A Safeguards Approach applied to control the Spent Fuel Assemblies Stored in a Spent Fuel Pond on an Unique On load Reactor

The traditional safeguards approach for On Load reactors is based on verification of the irradiated fuels that leave the core and to keep the knowledge of such items during interim or permanent storage. For doing that, tools such Core Discharge Monitor and Fuel Bundle Counter are used together with containment and surveillance. Besides, the verification of the original pool inventory shall be performed. The On load reactor, at Atucha, was built before the safeguards criteria require such rules and neither the tools to verify the spent fuel that goes from the reactor to the pond nor an instrument to verify the spent fuel inventory in the pond were available. To make even more complex the verification, the spent fuels are stored in a closed packed way, not allowing an easy way to verify the entire pool, and the installation of an item counter is difficult because of the long length of the fuel and the difficult access of the routes where the fuels travel. The paper presents the methodology to perform the pool inventory verification and an analysis of the obtained data. An underwater neutron counter, SFNC (Spent Fuel Neutron Counter), specially developed for application at the heavy water reactor was used to collect neutron signals in the gap between assemblies. A detector system was combined with underwater surveillance to act as an item counter when the fuels enter at the pond. Results of the spent fuel verification through SFNC are presented and analysis of the combined tools, detector system and surveillance, is made concerning to the safeguards approach fulfilment.