LEAK BEFORE BREAK

The objective of the project was to study whether the LBB concept can be applied to the piping system of VVER 440/230 and to assess the possibility to achieve, under certain conditions, a high reliability level. More precisely, it was analysed:

1. Whether a crack likely to be encountered will not reach a critical length and depth during the anticipated lifetime of the plant;
2. Whether a hypothetical through-wall crack will leak at a sufficient rate to ensure its detection before it can grow to a critical size at which major failure could occur.

Methodological steps taken:

• The first stage of the project was analysing and calculating whether the primary circuit of the VVER-230 for Kola and Novovoronezh meet the LBB application requirements.
• The second stage to determine the requirements for In-Service Inspection.
• The third one the Leak Detection System.
• The methodologies of EU-countries have been studied with the Russian project partner.

German: Accustomed, conservative methods (limit load and flow stress concept based, if possible, on available plant specific data (tensile and charpy values).
These methods are used by Siemens for screening and sensivity analysis.

French: More sophisticated elasto-plastic fracture mechanics methods (J-R-curve approach etc. according to NUREG 1061) based, if possible, on available plant specific data. If J-R-curves were not available for the plant specific piping, generic and mainly not plant specific data or data from the chemical composition specification were used.

These (German and French) procedures were adopted also for the EPR. Both methods are verified by tests. The German method is the basis for “Break Preclusion” (LBB) for all German PWR and BWR. The French method is and was the basis of LBB for all French LBB-studies.

Working procedure:

Collection, processing, presentation of initial data, experimental work, calculation of earthquake stresses (Safe Shut down Earthquake=SSE), proving the absence of corrosion-erosion damage, analysis of workshop and in-service-inspection with evaluation of defect-detectability.

Result Analysis 1: LBB is justified for Kola 1+2. Too high SSE-stresses occur at Novovoronezh 3+4 to justify LBB at Normal Operating Conditions (NOC) and SSE without seismic supports;
Result Analysis 2: With hypothetical supports, in case of NOC+SSE, LBB justified except for SL;
Result Analysis 3: With optimised supports, LBB justified for MCWL and SL at NOC+SSE.

Recommendation for remediation:

• NPP Kola 1+2: Results allow LBB;
• NPP NV3+4: Realization of LBB in 2 steps:
  1. Step 1: Integrity for NOC, as a leaking crack can be detected before it reaches the critical size.
  2. Step 2: LBB for NOC+SSE: Analysis of seismicity, seismic stresses and/or implementation of additional seismic supports.