Periodic Reporting for period 3 - APAL (Advanced PTS Analysis for LTO)
Période du rapport: 2023-10-01 au 2024-09-30
In the EU, most of the nuclear power plants (NPPs) are currently in the second half of their design life, making life extension an important aspect for countries planning to continue nuclear energy generation in the long term. To verify safe operation of existing NPPs undergoing long-term operation (LTO) upgrades, advanced methods and improvements are necessary. In addition, a more detailed quantification of implicit and explicit safety margins is necessary, including determination of the 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). The PTS is characterized by rapid cooling (i.e. thermal shock) of the reactor downcomer and internal RPV surface, accompanied in most cases by high pressure in the RPV. Thus, the PTS event presents a potentially significant challenge to the structural integrity of RPV in pressurized-water reactors (PWRs) and water-cooled water-moderated energy reactors (WWERs).
Currently, PTS analyses in the EU are based on deterministic assessments and conservative boundary conditions. PTS analyses of this type are reaching their limits in demonstrating the safety of NPPs facing LTO and need to be enhanced. Nevertheless, inherent safety margins exist, and several LTO improvements applicable to the NPPs, as well as advanced methods of PTS analyses, may be able to increase these safety margins. Additionally, the quantification of safety margins in terms of the risk of RPV failure using advanced probabilistic assessments becomes crucial, because probabilistic methods provide more comprehensive assessments in PTS analysis and allow for quantification of uncertainties of the results. The main objectives of the APAL project are to develop an advanced probabilistic PTS assessment method, quantify safety margins for LTO improvements and develop best-practice guidelines.
One of the most limiting safety assessments for LTO is the RPV integrity analysis of pressurized thermal shock (PTS). The PTS is characterized by rapid cooling (i.e. thermal shock) of the reactor downcomer and internal RPV surface, accompanied in most cases by high pressure in the RPV. Thus, the PTS event presents a potentially significant challenge to the structural integrity of RPV in pressurized-water reactors (PWRs) and water-cooled water-moderated energy reactors (WWERs).
Currently, PTS analyses in the EU are based on deterministic assessments and conservative boundary conditions. PTS analyses of this type are reaching their limits in demonstrating the safety of NPPs facing LTO and need to be enhanced. Nevertheless, inherent safety margins exist, and several LTO improvements applicable to the NPPs, as well as advanced methods of PTS analyses, may be able to increase these safety margins. Additionally, the quantification of safety margins in terms of the risk of RPV failure using advanced probabilistic assessments becomes crucial, because probabilistic methods provide more comprehensive assessments in PTS analysis and allow for quantification of uncertainties of the results. The main objectives of the APAL project are to develop an advanced probabilistic PTS assessment method, quantify safety margins for LTO improvements and develop best-practice guidelines.
WP1, titled “LTO improvements relevant for PTS analysis,” conducted an extensive literature survey and gathered experience to identify state-of-the-art LTO improvements (PTS analyses, NPP hardware, software, and operator actions) with potential impacts on PTS analysis results. WP1 consisted of five tasks:
- Residual stress distributions in welds (WRS) and in cladding
- Warm pre-stress (WPS) application in PTS analysis
- Improved thermal-hydraulic (TH) analysis
- Probabilistic PTS analysis
- Further potential LTO improvements for PTS mitigation
The state-of-the-art surveys included the collection of existing solutions, assessments, and identification of gaps for improvement. Technical questionnaires were completed by partners, and the results were summarized in a public report.
WP2, “Improved TH analysis,” quantified the impact of LTO improvements and human factors on TH boundary conditions. TH analyses were performed for the base case (SBLOCA with a 50 cm² break in the hot leg and loss of offsite power) and for six selected LTO improvements using various codes (RELAP5, ATHLET, TRACE, etc.), and for evaluations of three human interactions. TH data sets were exported for deterministic and probabilistic structural and fracture-mechanical analyses. Uncertainties related to TH models, plant parameters, and human factors were assessed. A Phenomena Identification and Ranking Table (PIRT) was created. A public summary report was issued.
WP3, “Deterministic margin assessment,” included calculations of temperature and stress fields for the base case and nine LTO improvements using 1D and 2D FE models. Results were published in Deliverables D3.1. In Task 3.3 fracture-mechanics benchmark calculations were conducted to determine stress intensity factors and maximum allowable reference temperatures, published in Deliverable D3.3. Further fracture-mechanics assessments of LTO improvements and TH uncertainties were carried out in Tasks 3.4 and 3.5 with results summarized in Deliverable D3.4. A public summary report was issued.
WP4, “Probabilistic margin assessment,” performed probabilistic analyses. Temperature and stress fields for 59 up to 130 TH data sets covering the TH uncertainties using Wilks approach were calculated using the 1D and 3D FE models. Probabilistic fracture-mechanics PTS analyses were performed in Tasks 4.3 - 4.5 for baseline benchmarks, LTO improvements and TH uncertainities. Results were published in Deliverables D4.3 - D4.5. Probabilistic margin assessments were performed in Task 4.6 with results published in Deliverable D4.6. A public summary report was issued.
