Thermal-hydraulic performance of confinement system of RBMK in case of accidents

摘要

Within the framework of a European Commission sponsored activity, an overall assessment of the deterministic safety technology of the 'post-Chernobyl modernized' Reactor Bolshoy Moshchnosty Kipyashiy (RBMK) has been completed. The accident analysis, limited to the area of design basis accident, constituted the key subject for the study; therefore, the notorious Chernobyl Unit 4 event was outside the scope of the investigation. The present paper deals with the evaluation of the accident performance of the confinement of the RBMK. Ignalina-2 and Smolensk-3 Nuclear Power Plants (NPP) are considered. The documented activity includes four main parts: (a) description of key features of the confinement for the two NPP; (b) identification and characterization of relevant accident scenarios part of the (DBA) area; (c) key features and qualification level of adopted computation tools primarily including codes and input decks; (d) results from the analysis of selected accident scenarios. In the first part of the paper, the layout complexity of the RBMK confinement systems, including the building on the top of the reactor cavity is pointed out together with main differences between Smolensk-3 and Ignalina-2 units. Accident scenarios suitable for the analysis, second part of the paper, are derived from recently issued International Atomic Energy Agency (IAEA) reports, from the available Safety Analysis Reports (SAR) of existing RBMK NPP and from the list of bounding accidents derived from the already mentioned EC Project. The features of Cocosys and Relap5 thermal-hydraulic system codes developed in Germany and US, respectively, are outlined in the third part of the paper, giving emphasis to the qualification level in the range of parameters of interest to RBMK confinement systems. The adopted nodalisations resembling Ignalina 2 and Smolensk 3 NPP are also described including the demonstration of the qualification level. The results, fourth part of the paper, basically show the overall RBMK confinement system robustness within the DBA area. The important role of the reactor cavity is confirmed and of the venting. The number of broken channels causing the displacement of the upper reactor cavity plate is calculated in the range 20-25.