Main aspects of the SEU fuel program at the Atuchai PHWR after five years of operating experience

Atucha I is the first nuclear station in Argentina and started operation in 1974. It is a pressure vessel type reactor, moderated and cooled with heavy water, and was designed by Siemens (Germany). Fuelling is on-power and the plant was originally fuelled with natural uranium. To reduce fuel costs a program was initiated in August 1993 to introduce gradually slightly enriched uranium (SEU) fuel (0.85 w% U235) with an associated burnup increase from 5900 MWd/tU to 11300 MWd/tU. The introduction of SEU fuel started in January 1995 and the program was divided in three Phases with an upper limit of SEU FA in the core: 12, 60 and 252 (full core). This paper describes the most important aspects of the project and the operating experience in the period 1995-1999. After five years of the program and with 86% of core positions (217 FA) loaded with SEU fuel, the operating experience has been fairly good. In particular, the new criteria to prevent PCI failures in power ramps for higher burnup SEU fuel in refueling operations, plant startups or power cycling has been effective. The average discharge burnup of the SEU fuel that completed their irradiation in 1999 was 11346 MWd/tU. The average discharge burnup of the natural fuel in the same year was 6549 MWd/tU, with an increase of about 11% of the original value for a natural fuel core. The average number of fresh fuel assemblies per full power day was being reduced from 1.31 to 0.92 in 1998 and 0.86 in 1999. The fuel costs dropped gradually during the program from 9.38 (with natural uranium fuel) to 6.81 U$S/MWh in 1999 (taking as reference the NU and SEU FA costs for 1998 and 1999). Because of this the SEU fuel program has been an important contribution to the reduction of Atucha I operating costs and to the improvement of the competitiveness of nuclear power generation against other sources of generation in the deregulated electrical market in Argentina. The use of SEU fuel also produced a small increase of the time to reach full power in plant startups or power cycling, and some increase to the susceptibility to axial xenon induced power oscillations. It is planned to explore in the future the technical feasibility and economical convenience to start an SEU fuel irradiation program in the Embalse Nuclear Station. (author)