Research On SiC Neutron Irradiation Temperature Monitor For Reactors

The novel experimental reactor possesses characteristics such as high temperature and intense radiation,rendering traditional temperature measurement methods inadequate for meeting its demands.High-purity silicon carbide(SiC)crystals,as passive sensors,offer advantages such as compact size,low gamma heating rate,and a wide temperature measurement range.They can be utilized for monitoring the irradiation temperature of reactor materials,enabling the evaluation of radiation damage.Following the Fukushima nuclear accident in Japan,along with the need for the development of new nuclear materials,SiC temperature measurement technology has regained significant attention.In this study,temperature measurements were conducted on 6H-SiC samples irradiated in the swimming pool-type reactor 49-2 at the China Institute of Atomic Energy.Various techniques including X-ray diffraction,Raman spectroscopy,electron microscopy,and nanoindentation were employed to analyze the structure and mechanical properties of neutron-irradiated SiC samples.The neutron-irradiation temperature of the SiC samples was measured using both the resistivity method and the lattice expansion method.The results revealed that for a neutron-irradiated SiC sample with a real-time recorded temperature of 550 ℃ and an irradiation dose of 0.02 dpa,the irradiation caused slight lattice expansion,resulting in a lattice strain of only 0.03%.Concurrently,the irradiation led to a 6% increase in SiC hardness.Thermal annealing was performed on the irradiated samples,and upon annealing at temperatures exceeding the irradiation temperature,the material’s hardness was restored.The resistivity of the neutronirradiated SiC sample after thermal annealing was measured using the four-point probe method.It was found that as the annealing temperature exceeded the irradiation temperature,the resistivity of the sample increased with the increasing annealing temperature.Raman spectroscopy analysis indicated that annealing of the carbon vacancies was the main cause for the increase in resistivity.Based on the relationship between resistivity and annealing temperature,the irradiation temperature of the sample was determined to be 615 ℃.For a neutron-irradiated SiC sample with a thermocouple recorded temperature of275 ℃ and an irradiation dose of 0.32 dpa,the main types of defects in the irradiated sample were invisible point defects and small defect clusters.Due to the formation of irradiation defects,the lattice strain in the sample increased to 0.445%,and the hardness increased by approximately 10%.Thermal annealing experiments were conducted on the irradiated SiC sample at different temperatures,and the variation of lattice expansion rate with annealing temperature was measured using X-ray diffraction.As the annealing temperature exceeded the irradiation temperature,the lattice strain in the sample linearly decreased with increasing annealing temperature.From this,the irradiation temperature of the sample was determined to be 408 ℃,which closely matched the irradiation temperature obtained by the resistivity method.Isothermal annealing experiments on the sample demonstrated that the recovery of defects in the neutron-irradiated SiC sample reached its maximum value after 30 minutes of annealing.According to empirical formulas,the theoretical lattice expansion rate of 0.32 dpa neutron-irradiated SiC at 400 ℃ was calculated to be 0.499%,consistent with the measured lattice expansion rate of SiC.This indicates that SiC crystals accurately measure the neutron-irradiation temperature,and the main reason for the discrepancy between SiC and thermocouple measurements during irradiation may be attributed to their lack of direct contact throughout the irradiation process.