HTGR Knowledge Base

Conference Article: In-pile corrosion of grade H-451 graphite by steam.

Richards, M.B.; Gillespie, A.G.; Hanson, D.L. (General Atomics, San Diego, CA (United States))

Abstract

The fuel elements for the Modular High-Temperature Gas-Cooled Reactor (MHTGR) are to be manufactured from a near-isotropic graphite, such as grade H-451. For economical plant operation, graphite corrosion must not impact the normal fuel-element replacement schedule and must not be the cause of extended reactor-shutdown periods. Graphite corrosion and the potential for increased fission-gas release from fuel hydrolysis are inherently coupled. The water vapor must first penetrate a web of graphite before reaching the fuel compacts that contain the small fraction of failed particles. Hence, graphite corrosion is an important issue for both plant design and safety. In general, previous calculations have not shown good agreement with data obtained from out-of-pile and in-pile experiments that involve both transport and reaction phenomena, unless significant adjustments were made to model parameters, including reaction-rate correlations. The currently used reaction-rate correlations, were developed from out-of-pile data. Under in-pile conditions, radiolysis of water vapor and neutron-induced damage to the graphite may enhance graphite corrosion. The recently completed irradiation of capsule HFR-B1 provided some unique data for in-pile corrosion of H-451 graphite. In this paper, the HFR-B1 graphite-corrosion data are analyzed and compared with model predictions. The calculations were performed using a new corrosion model that previously showed good agreement with burnoff-profile data for out-of-pile corrosion of grade 2020 graphite. Improvements in the new model include a correction in the mass-sink term for steam, and improved models for the burnoff factor and the effective diffusivity of steam in graphite. Predictions obtained suing the new model are also in good agreement with the HFR-B1 data, which indicates that the currently used reaction-rate correlation may be used to predict in-pile corrosion with good accuracy. The analyses described here are a logical first step to a more sophisticated model for predicting the coupled graphite-corrosion and fuel-hydrolysis phenomena.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Behaviour of gas cooled reactor fuel under accident conditions. Proceedings of a specialists meeting held in Oak Ridge, 5-8 November 1990
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--25, pp:157-162