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This project aims to establish a reference methodology for Leak Before Break (LBB) assessment in VVER 1000/320 units, with a focus on the Main Coolant Line (MCL) and the pressurizer Surge Line (SL). It includes the evaluation of diagnostic systems, improvement recommendations, and preclusion of water hammer, erosion corrosion, stress corrosion, and thermal stratification. The project also involves fracture mechanics calculations, materials testing, and LBB assessment using the results. The implementation and results are discussed in detail.
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Tacis R2.09/96, “LBB Application Review and Basic Implementation engineering for VVER 1000 Model 320 NPP” • Beneficiary: Rosenergoatom, Moscow • Consortium: Ansaldo and Empresarios, Agrupados • Local Subcontractor: Moht, Moscow • Budget: 1 M€ • Duration: 26 months • Contract expired: June 2000
Objectives of the project • Establish a reference “Leak Before Break” (LBB) assessment for the Main Coolant Line (MCL) and the pressurizer Surge Line (SL) of the VVER 1000/320 units • Definition of a reference methodology, including optimized fracture mechanics methods for cladded piping systems • Preparation of necessary materials data and validation files • Evaluation of the effectiveness of the diagnostic system, in particular of the ISI programs • Listing of recommendations for: • improvement to the existing information on diagnostic systems • ways of implementing recommended improvements
Rationale of LBB • The approach of the pipe break evaluation is aiming at demonstrating that for the piping satisfying the LBB criteria, a sudden Double Ended Guillotine Break (DEGB) and a subsequent catastrophic rupture of the piping is not a credible event • To demonstrate that a postulated surface crack does not grow by fatigue to a Through Wall Crack • The Leak Detection System (LDS) capability at the plant must be able to detect a leakage which is 10 times smaller than the leakage through a “reference leakage crack”. The critical crack size for a TWC must be 2 times larger than the reference leakage crack. • According to LBB standards such as US NRC SRP 3.6.3, Nureg 1061 etc. the LDS at a NPP must detect a leak of 3.8 l/min. The LDS must be even better if the above safety margin on 10 cannot be met. • Simultaneously demonstrate preclusion of water hammer, erosion corrosion, stress corrosion or thermal stratification • When a piping fulfils above LBB criteria emergency pipe whip restraints are not needed in order to provide access for ISI
Project implementation and results • Task 2; Preliminary LBB Analyses by using Russian approach • Collection of manufacturing and material data as well as the “as-built” drawings (Fig.) of the MCP and SL of the selected NPP Balakovo Unit 2 • Static and dynamic stress analyses in order to identify highly stressed and “weak” spots • Earthquake (SSE) and normal operation condition (NOC) stresses were evaluated • Critical TWC and under SSE and detectable flow rate estimates under NOC were established for all relevant regions • For MCP the LBB criteria and margins are fulfilled for LDS sensitivity 3.8 l/min • For the SL the required LDS sensitivity is 1.9 l/min in order to comply with the margin of 10 regarding leakage flow rate
Main Coolant Piping of VVER 1000/320 Pressurizer and SL
Task 3.1; Fracture mechanics (FM) calculations using different approaches • The objective was to complement the above preliminary LBB analyses for the most limiting regions by using western and new Russian Guidelines • NRCPIPE and SQUIRT (western), FRACTURE and CRACK-L (Russian) computer programs were used and compared • Data from international benchmark studies were used for the Crack Opening Area (COA), flow rate and critical TWC calculations • The calculations showed a good agreement between the western and Russian computer programs
Task 3.2; Formulation of a Materials Testing Program (MTP) • The aim of the MTP was to elaborate fracture toughness data for more advanced quantitative FM calculations (J-R curves and true stress strain curves) • The materials testing was carried out by ZNIITMASH, OKB Gidropress. In addition test results on the same material carried out by ZKTI on large scale tests was also provided • Base material of mark10GNMFA, weld PT-30 and EA 400/10T as well as bi-metal weld and HAZ was investigated • Compact Tension specimens of 1” thickness was qualified for LBB evaluations (results compatible with large scale tests) • Test results obtained on CT1 specimens showed considerable scattering; therefore lower bound envelope curves were used for elastic-plastic analyses • Based on the results of Tasks 3.1 and 3.2 a preliminary reference methodology for LBB application was elaborated (Task 3.3)
J-R curves for CT1 test specimens Fracture surfaces of CT1 test specimens
Task 4LBB assessment using the results from Task 3 • Task 4.1 Upgrading data collection • The experimental results from the MTP both for MCL and SL base metal, weld and HAZ compiled and compared to plant specific data of Task 2.1 • A more detailed stress analyses was made. The results showed that the criteria for static and seismic strength for all selected highly stressed and weak points were met • Fatigue cyclic strength calculations were carried out to compare designed and realized cumulative usage factor. The actually realized usage factor was systematically much lower than design • The exclusion factors (stratification, IASCC, water-hammer etc.) were analysed more in detail. It was confirmed that the piping is not sensitive to those phenomena
Task 4LBB assessment using the results from Task 3 • Task 4.2 and 4.3 LBB analyses and Reference Methodology • A final LBB analyses was carried out utilising the proposed methodology and updated material data from Task 3 • The analyses showed that for MCP the required crack length margins for TWC are met with the LDS sensitivity being 3,8 l/min • The medium crack length for a leakage of 38 l/min was 224 mm while the medium for critical crack length was 1195 mm (Fig.) • For SL the required margin are met only with the use of LDS sensitivity of 1,9 l/min • In the sub-task 4.3 the preliminary reference methodology in Task 3.3 was assessed in order to analyse applicability for Balakovo Unit 2. It was confirmed that the preliminary proposal did not need modifications.
Task 5Evaluation of existing LDS and ISI systems at Balakovo Unit 2 • The aim of this task was to analyze and evaluate the LDS and ISI systems in Balakovo Unit 2 • Furthermore the aim was to provide recommendations for improving and upgrading the LDS and ISI • The review of the LDS and ISI at Balakovo 2 showed that they do not fully meet the LBB approach • For new designs, however, new leak monitoring and inspection systems are being developed, which meet the LBB requirements on LDS and ISI • The new systems can be used in the VVER 1000 NPPs after trial, testing and validation • Regarding the ISI at Balakovo 2 it was recommended to use automated UT inspection of pipeline together with available inspection methods at site
Project goal and conclusions • A reference methodology for primary MCP/SL was developed as a handbook for LBB application in VVER 1000 NPPs • The “Handbook” was successfully applied for the pilot plant Balakovo Unit 2 and it can be applied to any VVER 1000 NPP • Most of the input data obtained, e.g. relevant material toughness properties, test results, water hammer, evaluation and thermal stratification can be applied to any other similar NPP • In order to fully meet the LBB criteria the LDS must be improved in Balakovo Unit 2. The ISI should also be improved by utilising automated UT method as a complement to the existing methods • The LDS in SL shall be capable to detect leaks of 1.9 l/min in order to fulfil the LBB criteria
