An Innovative Spherical Fuel Element to Inhibit the Infiltration of Liquid Fluoride Salt in Molten Salt Reactor

To inhibit the infiltration of liquid fluoride salt and easy to load and unload, fuel element in molten salt reactor (MSR) was isostatically pressed with an innovative design: A fuel-free low density graphite core of ≤ 30 mm diameter embedded in fuel-zone shell of ≥ 2.5 mm thickness, and then enveloped in a high density graphite shell of ≥ 5 mm thickness. Bulk density of the spherical fuel element can be designed from the range of 1.65–1.80 g/cm3, which is lower than the density of the liquid fluoride salt to make sure the fuel element can float in the MSR to load and unload. Characteristics of mercury infiltration and molten salt infiltration in graphite shell were investigated and compared with A3-3 graphite to identify the infiltration behaviors. The results indicated that the graphite shell has a low porosity about 9%, and an average pore diameter of 100 nm. The fluoride salt occupation of A3-3 was 10 wt% under 6.5 atm, whereas the salt gain did not infiltrate in graphite shell even up to 6.5 atm. It demonstrated that the outside graphite shell could inhibit the infiltration of liquid fluoride salt effectively. At the operating temperature of MSR (700 °C), thermal conductivity of graphite shell was 13.61 W/m K. The coefficient of thermal expansion (CTE) of outside graphite shell lied in 6.01×10−6 K−1 (α⫽) and 6.15×10−6 K−1 (α⊥) at the temperature range of 25–700 °C. The anisotropies factor of graphite shell calculated by CTE maintained below 1.12, which could meet the requirement of the spherical fuel element (below 1.30). The constant isotropic properties of graphite shell are beneficial for the integrity and safety of the spherical fuel element for a MSR.