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[en] In this thesis improved solutions for Green's functions are obtained. First the for this thesis essential techniques and concepts of QCD as euclidean field theory are presented. After a discussion of the foundations of the extended approach for the Feynman rules of QCD with a systematic approach for the 4-gluon vertex a modified renormalization scheme for the extended approach is developed. Thereafter the resummation of the Dyson-Schwinger equations (DSE) by the appropriately modified Bethe-Salpeter equation is discussed. Then the leading divergences for the 1-loop graphs of the resummed DSE are determined. Thereafter the equation-of-motion condensate is defined as result of an operator-product expansion. Then the self-consistency equations for the extended approaches are defined and numerically solved. (HSI)
[en] Conclusions and Outlook: The German government decided to terminate the use of nuclear energy production by the End of 2022. ▪ Renewable energy production is an increasing factor in Germany. ▪ GRS as the main German TSO will retain its function in supporting the German government in all relevant issues on nuclear safety and radioactive waste management in Germany and abroad. ▪ Research at GRS in reactor safety so far was and will be focused on code development and validation for existing / advanced / innovative NPPs and SMR. ▪ Various Challenges have been identified; some examples have been discussed. ▪ Other big Challenges are the conservation and transfer of “nuclear knowledge” to the next generation of nuclear engineers and physicists and the restructuring / modernisation of our “legacy codes”. ➢ GRS will have the necessary staff, competencies, know-how and validated tools for safety assessments for Advanced NPPs and SMR in future as well.
[en] In the frame of the reactor safety project RS1173, sponsored by the German Federal Ministry of Economics and Technology, analyses of international integral and separate effects tests have been performed for the validation of the code system ATHLET/ATHLET-CD. The work mainly comprised post-test calculations of selected experiments and the contributions to the working groups accompanying the experimental programs. For the assessment of the thermal-hydraulic models in ATHLET 8 integral tests and 4 separate effect tests have been considered. Together with the corroboration of the existing models, the validation analyses were mainly dedicated to the assessment of the modelling of non-condensable gases and their influence on two-phase natural circulation and on the primary heat removal through steam generators, as well as of the simulation of multi-dimensional flow processes. The validation calculations with respect to the simulation of multi-dimensional one- and two-phase flows aimed to investigate the range of applicability and limitations of the method of parallel channels in connection with the separate momentum equations for water and steam current used in ATHLET as well as to assess the status of the coupled version ATHLET/FLUBOX-3D. The ATHLET-CD validation analyses included the post-test calculations of 9 bundle tests, and was mainly focussed on the assessment of the improved and new models for core degradation, including the models for oxidation, melt formation and relocation for BWR components, as well as of the modelling of fission products and aerosol transport within the primary circuit taking into account chemical reactions within the module SOPHAEROS. As an additional contribution to code validation, the GRS methodology of uncertainty and sensitivity analysis was applied exemplarily to two validation calculations, one with ATHLET and one with ATHLET-CD. The results of these uncertainty analyses endorse the capability of the code system to reproduce adequately the main experimental outcomes. The measured values lay mostly within the tolerance limits of the calculated results. The performed analyses have identified the basic strengths but also weaknesses of the code system ATHLET/ATHLET-CD, leading to some proposals for modelling improvements and further developments.
[en] Highlights: • New release AC2 2019 contains ATHLET/ATHLET-CD 3.2 and COCOSYS 3.0. • Validation of AC2 codes and validation matrices are discussed. • Improvements for ATHLET 3D model, AIDA lower plenum module and SAFT FP transport. • For MCCI simulation in COCOSYS, set concrete decomposition temperatures to 1800 K. • Stable coupling at multiple locations of ATHLET to CFD-codes for single-phase flows. - Abstract: The system code package AC2 by GRS for safety analyses of nuclear reactors from normal operation to severe accidents has been updated with a new release. We briefly describe the main modules of AC2 2019: ATHLET 3.2, ATHLET-CD 3.2 and COCOSYS 3.0 and selected improvements in these codes. We illustrate the improved capabilities of AC2 with selected examples. The post-test-calculation of the flooding pool of the INKA test facility demonstrated the improved ATHLET 3D model with explicit mixture level model. For ATHLET-CD, we apply the AIDA module with improvements in wall ablation and heat transfer models to a generic AP1000 lower plenum filled with molten corium. And we apply the new SAFT fission product transport module simulating three release paths through steam generator U-tubes. For COCOSYS, we present uncertainty analysis results for two MCCI experiments which justify using an elevated concrete decomposition temperature of 1800 K. Finally, we illustrate the potential of coupling ATHLET to CFD codes for safety analysis with results of a post-test calculation of the ROCOM PKLIIIT1.1 test on 3D mixing phenomena in an RPV.
[en] This report provides a review of the main used risk measures for Level 1 and Level 2 PSA. It depicts their advantages, limitations and disadvantages and develops some more precise risk measures relevant for extended PSAs and helpful for decision-making. This report does not recommend or suggest any quantitative value for the risk measures. It does not discuss in details decision-making based on PSA results neither. The choice of one appropriate risk measure or a set of risk measures depends on the decision making approach as well as on the issue to be decided. The general approach for decision making aims at a multi-attribute approach. This can include the use of several risk measures as appropriate. Section 5 provides some recommendations on the main risk metrics to be used for an extended PSA. For Level 1 PSA, Fuel Damage Frequency and Radionuclide Mobilization Frequency are recommended. For Level 2 PSA, the characterization of loss of containment function and a total risk measure based on the aggregated activity releases of all sequences rated by their frequencies is proposed. (authors)
[en] The objective of this document is to identify some lessons learned from the Fukushima Dai-ichi accident for PSA. Based on the public information on the causes that have led to major radioactive release during the Fukushima Dai-ichi accident (initiating events, material and human response), the authors, ASAMPSA-E WP30 members have performed a review to examine the gaps/insufficiencies/incompleteness in the existing Level 1 and Level 2 PSAs. This is the aim of this report which is one of WP30 deliverables i.e. D30.2. The consideration of external initiating events for the different levels of defense-in-depth is one of the focal points in this review. Recommendations in the way of developing the different elements of PSAs have been proposed by the authors and were completed later during the ASAMPSA-E project. Moreover, first recommendations on the use of PSA information in decision making have been included as well. (authors)
[en] An extended PSA applies to a site of one or several Nuclear Power Plant unit(s) and its environment. It intends to calculate the risk induced by the main sources of radioactivity (reactor core and spent fuel storages) on the site, taking into account all operating states for each main source and all possible relevant accident initiating events (both internal and external) affecting one unit or the whole site. The combination between hazards or initiating events and their impact on a unit or the whole site is a crucial issue for an extended PSA. The report tries to discuss relevant methodologies for this purpose. The report proposes a methodology to select initiating events and hazards for the development of an extended PSA. The proposed methodology for initiating events identification, screening and bounding analysis for an extended PSA consists of four major steps: 1. A comprehensive identification of events and hazards and their respective combinations applicable to the plant and site. Qualitative screening criteria will be applied, 2. The calculation of initial (possibly conservative) frequency claims for events and hazards and their respective combinations applicable to the plant and the site. Quantitative screening criteria will be applied, 3. An impact analysis and bounding assessment for all applicable events and scenarios. Events are either screened out from further more detailed analysis, or are assigned to a bounding event (group), or are retained for detailed analysis, 4. The probabilistic analysis of all retained (bounding) events at the appropriate level of detail. (authors)