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[en] To investigate the cladding material FeCrAl which is one option of Accident Tolerant Fuel (ATF), the bundle experiment QUENCH-19 was performed at Karlsruhe Institute of Technology (KIT). ATF claddings could be used as one option to reduce e.g. the thermal impact by oxidation and to increase time margins for accident management measures. On the other hand, the neutron absorption is higher and the melting temperature is lower of FeCrAl compared to Zry based claddings. The test QUENCH-19 was accompanied by post test analyses at GRS with the AC² module ATHLET-CD. The experiment QUENCH19 was conducted similarly to the experiment QUENCH-15 for comparing the oxidation behaviour of FeCrAl and Zry at high temperatures. In contrast to test QUENCH-15, which showed a sharp escalation after start of quenching resulting in temperatures up to ≈1900°C, no escalation occurred during QUENCH-19 and peak cladding temperatures of only ≈1450°C were reached. For the post test simulations an oxidation correlation for KANTHAL APMT was available and also a second approach derived from an OECD/NEA report. Both approaches were implemented in ATHLET-CD with the assumption that only Al2O3 was generated but no other oxides. Compared to Zry oxidation both approaches give orders of magnitude lower oxidation rates and are only valid for one special composition, while different materials were used in the experiment. In general, the results of the post test simulation show good agreement of the thermal behaviour. While the maximum temperature well reproduced an underestimation of only app. 50°C, the radial temperature profile was significantly underestimated by 150200°C compared to the measured data. The comparison of the hydrogen production of in total 9 g in the test and less than 1 g for both oxidation approaches in the simulation shows that the oxidation model for FeCrAl in the code has to be improved. For a detailed evaluation of the calculated hydrogen generation the post test examination of the bundle is necessary to know which components contribute to the total value. (author)
[en] The bundle experiment QUENCH-19 was performed in August 2018 at Karlsruhe Institute of Technology (KIT) in cooperation with the Oak Ridge National Laboratory (ORNL) to investigate the cladding material FeCrAl under severe accident conditions. To support the experiment besides pre-test calculations also post-test analyses were performed with the GRS system code ATHLET-CD. The test QUENCH-19 was conducted similarly to the experiment QUENCH-15 to compare the oxidation behavior of FeCrAl and Zr at high temperatures. In contrary to test QUENCH-15, where a sharp escalation after start of quenching resulted in temperatures up to ~1900°C, no escalation occurred during QUENCH-19 and peak cladding temperatures of only ~1450°C were reached. After the implementation of two approaches for the oxidation kinetics of FeCrAl with the assumption that only Al2O3 and no other oxides will be generated, the post-test simulation with ATHLET-CD showed good agreement with the general thermal behavior within the heated length. While the maximum temperature was obtained with high accordance, the radial temperature profile was underestimated compared to the measured data. The comparison of the hydrogen production of totally 9 g resulting from the test and less than 1 g for both oxidation approaches in the simulation shows that the oxidation model for FeCrAl in the code has to be improved. For a detailed evaluation of the calculated hydrogen generation it is necessary to use the final results of the post-test examination of the bundle to know which components contributed to the total value. (author)
[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] The air ingress test PARAMETER SF4 was performed in July 2009 with the aim to investigate the behavior of a VVER bundle during reflooding after air ingress. Simulations are performed using the severe accident code ATHLET-CD. The results of the simulations show a good agreement in comparison to the experimental data during the pre-oxidation phase and in most bundle regions also during air ingress. The hydrogen generation is very close to the experimental data up to the beginning of quenching; afterwards the total amount of H_2 is underpredicted. Further improvement can be achieved by using a model to consider ZrN formation. (author)
[en] Thermal-hydraulic and core degradation phenomena play a decisive role for the course of severe accidents in light water reactors. Therefore, the simulation of such accidents with computer codes requires comprehensive and detailed modelling of these processes. The code ATHLET-CD is being developed for realistic simulation of accidents with core degradation and for evaluation of accident management measures. It makes use of the detailed and validated models of the thermal-hydraulic code ATHLET in an efficient coupling with models for core degradation and fission product behaviour. The capabilities of the coupled code are demonstrated by means of the calculation of the TMI-2 accident. The first three phases of the accident were successfully simulated in a reasonable computing time. The calculated system pressure and pressurizer level after pump trip, during the pump restart, and until core slump are in acceptable agreement with the measured data. The calculated hydrogen generation before the pump restart is in accordance with the deduced value. Contrary to estimates based on the system behaviour, no significant hydrogen generation was calculated during the quench phase. Further model improvements regarding the quenching of degraded core material, fracture and relocation of solid fuel rods, as well as the simulation of debris bed behaviour are necessary for better simulation. (authors)
