The Use of Small Graphite Specimen Test Data for Large Core Components for HTGR

Abstract

For nuclear reactor designs there is general recognition of the impracticality to directly test materials and components to determine their properties, which affect their performance. It becomes necessary therefore to determine design-significant physical, thermal, and mechanical properties from small specimens of the same material, representative of large components, by using consensus standard test methods and practices, such as those published by ASTM. Likewise, often it is not possible to duplicate the exact environment in testing, such as the neutron fluence, temperature, pressure, and their variation through component thickness and atmosphere, which, in the case of high temperature gas cooled reactors (HTGR), is a helium gas containing minimal impurities. Thus, the concern always exists about the use of limited test data on small specimens in the design of large graphite core components. In this paper, I discuss these concerns, with comparison to the practice used for light water reactors for metallic materials and concrete and conclude that this problem is not unique to graphite or high temperature gas cooled reactors and that past practices in the ASME design codes and standards, which reference ASTM materials and test specifications for metallic materials and the ACI codes and standards for concrete, provide reasonable assurance that adaptation of such practice for graphite would be eminently suitable. It is the author’s personal opinion that it is highly likely that such practice of using small size specimen data would also meet with general regulatory acceptance, provided such data are gathered by utilizing consensus standard practices and have sound science and fundamental understanding. Likewise, the use of data from non-standard tests may also be accepted on a case-by-case basis with regulatory staff taking the position, perhaps, that provisions are made available to substantiate such test data from operating experience using suitable online monitoring techniques, inservice inspections during shutdowns, and coupon testing.