Y. A. Kurakov

Minatom RF, Moscow, Russian Federation

Y. G. Dragunov, A. K. Podshibiakin, N. S. Fil,

S. A. Logvinov, Y. K. Sitnik

OKB Gidropress, Podolsk, Russian Federation

V. M. Berkovich, G. S. Taranov

Atomenergoproject, Moscow, Russian Federation

Abstract. Elaboration and introduction of NPP designs with improved technical and economic parameters are defined as an important element of the National Program of nuclear power development approved by the Russian Federation Government in 1998. This Program considers the designs of WWER-1000/V-392 and WWER — 640/V-407 power units as the priority projects of the new generation NPPs with increased safety. A number of passive systems based on natural circulation phenomena are used in V-392 and V-407 designs to prevent or mitigate severe accidents. Design basis, configuration and effect of some naturally driven systems of V-392 design sited at Novovoronezh are mainly reflected in the present paper. One of the most important mean for severe accident prevention in V-392 design is so called SPOT — passive heat removal system designed to remove core decay heat in case of station blackout (including failure of all diesel generators). This system extracts the steam from the steam generator, condenses it and returns water to steam generator by natural circulation. The SPOT heat exchangers are cooled by atmospheric air coming by natural circulation through a special direct action control gates which operate passively as well. Extensive experimental investigation of the different aspects of this system operation has been carried out to validate its functioning under real plant conditions. In particular, full-scale section of air heat exchanger-condenser has been tested with natural circulation steam, condensate and air paths modeled. The environment air temperature and steam pressure condensing were varied in the wide range, and the relevant experimental results are being discussed in this paper. The effect of wind velocity and direction to the containment is also checked by the experiments.

1. INTRODUCTION

The Program of Russian Federation nuclear power development for 1998-2005 years and for the period till 2010 (approved by Russian Federation Government Resolution No. 815 dated July 21, 1998) has defined that the elaboration and implementation of new generation NPP designs with enhanced safety is the necessary factor of nuclear power extension in Russia. The new generation NPP projects shall meet up-to-date national and international requirements and envisage: (1) the probability of limiting release and serious core damage at beyond-design accidents less than 10 and 10 per reactor-year, respectively; (2) reduction of urgent evacuation area to 300-500 meters and emergency planning area to protect the population in case of beyond-design accidents to 700-3000 meters.

The safety of new NPP is provided by consistently implementing the defense-in-depth principle, based on the application of a system of barriers in the way of ionizing radiation and radioactive substance release into the environment, and also by realizing the engineering and organizational activities to protect these barriers. The National Program of nuclear power development considers the design of 1000 MW power unit with WWER-1000/V-392 reactor to be the priority project of new generation plants. This unit is so designed that radiation effect on the population and the environment is considerably below the allowable values established by the up-to-date regulatory documentation. The permit of Russian Federal Nuclear and Radiation Safety Authority (Gosatomnadzor) was granted to construct the power units with V — 407 reactor plant on the Sosnovy Bor and Kola sites and two units with V-392 reactor plant on the Novovoronezh site.

Extensive application of passive safety means, using natural physical processes, along with the traditional active systems is a specific feature of both these designs. The IAEA Conference on “The Safety of Nuclear Power: Strategies for the Future” [1] has noted that the use of passive safety features is a desirable method of achieving simplification and increasing the reliability of the performance of essential safety functions, and should be used wherever appropriate. However, the application of passive means is connected with some problems, which have to be solved by each plant designer. The passive systems have their own advantages and drawbacks in comparison with the active systems both in the area of plant safety and economics. Therefore a reasonable balance of active systems and new passive means is adopted in V-392 design to improve safety and public acceptability of nuclear energy.

One important problem related to the implementation of the passive means is that, in the most cases, sufficient operating experience of the passive systems/components under real plant conditions does not exist. Besides, the existing computer codes for transient and accident analysis are not sufficiently validated for the conditions and phenomena which are relevant to the passive system functioning (low pressure, low driving pressure and temperature heads, increased effect of non-condensable, boron transport at low velocities, and the like). As a result, the time — and money-consuming research and development works may be needed individually for each reactor concept to validate the operability of the passive safety means proposed in the design. Therefore, the extensive experimental investigations and tests have been already performed and are being planned to substantiate the design of the safety features proposed for new units with WWER-1000/V-392 and WWER-640/V-407 reactor plants.