Radiolysis Studies in Supercritical Water
This dissertation will consist of three topics which fall under the general umbrella of studies of water radiolysis at supercritical temperatures and pressures for nuclear power applications. Introductory material common to all chapters in this dissertation relating to water radiolysis in supercritical water conditions is presented in the opening chapter.
Chapter 2 explores “The Application of Hydrogen Water Chemistry to Suppress Net Radiolysis in Supercritical Water.” One of the largest problems in all nuclear reactors (including the proposed Supercritical Water Reactor (SCWR) design) is stress corrosion cracking (SCC) of steel pipes. This is controlled by the corrosion potential, on which the O2 and H2O2 species formed in water radiolysis have a major impact.1,2 Even at low concentrations they strongly influence corrosion kinetics. In order to control the corrosion potential, a successful strategy employing hydrogen water chemistry inhibits radiolytic decomposition of the coolant by the minimum addition of hydrogen (the critical hydrogen concentration) to the primary cooling water.3 Experiments showing that this strategy can be successfully applied in supercritical water reactors are discussed. We have confirmed
that suppression of O2 in the bulk water is possible in the sub-and supercritical temperature regimes. However, results indicate that substantial hydrogen (no O2) is still generated by radiation-induced reactions on the walls of the metal flow system, an effect absent at lower (sub 350ÌÄåâÌâå¡C) temperatures. In fact, excess hydrogen seems to be generated at supercritical temperatures.
Chapter 3 explores “Solvated Electron (e-aq) Yields in Supercritical Water.” In support of an ongoing yield measurement program collaboration with the University of Wisconsin, Madison, radiolysis yields of H, H2, and e-aq were measured as a function of temperature and as a function of pressure (or density) in the mixed neutron/gamma radiation field from a nuclear reactor at the University of Wisconsin. Reactor data is compared with nearly identical experiments run with a 3MeV van de Graaff accelerator to measure the same yields in the absence of neutron radiolysis. Radiolysis was carried out using a beam of 2-3 MeV electrons from a van de Graaff accelerator and SF6 was used as a specific scavenger for the hydrated electron. Reduction of SF6 on the walls of the irradiation zone was apparent and subsequently investigated.
Chapter 4 explores “Carbonate Radical Formation in Radiolysis of Sodium Carbonate and Bicarbonate Solutions up to 250ÌÄåâÌâå¡C and the Mechanism of its Second Order Decay.” Pulse radiolysis experiments published several years ago raised the possibility that the carbonate radical formed from reaction of ÌÄå¢Ì¢åâåÂÌâå¢OH radicals with either HCO3- or CO32- might actually exist predominantly as a dimer form, e.g. ÌÄå¢Ì¢åâåÂÌâå¢(CO3)23-. In this work we re-examine the data upon which this suggestion was based, and find that the original data analysis is flawed. Upon re-analysis of the published data for sodium bicarbonate solutions and analysis of new transient absorption data we are able to establish the rate constant for this reaction up to 250C.
The complete mechanism of second order self-recombination of ÌÄå¢Ì¢åâåÂÌâå¢CO3- radicals has heretofore not been conclusively demonstrated. A simple optical measurement of the second order half-life does not properly describe the chemistry involved. A pre-equilibrium between ÌÄå¢Ì¢åâåÂÌâå¢CO3- and a short-lived dimer C2O62- is strongly indicated based on the slightly negative activation energy for the overall recombination found below 300oC. Existing conductivity data is only consistent with production of CO2 and CO42- from this dimer. The present work demonstrates the decay is autocatalytic thanks to reaction of ÌÄå¢Ì¢åâåÂÌâå¢CO3- with the peroxymonocarbonate HCO4- / CO42- product. In any pulse radiolysis experiment, particularly if OH radicals are incompletely scavenged, reaction of ÌÄå¢Ì¢åâåÂÌâå¢CO3- with hydrogen peroxide also contributes to the decay. Finally, the radicals ÌÄå¢Ì¢åâåÂÌâå¢O2- and ÌÄå¢Ì¢åâåÂÌâå¢CO4- formed respectively from the hydrogen peroxide and peroxymonocarbonate reactions, recombine with ÌÄå¢Ì¢åâåÂÌâå¢CO3-.
Finally, Chapter 5 will present a summary of findings and directions for further studies.
| Author ||Kyle Stewart Haygarth|
| Advisor ||Ian Carmichael|
| Contributor ||Ian Carmichael, Committee Member|
| Contributor ||David M. Bartels, Committee Member|
| Degree Level ||Master's Thesis|
| Degree Discipline ||Chemistry and Biochemistry|
| Degree Name
| Submission Date ||2009-07-23|
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