SEU Fuel In Atucha-1 NPP, A Valuable Experience for a CANDU-6 Core Enrichment

Abstract

The program to introduce SEU (0.85 % U235) fuel in Atucha-1 NPP started in 1993 and the first SEU fuels were loaded in 1995. Atucha-1 is a PHWR originally fuelled with natural uranium. Fuel Assemblies consist of 36 fuel rods. The active length is 5300 mm. At the present time 125 SEU Fuel Assemblies were irradiated without failures associated with the extended burnup or unfavorable influences on the operation of the power station. In the present stage of the program 72 % of core positions are loaded with SEU fuel. Mean fuel discharge burnup increased from 5900 MWd/tU to more than 1000 MWd/tU. The impact of the fuel on the cost of operation had an important reduction. This paper describes considerations about SEU fuel design analysis. The main concerns were related with the internal gas pressure, strains in fuel cladding and PCI-SCC sensitivity in power ramps at high burnup. A new burnup dependent criteria to prevent PCI failures was defined. The paper also details some aspects of SEU fuel performance data. Among them power ramps, discharge burnup and dwelling time data are discussed. In some cases the cladding axial growth after the irradiation was measured and the data obtained fits with a good agreement with model predictions. Despite the differences between Atucha-1 and Embalse fuel designs the experience accumulated in this program is encouraging and will be very helpful if a similar program is considered for a CANDU-6 core.