Thermal, structural, and radiological properties of irradiated graphite from the ASTRA research reactor - Implications for disposal

The release of Wigner energy from the graphite of the inner thermal column of the ASTRA research reactor has been studied by differential scanning calorimetry and simultaneous differential scanning calorimetry/synchrotron powder X-ray diffraction between 25 °C and 725 °C at a heating rate of 10 °C min-1. The graphite, having been subject to a fast-neutron fluence from 1017 to 1020 n cm-2 over the life time of the reactor at temperatures not exceeding 100 °C, exhibits Wigner energies ranging from 25 to 572 J g-1 and a Wigner energy accumulation rate of 7 × 10-17 J g-1/n cm-2. The shape of the rate-of-heat-release curves, e.g., maximum at ca. 200 °C and a fine structure at higher temperatures, varies with sample position within the inner thermal column, i.e., the distance from the reactor core. Crystal structure of samples closest to the reactor core (fast-neutron fluence >1.5-5.0 × 1019 n cm-2) is destroyed while that of samples farther from the reactor core (fast-neutron fluence <1.5-5.0 × 1019 n cm-2) is intact, with marked swelling along the c-axis. The dependence of the c lattice parameter on temperature between 25 °C and 200 °C as determined by Rietveld refinement for the non-amorphous samples leads to the expected microscopic thermal expansion coefficient along the c-axis of 26 × 10-6 °C-1. However, at 200 °C, coinciding with the maximum in the rate-of-heat-release curves, the rate of thermal expansion abruptly decreases indicating a crystal lattice relaxation. The 14C activity in the inner thermal column graphite ranges from 6 to 467 kBq g-1. The graphite of the inner thermal column of the ASTRA research reactor has been treated by heating to 400 °C for 24 h in a hot-cell facility prior to interim storage.