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

There is currently no consensus regarding the disposal of nuclear graphite. The two main problems include high activities of C-14 and H-3 as well as accumulation of Wigner energy (responsible for the Windscale Pile 1 fire in 1957). 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 o C and 725 o C at a heating rate of 10 o C.min -1 . The graphite, having been subject to a fast-neutron fluence from ~10 17 to ~10 20 n.cm -2 over the life time of the reactor at temperatures not exceeding 100 o C, exhibits Wigner energies ranging from 25 to 572 J.g -1 and a Wigner energy accumulation rate of ~7 x 10 -17 J.g 1 /n.cm -2 . The shape of the rate-of-heat-release curves, e.g., maximum at ca. 200 o 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 x 10 19 n.cm -2 ) is destroyed while that of samples farther from the reactor core (fast-neutron fluence < 1.5 – 5.0 x 10 19 n.cm -2 ) is intact. The dependence of the c lattice parameter on temperature between 25 o C and 200 o C as determined by Rietveld refinement for the nonamorphous samples leads to the expected microscopic thermal expansion coefficient along the c axis of ~26 x 10 -6 o C -1 . However, at 200 o 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 C-14 activity in the inner thermal column graphite ranges from 6 to 467 kBq.g -1 . Prior to interim storage or final disposal, thermal treatment of graphite irradiated under similar conditions to a fast-neutron fluence higher than ~5 x 10 19 n.cm -2 should be considered. The graphite of the inner thermal column of the ASTRA research reactor has been treated by heating to 400 o C for 24 h prior to interim storage. This effectively removes 90 - 100% of the Wigner energy. Generally, incineration might still be the most reasonable disposal option for irradiated graphite considering the large waste volume reduction factor and the literal disappearance of the Wigner energy issue. However, questions concerning releases of C-14 need to be addressed.