradiation-induced volume expansion of the unit cell is less than 0.2%. The results are compared with available neutron-irradiation data, and the differences in observed behavior discussed. 18 references

No abstract available

[en] Irradiation of Gd₂Ti₂O₇ with 3 MeV Ar⁺ ions results in expansion of the unit-cell volume from defect accumulation as well as irradiation-induced amorphization. The damage cross-section for amorphization determined from X-ray diffraction analysis is 0.023 nm². Transmission electron microscopy and electron diffraction have confirmed the irradiation-induced amorphous character at high fluences. Raman spectroscopy indicates a decrease in scattered intensity and an increase in linewidth with fluence for the principal vibrational modes, consistent with increasing irradiation-induced disorder. The wavenumber of the most intense Raman peak at ∼ 310 cm^-1 (attributed to the O-Gd-O bending mode) decreases with fluence, indicating a change in bond Angle. The increase in the wavenumber of the Raman peak at ∼ 515 cm^-1 (attributed to the Gd-O stretching mode) with fluence suggests a decrease in bone length. At intermediate fluences, a forbidden peak at 765 cm^-11 becomes slightly activated by irradiation-induced disorder on the cation sites

[en] The purpose of this second workshop on irradiations effects was to continue the discussions initiated at the first workshop and to obtain guidance for the Materials Characterization Center in developing test methods. The following major conclusions were reached: Ion or neutron irradiations are not substitutes for the actinide-doping technique, as described by the MCC-6 Method for Preparation and Characterization of Actinide-Doped Waste Forms, in the final evaluation of any waste form with respect to the radiation effects from actinide decay. Ion or neutron irradiations may be useful for screening tests or more fundamental studies. The use of these simulation techniques as screening tests for actinide decay requires that a correlation between ion or neutron irradiations and actinide decay be established. Such a correlation has not yet been established and experimental programs in this area are highly recommended. There is a need for more fundamental studies on dose-rate effects, temperature dependence, and the nature and importance of alpha-particle effects relative to the recoil nucleus in actinide decay. There are insufficient data presently available to evaluate the potential for damage from ionizing radiation in nuclear waste forms. No additional test methods were recommended for using ion or neutron irradiations to simulate actinide decay or for testing ionization damage in nuclear waste forms. It was recognized that additional test methods may be required and developed as more data become available. An American Society for Testing and Materials (ASTM) Task Group on the Simulation of Radiation Effects in Nuclear Waste Forms (E 10.08.03) was organized to act as a continuing vehicle for discussions and development of procedures, particularly with regard to ion irradiations

[en] Atomistic simulations of coupled anion/cation disorder in the pyrochlore structure of Gd₂(Zr_xTi_1-x)₂O₇ show that homoatomic cation vacancy migration energies range from 3.8 to 7.9 eV, and increase with the addition of anion defects. For ordered materials, the energies for heteroatomic Ti or Zr vacancy migrations to Gd sites are 1.4 and 1.9 eV, respectively, but the addition of neighboring oxygen defects generally reduces these energies to less than 1 eV. The heteroatomic migration of Gd vacancies to Ti or Zr sites remains rather large (>2.7 eV) even with the presence of oxygen defects

[en] Ca₂La₈(SiO₄)₆O₂ thin crystals become amorphous under ion beam irradiation. The ion dose required for complete amorphization of the thin crystal (critical amorphization dose, D_c) increased with the increasing irradiation temperature and decreased with ion mass at elevated temperatures. Samples irradiated with 1-1.5 MeV Ar⁺, Kr⁺ and Xe⁺ ions to doses much lower than Dc, in the temperature range from 20 to 498 K were used for a detailed HRTEM study to better understand the amorphization process. The residual collision cascade damage after irradiation appeared as manometer scale amorphous domains. The images of these domains are extremely sensitive to the sample thickness. Small domains of cascade size were only found at the very thin edge of the sample. In thicker regions, amorphous domains appear after higher doses as the result of cascade overlap in projection. At higher temperatures, the observed amorphous domains are smaller indicating thermal recovery at the amorphous/crystalline interface. The amorphous domains are also larger in size after irradiation with ions of higher mass at a fixed ion dose. These results are consistent with the Dc-temperature curves determined by in situ TEM with the HVEM-Tandem Facility at Argonne National Laboratory. The width of the amorphous rim along the edge of the specimen grew with increasing ion dose suggesting that amorphization also proceeds from the sample surface. Images of the collision cascade damage were compared to the cascade sizes calculated with the TRIM code. Some digitally acquired HRTEM images of the cascade damage were processed to reveal more detailed information

[en] The effects of atmospheric moisture and radiation damage on fracture properties of nuclear waste glasses and ceramics was investigated by indentation techniques. In nuclear waste glasses, atmospheric moisture has no measurable effect on hardness but decreases the fracture toughness; radiation damage, on the other hand, decreased the hardness and increased the fracture toughness. In nuclear ceramics, self-radiation damage from alpha decay decreased the hardness and elastic modules; the fracture toughness increased with dose to a broad maximum and then decreased slightly with further increases in dose