Safety enhancement concept for NPP of new generation with VVER reactors

With the present day conditions, in order to successfully promote new NPP designs in the electric power markets, it is necessary to ensure enhanced technical/economic performances provided that international safety requirements are properly adhered to. When compared with high-powered nuclear power plants, NPP VVER-640 design (medium powered) possesses a number of advantages for the regions with undeveloped energy systems. Reduced specific energy intensity of the core adopted in this type of reactor allows to ensure the emergency cooldown of the reactor plant by passive means and to minimize the 'human factor' risk and external effects and provide sound substantiations as to how to retain corium inside RPV in case of severe accidents. At the same time, high-powered NPPs seem to be promising for regions with developed energy systems. Among such designs, NPP VVER-1000 and VVER-1500 designs are the most desirable. Configuration of new generation NPP with VVER-1500 is to be selected based on the gained experience in designing NPPs of previous generations considering the latest safety requirements and situation in the domestic and global energy markets for the time being and in the short run. Recent IAEA publications and latest EUR requirements insist that the following key safety indices should be established for new NPP designs: - aggregated frequency of core melting is 10-6 (1/year); - frequency of maximum accident release is 10-7 (1/year). To meet the aforementioned criteria, it is necessary to implement some safety assurance principles recommended by IAEA (in-depth defence, single failure, redundancy, diversity, etc.), application of deterministic and probabilistic methods for selection of safety assurance activities and means and use of reasonable combination of active and passive systems. Application of VVER-640 concept to high-powered NPPs seems to be a formidable task due to a number of reasons, namely, it is quite difficult to carry out cooldown process through a fuel pool during accidents accompanied by leaks of primary coolant. Moreover, if this is a case, no detention of corium within RPV is guaranteed during severe accidents. Anyhow, many of the useful properties of other passive systems adopted for NPP VVER-640 design may be used in high-powered NPP designs. In the course of selecting the configuration of NPP VVER-1500, it is advisable to take the latest NPP VVER-1000 designs as a basis. The design is proposed to be developed on a stage-by-stage scheme. In this context, the first stage would include the elaboration of a design of a prototype power unit on the basis of existing NPP VVER-1000 designs with the supplement of a passive containment heat removal system. Furthermore, the design shall provide for a possibility of implementing additional passive safety systems without drastic changes in the design. During further stages, it is necessary to finalize and approve a technique for assessment of probabilistic safety indices as regards inter-group common cause failures. The next step is a substantiation of implementation of additional passive safety systems and selection of their properties. Despite of the formidability of problems arising in the course of elaborating new NPP VVER-1500 designs, a holistic analysis of such designs which are under elaboration now both in Russia and abroad, as well as consideration of the latest safety requirements will create a sound basis for advanced technical approaches to safety assurance, thus allowing to reach the best safety indices in the world. (author)