Reaction Kinetics in Supercritical Water as Probed with Muonium

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We propose to use muon spin spectroscopy to study chemical reactions in superheated water over the full range of temperatures and pressures envisaged for a supercritical-water-cooled nuclear reactor (SCWR). In such experiments we use muonium (Mu) as a surrogate for atomic hydrogen (H); chemically Mu can be viewed as a light isotope of H. TRIUMF, in Vancouver B.C., is one of only four locations in the world with intense muon beams suitable for such work. In phase I of the GenIV project we extended our previous work on Mu decay kinetics of bimolecular reactions in superheated water: rate constants exhibit a broad maximum with increasing temperature below the critical point, followed by a sharp minimum at the compressibility maximum. A new target cell has been designed and fabricated to extend studies to 650°C. At constant pressure (e.g. 25 MPa) the density of water falls by a factor of ten from the subcritical liquid at 300°C to the supercritical fluid at 650°C. We will pay particular attention to the reaction of H with water, since this is the major source of hydrogen gas at high temperature, and is a key reaction for the development of a chemistry control strategy for the SCWR.