Assessment of the Fingerprinting Method for Verification of Spent Fuel in MACSTOR KN-400 CANDU Spent Fuel Dry Storage

摘要

Wolsong site in republic of Korea currently has four CANDU reactors. Spent fuel bundles discharged from the reactors are transferred to an interim dry storage facility after a few years of cooling in the water pool. However, the site does not have enough space for further extension of its existing interim storage facility. Hence, Korea Hydro and Nuclear Power has built a new modular type of dry storage facility, MACSTOR KN-400, which has the capacity to store up to 24000 CANDU spent fuel bundles. The safeguards regulations of the IAEA demand an effective method of spent fuel verification at the MACSTOR KN-400 storage facility in the event of any loss of continuity of knowledge. Central and corner structures of the MACSTOR KN-400 along with the re-verification tube were modeled using the radiation transport code, MCNP. Central and corner structures have different types of re-verification systems. Both, gamma and neutron simulations were carried out using these MCNP models developed for the MACSTOR KN-400. CANDU spent fuel bundles with a burnup of 7.5 GWD/t (burnt at specific power of 28.39 MW/t) and 10 years cooling time were considered for the radiation source term. MCNP gammatransport simulations were done by including a CZT detector inside the re-verification tube. Gamma-transport simulations for different spent fuel diversion scenarios were carried out. It was observed that for diversion scenarios wherein most of the bundles are removed from the inner portions of the basket (opposite side of the collimator to the re-verification tube), it was difficult to conclude whether diversion has taken place based on the change in gamma radiation signals. Similarly neutron transport simulations were also carried out by including a helium-3 detector inside the re-verification tube. Results obtained from neutron-transport simulations for various spent fuel diversion scenarios were encouraging and indeed indicated higher sensitivity for diversion of fuel bundles. The study concludes that as the fingerprinting method based on gamma signals alone does not adequately protect diversion of fuel bundles inside baskets, alternative fingerprinting method such as the one with neutron signals should be explored.