Ageing Management, Monitoring and Inspection of Spent Fuel Storage by Canister System

Ageing Management Programme (AMP) for the storage system over the period of extended storage will address uncertainties in the safety-relevant functions of the system that may otherwise be impaired by ageing mechanisms. The AMP identifies System, Structure and Components (SSCs) that need specific actions to mitigate ageing and ensures that no ageing effects result in a loss of their intended function during an intended licensed period. AMPs generally include prevention, mitigation, monitoring, inspection, and maintenance programmes. An example of monitoring to detect confinement loss of (Helium leakage from) canister is as follows. In a concrete cask storage system, spent fuel assemblies are placed and weld-sealed in a canister filled with Helium gas. If the Helium gas leaks due to stress corrosion cracking of the weld, for instance, the effect of Helium convection is lost in the canister, causing the temperature profile on the canister surface to change. It was found that the temperatures difference between the bottom and the top of the canister surface changed remarkably with the Helium gas leak. Monitoring the temperature difference enables confirmation of the integrity of the canister containment. An example of inspection to detect spent fuel integrity in canister is as follows. When a spent fuel rod lost its integrity, gaseous fission products were discharged and diffused in the canister. Among them, Krypton- 85 emits gamma rays of 514 keV. Detection of this gamma ray from outside of the canister enables identification of a loss of integrity of spent fuel rods without opening the canister lid. Experiments were performed using a small-scale mock-up canister. The Krypton-85 leak of about 1011 Bq - about 10% of the Krypton-85 inventory in a fuel rod - could be detected by Ge gamma ray detectors. This technique can be used as an inspection method of integrity or damage of spent fuel. It is noted that Krypton-85 decays out with the half-life of approximately 11 years. Accordingly, time duration of the inspection measurements has to be extended for aged fuel in the canister. (authors)