The proposed experiments allow for further investigation of nuclear graphite samples tested during the 2016 molten flibe salt in-core irradiation at the Massachusetts Institute of Technology Reactor (MITR). Measurements in this proposal can also expand the current understanding of hydrogen trapping in neutron irradiated graphite and produce useful results relevant for advanced reactors. The graphite samples included will first be characterized to examine the microstructure changes caused by high-temperature molten salt and neutron irradiation. Irradiated and as-received graphite samples will then be monitored for hydrogen solubility at elevated temperatures using constant pressure gas charging. The motivation of this work is to directly measure hydrogen retention in graphites after neutron irradiation and understand the relevant mechanisms responsible for the changes in solubility.
A main graphite grade of interest is IG-110 because of its availability in previous literature as well as its use in previous MITR irradiations. High-purity IG-110U discs 8mm diameter and 2mm thick have been irradiated at MITR up to a fast fluence of 3.4E20 n/cm2 (E>0.1 MeV), which is expected to create a noticeable change in graphite microstructure and hydrogen solubility based on studies in literature. The graphite interplanar distance (d002) will be measured using X-ray diffraction, which can be correlated to the overall degree of graphitization. Defects in the graphite structure created by neutron damage are believed to decrease graphitization after irradiation as well as the increase the overall trapping of hydrogen. Samples will also be examined with a scanning electron microscope (SEM) to observe the grain structure and pore size distribution with and without neutron damage. Analysis with SEM can help determine whether other microstructural metrics can explain differences in graphite hydrogen retention. In addition to the IG-110U samples, one other grade of nuclear graphite will be selected for the baseline characterization.
Direct measurements of hydrogen retention in irradiated and as-received graphites will be made with a gas sorption analyzer equipped with a sample heater. The hydrogen retention measurements will be carried out over a representative range of expected fluoride-salt-cooled high-temperature reactor (FHR) temperatures and tritium partial pressures, namely 500-700ᵒC and 0.1-10kPa H2. The relationship between total retention and hydrogen pressure can be used to determine whether hydrogen is retained in a molecular form or whether dissociative retention occurs. An intermediate vacuuming and recharging step can be added to the procedure to differentiate between high-energy, persistent retention, and easily removable hydrogen. The weak/strong retention procedure is useful to examine the extent to which neutron irradiation increases concentration of low energy and high energy trapping sites. If successful, these measurements aim to be the most representative hydrogen solubility data for the FHR because of the relevant graphite grades, irradiation temperatures, hydrogen charging temperatures and hydrogen partial pressures.