The effect of thermal annealing on thermal conductivity of graphite GR-280 irradiated up to high neutron fluence

Background. The GR-280 grade graphite is used as the core stacking material in PTR nuclear power reactors. Thermal conductivity of graphite affects both radiation and thermal stresses attained by the graphite blocks during the reactor operation and it is one of the key parameters affecting stress and strain states of graphite stacking. Thermal conductivity of the GR-280 graphite irradiated up to high neutron fluence is scarcely studied in the operating temperature range of graphite stacking and higher. As measurements at high temperatures are rather complex, thermal conductivity has been measured mainly at a room temperature. The main objective was to investigate the annealing effect on the thermal conductivity of the GR-280 graphite irradiated up to high neutron fluence at temperatures from Т$_{irrad}$ to 1200$^{\circ}С$. Materials and methods. The samples of the GR-280 graphite, irradiated at 450-650 $^{\circ}С$ up to neutron fluence (0.5-1.5)$\cdot 10^{26}$ m$^{-2 }$, were subjected to post-irradiation examinations. Its thermal conductivity coefficient was measured by the laser flash method. Annealing was carried out at 600 $^{\circ}С$, 800 $^{\circ}С$, 1000 $^{\circ}С$ and 1200 $^{\circ}С$ in argon for 2h. Results. This work made it possible to obtain the thermal conductivity of the GR-280 graphite, irradiated within a temperature range of 450-650 $^{\circ}С$ up to neutron fluence (0.5-1.5)$\cdot 10^{26}$ m$^{-2 }$, as a function of temperature and annealing time. Conclusions. It was demonstrated that the thermal conductivity began recovering for irradiated graphite specimens at an annealing temperature of 800-900 $^{\circ}С$. The difference of the thermal conductivity is within a range of 19-39 W/(m$\cdot$K) for the samples before and after annealing at 1 200 $^{\circ}C$ and it becomes lower as the neutron fluence and irradiation temperature increase. The thermal conductivity of the irradiated graphite increases by a factor of 2.3 after annealing at 1200 $^{\circ}С$ regardless of irradiation parameters.