Experience and development of WWER-1000 control rod materials

Experience in operation of absorbing assemblies of the WWER-1000 and PWR reactors with similar designs and operating conditions in the core was discussed in details during special IAEA Technical Committees in 1993 and 1998 as well as in the frame of two-side Collaboration Program between Minatom, Russia and CEA, France (1998-2003). This was done in order to increase the reliability and safety of the cluster assemblies operation and use more effective and radiation resistant absorbing materials instead of boron carbide and Ag-In-Cd alloy. The main difference of the new design of the WWER-1000 absorbing elements consists in the application of non-swelling absorbing materials based on dysprosium or hafnium oxides as well as of claddings made of chromium-nickel alloy CNM-1 that are not affected by corrosion cracking and have high resistance to damage under strong strains during operation. This design provides a long-term safe operation and Dy2O3·TiO2 (or Dy2O3·HfO2) pellets can be suggested as absorbing materials for the PWR cluster absorbing assemblies instead of the Ag-In-Cd alloy. This report presents the main results of the investigation of different absorbing elements of the WWER-1000 control and safety rods, stages of their upgrading and future perspectives. In conclusion large experience has been accumulated in the development and operation of the WWER-1000 absorbing assemblies with different modifications. No failures have been observed. A new design of absorbing elements with Dy2O3·HfO2 (Dy2O3·TiO2) and B4C pellets covered with chromium-nickel CNM-1 alloy cladding can be used for a significant increase of time period of the safe operation of the WWER-1000 absorbing assemblies. This design can be recommended for PWR cluster assemblies. (authors)