WWER fuel: Results of post irradiation examination

Experience in the field of fabrication, operation, testing and post-irradiation examinations (PIE) made it possible to settle the following requirements for a new generation of WWER nuclear fuel: - For WWER-1000 FA, the service life is no less than 5 years, 3 alternative fuel cycles (FC): 12 months x 4 FCs, 12 months x 5 FCs and 18 months x 3 FCs; - For WWER-440 FA, fuel cycle is 12 months x 5 FCs and a part of operating assembly is left for the 6. year; - High fuel burnup - up to 70 MWd/kgU; - Dimensional stability of FA and its components; - FA repairability; - Adaptability of fuel cycles; - Maintenance of maneuvering operating conditions at the NPP; - Reliability of control rod operation; - High serviceability level - FE leakage is no worse than 10-5 l/year. In order to provide the fulfillment of the above-given requirements, designers and production engineers have worked out cumulative measures and engineering solutions, which are introduced in development of a new generation fuel. Currently old design FA-M assemblies provided with steel skeleton are being operated in WWER-1000 reactors at Ukrainian and Bulgarian NPPs. As for Russian NPPs, new-type FAs are operated. These are advanced FAs (AFA), FA-A and FA-2 provided with zirconium alloy skeletons. A design of the second generation of WWER-440 operating assemblies was developed with respect to changes in some geometrical parameters, fastening of FEs in the lower grid (splinting was substituted for collet), usage of reinforcing rib under the lower grid, anti-debris filter and hafnium elements of junction unit as well as hafnium content decrease from 0.05 % mass down to 0.01% mass in zirconium materials. They are basic designs of FAs in order to be introduced in a five-year fuel cycle of WWER-440 NPPs in Czech Republic and Slovakia since 2005 and have got prospects for development. The operating experience of dismountable operating assemblies at the Loviisa NPP, vibration-proof operating assemblies at the Kozloduy NPP and advanced working assemblies at the Kola NPP demonstrate this. Now a high-priority task is to prolong the service life and increase a burnup of WWER fuel without decrease of high reliability. Design and engineering innovations in the course of fuel development require a fundamental analysis from the viewpoint of efficiency. Innovations are checked at the final stage based on the results of PIE of FAs after semi-commercial operation in the commercial reactor. PIE make it possible to get a detailed and unprejudiced evaluation of a new generation fuel for consistency with the above stipulated requirements. The present paper describes the results of FA PIE with regard to its main parameters and properties, which affect the serviceability of products and their structural components. The paper has the following contents: Introduction; Post-irradiation examination results; FA geometries and rigidity; State of fuel elements; Fuel- cladding interaction; Cladding corrosion; Fission gas release; Zr-spacer grid state; Fuel failure; Corrective actions; Conclusion. In conclusion the authors underline that the WWER-440 FAs provided with shroud demonstrate a high geometrical stability in the course of their operation up to 6 fuel cycles. The problem of FA bending in the WWER-1000 reactor core has been solved at the expense of FA-A and FA-2 development. These designs are provided with rigid skeleton that demonstrates permissible deformation if they are operated up to 6 fuel cycles. As for other parameters affecting the FE serviceability (geometries, cladding corrosion, mechanical properties, fission gas release, etc), the service life is not exhausted up to a fuel burnup of 70 MWd/kgU. The state of ZSGs is satisfactory on the whole, but there are some cases of higher corrosion demonstrating the necessity of further design and engineering work. E635 products (guide tubes, central tubes and FE claddings) demonstrate geometrical stability and strength properties as opposed to the E110 products