Full Project Title:
Services required for detailed design studies aiming at defining the optimal solution for the qualification of the LBB (Leak Before Break) concept of the primary circuit including the surge line at the Unit 2 of ANPP.

Part 2: Final LBB Concept Application with Complete Design on Supports Positioning and Anchoring to Civil Structures; Solution of Regulatory Requirements related to Hydraulic Snubbers Failures. Recommendations on ISI Programme, the LDS (Leak Detection System) and NPP Operator Instruction be elaborated.

Objectives

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 objectives of part 2 of the project were:

• To analyse the integrity of the support system for the MCL (Main Coolant Line) and SL (Surge Line) by assuming 15 different cases of failure of one snubber in SSE (Safety Shut Down Earthquake). In all cases the installation of 3 additional viscous dumpers (GERB 40) in the SL (as from Part 1) be taken into account. Furthermore to carry out LBB calculations for the MCL and SL under similar snubbers failure conditions assuming installation of the GERBs.
• To confirm stability of heavy primary components support structures (RPV, SG, pressurizer, main circulation pump, main gate valve etc) during LOCA and SSE.
• Justification of upgrading of ISI (In-service Inspection) procedures for MCL and SL weld.
• Review of the existing LDS monitoring at Medzamor unit 2, evaluate its compatibility with normative LBB requests and provide recommendations on application.
• Detailed FEM integrity calculations of the longitudinal welds of DN 500 and DN 200 elbows.

Project Description

• The most important issue of this project was to confirm that the LBB criteria can be met under the assumptions that one hydraulic snubber fails. In this project the failure one hydraulic snubber in 15 different locations of one loop were analysed. In all cases the mounting of 3 viscous dampers (GERB 40 type) in the SL was assumed as given in Handbook nr. 7 of project part 1. The aim of the snubber failure calculations were to assess whether all the other snubbers in the loop remain functioning and accordingly the LBB criteria of the piping can still be met. The forces acting on each hydraulic damper was calculated and it was confirmed that the allowable forces were not exceeded in any single case. Forces acting on the GERBS were also analysed in a similar way. The probability of failure of some snubbers with the lowest safety margins were calculated during overloading. It was shown that the probability of snubbers failure can be rather high, especially for the 20 tons snubbers.
• LBB calculations were carried out for circumferential cracks in MCL and SL, assuming failure of one hydraulic snubber each time in 15 different locations and assuming the installation of 3 GERB 40 viscous dampers in the SL. The calculations showed that the LBB criteria could be fulfilled in each case with sufficient safety margins.
• The stability of the supports of a.m. heavy components (RPV, SG, pressurizer, main circulation pump, main gate valve etc) during SSE and LOCA conditions was analysed as given above. In all cases it was confirmed that the supports are sustainable and no overloading with subsequent movement of components would occur.
• The fatigue crack growth of a postulated flaw of 20% of the wall thickness was carried out according to ASME Code Section XI. In these calculations the design number of transients as well as thermal stratification was taken into account. The temperature difference in stratification loading was assumed at 60 oC. The results of the calculations showed that the maximum end of life crack would be 21,31% of the wall thickness. This can be considered rather insignificant and acceptable.
• In addition to the above LBB evaluation the integrity of some selected longitudinal welds in MCL and SL elbows (NOC+SSE condition) was analysed with more detailed FEM based analyses (SYSTYS, PICEP). The results of the FEM calculations showed that LBB criteria were fulfilled in all cases as was expected.
• In the LDS review the following findings were reported:

o RMS (Radiation Monitoring System), HMS (humidity Monitoring System) and AMS (Acustic Monitoring System) are currently installed in Medzamore 2.
o The RMS is good for detection and monitoring of leakage in the MCL and SL, but not good enough for localisation of the leakage as requested
o The HMS is good for detection but not good enough for localisation.
o The AMS has been installed since 1995 but has not been upgraded. At present it does not fulfil LBB/LDS requirements. Accordingly it is necessary to verify the status of existing sensors and cables and replace as necessary.
o Operator instructions for LDS operation were also provided as preliminary users manual

• The Consultant NRI Rez has been in activities related to ISI in Medzamor 1 since the year 2000. Accordingly they had a good understanding of the status in this issue at the plant. In this project a description of the state of the art as well as the future plans were reported. The main areas covered were inspection areas, Inspection procedures, description of UT (ultrasonic testing), mechanised UT procedures and qualification as well as implementation methods were elaborated and reported.

Project Results

The results of the project showed that the LBB approach could be met for the MCL without any plant modifications. For the SL the LBB approach could be achieved only after installation of 3 viscous dampers in the SL. It was confirmed that the LBB criteria could also be fulfilled when assuming failure of one hydraulic snubber for 15 different selected locations, provided that the 3 viscous GERB dampers are installed. It was also confirmed that remaining snubbers and the GERBs will sustain the loading in these cases. Furthermore the results showed that the support structures of the main primary heavy components will sustain loading imposed in LOCA and SSE events and thus the components will remain stable. Fatigue crack calculations showed that crack growth in the MCL and SL during the design lifetime will be limited. The results of more detailed FEM calculations showed that the integrity of MCL and SL elbow longitudinal welds can be confirmed during NOC and SSE. The LDS review showed that the LDS criteria can not be fulfilled according to LBB requests and an upgrading of the systems is needed. Finally the status, plans, needs and recommendations regarding ISI at Medzamor 2 were reported.