The Very High Temperature Reactor (VHTR) has been selected by the U.S. as the Generation IV technology for the Next Generation Nuclear Plant (NGNP), and also by the Republic of Korea for the Nuclear Hydrogen Development and Demonstration (NHDD) project. One of the key long-lead items for the VHTR is the Reactor Pressure Vessel (RPV). In the absence of active vessel cooling, the RPV temperature during normal operation is determined by the design point selected for the primary coolant inlet temperature and the design of the reactor internal components, including the physical location of riser channels that route the coolant flow to the plenum above the reactor core. For the VHTR, the primary coolant (helium) inlet temperature is expected to be in the range 490°C to 590°C and the outlet temperature is expected to be in the range 850°C to 950°C. For the RPV, both SA-508/533 steel and higher alloy steels with higher temperature capability (e.g., 9Cr-1Mo-V steel) are being considered. Because of its extensive experience base as an ASME Section III code-approved material for Light Water Reactor (LWR) pressure vessels, SA-508/533 steel is emerging as a strong candidate for the VHTR RPV. However, in order to use this material, the RPV temperature must be maintained below ASME code limits, which are 371°C during normal operation and 538°C for up to 1000 h during accident conditions.

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