Reaction of nuclear-grade graphite with low concentrations of steam in the helium coolant of an MHTGR

Abstract

The fuel elements for the MHTGR are manufactured from nucleargrade graphite and have core-residence times of about 1000 days that are determined from considerations based on nuclear-design criteria. To ensure structural integrity and minimize the risk to plant investment, a potential limitation of fuel-element life may result from graphite corrosion in the high-temperature regions of the core ( T ≅ 1100 °C). This corrosion is caused primarily by reactions of graphite with the low concentrations of steam (0.01–0.1 ppm) that are normally present in the helium coolant Economical operation of the reactor requires that the steam concentrations be maintained at such low levels that corrosion rates will not impact the normal fuel cycle. In this paper, we develop a graphite-corrosion model and address this important practical problem.