The role of Th-U minerals in assessing the performance of nuclear waste forms

Abstract

Materials designed for nuclear waste disposal include a range of ceramics, glass ceramics and glass waste forms. Those with crystalline phases have provided the momentum for studies of minerals as a means to understand aspects of waste-form crystal chemistry, behaviour in aqueous systems and radiation damage over geological periods of time. Although the utility of natural analogue studies varies, depending upon the degree of analogy to the proposed geological repository and other factors such as chemical composition, the available data suggest that Th-U host phases such as brannerite, monazite, pyrochlore, zircon and zirconolite are resistant generally to dissolution in aqueous fluids at low temperatures. Geochemical durability may or may not extend to hydrothermal systems depending on the specifics of fluid composition, temperature and pressure. At elevated temperatures, for example, davidite may break down to new phase assemblages including titanite, ilmenite and rutile. Perovskite is generally less resistant to dissolution at low temperatures and breaks down to TiO2, releasing A-site cations to the aqueous fluid. Studies of radiation damage indicate that the oxide and silicate phases become amorphous as a result of the gradual accumulation of alpha-recoil collision cascades. Monazite tends to remain crystalline on geological time scales, a very attractive property that potentially eliminates major changes in physical properties such as density and volume, thereby reducing the potential for cracking, which is a major concern for zircon. In spite of recent success in describing the behaviour of Th-U minerals in geological systems, considerable work remains in order to understand the P-T-X conditions during alteration and T-t history of the host rocks. ABSTRACTMaterials designed for nuclear waste disposal include a range of ceramics, glass ceramics and glass waste forms. Those with crystalline phases have provided the momentum for studies of minerals as a means to understand aspects of waste-form crystal chemistry, behaviour in aqueous systems and radiation damage over geological periods of time. Although the utility of natural analogue studies varies, depending upon the degree of analogy to the proposed geological repository and other factors such as chemical composition, the available data suggest that Th-U host phases such as brannerite, monazite, pyrochlore, zircon and zirconolite are resistant generally to dissolution in aqueous fluids at low temperatures. Geochemical durability may or may not extend to hydrothermal systems depending on the specifics of fluid composition, temperature and pressure. At elevated temperatures, for example, davidite may break down to new phase assemblages including titanite, ilmenite and rutile. Perovskite is generally less resistant to dissolution at low temperatures and breaks down to TiO 2 , releasing A-site cations to the aqueous fluid. Studies of radiation damage indicate that the oxide and silicate phases become amorphous as a result of the gradual accumulation of...

Determination of the REE content, geological age, and absorbed alpha dose of allanite mineral from Palawan, Philippines

“…In this study, the geologic age was approximated using the U–Th–Pb radiometric dating. The method of age determination based on radioactive disintegration of Th and U is based ultimately on the fact that these radioactive elements disintegrate spontaneously at constant rates and that the multiple disintegration phenomenon is assumed to result in the production of lead . Figure shows the decay chain of 238 U, 235 U and 232 Th, and their corresponding half‐lives patterned from Gilmore.…”

“…Because of its mineral structure and composition, the allanite mineral structure is one of the proposed matrices for high‐level waste immobilization and long‐term storage material for actinide‐ and lanthanide‐bearing waste forms . In the issue of long‐term disposal of high‐level radioactive wastes, the allanite and similar minerals could serve as natural analogues for evaluating the long‐term effects of radiation to the stability and integrity of disposal methods …”

“…The α‐recoil nucleus, on the other hand, with a lower energy of about 70–100 keV transfers its energy by creating ~1,000–2,000 atomic displacements along ~10‐ to 40‐nm track . The extent of the radiation damage effect in the mineral's crystalline structure is dependent on the α‐radiation dose due to the progressive overlap of α‐recoil collision cascades . The α‐radiation dose received, which causes damage to the mineral, is strongly dependent on the amount of the α‐emitters present and the length of time that the mineral has been exposed to it.…”

“…[3,4] In the issue of long-term disposal of high-level radioactive wastes, the allanite and similar minerals could serve as natural analogues for evaluating the long-term effects of radiation to the stability and integrity of disposal methods. [5][6][7] The α-decay of U, Th, and their daughter nuclides in the allanite may result in radiation damage to the mineral's crystal structure and can cause the transformation from a crystalline to amorphous structure termed as metamictization. [8][9][10] The energy of the α-particle, which ranges from 4.0 to 6.0 MeV, is dissipated by ionization over a 10-to 20-μm path and yields~100-200 atomic displacements.…”

“…[1,9,11,12] The extent of the radiation damage effect in the mineral's crystalline structure is dependent on the α-radiation dose due to the progressive overlap of α-recoil collision cascades. [3,5,6] The α-radiation dose received, which causes damage to the mineral, is strongly dependent on the amount of the α-emitters present and the length of time that the mineral has been exposed to it. The chemical composition and complexity of the crystal structure also affect the metamictization process.…”