R&D of lifetime extension for primary materials by active chemistry control for coolant helium; A Study of chemical equilibrium in the HTGR core

Sakaba, Nariaki ; Hamamoto, Shimpei  ; Takeda, Yoichi*

Inherent safety advantages of the helium gas cooled reactors over other types of reactors depend importantly on the chemical stability of the actual helium coolant gas itself. Although helium being an inert gas does not react with fuel and components, chemical impurities which exist in the actual helium coolant can react with the surface of high-temperature materials such as the heat transfer tubes of the intermediate heat exchanger. The chemical effect of the impurities strongly influences to shorten the lifetime of the high-temperature materials. The dominant chemical reactions occurring in the core have not previously identified due to the complicated effects of not only high temperatures but also radioactivity during power operation of the helium gas cooled reactors. As such the methodology to control the high-temperature material lifetime has not been established because of the lack of knowledge and active control of the carbon activity and partial pressure of the oxygen which determine the creep fatigue of the high-temperature materials. The present study of the chemical equilibriums in the HTGR core is an initial effort to establish the lifetime extension methodology. The study examines the effects of high temperature and irradiation on the chemical equilibriums in the core by using an analytical code and basing on the chemical impurity data obtained in the HTTR operations.