The high temperature isolation valve (HTIV) is a key component to assure the safety of a high temperature gas cooled reactor connected with a hydrogen production system for protections of radioactive material release from the reactor to the hydrogen production system as well as of combustible gas ingress to the reactor at the accident of fracture of an intermediate heat exchanger and the chemical reactor. However, the HTIV has not been made for practical use in the helium condition over 900°C
yet. The conceptual structure design of an angle type HTIV was carried out. A seat made of Hasteloy-XR is welded inside a valve box. Internal thermal insulation is employed around the seat and a liner because the high temperature helium gas flows inside the valve. The inner diameter of the top of seat was set 445 mm based on fabrication experiences of valve makers. A draft overall structure was proposed based on the diameter of the seat. The numerical analysis was carried out to estimate the temperature distribution and stress of metallic components by using a three-dimensional finite element method code. Numerical results showed that the temperature of the seat was simply decreased from the top around 900°C
to the root, and the thermal stress locally increased at the root of the seat, which was connected with the valve box. The stress was lowered below the allowable limit 120 MPa by decreasing the thickness of the connecting part and increasing the temperature of the valve box to around 350°C
⁠. The stress also increased at the top of the seat. Creep analysis revealed that the intactness of the HTIV is kept after the assumed operation cycles of the plant life as well as at the depressurization accident.