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

The Korea Hydro and Nuclear Power has built a new modular type of dry storage facility, known as MACSTOR KN-400 at Wolsong reactor site. The building has the capacity to store up to 24000 CANDU spent fuel bundles in a 4 rows by 10 columns arrangement of silos. The MACSTOR KN-400 consists of 40 silos; each silo has 10 storage baskets, each of which can store 60 CANDU spent fuel bundles.

The development of an effective method for spent fuel verification at the MACSTOR KN-400 storage facility is necessary in order for the International Atomic Energy Agency (IAEA) to meet with safeguards regulations. The IAEA is interested in having a new effective method of re-verification of the nuclear material in the MACSTOR KN-400 dry storage facility in the event of any loss of continuity of knowledge, which occasionally happens when the installed seals fail.

In the thesis work, MCNP models of central and corner structures of the MACSTOR KN-400 facility are developed, since both have different types of re-verification system. Both gamma and neutron simulations were carried out using the MCNP models developed for MACSTOR KN-400. The CANDU spent fuel bundle with discharge burnup of 7.5 GWD/t (burned at specific power of 28.39 MW/t) and 10 years cooled was considered for radiation source term estimation.

For both the structures, MCNP simulations of gamma transport were done by including Cadmium-Zinc-Telluride (CZT) detector inside the re-verification tube. Gamma analyses for different spent fuel bundle diversion scenarios were carried out. It was observed that for diversion scenarios wherein the bundles are removed from the inner portions of the basket (opposite side of the collimator of the re-verification tube), it was difficult to conclude whether diversion has taken place based on the change in gamma radiation signals. Similar MCNP simulations of neutron transport were carried out by integrating helium-3 detector inside the re-verification tube and the results obtained for various diversion scenarios were encouraging and can be used to detect some spent fuel diversion cases. In the central structure, it was observed that addition of moderating material between the spent fuel and the detector increased the sensitivity of the detecting system for various diversion cases for neutron simulations.

In the worst scenario, the diverting state could divert 14 spent fuel bundles from each of 10 baskets in a silo from the basket region opposite to the collimator of the re-verification tube. The non-detection probability for this scenario is close to 1. This diversion cannot be easily detected using the currently designed detection system. In order to increase the detection probability, either the design of the facility must be changed or other safeguard methods, such as containment and surveillance methods must be used for safeguarding the nuclear material at the facility.