Sen. Sci. Dr. T. Apostolov, Sen. Sci. Dr. Kr. Ilieva, Sen. Sci. Dr. S. Belousov, Sen. Sci. Dr. Iv. Penev, Sci .Res. Dr. T. Petrova, Sci .Res. L. Kinova, Dr. S. Antonov, Sci .Res.. I. Popova, Phys. A. Trifonov, Sci .Res. R. Prodanova, Sci .Res.. C. Peneva
The reactor pressure vessel (RPV) embrittlement caused by the impact of neutron irradiation (neutron fluence) from the reactor core during the NPP operation could be the reason of the dangerous break of the vessel integrity (thermal shock) when accident shut down of the unit is performed. That is why to guarantee the reactor nuclear safety it is needed to know the state of the vessel metal degradation due to its neutron embrittlement. This determines the accurate definition of the RPV neutron fluence as necessary requirement for reactor lifetime assesment. The problem of RPV ageing is especially actual for the units with VVER-440/230 at Kozloduy NPP, which are under operation about twenty years.
The INRNE specialists have developed original calculation and experimental methodologies for accurate determination of the RPV neutron fluence for VVER-440 and VVER-1000. It provides a reliable current RPV metal embrittlement estimation as well as is a base for prognosis of the RPV metal degradation during the following reactor operation. The results obtained for neutron fluence have been applied in practice many times at Kozloduy NPP as well as for taken out of management decisions.
The RPV reactor dosimetry investigations for Units 1-6 Kozloduy NPP have been carried out by INRNE from 1989 for the whole period of the units operation (over 22 contracts with Kozloduy NPP have been fulfilled). The RPV neutron fluence results have been used as a base for Unit 2 RPV lifetime evaluation in 1992, and for Unit 1 in 1995-1996.
The INRNE specialists have developed a methodology for the most reliable estimation of the neutron fluence at the surveillance specimen locations of VVER-1000 reactors. Data for the neutron fluence onto the metal specimens from surveillance assemblies 1L and 2L of Unit 5 have been obtained for an assessment of the current embrittlement and prognosis for next three years.
The high reliability and validity of the developed methodologies for the RPV neutron fluence determination have been approved within world-wide scientific society - AWGRD (European Working Group of Reactor Dosimetry), IAEA (International Atomic Energy Agency), NRC (Nuclear Regulatory Commission of the USA).
The results of these activities are presented in more then 40 scientific publications in such journals as Nuclear Science and Engineering, Nuclear Engineering and Design, proceedings of the international meetings of EWGRD, ASTM (American Society for Testing of Materials) and ASJ (Atomic Society of Japan) in USA, Germany, Russia, Czech Republic, Israel, as well as in the proceedings of Bulgarian Nuclear Society etc. Home page Back