A Heuristic for Designing Hybrid Compact Heat Exchangers for Nuclear Applications

Hybrid compact heat exchangers (HCHEs) are a potential source of innovation for intermediate heat exchangers in nuclear industry, with HCHEs being designed for generation IV nuclear power applications. Compact heat exchangers are commonly fabricated using diffusion bonding (DB), which can provide challenges for HCHEs due to resultant nonuniform stress distributions across hybrid structures during bonding, leading to variations in joint properties that can compromise performance and safety. In this paper, we introduce a heuristic for determining whether a feasible set of DB conditions exist for producing HCHE designs capable of meeting regulatory requirements under nuclear boiler and pressure vessel codes. A DB model was used to determine DB parameters for predicting percent bonded area, which determines the lower threshold of DB parameters. Structural analysis was performed on the heat exchanger design for 316 stainless steel to determine higher threshold on the DB parameters to avoid failure modes due to creep, buckling, and yield in the HCHE structure during DB. A set of DB conditions were identified and validated experimentally by producing various test coupons for evaluating bond strength, ductility, porosity, grain size, creep rupture, creep fatigue, and channel deviation. A five-layer hybrid compact heat exchanger structure was fabricated and tensile tested demonstrating that the bonding parameters satisfy all criteria in this paper for DB HCHEs with application to the nuclear industry.