[en] The QUENCH experimental programme at FZ Karlsruhe investigates phenomena associated with reflood of a degrading core under postulated severe accident conditions, in the early phase where the geometry is still mainly rod-like. The latest large-scale bundle test, QUENCH-13, is the first in this programme to include a silver/indium/cadmium (SIC) control rod of prototypic PWR design. The effects of the presence of the control rod on early-phase degradation and on reflood behaviour are examined under integral conditions, while the opportunity is taken to measure, in realistic geometry, release of SIC aerosols following control rod rupture. These materials can affect the chemistry of fission products in the reactor circuit, and hence the radioactive source term to the environment in the event of containment failure. In particular, the sharp release of cadmium on control rod failure, which can involve some tens of percent of the inventory, is ill-defined experimentally. Pre-test calculations were performed to determine the test boundary conditions, such as the electrical power history to the bundle, the coolant flow, and the reflood timing and rate. The aim was to stabilise the bundle at maximum temperature of 1250 K, then ramped at about 0.25 K/s to give the best chance to measure the control rod aerosol release under controlled conditions, then to reflood, without provoking an oxidation excursion, at maximum bundle temperature of 1800-1850 K A further aim was to check thermal conditions in the offgas pipe, where the aerosol instrumentation was situated. The calculational support was organised through the Source Term area of the EU 6' Framework Network of Excellence SARNET, linking the experimental team at FZK with modellers at PSI, GRS and EdF. Following agreement of the target test conditions, the modelling teams used SCDAP-based codes, ATHLET-CD and MAAP4 respectively to help the definition of the test boundary conditions, and in the latter two cases to estimate the control rod aerosol release. The facility models used were benchmarked against data from previous QUENCH tests, while also the ATHLET-CD release modelling was checked against Phebus FP data. The experimental protocol took account of the recommendations from the pre-test studies. Benefit was gained in the cooperation through the use of independent codes by different organisations, in lending confidence to the test predictions, and in obtaining different perspectives on the test conduct. The experiment was successfully performed according to the agreed specification on 7 November 2007, and the results are to be analysed on a collaborative basis. Post-test calculations are planned following release of the definitive results. (authors)
[en] Highlights: • A state-of-the-art for air oxidation modelling in the frame of severe accident is done. • Air oxidation models from main severe accident codes are detailed. • Simulations from main severe accident codes are compared against experimental results. • Perspectives in terms of need for further model development and experiments are given. - Abstract: Air ingress is a potential risk in some low probable situations of severe accidents in a nuclear power plant. Air is a highly oxidizing atmosphere that can lead to an enhanced Zr-based cladding oxidation and core degradation affecting the release of fission products. This is particularly true speaking about ruthenium release, due to its high radiotoxicity and its ability to form highly volatile oxides in a significant manner in presence of air. The oxygen affinity is decreasing from the Zircaloy cladding, fuel and ruthenium inclusions. It is consequently of great need to understand the phenomena governing cladding oxidation by air as a prerequisite for the source term issues in such scenarios. In the past years, many works have been done on cladding oxidation by air under severe accident conditions. This paper with in addition the paper “Cladding oxidation during air ingress – Part I: Synthesis of experimental results” of this journal issue aim at assessing the state of the art on this phenomenon. In this paper, the modelling of air ingress phenomena in the main severe accident codes (ASTEC, ATHLET-CD, MAAP, MELCOR, RELAP/SCDAPSIM, SOCRAT) is described in details, as well as the validation against the integral experiments QUENCH-10, QUENCH-16 and PARAMETER-SF4. A full review of cladding oxidation by air is thus established.