INTRODUCTION
A concerted 3-year R&D program has been completed in France and in the US on materials for heat exchangers of next generation nuclear gas-cooled reactors. The scope was to gain data on the mechanical and corrosion behavior of candidate heat exchanger alloys in the range of service conditions expected for a Very High Temperature Reactor (VHTR), specifically the temperature, the load and cycling, and the chemical environment.
As far as corrosion was concerned, the main objectives of the program were to compare the performances of materials and to improve the understanding of their properties. Oxidation, carburization and decarburization modes were evidenced depending on the coolant impurity content. A special emphasis was placed on the consequences of corrosion on alloy properties: microstructure, ductility, tensile strength. Next step will involve the study of the environmental effect on fatigue and creep-fatigue life. This paper presents the main achievements on corrosion of the R&D program.
As part of the US Department of Energy bilateral International-Nuclear Energy Research Initiatives (I-NERI), a 3-year concerted R&D program has been completed in April 2009 (1). The scope was the materials for heat exchangers of Generation IV high-temperature gas-cooled nuclear reactors (HTGR) such as the Next Generation Nuclear Plant NGNP, High Temperature Reactors HTR, the Gas-cooled Fast Reactor GFR, or Very High Temperature Reactors VHTR. HTGR systems may attain high yields for electricity generation and have the capability to efficiently produce hydrogen from the high temperature process heat produced in the core. Table 1 gives basic features of some of these concepts of nuclear plant. Except for the long-term GFR, the technology is based on the design of HTGR plants which have been built and operated in the past. The primary in-core structures are made of graphite, either as a prismatic block or as pebble-bed core. The key out-of-core structures include the reactor pressure vessel (made of 2 ¼ Cr steel or F/M steel), the cross-vessel component and the intermediate heat exchangers (IHX) which receive gas of the highest possible temperature for delivery to the hydrogen production plant. Such components place great demands on their materials of construction.
INL(A) for the US and CEA(B) for France were the leading organizations of the collaboration. The scope was to gain data on the mechanical and corrosion properties of IHX materials in the range of service conditions expected for these reactors, specifically the temperature, the load and cycling, and the chemical environment.
(Table in full paper)
Regardless of the IHX design - compact heat exchangers would be preferred from an efficiency standpoint but the manufacture technologies are not mature yet - material selection for this component is critical. The material must be available in the appropriate product form - both plate and sheet, weldable and suitable for use up to 750°C or above. Thin products must also sustain a differential pressure of 5 to 7 MPa during off-normal events. Therefore, mechanical strength at high temperature, as well as creep, fatigue, creep-fatigue and corrosion resistance is required for extended lifetime (tenths of years).
