Detailed design studies aiming at defining the optimal solution for the qualification of the LBB concept of the Primary Circuit incl. the Surge Line at the ANPP Unit 2

The LBB project "Services required for detailed design studies aiming at defining the optimal solution for the qualification of the LBB concept of the primary circuit including the surge line at the Unit 2 of ANPP" was split in two parts due to budgetary, non-technical reasons. In Part 1, "Basic LBB Concept Application with preliminary Design Files for Supports and Hydraulic Snubbers Positioning" the basic calculations regarding feasibility of LBB without postulation of any failure of snubbers was carried out for the MCL (Main Coolant Line) and SL (Surge line). The aim was at calculating the critical crack length of selected pipe line locations and evaluate whether the leak detection capacity will meet the standard requirements (3.8 l/min for a crack which is 2 times smaller than the calculated critical crack length). The first calculation round showed that the LBB request could be met for the MCL but not for the SL. Several sensitivity studies were carried out in order to iterate an acceptable solution for the LBB approach for the SL. For each step a "handbook" was written were the results were reported. Totally 7 handbooks were elaborated in Part 1. At first stiffness values given by the manufacturer of the snubbers were used in the stress analyses of the piping. In the next step the stiffness values were measured and the real values were used in the calculations. In later steps "as built" real dimensions of the pressurizer nozzles as well as of SL elbows with longitudinal welds were used instead of design values. The LBB approach could finally be achieved only after installation of 3 viscous dampers (GERB) on selected locations of the SL (handbook 7).

The overall objective of the project was to define a relevant solution, approved by ANRA, and provide the technical background to ANPP for upgrading the layout of the main coolant lines, including the surge line, as well as the inspection and surveillance practices, in order to ensure that those piping systems are compliant with the criteria for LBB on the basis of currently available international methodology and practice.

The results of this project showed that the LBB approach could be met for the MCL without any major plant modifications. For the SL the LBB approach could be achieved only after installation of 3 viscous dampers in the SL.