Material selections and R and D requirements for CANDU-SCWR fuel channels

One of the six reactor concepts being considered by the Generation-IV International Forum (GIF) for international collaborative R and D is the Supercritical Water Reactor (SCWR). With SCW coolant, the thermodynamic efficiency is increased to over 40%, depending on the temperature. The CANDU-SCWR concept falls under this category. Like the present CANDU reactors, the CANDU-SCWR is a horizontal pressure tube design. Fuel bundles residing inside the pressure tubes are cooled by light SCW. The high temperature and high pressure of the coolant demand a lot from the reactor core materials. At high temperatures, mechanical strengths are reduced, plus the corrosion (oxidation) rates, and the associated hydrogen ingress rate for zirconium alloys, are increased. Thus, the standard CANDU pressure tube design cannot be used. A way of overcoming these difficulties is to thermally insulate the hot coolant from the pressure tube so that it operates at a lower temperature. This paper presents one of the insulated pressure tube designs being considered for CANDU-SCWR. The pressure tube is thermally insulated from the hot coolant inside by a porous ceramic insulator. A perforated metal liner protects the insulator from being damaged by the fuel bundles and erosion by the coolant. The coolant pressure is transmitted through the perforated metal liner and insulator and applied to the pressure tube directly. The thermally insulated pressure tube is in contact with the moderator, which operates at about 80 degree C. The insulator reduces the flow of heat from the coolant, thereby allowing the pressure tube to operate at ∼100 degree C. The insulator does not need to support the coolant pressure, although it must be able to withstand the weight of the fuel bundles. The material selection for each component depends on its function. The fuel sheaths and the perforated liner must have high corrosion resistance in SCW, although their resident times are significantly different. The insulator must have high thermal resistance and corrosion resistance in SCW, plus sufficient strength to bear the weight of the fuel bundles without significant thickness reduction during its design life. The pressure tube is the pressure boundary material, so it must have high strength to contain the coolant. One common requirement for all fuel channel components is that they should be as neutron transparent as possible. Irradiation deformation of all the components must also be considered in design. The paper will review existing data for CANDU-SCWR fuel channel materials, indicate where more data are required, and our plans to obtain them. (authors)