Diffusion Welding of Compact Heat Exchangers for Nuclear Applications

The next-generation nuclear plant (NGNP) is designed to be a flexible source of energy, producing various mixes of electrical energy and process heat (for example, for hydrogen generation) on demand.  Compact heat exchangers provide an attractive way to move energy from the helium primary reactor coolant to process heat uses.  For process heat efficiency, reactor outlet temperatures of 750-900°C are desirable. There are minor but deleterious components in the primary coolant; the number of alloys that can handle this environment is small.  The present work concentrates on Alloys 800H and 617.

Diffusion welding parameters, including surface preparation, filler interlayers, and time/temperature profiles, were developed in the Gleeble^¨, a thermomechanical testing machine.  This included welding several sheets of material in diameters up to one inch.  These welds were characterized by the usual methods (optical, SEM, EDX, EBSD) and used for scaling up to 2 x 2 in. stacks of up to 32 sheets, welded in a vacuum hot press.  Samples of the welds are being subjected to a simulated, high temperature, NGNP environment (He plus various minor constituents) for hundreds to thousands of hours, and the oxidation characteristics of the welds will be compared with those of the base material. 

In addition, Thermocalc/DICTRA modeling software was used to model the diffusion welding process for these particular alloys.  Agreement with experiment was generally good, suggesting that such modeling is a useful tool for reducing the experimental matrix required to set diffusion welding parameters. 

The attached figures include an EBSD image and a comparison of modeled and experimental data for Alloy 617 diffusion welds. 

Abstract Figure 1.  Model and experimental data comparison as indicated. 

Abstract Figure 2. EBSD image of a diffusion weld in Alloy 617, 1150°C, 3 h, 5 MPa, including a 15 µm Ni foil interlayer at image center.