Reaction Kinetics In Supercritical Water as Probed with Muonium

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We propose to make measurements of muonium reaction rates in supercritical water to provide unique data necessary for the development of a supercritical-water-cooled nuclear reactor. Muonium (Mu) behaves as a light isotope of atomic hydrogen (H). TRIUMF, in Vancouver, B.C. is one of only four locations in the world with intense muon beams suitable for such work. We have already made measurements as high as 470°C, but for this new project it will be necessary to engineer new apparatus capable of higher temperatures, ideally up to 650°C. A common feature of our measurements to date is that muonium rate constants initially increase with temperature (the expected behaviour) but then go through a maximum. For one reaction (Mu + methanol) the rate constant was found to go through a minimum close to the critical point (374°C) and then rise again. It is our belief that the key factor is the solvent-driven dynamics, in particular the number of collisions per encounter of a pair of reactants. Recent work on H, OH and hydrated electrons by other authors shows similar behaviour, thus supporting our model. Therefore Mu reaction data can be used to simulate the behaviour of H and other radiolysis transients under such conditions, or at the very least provide data which can be used to model the behaviour of the solvent. It is relatively easy to carry out Mu kinetics experiments on samples under extreme conditions. This is because muon spin spectroscopy is sensitive only to the transient species under study and is unaffected by environmental effects. It is possible to produce a beam of muons with sufficient energy to penetrate a thick metal window of a pressure vessel, avoiding the problems associated with optical windows needed for competing methods. Muon spin spectroscopy operates on the microsecond timescale, so optimal Mu decay rates can be achieved with reactant concentrations of 0.01-100 mM for the fast rate constants expected. The choice of reactions to be studied will be made with the guidance of AECL scientists and will include key products of the radiolysis of water (hydrogen and oxygen) as well as studies as a function of pH and ionic strength.