Patent Number: 
Section: claims

1. A method for detecting gas leakage from a radioactive material sealed container, adapted to detect leakage of inactive gas from a metallic sealed container of the radioactive material sealed container that includes: the metallic sealed container configured to store and seal spent fuel and the inactive gas; and a non-sealed concrete-made storage container having a shielding function and configured to store the metallic sealed container, the method comprising:measuring a temperature at a bottom portion of the metallic sealed container;also measuring a feeding air temperature of external air passing between the metallic sealed container and the concrete-made storage container; anddetermining occurrence of leakage of the inactive gas when significant change appears in correlation between the temperature at the bottom portion of the metallic sealed container and the feeding air temperature or when change appears in a physical amount related to leakage of the inactive gas calculated by using the temperature at the bottom portion of the metallic sealed container and the feeding air temperature. 2. The method for detecting gas leakage from a radioactive material sealed container according to claim 1, whereinthe physical amount is a temperature of the inactive gas in vicinity of a bottom surface inside the metallic sealed container,the temperature of the inactive gas is calculated from the temperature at the bottom portion of the metallic sealed container and the feeding air temperature, andwhen the temperature of the inactive gas rises while the feeding air temperature decreases, occurrence of leakage of the inactive gas is determined. 3. The method for detecting gas leakage from a radioactive material sealed container according to claim 1, whereinthe physical amount is a heat flux emitted from the bottom portion of the metallic sealed container to the external air,a heat flux is calculated by multiplying a heat transfer coefficient at a bottom surface of the metallic sealed container by a difference between the temperature at the bottom portion of the metallic sealed container and the feeding air temperature, andwhen fluctuation of the heat flux is larger than threshold heat flux fluctuation, occurrence of leakage of the inactive gas is determined. 4. The method for detecting gas leakage from a radioactive material sealed container according to claim 3, wherein a fluctuation amount of the heat flux caused by fluctuation of the feeding air temperature is excluded by adding, to the heat flux, a heat flux calculated by multiplying a coefficient α by the fluctuation amount of the feeding air temperature. 5. The method for detecting gas leakage from a radioactive material sealed container according to claim 1, wherein the temperature at the bottom portion of the metallic sealed container is a temperature at a bottom portion center of the metallic sealed container. 6. An apparatus for detecting gas leakage from a radioactive material sealed container, adapted to detect leakage of inactive gas from a metallic sealed container of the radioactive material sealed container that includes: the metallic sealed container configured to store and seal spent fuel and the inactive gas; and a non-sealed concrete-made storage container having a shielding function and configured to store the metallic sealed container, the apparatus comprising:a first temperature sensor configured to measure a temperature at a bottom portion of the metallic sealed container;a second temperature sensor configured to measure a feeding air temperature of external air passing between the metallic sealed container and the concrete-made storage container; anda gas leakage estimation unit configured to estimate occurrence of leakage of the inactive gas when significant change appears in correlation between the temperature at the bottom portion of the metallic sealed container and the feeding air temperature or when change appears in a physical amount related to leakage of the inactive gas calculated by using the temperature at the bottom portion of the metallic sealed container and the feeding air temperature. 7. The apparatus for detecting gas leakage from a radioactive material sealed container according to claim 6, whereinthe physical amount is a temperature of the inactive gas in vicinity of a bottom surface inside the metallic sealed container,the gas leakage estimation unit calculates the temperature of the inactive gas from the temperature at the bottom portion of the metallic sealed container and the feeding air temperature, andwhen the temperature of the inactive gas rises while the feeding air temperature decreases, occurrence of leakage of the inactive gas is determined. 8. The apparatus for detecting gas leakage from a radioactive material sealed container according to claim 6, whereinthe physical amount is a heat flux emitted from the bottom portion of the metallic sealed container to the external air,the gas leakage estimation unit calculates a heat flux by multiplying a heat transfer coefficient at a bottom surface of the metallic sealed container by a difference between the temperature at the bottom portion of the metallic sealed container and the feeding air temperature, andwhen fluctuation of the heat flux is larger than threshold heat flux fluctuation, occurrence of leakage of the inactive gas is determined. 9. The apparatus for detecting gas leakage from a radioactive material sealed container according to claim 8, wherein the gas leakage estimation unit excludes a fluctuation amount of the heat flux caused by fluctuation of the feeding air temperature by adding, to the heat flux, a heat flux calculated by multiplying a coefficient α by the fluctuation amount of the feeding air temperature. 10. The apparatus for detecting gas leakage from a radioactive material sealed container according to claim 6, wherein the first temperature sensor measures a temperature at a bottom portion center of the metallic sealed container as the temperature at the bottom portion of the metallic sealed container. 11. The apparatus for detecting gas leakage from a radioactive material sealed container according to claim 6, wherein the first temperature sensor is inserted from an air inlet port of the metallic sealed container and also installed at a back surface of a metal plate mounted on a lifter movable up and down relative to the bottom portion of the metallic sealed container, and configured to measure a temperature at the back surface of the metal plate having a surface in a state of contacting the bottom portion of the metallic sealed container.