PuO₂ and Fuel Residues

Zirconium based glass-ceramics and full ceramics are being studied as future wasteforms for plutonium residues. High fraction zirconolite glass-ceramics incorporate a wide range of actinides and rare earth elements in the zirconolite structure whilst retaining remaining miscellaneous material in the glass phase. This makes them very favourable for plutonium residues where the exact composition is unknown and requires a matrix flexible to a vast range of elemental incorporations. The fraction of zirconolite formed has been seen to change with the glass composition whereby a more aluminous glass phase promotes a higher yield of zirconolite.1 This project aims to understand of the mechanisms controlling the crystalline phase formation, in order to find an optimum formulation for the wasteform.

Actinides readily partition into the crystalline phase and CaF2 has been shown to aid the process of waste digestion. However, α-decay of plutonium induces problematic alpha-neutron reactions which are substantially increased by the presence of 19F ions from CaF2. Alpha particles generated by decay react with the 100% abundant 19F ions to produce a high energy neutron and an energetically unstable product. The neutron causes further damage through neutron irradiation and by generating a chain reaction of alpha-neutron reactions. Similarly, the excited ion stabilises itself through the emission of a high energy gamma ray making the overall wasteform difficult to handle during production and would require additional safety measures. This project aims to establish a mechanism underpinning the role of CaF2 as a mineralising agent and thus be able to reduce, if not eliminate, the impact of alpha-neutron reactions by either reducing the concentration of CaF2 or by selecting an alternative.

As a whole, this project aims to develop an understanding of the mechanisms controlling the ceramic phase formation and the partitioning of actinides within. After optimising the formulation, the maximum waste loading without detrimental effects to the structure can be found. The primary consolidation technique throughout the course of the project is hot isostatic pressing (HIPing). HIPing achieves near theoretical density by applying heat and pressure simultaneously. The use of both conditions means lower temperatures can be used and a finer grain structure can be achieved, thus improving the strength and durability of the whole wasteform. Other advantages of HIPing for nuclear wasteforms include minimal off-gas production, homogeneous incorporation of radionuclides and the production of a hermetically sealed wasteform ready for long-term disposal without the addition of another barrier.2

References

1          E. Maddrell, S. Thornber, and N. Hyatt, “The influence of glass composition on crystalline phase stability in glass-ceramic wasteforms,” J. Nucl. Mater., (2014).

2          E.R. Vance, M.W.A. Stewart, and S.A. Moricca, “Progress at ANSTO on SYNROC,” J. Aust. Ceram. Soc., 50 [1] 38–48 (2014).