Periodic Reporting for period 2 - APAL (Advanced PTS Analysis for LTO)
Período documentado: 2022-04-01 hasta 2023-09-30
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, more detailed quantification of implicit and explicit safety margins is necessary, 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). 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 in the EU, PTS analyses are based on deterministic assessments and conservative boundary conditions. The PTS analyses of this type are reaching their limits in demonstrating the safety of NPPs facing LTO, and they 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 the safety margins in terms of 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 in the EU, PTS analyses are based on deterministic assessments and conservative boundary conditions. The PTS analyses of this type are reaching their limits in demonstrating the safety of NPPs facing LTO, and they 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 the safety margins in terms of 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.
Within WP1 “LTO improvements relevant for PTS analysis” an extensive literature survey and experience gathering was performed to identify state-of-the-art LTO improvements (both hardware and software) with potential beneficial or adverse impact on the results of PTS analysis.
Four LTO improvements concerning PTS analyses 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 of NPPs relevant for PTS mitigation.
The state-of-the-art surveys 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 was summarized.
Within WP2 “Improved TH analysis”, the work on “Quantification of impact of LTO improvements and human factor on TH analysis boundary conditions” was completed and Deliverable D2.1 was submitted. TH analyses for the base case (SBLOCA with 50 cm2 break in hot leg and with loss of offsite power) and for 6 selected LTO improvements were performed with different computer codes. Further, the effects of 3 selected human interactions were evaluated. TH data sets representing selected LTO improvements or human interactions were exported to WP3 and WP4 for deterministic and probabilistic fracture-mechanical analyses. System and mixing codes used in the TH simulations were RELAP5, ATHLET, TRACE, KWU-MIX, GRS-MIX and ECC-MIX. The applicability of the computational fluid dynamics 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 completed, and Deliverable D2.2 was submitted. Uncertainties in thermal-hydraulic analyses for the purpose of PTS evaluation were assessed. Three types of uncertainties were investigated: model uncertainties, plant uncertainties covering initial and boundary conditions as well as parameters of the NPP systems, and uncertainties associated with human interaction. Determination of the most relevant and important phenomena and parameters for the TH loading during PTS was performed, and the results were compiled in a Phenomena Identification and Ranking Table (PIRT).
TH uncertainty analyses (for statistical distributions of selected TH parameters defined in Deliverable D2.2) were performed using above mentioned TH codes within Task 2.3 using Wilks approach. This includes 59, 93 or 130 simulations of the selected PTS transient. The resulting data were exported to WP 3 and WP 4 and are summarised in D2.3.
The Public summary report of WP2 was issued.
Within WP3 “Deterministic margin assessment”, the temperature and stress fields for the base case and for 9 LTO improvements were calculated by the partners using 1D and 2D FE models. Comparison of the results was published in D3.1.
All parts of the benchmark to be performed within WP3 were defined and all parameters were set in D3.2.
In Task 3.3 The first fracture-mechanics benchmark calculations were performed by all involved partners. Stress intensity factors and maximum allowable reference temperatures were calculated within this Task. The fracture-mechanics assessments of LTO improvements and TH uncertainties were performed within Tasks 3.4 and 3.5. All results will be compared and the conclusions for further assessment will be drawn in D3.3 and D3.4 which will be published in the beginning of 3rd reporting period.
Within WP4 “Probabilistic margin assessment”, the temperature and stress fields for 59 up to 130 TH data sets were calculated by the partners using 1D and 3D FE models.
The complete description of the benchmarks, i.e. “Definition of a probabilistic benchmarks” was published in D4.2. The benchmark calculations started, the probabilistic margin assessment methods were defined and results of pre-requisite tasks were compared.
Four LTO improvements concerning PTS analyses 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 of NPPs relevant for PTS mitigation.
The state-of-the-art surveys 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 was summarized.
Within WP2 “Improved TH analysis”, the work on “Quantification of impact of LTO improvements and human factor on TH analysis boundary conditions” was completed and Deliverable D2.1 was submitted. TH analyses for the base case (SBLOCA with 50 cm2 break in hot leg and with loss of offsite power) and for 6 selected LTO improvements were performed with different computer codes. Further, the effects of 3 selected human interactions were evaluated. TH data sets representing selected LTO improvements or human interactions were exported to WP3 and WP4 for deterministic and probabilistic fracture-mechanical analyses. System and mixing codes used in the TH simulations were RELAP5, ATHLET, TRACE, KWU-MIX, GRS-MIX and ECC-MIX. The applicability of the computational fluid dynamics 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 completed, and Deliverable D2.2 was submitted. Uncertainties in thermal-hydraulic analyses for the purpose of PTS evaluation were assessed. Three types of uncertainties were investigated: model uncertainties, plant uncertainties covering initial and boundary conditions as well as parameters of the NPP systems, and uncertainties associated with human interaction. Determination of the most relevant and important phenomena and parameters for the TH loading during PTS was performed, and the results were compiled in a Phenomena Identification and Ranking Table (PIRT).
TH uncertainty analyses (for statistical distributions of selected TH parameters defined in Deliverable D2.2) were performed using above mentioned TH codes within Task 2.3 using Wilks approach. This includes 59, 93 or 130 simulations of the selected PTS transient. The resulting data were exported to WP 3 and WP 4 and are summarised in D2.3.
The Public summary report of WP2 was issued.
Within WP3 “Deterministic margin assessment”, the temperature and stress fields for the base case and for 9 LTO improvements were calculated by the partners using 1D and 2D FE models. Comparison of the results was published in D3.1.
All parts of the benchmark to be performed within WP3 were defined and all parameters were set in D3.2.
In Task 3.3 The first fracture-mechanics benchmark calculations were performed by all involved partners. Stress intensity factors and maximum allowable reference temperatures were calculated within this Task. The fracture-mechanics assessments of LTO improvements and TH uncertainties were performed within Tasks 3.4 and 3.5. All results will be compared and the conclusions for further assessment will be drawn in D3.3 and D3.4 which will be published in the beginning of 3rd reporting period.
Within WP4 “Probabilistic margin assessment”, the temperature and stress fields for 59 up to 130 TH data sets were calculated by the partners using 1D and 3D FE models.
The complete description of the benchmarks, i.e. “Definition of a probabilistic benchmarks” was published in D4.2. The benchmark calculations started, the probabilistic margin assessment methods were defined and results of pre-requisite tasks were compared.
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.
A best-practice guidance for performing deterministic and probabilistic RPV integrity assessment will be formulated at the end of APAL project, considering improved methodologies and recommendations for the assessment of LTO improvements. Some new features will be 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 methods, 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.
A best-practice guidance for performing deterministic and probabilistic RPV integrity assessment will be formulated at the end of APAL project, considering improved methodologies and recommendations for the assessment of LTO improvements. Some new features will be 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 methods, 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.