HTGR Knowledge Base

Conference Article: Radiolytic graphite oxidation revisited

Minshall, P.C.; Sadler, I.A.; Wickham, A.J. (Nuclear Electric plc, Berkeley (United Kingdom). Berkeley Technology Centre)

Abstract

The importance of radiolytic oxidation in graphite-moderated CO2-cooled reactors has long been recognised, especially in the Advanced Gas-Cooled Reactors where potential rates are higher because of the higher gas pressure and ratings than the earlier Magnox designs. In all such reactors, the rate of oxidation is partly inhibited by the CO produced in the reaction and, in the AGR, further reduced by the deliberate addition of CH4. Significant roles are also played by H2 and H2O. This paper reviews briefly the mechanisms of these processes and the data on which they are based. However, operational experience has demonstrated that these basic principles are unsatisfactory in a number of respects. Gilsocarbon graphites produced by different manufacturers have demonstrated a significant difference in oxidation rate despite a similar specification and apparent equivalence in their pore size and distribution, considered to be the dominant influence on oxidation rate for a given coolant-gas composition. Separately, the inhibiting influence of CH4, which for many years had been considered to arise from the formation of a sacrificial deposit on the pore walls, cannot adequately be explained by the actual quantities of such deposits found in monitoring samples which frequently contain far less deposited carbon than do samples from Magnox reactors where the only source of such deposits is the CO. The paper also describes the current status of moderator weight-loss predictions for Magnox and AGR Moderators and the validation of the POGO and DIFFUSE6 codes respectively.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Specialists meeting on graphite moderator lifecycle behaviour. Bath (United Kingdom). 24-27 Sep 1995.
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--901, pp:181-191