Report of a consultants' meeting on containment and confinement performance in NPPS with WWER 440/213 and 440/230 reactors

The reactors WWER 440 model V-230 being operated in several countries of central and eastern Europe were designed in the 1960-ies assuming that the design basis accident can be limited to a break of a pipe of 100 mm diameter fitted with an orifice of 32 mm diameter. This assumption presently judged to be inadequate according to the international safety requirements, was used to determine the Emergency Core Cooling System. For this size of the break, the spray system of the reactor building was designed to prevent uncontrolled releases to the environment. For larger breaks, till full rupture of one of two pressurizer surge lines of 200 mm diameter each, large flaps were designed to open during the initial pressure peak after the accident and release to the environment large amounts of steam escaping from the broken reactor coolant system (RCS). In view of this initial release of radioactive medium and due to its other unsatisfactory features, the building surrounding the WWER 440/230 reactor is not called a containment, but is said to fulfill a confining function, in short it is termed a confinement. The shortcomings of the WWER 440/230 confinements have been long recognized and the newer model of WWER 440 units, called model V-213, has been provided with an improved building of higher leaktightness and higher strength, containing a complicated system of water trays and air traps designed to condense steam and remove a part of the air from the reactor compartments after large break loss of coolant accident (LB LOCA). For such case, the system provides fast pressure decrease in the containment to subatmospheric values and thus prevents any significant releases of radioactivity to the environment. As the main process is that of steam bubble condensation in water, the system is called bubbler condenser containment. The bubbler condenser containment was the answer to the problem of providing the old WWER 440 model 230 units with a reliable containment without excessive changes in design of original hermetic compartments and without significantly increasing the maximum pressures and temperatures possible inside the containment, which would mean the necessity of exchanging a significant part of reactor equipment unable to stand high environmental demands typical e.g. for large dry containments. However, although the bubbler condenser containment could have been a good solution for the newly built units, it was rather difficult to adapt it to the old WWER 440 V-230 reactors already in operation.Therefore the WWER 440/230 units continued to be operated with previously built confinements, even though the contemporary safety rules clearly indicated inadequacy of such designs. The IAEA has been studying the safety of WWER 440/230 reactors for more than 3 years. In February 1992 the ranking of safety issues identified in the Extrabudgetary Programme on WWER-440 model 230 NPPs was published, and in May 1992 the major findings of the Programme were summarized. The issues related to radioactive materials confinement in case of LOCA were ranked as being of high safety concern, and those related to ECCS redundancy and physical separation of highest safety concern. The main safety issue was connected with breaks larger than 32 mm, which in WWER 440 model 213 reactors were considered as being within DBA limits, but in WWER 440/230 units were equivalent to Beyond DBA events. Recognizing the importance of improvement of the final barrier against fission product release in WWER 440/230 reactors and the necessity to upgrade it so that it could withstand breaks larger than 32 mm without uncontrolled releases to environment, the IAEA took the initiative of preparing a report on WWER 440/230 units confinement status and possible improvement, and obtained the assistance of the Swiss Government in financing the work which was eventually done by Elektrowatt Engineering Services Ltd. The report 'Technical Basis for Improving the Confinement Function in WWER 440/230 NPPs' provided a review of containment design requirements and containment designs in various countries and a detailed analysis of various measures possible to be implemented in WWER 440/230 confinements with the view to upgrade them for accident conditions. This report was provided to all participants of the present Consultants Meeting as the basis for further discussions.