Dry Powder Processing Methods for Hot-Isostatically Pressed Zirconolite Wasteforms

The UK currently holds the largest stockpile of civilian-owned plutonium dioxide in the world which imparts a significant economic cost and security risks . At present the NDA has presented two key options for managing the stockpile and in turn mitigate the costs and risks. The preferred option is reusing the PuO2 as fuel for burn-up in reactors and subsequent disposal into GDF(Geological Disposal Facility). The second option is converting the Puo2 into a wasteform and then entombing it into a GDF. However, at present, around 12% of the overall stockpile is unsuitable for conversion into fuel and as such will need to be made into a wasteform, this is the portion of interest to this project.

Presently, the key wasteforms of interest are a mixed oxide (U,Pu)O2 called Disposal-MOX and a ternary oxide (Ca,Pu)(Zr,Pu)(Ti)O7 called Zirconolite. This project will be focusing on Zirconolite and specifically the methods of manufacturing the wasteform. These wasteforms were conventionally synthesised at lab-scale by grinding down the precursor oxides with surrogate waste material and then sintering at temperatures up to 1400 0C. The grinding stage is nominally carried out via the use milling fluids such as isopropyl alcohol, acetone, anhydrous ethanol or deionized water to allow for extending grinding without agglomeration/caking of powders within the mill. However, this project seeks to explore if it is possible to creates these wasteforms using a dry process i.e. without the use of these milling liquids. The benefits of removing the liquid from the process are overall process simplification, reduction in overall plant size as well as reducing overall waste effluent generated.

However, the key challenge this introduces is how to manage the agglomeration occurring within the mill during the grinding process. If this agglomeration is unchecked, it could result in the formation a hard powder coating on the interior of the mill. This may lead to discrepancies in PuO2 accountancy, as well as an inhomogeneous powder blend which could impair desired phase development. Thus, a suitable method to control agglomeration is needed.
Thus far, this project has found several means of controlling agglomeration via innovative uses of lubricants, heat treatments and alternative milling methods.