Development of Improvements for the Argentine PHWR Fuels to Increase the Reliability at Extended Burnup and Other Operating Conditions. A Summary Report of Research Contract with National Commission for Atomic Energy (CNEA), Argentina. Annex 1

Argentina is carrying on an advanced cycle with increased burnup in the Atucha-1 nuclear power plant (NPP) since 1995. An increase of the U-235 enrichment from natural uranium (NU) to 0.85% allowed increasing the average discharge burnup of the fuel from 5900 MWd/tU to more than 11 000 MWd/tU. The main consequence of this improvement is an important reduction of the fuel consumption and a positive impact on the reduction of the cost of power generation. A detailed fuel design analysis was performed by the National Commission on Atomic Energy (CNEA) to evaluate the behavior of the fuel in the new operating conditions and to define initial design modifications to reduce the impact of the changes in the response of the fuel in such new conditions. Currently a SEU fuel design for use in Atucha-2 is under development at CNEA. The discharge burnup of the Atucha-2 SEU fuel will be higher than that of the Atucha-1 SEU fuel. Due to the extended discharge burnup, the plant operation will be limited to stay within power ramps which fuel assemblies can withstand without failure. Fuel analysis that supports the operational limits are associated with cladding materials, fuel behavior during power ramps and a buildup of internal gas pressure in the fuel element due to the increase of fission gases released from the fuel pellets. As a participant of this CRP, CNEA is expected to define a set of potential improvements of the fuel for Atucha type PHWRs to increase the reliability at extended discharge burnup. The specific topics to be addressed with the potential design optimizations included: • life limiting aspects of higher burnup fuels; • operational margins and reliable calculation tools to verify these margins; • fuel resistance to high burnup conditions, power ramps and eventual accident situations. The development of the potential improvements also included aspects to facilitate application at industrial stage and the feasibility of demonstration irradiations. This Annex describes the activities that were performed by CNEA under the framework of the CRP on Reliability of Extended Burnup and Advanced PHWR Fuels.