The very high temperature reactor (VHTR), using gas-cooled reactor technology, is one of the
six reactor concepts selected by the Generation IV International Forum and is anticipated to be
the reactor type for the next generation nuclear plant (NGNP). In this type of reactor 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
secondary fluid for electricity production, process heat, or hydrogen cogeneration. The current
Technology Readiness Level status issued by NGNP to all components associated with the IHX
for reactor core outlet temperatures of 750-800oC is 3 on a scale of 1 to 10 with 10 being the
most ready. At present, there is no proven high-temperature IHX concept for VHTRs. Amongst
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 is the primary focus of this paper.
In the current study, two PCHEs were fabricated using Alloy 617 plates and will be
experimentally investigated for their thermal-hydraulic performance in a high-temperature
helium test facility (HTHF). The HTHF was primarily designed and constructed to test the
thermal-hydraulic performance of PCHEs The test facility is primarily of Alloy 800H
construction and is designed to facilitate experiments at temperatures and pressures up to 800oC
and 3 MPa, respectively. The PCHE fabrication related processes, i.e., photochemical
machining and diffusion bonding are briefly discussed for Alloy 617 plates. Diffusion bonding
of Alloy 617 plates with and without a Ni interlayer is discussed. Furthermore, preliminary
microstructural and mechanical characterization studies of representative diffusion bonded
Alloy 617 specimens are presented.