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The effects of neutron irradiation to 10 dpa at 500 and 800 °C on a high-quality three-dimensional balanced weave composite (FMI-222) is presented. Strength and dimensional stability for this system is compared to earlier work on this material, at lower dose, and contrasted with that of a well studied isotropic graphite (POCO AXF-5Q) irradiated at identical conditions. For both irradiation temperatures the composite strength in bending is substantially increased. While both irradiation temperatures cause contraction along the bend bar axis, the amount of contraction is greater for the higher temperature irradiation. Moreover, for the 500 °C irradiation the corresponding decrease in volume is observed, though an apparent large increase in volume occurs for the 800 °C irradiated composite. This departure from isotropic dimensional change is explained in terms of fiber dimensional stability model previously presented.

Specimens of pure copper, a high purity austenitic stainless steel, and V–4Cr–4Ti were exposed to eight cycles of either constant temperature or periodic temperature variations during neutron irradiation in the High Flux Isotopes Reactor to a cumulative damage level of 4–5 displacements per atom. Specimens exposed to periodic temperature variations experienced a low temperature (360 °C) during the initial 10% of accrued dose in each of the eight cycles, and a higher temperature (520 °C) during the remaining 90% of accrued dose in each cycle. The microstructures of the irradiated stainless steel and V–4Cr–4Ti were qualitatively similar to companion specimens that were continuously maintained at 520 °C during the entire irradiation. The microstructural observations on pure copper irradiated at a constant temperature of 340 °C in this experiment are also summarized. The main radiation-induced microstructural features consisted of dislocation loops, stacking fault tetrahedra and voids in the stainless steel, Ti-rich precipitates in the V alloy, and voids (along with a low density of stacking fault tetrahedra) in copper.

Post-irradiation examination has started and initial results are available for the varying temperature irradiation experiment conducted in the high flux isotope reactor (HFIR), carried out in the framework of the Japan–USA Fusion Cooperation Program (JUPITER project). This experiment is an unprecedented controlled irradiation experiment designed for symmetrical comparison of isothermal and temperature-variant neutron irradiation effects. This paper summarizes the irradiation experiments and initial results obtained from the post-irradiation examination of an austenitic alloy, vanadium, a vanadium alloy and copper. These preliminary results extend the trends observed in earlier, lower fluence experiments, with some effects of the low temperature phase of variable temperature irradiation on evolving microstructures.

A temperature controlled instrumented capsule has been irradiated in the RB* position of the high flux isotope reactor (HFIR) at the Oak Ridge National Laboratory in order to perform in situ thermal conductivity measurements of monolithic ceramics and composite materials during neutron irradiation. This determination of thermal conductivity utilizes a temperature gradient technique whereby two thermocouples measure the absolute temperature along a cylindrical sample which is constrained to one-dimensional thermal conduction. The heat source is the intrinsic gamma heating of the core region of the HFIR. This paper provides an overview of the experiment and gives preliminary results on the degradation in thermal conductivity of a few ceramic specimens measured as a function of irradiation temperature (200–700°C) and fast neutron fluence up to ∼3.4 × 1025 n/m2 (E>0.1 MeV ) .

An experiment to directly compare the effects of neutron fluence received at steady temperature on microstructure and mechanical properties of candidate fusion reactor structural materials to the effects of receiving 10% of the fluence at reduced temperature has been designed. The Varying Temperature Experiment is being fabricated for irradiation in a large Removable Beryllium (RB) position of the High Flux Isotope Reactor (HFIR). For this experiment, specimen temperatures are elevated and controlled whenever there is substantial neutron fluence to the specimens (reactor power > 10%). Half of the specimens are irradiated at two distinct temperatures, and half are irradiated at constant temperature. The capsule features to meet these requirements are described. The experiment will operate for ten reactor cycles beginning in early 1998. This project is a collaborative effort under the Japan-USA cooperative program (Jupiter Project).