Investigating the Solubility of Spent Nuclear Fuel

Sasikumar, Yadukrishnan (2022). Investigating the Solubility of Spent Nuclear Fuel. PhD thesis The Open University.



The safe management of spent nuclear fuel (SNF) is a key challenge facing the nuclear industry today. In the potential event of a premature pinhole cladding failure, fuel pond water can come into contact with highly active spent fuel and remain unnoticed going into storage. In such scenarios, there are risks from reactions including water radiolysis on the fuel surface leading to an accelerated dissolution of the fuel matrix. While this mechanism has been widely studied, there still exist gaps in the understanding of water carryover on dried fuel, precise corrosion mechanisms, and the corrosion rate dependence on lattice modifications in the fuel matrix. Also, the long-term safety implications of such fuel-water interactions are yet to be determined.

This thesis addresses the scenario by looking at two pathways of this fuel-water interaction - adsorption and radiolytic dissolution: in three separate strands. In the first strand, wetting of SNF surfaces is considered at high temperatures to evaluate the hygroscopic nature of failed SNF, post drying. This is studied by conducting adsorption isotherm experiments in an experimental rig that was designed and assembled by the author. In the second strand, the radiolytic corrosion mechanism of highly active idealistic SNF surfaces is studied by using epitaxial thin film samples in artificial radiolytic environments. The samples included (U, Ce)O2 in various U:Ce percentage compositions in order to probe the dissolution characteristics of (U, Pu)O2 mixed oxide (MOX) surfaces as a function of induced lattice modifications in the SNF (UO2) matrix. The last strand addresses the scenario of a failed fuel exposed to spent fuel pond water, resulting in wet, waterlogged fuel going into storage. Here static de-ionized (DI) water leaching tests are performed on High Burnup SNF to understand the trends in fuel dissolution as a function of time and surface area of fuel exposed. Further, the fuel surfaces, post leaching, are metallographically analysed to determine the changes caused due to corrosion and elucidate potential safety implications in the long term.

In conclusion, this research will give insights and assess the short- and long-term behaviour of SNF in the presence of water, in the event of a pinhole cladding failure.

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