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The Radiation Chemistry of Gases at the Interface with Ceramic Oxides
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
As of 2011, the UK had 112 tonnes (t) of plutonium dioxide PuO2 in interim storage at the Sellafield site and this is increasing by approximately 5 t per annum with the continued reprocessing of spent nuclear fuel. PuO2 is stored in small quantities in a sealed multi-canister system for security and ease of handling. During long term storage, radiolysis of the gas phase and adsorbed species could potentially lead to canister pressurisation and/or failure. It is of great importance to understand the mechanisms occurring in the gas phase and to understand the resulting gas phase composition after decades of storage.This research investigates the radiation chemistry of two gas phase systems in the presence or absence of inactive PuO2 surrogate material (namely cerium dioxide (CeO2) and zirconium dioxide (ZrO2)).The systems of interest are, firstly, radiolysis of hydrogen (H2), oxygen (O2) and argon gas mixtures utilising both 60Co gamma rays and He(2+) accelerated ions. Depletion of H2 and O2 has been investigated using gas chromatography. A bespoke manifold has been designed to mix these gases in various ratios, suitable reaction vessels and a subsequent sampling system has been developed to undertake this research. The rate of H2 depletion is independent of initial H2 concentration and radiation type. In the presence of an oxide surface, the rate of H2 depletion is vastly increased when compared to homogeneous studies using 60Co gamma rays. Depletion is greatest in the presence of ZrO2. In all systems, depletion of H2 is linear with increasing absorbed dose.The second system of interest is the radiolysis of moist air utilising 60Co gamma rays. Formation of nitric acid (HNO3) has been investigated using ion chromatography to determine nitrate (NO3-) anion production. Nitrate production increases linearly with absorbed dose and is greater in the presence of an oxide powder. The rate of production increases with increasing mass of oxide. Oxalate (C2O42-) was produced radiolytically from dimerisation of carbon dioxide and was greatest in the presence of ZrO2. Reducing the specific surface area of CeO2 reduced the concentration of nitrate formed when compared to higher surface area CeO2.