R.2.31/94: LEAK BEFORE BREAK CONCEPT

To assess the application of the concept of LBB to the RBMK primary piping on the basis of analytical and experimental validation.
The main objective of the project was to accomplish a complete deterministic evaluation of leak-before-break (LBB) safety for the large (800 mm) main coolant circuit piping of a selected RBMK Reference Unit.
The project also should evaluate the LBB concept for the large (800 mm) main coolant circuit piping of the following RBMK-1000 type NPP units taking into account the different design lay-outs for these NPPs but using the verified design data for one of the units (Reference Unit) based on actual loads and condition of metal:
Of the 1st generation: Kursk NPP Units 1 & 2;
Or of the 2nd generation: Kursk NPP Units 3 & 4, Smolensk NPP Units 1 & 2;
Or of the 3rd generation: Smolensk NPP Unit 3.
The project should include a study of materials crack resistance properties, detailed stress and fracture mechanics analyses.

The project has been completely performed as specified in the Technical Terms of Reference of the project. All the deliverables specified in the TOR were prepared and the objectives of the project were accomplished.
A complete deterministic evaluation of the LBB safety concept was performed for the large diameter (800 mm) main coolant circuit (MCC) piping of the selected Reference Unit, Smolensk-3.
The LBB concept is considered applicable for this studied case. The European (German) LBB procedure was chosen as the methodology to be applied and evaluated for use in the reference case for a RBMK 1000 unit.
The methodology was used for comparison.
All the structural materials used for large diameter MCC piping fabrication are ductile at operational temperature ranges. The results compiled indicate that materials show ductile behaviour throughout the operational temperature range, and that the mechanical properties meet the requirements defined by the specifications set in the relevant Russian regulations.
Comparison of RDIPE and Siemens results on fracture resistance J-R-curves showed that the results are consistent and within a common scatter band.
Environmentally assisted cracking is considered to be very unlikely; this is confirmed by operational experience of all the RBMK plant units.
However, it would be of further interest to confirm whether stress corrosion cracking or corrosion fatigue could be possible in unexpected adverse combinations of metallurgical structure of materials, water chemistry and loading conditions.
The LBB analysis showed adequate margins for the studied component, on crack size and leak rate for the actual plant piping materials. This conclusion was supported by the results of fracture mechanics, thermal-hydraulic and leak rate analyses. The adequacy of the incorporated current margin of 2 on through-wall detectable crack length (against failure) and the current (total) margin of 10 on leak rate and leak detection capability were evaluated by considering the uncertainties. A residual margin of 1.3 remains to account for additional uncertainties in estimates of maximum load-carrying capacity of the cracked pipes, and a reserve margin of 3.1 remains to account for additional factors and uncertainties affecting leak rate. A major effort was expended on a structural validation test, the loading conditions of which were more severe than those experienced in the plant under normal operation.
The results obtained demonstrated LBB behaviour for the actual Smolensk 3 main circulation pump (MCP) piping with sufficient safety margins in relation to fatigue crack growth, crack initiation and catastrophic failure. In fact the experiment demonstrated enhanced toughness due to tearing by a factor of at least 1.5 on the initiation load.
This experiment provided additional validation information to that obtained from many (about 20) structural tests previously carried out in Russia on the 800 mm RBMK piping materials. The inspection maps provided were well documented, and automated inspection systems are used wherever possible on the MCC piping. Comparison of the Western and Russian automated defect detection and sizing capabilities showed relatively good compliance. Urgent delivery of appropriate inspection equipment for 800 mm piping in the Smolensk NPP Unit 3 and all other RBMK units was recommended. Further work should be carried out to establish the effectiveness of automated inspection systems, techniques and procedures for all other RBMK components.
For leak detection of the 800 mm pipelines and the suction header as well as the lower parts of the down comer, the following leak detection systems were recommended:
Humidity monitoring (also in combination with monitoring of the water inventory in the containment air);
Radio particulate activity monitoring and radio gas activity monitoring;
Moisture Leakage Detection System with Sensor Tube (FLUES).
Another important aspect of the project was the exchange of knowledge between the EU and Russian experts and organizations.