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Conference Article: Thermodynamic analysis of behaviour of HTGR fuel and fission products under accidental air or water ingress conditions

Minato, K.; Fukuda, K. (Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment)

Abstract

A core of high temperature gas-cooled reactor (HTGR) consists of coated fuel particles, graphite and absorber of boron carbide (B4C). Silicon carbide (SiC) as well as pyrolytic carbon (PyC) are used as coating materials for TRISO-coated fuel particles. Zirconium carbide (ZrC) is a candidate coating material which is expected to be used at higher temperatures than SiC. Thermodynamic analysis was carried out to understand the chemical behaviour of these materials and fission products under accidental air or water ingress conditions. The analysis on the systems of SiC-C-(O2 or H2O)-He, ZrC-C-(O2 or H2O)-He, and B4C-C-(O2 or H2O)-He showed that the active-to-passive transitions of oxidation of SiC+C, ZrC+C and B4C+C occurred at lower temperatures than those of SiC, ZrC and B4C, respectively, at a given initial O2 or H2O pressure. Under most of the accident conditions the passive oxidation of these materials was expected to occur. In the passive oxidation of SiC+C, ZrC+C and B4C+C preferential oxidation of SiC, ZrC and B4C occurred, respectively, while C was not oxidized as long as SiC, ZrC and B4C were present. B4C was oxidized to B2O3 under the conditions of interest, where B2O3 was liquid with high vapor pressure of B-containing species. The analysis on the system of Cs-I-B4C-C-(O2 or H2O)-He showed that cesium would form CsBO2 if boron was available. Melting point of CsBO2 is 989.7 K, which is higher than those of Cs and CsOH. This suggests that cesium should be deposited as CsBO2 at higher temperature regions than as Cs or CsOH, which results in decrease in cesium release to the environment.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Technical committee meeting on response of fuel, fuel elements and gas cooled reactor cores under accidental air or water ingress conditions. Beijing (China). 25-27 Oct 1993
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--784, pp:86-91