Unattended Monitoring of Spent Fuel Transfers to Dry Storage at the Atucha 1 Nuclear Power Plant in Argentina

In order to meet verification obligations established in relevant safeguards agreements, the Brazilian-Argentine Agency for Accounting and Control of Nuclear Materials (ABACC) and the International Atomic Energy Agency (IAEA) have implemented unattended monitoring at the President Juan Domingo Perón (Atucha 1) Nuclear Power Plant in Argentina to verify the transfer of spent fuel assemblies from spent fuel ponds to a dry storage, avoiding typically intrusive and costly approaches based on permanent presence of inspectors on site. Once in the dry storage, those spent fuel assemblies will not be available for periodic visual access nor physical verification by inspectors. This difficultto-access condition requires that at least two independent containment and/or surveillance systems with different working principles (dual C/S) be applied to provide a more reliable continuity of knowledge (CoK) over the transferred assemblies. Different unattended systems were used aiming at maintaining CoK over all transfer steps. The monitoring covers since the appropriate positioning and counting of selected spent fuel items in a maneuvering pond, their insertion into a transfer cask and the final unloading into specific dry silos located in a dedicated building. The systems are based on non-destructive (NDA) gamma-ray and neutron detection to ensure the presence, loading or unloading of spent fuel during each transfer step. Surveillance cameras support all transfer operations, as well as maintain CoK over the already transferred items stored in the dry silos. A 2D laser scanning system is also used in combination with surveillance cameras as the containment element of the dual C/S in the dry storage. This is an innovative development for this type of application. This paper discusses the main technical challenges faced during the first transfers and the identified improvements that permitted the successful verification of all completed transfers by ABACC and the IAEA since the first one in 2022. It is worth to note that the cooperation from the Operator (Nucleoelectrica Argentina S.A.) and the Regulatory Nuclear Authority of Argentina (ARN) in installing and testing of equipment, as well as carefully planning of transfer activities, was of fundamental importance to the positive results obtained so far.