Justification of conditions of «Leak before break» concept application for Mochovce NPP units 3, 4

Calculational-and-analytical and experimental researches towards safe operation of the main circulating pipeline (Dn500) (MCP) and surge line (Dn200) (SL) of WWER-440 reactor of Mochovce NPP units 3, 4 are carried out. The researches are done with account for the actual state of the pipelines, including their evaluation, as well as the assessment of the forthcoming operation conditions, maximum design earthquake mode and stratification mode for SL. It is shown that safe operation of MCP and SL is ensured in full scale for welds of these pipelines and for the necessary operation modes. It is determined minimum sensitivities for all MCP Qs = 6 l/min necessary for leak detection through stable through-thickness cracks in pipelines, which are considerably more than minimum permissible sensitivities accepted in international practice. It is recommended to install leakage control systems ensuring sensitivity of MCP - 6, of SL - 4.3 l/min and to use minimum two independent leakage control systems within the «Leak before break» concept framework