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Advanced PTS Analysis for LTO

Periodic Reporting for period 1 - APAL (Advanced PTS Analysis for LTO)

Période du rapport: 2020-10-01 au 2022-03-31

In the EU, most of the nuclear power plants (NPPs) are currently in the second half of their designed lifetime, making lifetime extension an important aspect for countries planning to continue producing nuclear energy in the long term. To verify safe operation of existing NPPs going through long-term operation (LTO) upgrades, advanced methods and improvements are necessary. In addition, evolution of safety requirements concerning LTO upgrades will result in more detailed quantification of implicit and explicit safety margins including determination of risk of reactor pressure vessel (RPV) failure.
One of the most limiting safety assessments for LTO is the RPV integrity analysis of pressurized thermal shock (PTS) to prevent severe accident. The PTS is characterized by rapid cooling (i.e. thermal shock) of the reactor downcomer and internal RPV surface, followed sometimes by re-pressurization of the RPV. Thus, the PTS event poses a potentially significant challenge to the structural integrity of RPV in pressurized-water reactors (PWRs) and water-cooled water-moderated energy reactors (WWERs).
Currently in the EU, PTS analyses are based on deterministic assessments and conservative boundary conditions. This type of PTS analyses is reaching their limits in demonstrating the safety of PWRs and WWERs facing LTO and they need to be enhanced. However, inherent safety margins exist, and several LTO improvements applicable to the NPP, as well as advanced methods of PTS analyses, may be able to increase these safety margins. Additionally, the quantification of the safety margins in terms of risk of RPV failure by advanced probabilistic assessments becomes crucial, because the probabilistic methods ensure more comprehensive assessments in PTS analysis and they enable the quantification of uncertainties of the results.
The main objectives of the APAL project are to develop advanced probabilistic PTS assessment method, to quantify safety margins for LTO improvements and development of best-practice guidance. The project will address multidisciplinary and multi-physics challenges related to RPV safety assessment of PTS mitigation.
Within WP1 “LTO improvements relevant for PTS analysis” an extensive literature review and collection of experience to identify the state-of-the-art of LTO improvements (hardware and software) with potential beneficial or adverse impact on the results of PTS analysis was performed.
Four LTO improvements have been defined to be investigated in more detail:
- Residual stress distributions for welds (WRS) and cladding
- Warm pre-stress (WPS) approach applied in PTS
- Thermal-hydraulic (TH) analysis (including definition of human factor)
- Probabilistic PTS analysis
Task 1.5 was dedicated to the investigation of further potential LTO improvements relevant for PTS analysis.
The state-of-the-art reviews include:
- Collection of existing solutions/approaches for assessment of LTO improvements.
- Collection of existing assessments.
- Identification of gaps and possible improvements. The process was carried out through the preparation of technical questionnaires that were completed by partners according to their experience. Based on the compilation of answers and discussions, the state-of-the-art of the investigated LTO improvements have been summarized.

Within WP2 “Improved TH analysis”, the work on “Quantification of impact of LTO improvements and human factor on TH analysis boundary conditions” was almost finished within the 1st reporting period. The Deliverable D2.1 was submitted at the beginning of 2nd reporting period. TH analyses for the base case (SBLOCA with 50 cm2 break in hot leg and with loss of offsite power) and for the six selected LTO improvements were performed with different computer codes. Further, the effect of three selected human interactions was evaluated with the help of TH simulations. TH data sets representing selected LTO improvements or human interactions were delivered to WP3 and WP4 for deterministic and probabilistic fracture-mechanical analyses. System and mixing codes used in the simulations were RELAP5, ATHLET, TRACE, KWU-MIX, GRS-MIX and ECC-MIX. The applicability of the CFD codes (Fluent) for the selected case was tested too.
The work on the determination of uncertainties in TH analyses related to computer code models, plant parameters and human factors was almost finished within the 1st reporting period. D2.2 was submitted at the beginning of 2nd reporting period. Uncertainties in thermal-hydraulic analyses for PTS were assessed. Three types of uncertainties were investigated: model uncertainties (connected with computer-code models and the correct prediction of relevant phenomena), plant uncertainties covering initial and boundary condition and parameters of the NPP systems, and uncertainties connected with human interaction. The determination of the most relevant and important phenomena and parameters for the TH loading during PTS were compiled in a phenomena identification and ranking table (PIRT). Specified and quantified TH input uncertainties related to each computer code model used, plant parameters, and human factors were summarized in the report to D2.2.
Work on the Performance of the TH uncertainty analysis and export of TH data sets was started at the end of the 1st reporting period and will be done mostly within the 2nd reporting period.
Within WP3 “Deterministic margin assessment”, the temperature and stress fields for the preliminary base case were calculated by the partners by 1D FE models and compared to ensure consistency between several codes and applications. The final base case is ready to be assessed after conclusions from first application are drawn. Some other tasks (like LTO improvements) are waiting for TH data from WP2.
All parts of the benchmark to be performed within WP3 were defined and all parameters were set.
The first part of the benchmark (based on pre-defined input data) has started (Task 3.3). Stress intensity factor solutions (reference solution and preferred) were assessed, and the results will be compared and the conclusions for further assessment will be drawn.

Within WP4 “Probabilistic margin assessment” the main part of the work will be performed when the TH data sets are delivered from WP2. the work to produce a complete description of the benchmarks, i.e. “Definition of a probabilistic benchmarks” is ongoing.
A best-practice guidance for deterministic and probabilistic RPV integrity assessment will be formulated at the end of APAL project considering improved methodologies and also recommendations for the assessment of LTO improvements. It will describe the advanced methods assessed in the project (both deterministic and probabilistic). Some new features will be addressed like treating of various types of uncertainties in TH analysis, propagation of uncertainties in the entire PTS assessment, human factor, weld residual stress solutions, warm pre-stressing methods, etc. The guidance on best-practice for advanced RPV integrity assessment will be beneficial to increase the regulatory acceptance of margin justification. Its application on PTS assessment can assure safe long-term operation of European NPPs.
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