R2.09/96 LBB Applicability

VVER-1000 (V-320) were designed at the end of the seventies and met the regulatory documents and operational safety requirements valid at that time. As the most severe design basis accident postulated breaks of pipelines up to double ended guillotine breaks of the main coolant lines (MCL) were assumed. Safety systems were designed to withstand all design basis accidents.

To protect the systems against mechanical loads due to breaks of pipelines (reactive and active forces on equipment, pipelines and civil structures under coolant blow-down, emergency pipe-whip restraints were installed on the main coolant lines.

Improvement of the safety level requires application of new approaches: assessment of possible pipeline breaks, assessment of consequences of breaks and measures on protection against breaks. One of the most promising is the application of the leak-before-break (LBB) concept.

The following general summary conclusions can be drawn:

• The "reference" methodology for primary MCP/SL piping systems (with cladding) was developed and fully defined in a comprehensive document;
• The verification of the possibility to apply the LBB approach to the MCP/SL according to the defined "reference" methodology was successfully achieved and the corresponding results were provided;
• The whole set of necessary MCP/SL materials properties, loading conditions, piping stresses and validation files, supplemented also with the Material Testing Programme experimental results, are fully provided;
• The evaluation of the effectiveness and the reliability of the Leak Detection System (LDS), installed in Balakovo NPP Unit 2, taking into account the In-Service Inspection (ISI) programs also, were thoroughly performed and the results are described.

The assessment has demonstrated that the presently installed LDS cannot be used for LBB concept application and the piping metal ISI system does not meet the LBB requirements to a full extent;

• A list of recommendations to improve the Leak Detection (and ISI) systems has been provided. Among the activities performed the new leak monitoring systems, presently under development for the new design of VVER-1000 NPP’s, have been assessed with positive results; suggestions and plans to implement the recommended improvements have been described;
• The transfer of Western experience in the application of the LBB concept from the Consortium to the Local Sub-Contractor was continuously performed during the Project development and the implementation in Spanish NPP’s explained and described in detail during the performance of the project.

The Western computer programs, dealing with determination of elastic-plastic critical and detectable crack lengths, have been transferred to OKB Gidropress and the corresponding Russian codes benchmarked.