Aleksej Popel

Sponsor: UK EPSRC (grant EP/I036400/1) and Radioactive Waste Management Ltd (formerly the Radioactive Waste Management Directorate of the UK Nuclear Decommissioning Authority, contract NPO004411A-EPS02)

The effect of radiation damage by fission fragments on the structural stability and dissolution of the UO2 fuel matrix

The aim of my PhD work was to study the separate effect of in-reactor radiation damage on the structural integrity and dissolution of the uranium dioxide matrix in water. Radiation damage effects were studied by irradiating sets of samples with Xe and U ions. The samples were plain and doped (simfuel) bulk UO₂, thin films (90 150 nm) of UO₂ on single crystal LSAT (Al₁₀La₃O₅₁Sr₁₄Ta₇) and YSZ (yttria-stabilised zirconia) substrates with (001), (110) and (111) orientations, bulk and thin film CeO₂ on single crystal silicon (001) substrates. Cerium dioxide samples were used to assess the suitability of using CeO₂ as an analogue to UO₂.

A portion of each of the samples was irradiated with 92 MeV¹²⁹Xe²³⁺ ions to a fluence of 4.8 × 10¹⁵ ions/cm² to simulate the damage produced by fission fragments in uranium dioxide nuclear fuel. Some of the samples were also irradiated with 110 MeV ²³⁸U³¹+ ions to fluences of 5 × 10¹⁰, 5 × 10¹¹ and 5 × 10¹² ions/cm² to study the accumulation of the induced damage.

Irradiated and unirradiated samples were characterized using a range of analytical techniques before and after dissolution tests. Extended, controlled atmosphere batch dissolution experiments were conducted to measure uranium concentrations in water in contact with irradiated and unirradiated UO₂ thin films on LSAT substrates.

It was generally observed, before dissolution, that the ion irradiations induced microstructural and structural changes in the samples. The most significant finding of the dissolution tests is the lower concentration of uranium in the batch test of irradiated thin films of UO₂ on LSAT substrates, as compared to the unirradiated films. A hypothesis that the irradiation induced chemical mixing of tri-valent substrate elements stabilised the UO₂ lattice was suggested. The results of the microstructural response and dissolution of the irradiated CeO₂ thin films cast doubt on the usefulness of using CeO₂ as an inactive analogue for UO₂, although further work is needed to clarify this issue.

Achievements

In total, including postdocs: 13 papers published and more than 30 workshops and conferences attended, The Seah Award (UKSAF Young Surface Analyst Award 2016), The Castle Award (UKSAF Young Surface Analyst Award 2014), Cambridge Society for the Application of Research (CSAR) award winner 2014 (£1k)

Since graduation

Postdoc at the Technical University of Denmark (DTU)