Warm prestressing is widely acknowledged as being able to enhance material toughness, especially in steels that exhibit lower shelf cleavage fracture. The enhancement in toughness has a significant impact on the integrity of pressure vessels, particularly during severe loading conditions, such as pressurised thermal shock. In this paper, we undertake detailed statistical analyses of experimental data provided via a comprehensive programme of fracture tests at UJV (Ústav jaderného výzkumu Řež a.s.). A warm prestressing model, developed by Chell, is used to predict the change in toughness probabilistically, using Monte-Carlo methods to predict the distribution in toughness following different warm prestressing cycles. The results obtained from this model are also compared to predictions made by the Wallin approach. Experimental data was generated, at UJV for WWER 440 RPV steel, using small single-edge-notched bend SEN(B) specimens (or pre-cracked Charpy) across a range of different fracture temperatures, warm pre-stress temperatures, and levels of preload, in both as-received and irradiated conditions. In this paper, experimental data obtained only from tests on unirradiated specimens were statistically treated. A three parameter Weibull distribution was used to map the scatter observed in the virgin toughness. The statistical significance of increase in apparent fracture toughness due to warm prestressing was evaluated using the Mann-Whitney test. It was further shown by Monte-Carlo simulations that the Chell and Wallin models provide slightly conservative predictions of the resulting fracture toughness. Both, the experimentally measured and predicted values of the resulting fracture toughness, depend on the specific tests conditions, especially on the level of preload.
Rector pressure vessel (RPV) is a key component of all PWR and VVER nuclear power plants (NPPs). Assuring its integrity is therefore of high importance. Due to high neutron fluence the RPV material is embrittled during NPP operation. The embrittled RPV may undergo severe loading during potential events of the type of pressurised thermal shock (PTS), possibly occurring in the NPP. The resistance of RPV against fast fracture has to be proven by comprehensive analyses. In most countries (with exception of the USA), proving RPV integrity is based on the deterministic PTS assessment. In the USA, the "screening criteria" for maximum allowable embrittlement of RPV material, which form part of the USA regulations, are based on the probabilistic PTS assessment. In other countries, probabilistic PTS assessment is performed only at research level or as supplementary to the deterministic PTS assessment for individual RPVs. In this paper, description of complete probabilistic PTS assessment for a VVER 1000 RPV is presented, in particular, both the methodology and the results are attached. The methodology corresponds to the Unified Procedure for Lifetime Assessment of Components and Piping in WWER NPPs, "VERLIFE", Version 2008. The main parameters entering the analysis, which are treated as statistical distributions, are as follows:-initial value of material reference temperature T 0 ,-reference temperature shift ∆T 0 due to neutron fluence,-neutron fluence,-size, shape, position and density of cracks in the RPV wall,-fracture toughness of RPV material (Master Curve concept is used). The first step of the analysis consists in selection of sequences potentially leading to PTS, their grouping, establishing their frequencies, and selecting of representative scenarios within all groups. Modified PSA model is used for this purpose. The second step consists in thermal hydraulic analyses of the representative scenarios, with the goal to prepare input data for the structural analyses (pressure and temperature variations in the reactor downcomer). The third step consists in performing structural analyses, covering (deterministic) temperature and stress fields calculations for all representative scenarios, and finally, probabilistic fracture mechanics analyses. The results of the third step analyses are conditional probabilities of initiation of fast fracture for all selected representative scenarios. Combining them with frequencies of all groups, the final value of (unconditional) frequency of initiation of fast fracture of the RPV is established. In this paper, examples of both input data and the results are presented.
In this paper the project DEFI-PROSAFE is presented. In the frame of Nugenia+ project, a work package was dedicated to the “DEFInition of reference case studies for harmonized PRObabilistic evaluation of SAFEty margins in integrity assessment for long-term operation of reactor pressure vessel” (acronym DEFI-PROSAFE). A methodology is proposed to assess safety margins in RPV integrity accounting for uncertainties propagation, because no commonly accepted European approach exists for probabilistic assessment of RPV fast fracture risk. The DEFI-PROSAFE methodology, which is based on the comparison between deterministic and probabilistic assessment, will be detailed. The experience gained from the US Screening Criteria (NUREG-1806) and past projects (ICAS, PROSIR), and guideline (IAEA TecDoc 1627) as well as aspects specific to European deterministic integrity approach have been considered. Usually for probabilistic fracture assessment of the RPV, the parameters (describing flaws, material and neutron fluence) are sampled and the RPV cylindrical region is assessed using deterministic thermal hydraulic loading for evaluation of initiation or failure conditional probability. Within DEFI-PROSAFE methodology the uncertainties in thermal-hydraulic input parameters are taken into account and their propagation in the structural assessment is considered. Comparison of RELAP5 thermal hydraulic results and mixing calculation results (KWU-MIX) with experiment relevant to PTS assessment was performed. Furthermore the DEFI-PROSAFE methodology considers RPV discontinuity regions (like RPV nozzle) and specific PIRT analysis has been performed for selection of the TH-parameters. A new benchmark for probabilistic assessment of RPV was defined within the DEFI-PROSAFE project. The benchmark definition and case studies based on previous R&D project (ICAS, PROSIR) will be presented. The performance of the new benchmark may be submitted as new European project.
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