Model analysis of the influence of coolant impurity on the properties of high temperature alloys of HTGR

[Background] The small amount of CO, H₂, H₂O and CH₄ and other impurities existed in the primary coolant of high temperature gas-cooled reactor (HTGR) has important influence on the high temperature properties of high temperature alloys for HTGR. A great number of studies have been carried out on this aspect. Due to the difficulty in controlling the trace impurity content in experimental helium gas during the research process, the distribution of relevant research results is relatively scattered, so appropriate models need to be induced analysed. [Purpose] This study aims to review two research methods and their applications for the influence of helium impurity in coolant on the properties of high temperature alloys under the very-high temperature operation of HTGR. [Methods] First of all, the composition and source of impurities in the primary coolant of HTGR were introduced, and the corrosion mechanism of the alloy in the impure helium of HTGR was analyzed. Then two widely used theoretical models, i.e., the stability diagram for chromium model and the ternary environmental attack (TEA) diagram model, were compared and investigated, and some applications of the two methods were reviewed. Finally, the oxygen partial pressure and carbon activity of helium gas of three HTGR coolants were calculated based on the impurity limit data of high temperature reactor using the stability diagram for chromium model. [Results and Conclusions] The stability diagram for chromium model has been widely used to analyze the effect of helium impurity content on the properties of nickel-chromium alloy materials, and its rationality has been verified. The TEA diagram model can specify the allowable impurity levels and provide the basis for reactor gas impurity control. However, it has not been widely used due to the need for many tests in determining the boundary. (authors)