Analysis of Justification for Extension of Operation of Russian NPP With VVER in Aspect of Pressure Vessel Radiation Embrittlement

Abstract

The reactor pressure vessel (RPV) is one of the most key components of equipment of Nuclear Power Plant (NPP) with VVER type reactors. The issue of the plant lifetime extension is actual especially in aspect of RPV embrittlement. An analysis of influence of RPV embrittlement on the evaluation of RPV lifetime and by means of that on an opportunity of operation extension beyond the design life within the framework of the license renewal is presented. The Regulatory Standards Base, existing in Russia and guiding a process of justification of RPV lifetime, is surveyed. SEC NRS, realizing a scientific and engineering support of The Federal Nuclear and Radiation Safety Authority of Russia (Gosatomnadzor of Russia), takes part in such Regulatory Standard development. The role of SEC NRS in this process has been shown. Besides, the activity of SEC NRS in analysis of justification of RPV service lifetime prolongation by expertise of the utility documentation within the framework of license process is demonstrated. Different measures aimed at the management of RPV steel degradation and thus providing the prolongation of RPV operation of the first generation of VVER-440 are considered. Among such measures the thermal annealing and regulation of radiation load of pressure vessels are reviewed. A special attention is drawn to a task of RPV embrittlement prediction in the extended operation period. SEC NRS approach to the analysis of efficiency of such measures and their influence on justification of RPV lifetime is demonstrated. The conservative approach based on safety margins concept is the key aspect in the analysis. The results of expert analysis performed in SEC NRS particularly towards the application of the first generation of VVER-440 reactors are presented. On the basis of such analyses, the priority issues, solution of which considerably increases a confidence of justification of VVER-440 and VVER-1000 RPV lifetime, are assigned. Evaluation of RPV surveillance specimen and template programs is presented. The topicality of RPV weld metal embrittlement tests of specimens, obtained from templates cut out from operated reactors and trepans of RPV of shut-down VVER (for example, Greifswald NPP) is discussed. The results of such tests are indispensable both for replenishing database with reliable experimental data, and for justifications of RPV service life prolongation.