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SAFETY ASSURANCE APPROACH TO SEVERE ACCIDENTS AND SEVERE ACCIDENT MANAGEMENT AT NUCLEAR POWER PLANTS WITH WWERDOI: 10.1615/ICHMT.1995.RadTransfProcHeatMassTransfSevNuclReactAcc.40 V. A. Sidorenko SinopsisDuring 30 years passed since the startup of the first WWER reactor in Novovoronezh, 55 WWERs have been put into operation at 17 NPPs sited on the ex-USSR territory, in the East European countries and Finland. As of 01.01..95, the WWERs of different designs have the total of reactor years exceeding 600. Beginning from the first WWER a great importance was attached to elimination of phenomena and events that could cause a severe accident or aggravate development of an accident making it eventually a severe one. The first NPPs with the WWERs were built in the regions with the least possible external impacts (in non-seismic regions, far away from the aviation air corridors, harmful production, etc.). In designing the WWERs the materials and structures resistant to rapid destruction were used. Potentially harmful phenomena such as reactor vessel embrittlement, initiation of cracks in the SG headers etc., were under continuous observation during the operation. As the experience showed, this permitted timely measures to be taken against propagation of failures in severe accidents. The WWER equipment characteristics (secondary water inventory in the SG, pressurizer capacity etc.) were chosen so that the accident would slowly develop, and the operational personnel could take effective measures for its mitigation. An example of successful employment of the WWER capabilities by the personnel is the minimisation of the consequences of a fire at the Armenian NPP resulting in complete loss of the SG feedwater. A serious impetus to redouble efforts at investigating severe accidents was the Three Mile Island and Chernobyl accidents. The normative documentation adopted in Russia in the late 80s stipulates the proof of the low probability of severe accidents in the designs. For operating NPPs special guides and instructions, determining the personnel's actions during the beyond-design accidents, were worked out. The measures for increasing their safety stipulate the modernisation of the old NPPs and introduction of new systems and equipment for the beyond-design accidents management. The efficiency of these measures is substantiated by the use of the PSA methods. The designs of new power units with LWR envisage special measures both for the elimination of severe accidents and mitigation of their consequences. In the design of the NPP with WWER-640, which continue the traditional WWER line, passive systems are used for the long-term core cooldown during the accident. In spite of the low core melting probability measures are taken to keep the corium within the reactor vessel or at least inside the containment. The WPBER-600 design extends the line of safety enhancement, which has been realised in the design of the Soviet nuclear district heating plant. The integral layout of the NSSS equipment, low heat loads, second (guard) vessel make the core melting practically impossible. However, just as in the WWER-640 design, the containment is provided with an additional facility for catching the melted core. The Russian investigations of processes occurring in the severe accidents and measures for their management are currently closely connected with the international efforts in this field. One of the international severe accident projects directed to the confirmation of the possibility of corium retention inside the reactor vessel (RASPLAV) is being carried out on the basis of the experimental complex now under construction at the RRC “Kurchatov Institute”. The close connection of the new engineering approaches undertaken for LWR safety enhancement in each nuclear country with the international program of investigations permits the desired results to be reached in an optimal way. |
