### abstract

- Korea Hydro and Nuclear Power has built a new modular type of CANDU spent fuel bundle dry storage facility, MACSTOR KN-400, at the Wolsong reactor site in the Republic of Korea. Four CANDU reactors operate at the Wolsong site, and the MACSTOR KN-400 has the capacity to store up to 24000 CANDU spent fuel bundles. The International Atomic Energy Agency safeguards regulations demand an effective method for spent- fuel re-verification at the MACSTOR KN-400 facility in the event of any loss of continuity of knowledge. A radiation signal dependent spent-fuel re- verification design of the MACSTOR KN-400 is scrutinized through mathematical model development and Monte Carlo radiation transport simulations using the state-of-the-art computer code MCNP. Both gamma and neutron transport simulations for various spent fuel bundle diversion scenarios are carried out for the central and corner re-verification tube structures. The CANDU spent fuel bundles with a burnup of 7500 MWd/tonne U (burned at a specific power of 28.39 MW/tonne) and 10 years of cooling time are considered for the radiation source term. Results of the gamma transport simulations incorporating cadmium-zinc-relluride detectors inside the re-verification tube show that spent fuel bundles diverted from the inner locations of the storage basket cannot be detected by observing a gamma radiation signal change. Neutron transport simulations consisting of a 3He detector inside the re-verification tube show that certain spent fuel bundle diversions could be detected. However, inverse MCNP neutron transport simulations show that the possibility of detecting diversion of 67% of spent fuel bundles stored in the basket region on the opposite side from the collimator of the re-verification tube is small, assuming a neutron detection counting time of 1 h per re-verification tube. It is also observed that the nondetecrion probability for most of the diversion scenarios considered is large. Nondetection probability here is defined as the probability of not detecting the diversion of spent fuel bundles from the baskets by observing radiation signal reduction from the removal of the bundles. Containment and surveillance methods are being employed for safeguards purposes at the facility, supplemented by periodic axial profile fingerprinting. However, since the nondetection probability is large for most scenarios, the facility should consider alternatives to this method in case loss of continuity of knowledge occurs.