Role of transition and lanthanide metal carbonyls in various HTGR safety-related problems

While vapor phase transport of fission metals and metal catalysis of graphite oxidation are two processes of significant importance to HTGR safety evaluation, they have not yet been adequately incorporated into codes which model fission product transport and release during normal and off-normal reactor operation because: (1) little is known concerning the effect of enhanced vapor phase transport by volatile compound formation in HTGR environments, and (2) the atomic mechanism of metal catalyzed graphite oxidation by either steam or air is not well understood. The present paper presents a cursory look at a highly volatile class of compounds, the metal carbonyls, in order to determine their importance in the two above mentioned HTGR safety-related processes. This paper shows that: (1) formation of gaseous metal carbonyls may provide a means for enhancing fission metal redistribution in and release from HTGR coated oxide fuel particles, and (2) decomposition of surface metal carbonyls formed on fission metal surfaces in graphite environments is a plausible rate determining step in the mechanism for transition metal catalyzed graphite oxidation