Fabrication and design aspects of high-temperature compact diffusion bonded heat exchangers

► We examine diffusion bonding and photochemical machining of Alloy 617 for high-temperature printed circuit heat exchangers (PCHEs). ► Two PCHEs were successfully fabricated using Alloy 617 plates for performance testing in a high-temperature helium facility. ► Simplified stress analysis indicates a conservative design for the PCHEs fabricated at temperatures and pressures up to 800̊C and 3MPa. ► Microstructural and mechanical characterization results of diffusion bonded specimens provide additional confidence for PCHE operation up to 650̊C and 3MPa. The Very High Temperature Reactor (VHTR) using gas-cooled reactor technology is anticipated to be the reactor type for the Next Generation Nuclear Plant (NGNP). In this reactor concept with an indirect power cycle system, a high-temperature and high integrity Intermediate Heat Exchanger (IHX) with high effectiveness is required to efficiently transfer the core thermal output to a secondary fluid for electricity generation, hydrogen production, and/or industrial process heat applications. At present, there is no proven IHX concept for VHTRs. The current Technology Readiness Level (TRL) status issued by NGNP to all components associated with the IHX for reduced nominal reactor outlet temperatures of 750–800°C is 3 on a 1–10 scale, with 10 indicating complete technological maturity. Among the various potential IHX concepts available, diffusion bonded heat exchangers (henceforth called printed circuit heat exchangers, or PCHEs) appear promising for NGNP applications. The design and fabrication of this key component of NGNP with Alloy 617, a candidate high-temperature structural material for NGNP applications, are the primary focus of this paper. In the current study, diffusion bonding of Alloy 617 has been demonstrated, although the optimum diffusion bonding process parameters to engineer a quasi interface-free joint are yet to be determined. The PCHE fabrication related processes, i.e., photochemical etching and diffusion bonding are discussed for Alloy 617 plates. In addition, the authors’ experiences with these non-conventional machining and joining techniques are discussed. Two PCHEs are fabricated using Alloy 617 plates and are being experimentally investigated for their thermal-hydraulic performance in a High-Temperature Helium Facility (HTHF). The HTHF is primarily of Alloy 800H construction and is designed to facilitate experiments at temperatures and pressures up to 800°C and 3MPa, respectively. Furtherm