Answers to remaining questions on bubbler condenser - Activity Report of the OECD NEA Bubbler-Condenser Steering Group

The Russian design VVER-440 (type 213) pressurised water reactors are fitted with a pressure-suppression containment structure, called bubbler-condenser, having the function to reduce the pressure of the entire containment in case of a design basis accident (DBA), such as a loss of coolant accident (LOCA). This device consists of a tower of typically 12 floors communicating with the reactor containment building. Each floor is flooded with a pool of cold water (at room temperature) and includes gap-cap inlet openings. In the unlikely case of a LOCA, the steam from the primary circuit of the reactor and air enter the bubbler condenser tower and are forced by the gap-cap system to bubble into the cold water present at each floor of the bubbler-condenser. This causes the steam to condense, thus maintaining both temperature and pressure within containment below given limits during the entire course of a postulated design basis accident The bubbler-condenser was designed to withstand design basis accident conditions and to maintain its integrity in order to fulfil its safety function. Nevertheless, particularly for design basis accidents, detailed analyses identified the need to improve the modelling of accidents and to extend the knowledge of integral and separated effects. There was also a need to produce qualified experimental data in order to strengthen the basis for computer codes validation. During the 1990's, a number of investigations, including analyses and experiments by the utilities, as well as EU PHARE projects and related OECD NEA Expert Group activities, have been performed in order to fully ascertain the capabilities of the VVER-440/213 bubbler-condenser. These investigations consisted among others of experiments intended to simulate large break LOCA conditions and provided adequate answers to the most important issues related to the bubbler-condenser function. However, certain questions still remained open and needed further assessment. In response to a request of their safety authorities to answer the remaining questions and complete the bubbler-condenser assessment, the Hungarian, Czech and Slovak utilities took the initiative in 2001 to perform a joint experimental programme. This was to be realised at a specialised facility, the EREC facility located at Electrogorsk near Moscow, which had also been used for earlier bubbler-condenser experimental work. The utilities involved were the Hungarian Paks NPP, the Slovak Bohunice NPP and Mochovce NPP and the Czech Dukovany NPP. Parallel to the initiative for establishing this consortium, the Hungarian Safety Authority (HAEA) requested the assistance of the OECD NEA Committee on the Safety of Nuclear Installations (CSNI) for the preparatory phase of the experimental work as well as for the analyses of code calculations and experimental results. The CSNI supported the HAEA request of assistance and approved the establishment of a Bubbler-Condenser Steering Group (BC SG) to carry out given tasks according to an agreed mandate. The Steering Group consisted of a representative of each of the Czech, Hungarian and Slovak regulatory bodies and of each of the utilities involved, as well experts from the German GRS, the French IRSN, the US DOE and the EU. These organisations had all been involved with previous bubbler-condenser work. The objectives of the Steering Group were: - to produce convincing evidence that the VVER-440/V213 type bubbler-condenser works during DBAs as designed; - To help in the planning of the new EREC experiments and in the interpretation of the results; - to provide well qualified experimental results serving as basis for the validation of best estimate calculation tools. The present Project Activity Report summarises the activity and conclusions of the Steering Group, and is primarily meant for the Nuclear Regulatory Bodies of the Czech Republic, Hungary and Slovak Republic. However, it can be of interest also for the regulatory bodies in other CSNI member countries and for various institutions in the EU member states