Programme for Study of the Effects of Helium Exposure on the Mechanical Properties of Primary Circuit Materials for HTR's

Most if not all alloys undergo structural changes during exposure at elevated temperature (i.e., they are not thermally stable). These changes may take place in only a few hours or may still be in progress after thousands of hours but, in either event they are accompanied by changes in mechanical properties and behaviour. In addition to time and temperature of exposure, the environment in which the exposure is performed can cause modifications to metallurgical structure and, thereby, further affect the properties of the material. In order to provide assurance that components of HTR systems will not fail during their design lifetime as a result of degradation of time-independent mechanical properties (i.e., tensile strength, ductility, impact strength, etc.), it is necessary to know, for each material involved the effects of time — temperature — environment on these properties. Therefore, the study described in this report has the following objectives: 1) To determine the effects of aging (i.e., time at temperature) in HTR helium on the mechanical properties of HTR primary circuit materials 2) To identify any deleterious effects specifically induced by HTR helium as opposed to air, and 3) To suggest materials which, from the standpoint of the above, should be acceptable for long-term service in the HTR environment. Alloys pertinent to all HTR systems are included in the programme. Temperatures to be studies were chosen relative to each alloy and its proposed use and cover the range 400 to 1000°C. Exposure times of up to 25,000 hr. in helium and 10,000 hr. in air are to be employed. Tensile, notch tensile, and impact tests will be used for post-exposure evaluation. Additionally, the post exposure testing of a large number of creep specimens which achieved only small strains in earlier work is described in Appendix B. (author)