Nuclear Graphite Inhomogeneity as Revealed by Nonuniformity of Oxidation Rates and New Billet Qualification Requirements

This study shows that several nuclear graphite grades developed for HTGR exhibit scatter in OR (oxidation rate) much like the physical and mechanical properties of the same grades. The implication of OR scatter on the design and safe operation of GCC may need further investigation. As an example, regarding the HHA-II- 2000 MDS on oxidation, if the OR distribution in a grade is confirmed, then it should be determined whether the specimens used for MDS are of same OR or not. For a safe design and operation of GCC, the inclusion of new requirements in the relevant ASTM graphite materials specification (ASTM D-7219 or D 7301) or ASME GCC design rule (Section III, Div. 5 HHA-3141) is suggested in order to help screening out billets of unusual oxidation behavior (OR) from blocks used for GCC manufacture.