Investigation of chemical characteristics of primary helium gas coolant of HTTR (high temperature engineering test reactor)

The technical basis of helium gas purification control for HTGRs was established by verifying the design of the Primary Helium Purification System (PHPS) of the HTTR by showing that the measured concentrations of impurities of the primary helium coolant were restricted below the criteria of control to protect the graphite oxidation, and that the carburization atmosphere was maintained to keep intact of metallic high temperature components, in the 30-day continuous operation and the 50-day long-term high temperature operation. The analytical model, which was newly established by improving the conventional method that predicted the impurity concentrations conservatively higher than the measured values, predicted the composition of the impurities such as H2, CO, H2O and CO2, which is determined by the temperature dependency of release of impurities during the rated power operation adequately. In contrast, it was revealed that the measured concentration of H2O remarkably decreased while the concentration of CO increased in the primary helium coolant in the long-term high temperature operation.