Zirconolite for Minor Actinide Containment and Alpha Irradiation Resistance

Abstract

Zirconolite is a potential matrix for the immobilization of the minor actinides: neptunium, curium, americium, and small quantities of unrecyclable plutonium, produced by the reprocessing of the spent fuel.

In order to check the incorporation of actinides into the structure, zirconolite ceramic pellets doped with 10 wt% of ²³⁹PuO₂ were sintered. Characterization by scanning electron microscopy, X-ray diffraction, and X-ray Absorption Near-Edge Structure (XANES) spectroscopy have been done on this material. The microstructure of the pellets is homogeneous, and their relative density is higher than 90% of the theoretical density. XANES spectroscopy shows that Pu is at oxidation state (IV) in this material.

To investigate the effects of radiation damage on zirconolite structure, pellets doped with 10 wt% of ²³⁸PuO₂ were fabricated. ²³⁸Pu accelerates the radiation damage relative to the ²³⁹Pu because of its much higher specific activity (632 × 10⁹ Bq/g for ²³⁸Pu versus 2.2 × 10⁹ Bq/g for ²³⁹Pu). Some pellets are stored at ambient, 250 and 500°C. Up to 2.2 × 10¹⁸ α·g⁻¹, macroscopic swelling of the samples stored at ambient is ~2.2%/10¹⁸ α·g⁻¹, and the microscopic one near 1.3%/10¹⁸ α·g⁻¹. Some microcracks are observed on these pellets. The samples started to become amorphous at 2.2 × 10¹⁸ α·g⁻¹. The swelling strongly decreases with the storage temperature of the samples.

Swelling results are interpreted in terms of alpha radiation damage on the structure; at this time helium accumulation does not appear to have a major role in this process.