There is significant current interest in the commercial power industry in developing nuclear reactor concepts using molten salt as a coolant or as a fuel-bearing medium and coolant. The Molten Salt Reactor Experiment (MSRE), which was performed at Oak Ridge National Laboratory from 1964-1969, established the feasibility of this reactor design and performed some of the early work in qualifying appropriate structural materials and coolant salts. However, modern versions of this design often incorporate aspects that are beyond the design basis of the MSRE. For example, some designs intend to operate at a higher temperature, use a different structural material, or a different salt altogether. These new designs will eventually require irradiation testing in a variety of reactors and conditions. This document specifically evaluates the possibilities and requirements associated with the most difficult of these conditions: a flowing salt irradiation under high neutron flux.

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The HTV capsule is a High Flux Isotope Reactor (HFIR) target-rod capsule designed to operate at very high temperatures. The graphite containing section of the capsule (in core) is approximately 18 in. (457.2 mm) long and is separated into eight temperature zones. The specimen diameters within each zone are set to achieve the desired gas gap and hence design temperature (900, 1200, or 1500°C). The capsule has five zones containing 0.400 in. (10.16 mm) diameter specimens, two zones containing 0.350 in. (8.89 mm) diameter specimens and one zone containing 0.300 in. (7.62 mm) diameter specimens. The zones have been distributed within the experiment to optimize the gamma heating from the HFIR core as well as minimize the axial heat flow in the capsule. Consequently, there are two 900°C zones, three 1200°C zones, and three 1500°C zones within the HTV capsule. Each zone contains nine specimens 0.210 ± 0.002 in. (5.334 mm) in length. The capsule will be irradiated to a peak dose of 3.17 displacements per atom. The HTV specimens include samples of the following graphite grades: SGL Carbon’s NBG-17 and NBG-18, GrafTech’s PCEA, Toyo Tanso’s IG-110, Mersen’s 2114, and the reference grade H-451 (SGL Carbon). As part of the pre-irradiation program, the specimens were characterized using ASTM Standards C559 for bulk density, and ASTM C769 for approximate Young’s modulus from the sonic velocity. The probe frequency used for the determination of time of flight of the ultrasonic signal was 2.25 MHz. Marked volume (specimen diameter) effects were noted for both bulk density (increased with increasing specimen volume or diameter) and dynamic Young’s modulus (decreased with increasing specimen volume or diameter). These trends are extended by adding the property versus diameter data for unirradiated AGC-1 creep specimens (nominally 12.5 mm-diameter by 25.4 mm-length). The relatively large reduction in dynamic Young’s modulus was surprising given the trend for increasing density with increasing volume. The graphite-filler particle size was noted to be influential in the volume dependency data, with finer grained graphites showing the least specimen volume/diameter effect. Here the volume dependency trends are discussed in terms of the graphite’s filler-particle size and texture.