WP5, “Definition of best-practice for advanced PTS analysis,” focused on gathering conclusions and recommendations from WP1 to WP4. Feedback from the Advisory Board and End-users was collected throughout the project, especially during the End-user workshop and Final seminar. The "Final report on guidance on best-practice for deterministic and probabilistic RPV integrity assessment" was published.
WP2-4 analyses and the preparation of best-practice guidance were supported by WP6, ensuring communication, visibility, and dissemination of project results. WP6 also contributed to strategic planning and operational support for the project’s exploitation.
- Residual stress distributions in welds (WRS) and in cladding
- Warm pre-stress (WPS) application in PTS analysis
- Improved thermal-hydraulic (TH) analysis
- Probabilistic PTS analysis
- Further potential LTO improvements for PTS mitigation
The state-of-the-art surveys included the collection of existing solutions, assessments, and identification of gaps for improvement. Technical questionnaires were completed by partners, and the results were summarized in a public report.
WP2, “Improved TH analysis,” quantified the impact of LTO improvements and human factors on TH boundary conditions. TH analyses were performed for the base case (SBLOCA with a 50 cm² break in the hot leg and loss of offsite power) and for six selected LTO improvements using various codes (RELAP5, ATHLET, TRACE, etc.), and for evaluations of three human interactions. TH data sets were exported for deterministic and probabilistic structural and fracture-mechanical analyses. Uncertainties related to TH models, plant parameters, and human factors were assessed. A Phenomena Identification and Ranking Table (PIRT) was created. A public summary report was issued.
WP3, “Deterministic margin assessment,” included calculations of temperature and stress fields for the base case and nine LTO improvements using 1D and 2D FE models. Results were published in Deliverables D3.1. In Task 3.3 fracture-mechanics benchmark calculations were conducted to determine stress intensity factors and maximum allowable reference temperatures, published in Deliverable D3.3. Further fracture-mechanics assessments of LTO improvements and TH uncertainties were carried out in Tasks 3.4 and 3.5 with results summarized in Deliverable D3.4. A public summary report was issued.
WP4, “Probabilistic margin assessment,” performed probabilistic analyses. Temperature and stress fields for 59 up to 130 TH data sets covering the TH uncertainties using Wilks approach were calculated using the 1D and 3D FE models. Probabilistic fracture-mechanics PTS analyses were performed in Tasks 4.3 - 4.5 for baseline benchmarks, LTO improvements and TH uncertainities. Results were published in Deliverables D4.3 - D4.5. Probabilistic margin assessments were performed in Task 4.6 with results published in Deliverable D4.6. A public summary report was issued.
WP5, “Definition of best-practice for advanced PTS analysis,” focused on gathering conclusions and recommendations from WP1 to WP4. Feedback from the Advisory Board and End-users was collected throughout the project, especially during the End-user workshop and Final seminar. The "Final report on guidance on best-practice for deterministic and probabilistic RPV integrity assessment" was published.
WP2-4 analyses and the preparation of best-practice guidance were supported by WP6, ensuring communication, visibility, and dissemination of project results. WP6 also contributed to strategic planning and operational support for the project’s exploitation.
Combination of probabilistic approach (treatment of many input parameters as statistical distributions) in both thermal-hydraulic and fracture-mechanics parts of PTS analyses is a new feature, for the first time used in APAL project.
The best-practice guidance for performing deterministic and probabilistic RPV integrity assessments was formulated during the APAL project, considering improved methodologies and recommendations for the assessment of LTO improvements. Some new features were addressed, such as 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 approach, etc. The guidance on best practice for advanced RPV integrity assessment will be beneficial in increasing the regulatory acceptance of margin justification. Its application to PTS assessment can assure safe long-term operation of European NPPs. Most of the APAL partners are the main drivers for further application of APAL results in their countries, as they are performing PTS analyses or plant assessments, conducting state reviews of PTS analyses, or transferring innovations to the national regulatory body. Other partners will exploit the results via further research, training and education activities. Accordingly, there will be different modes of impact at national levels specific to the local context of nuclear regulation and role of the APAL partners.
In addition to dissemination and exploitation activities planned at national level, the APAL consortium members plan presentations of APAL results at international conferences, in journals and within various expert groups.
The best-practice guidance for performing deterministic and probabilistic RPV integrity assessments was formulated during the APAL project, considering improved methodologies and recommendations for the assessment of LTO improvements. Some new features were addressed, such as 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 approach, etc. The guidance on best practice for advanced RPV integrity assessment will be beneficial in increasing the regulatory acceptance of margin justification. Its application to PTS assessment can assure safe long-term operation of European NPPs. Most of the APAL partners are the main drivers for further application of APAL results in their countries, as they are performing PTS analyses or plant assessments, conducting state reviews of PTS analyses, or transferring innovations to the national regulatory body. Other partners will exploit the results via further research, training and education activities. Accordingly, there will be different modes of impact at national levels specific to the local context of nuclear regulation and role of the APAL partners.
In addition to dissemination and exploitation activities planned at national level, the APAL consortium members plan presentations of APAL results at international conferences, in journals and within various expert groups.