Beneficiary Organisation Details

Kozloduy NPP:
http://www.kznpp.org

Project Aims

The goal of the investigation was to assess the applicability of leak-before-break (LBB) concept for primary circuit and to show that the leakage detection system ALUS, installed at Kozloduy NPP ensures that leakages can be detected long before critical defect sizes are reached.

Project Results

LBB-analysis was performed for Kozloduy NPP unit 2. The results were considered to be applicable to the other units only in case the data concerning materials, stresses and defects were comparable, which had to be a subject of special further evaluations.
The results of the fracture mechanics analyses performed were as follows:

• The propagation of the assumed detectable defects by NDE was found to be negligibly small for one service cycle;
• There were many service cycles necessary to reach the critical crack depth;
• The resulting stable leaks propagated very slowly. There were several cycles necessary to reach the critical through-wall crack lengths.

The results of the leak rate calculations together with the assessment of the sensitiveness of installed leakage detection system ALUS were as follows:

• For normal operation conditions the ALUS ensures that leakages in the main coolant line (MCL) and steam line (SL) can be detected long before critical dimensions are reached;
• For normal operation + save shut down earthquake( SSE) loads conditions this holds for the MCL, too. For the SL backfitting of the support structure is mandatory, in order to lower stresses in such a way, that LBB can be applicable and the ALUS is able to work effectively.

The following conclusions were drawn with respect to LBB-behaviour of MCL and SL:

• LBB-concept was applicable and could be demonstrated for the main pipework for normal operation. The sensitiveness of the leak detection system ALUS ensured that the stable leaks could be detected;
• Concerning the additional loading case SSE this conclusion holdedfor the MCL, too. The critical through-wall crack lengths decreased only by a small amount when taking into account normal operation + SSE.

Further Project Results

Further information on the project results could be sought from the beneficiary organisations.