Helium Impurity Levels for Compatibility Testing of HTR Primary Circuit Materials

This introductory paper surveys the possible sources of gaseous impurities in the helium of the primary circuit of HTGR's and discusses some operational helium impurity measurements made on the prototype HTR's Dragon, Peach Bottom and AVR. The thermodynamic aspects are covered in terms of oxidation and carburising potentials of the impurity ratios and levels with respect to metal primary circuit components. The importance of chemical ratios is stressed. On this basis suggested impurity levels for primary circuit materials test facilities are put forward and some observations made on the design of these test facilities. (author)