Infrared spectroscopic and mass spectrometric studies of high-temperature molecules relevant to severe nuclear reactor accidents

This thesis describes the use of mass spectrometry and matrix isolation - infrared spectroscopy to characterise a number of high-temperature vapour species. The compounds studied were selected because of their possible roles in influencing the transport of fission products during a severe accident in a nuclear power reactor. The most important fission products, on the basis of abundance, volatility and radiobiological impact, are isotopes of iodine, caesium and tellurium, and some of the major uncertainties in severe accident analysis relate to the possible interactions between fission product vapours and other reactor materials. Three of the compounds studied, tin telluride, indium telluride and indium iodide, are possible products of such reactions. Caesium molybdate is postulated to form within the fuel, and may be important in determining the release and transport of both caesium and molybdenum isotopes. Tin telluride, lead selenide and lead telluride vaporise mainly as MX molecules, together with small quantities of M2X2. The infrared spectra of the dimers have been interpreted on the basis of a planar ring structure. The tellurides and selenides of gallium and indium vaporise mainly as M2X molecules. The infrared spectra of these molecules were consistent with non-linear structures, although considerable variation in the M-X-M angle was inferred. The low-frequency infrared-active vibrations of the caesium molybdate molecule have been established. These were interpreted in terms of a D2d structure. Indium tri-iodide vaporises as the dimer at low temperatures, with increasing dissociation to InI3 and eventually to InI and I2 with increasing temperature. The infrared spectrum of matrix-isolated In2I6 was consistent with the established structure of this molecule in the solid and liquid phases. These studies also verified that the InI3 molecule has a planar structure. The infrared data have been used to estimate the non-infrared active frequencies (where appropriate) of the matrix-isolated molecules, and hence to calculate thermodynamic quantities for the vapour species over a wide range of temperatures.