Phebus-FP is an international integral experimental programme initiated by the French Nuclear Protection and Safety Institute, the European Commission and 'Electricité de France'. Its objective is to investigate the main phenomena involved in LWR severe accidents through six in-pile tests. The programme is also sponsored by USA, Japan, Canada, Korea and Switzerland. Tbe FPT0 and FPTI tests were the first two experiments carried out. The post test analysis revealed that rhenium released by thermocouples could play an important role in the sump cbemistry due to tbe formation of the strong acid HReO4. A reassessment of tbe rhenium cbemistry focused on its release and transport in mixtures steam/hydrogen witb hydrogen concentration typical of the experimental conditions bas been consequently executed. Vapour pressures were determined by chemical tbermodynamics for tbe condensed (metallic rhenium) and gaseous phase Re2O7-ReOH-Re(OH)2. The results obtained have indicated that: Condensed state: ReO2(cr) is tbe stable phase only for very low H2/H2O ratios and decomposes at circa 1000 °K. For H2 greater than 0.1 % the metallic rhenium becomes stable for all temperatures. Tbe metallic rhenium pressure is very low for all situations of interest. Gas phase: Re2O7(g) can give very high pressures even at low temperature (for instance 1000 °K) if the concentration of H2 is comprised in the range 0.001-0.01 % . For higher concentration of H2 (circa O.5% and more) the pressure of Re2O7(g) can increase greatly as temperature increases. The presence of other dominant gas species can be excluded. As a consequence, tbe transport of this element as aerosol and vapour would occur essentialIy by tbe species metallic rhenium and Re2O7(g).
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