QUALIFICATION OF THE SYSTEM CODE AC² (SUBMODULE ATHLET) FOR THE SAFETY ASSESSMENT OF PASSIVE RESIDUAL HEAT REMOVAL SYSTEMS

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1 QUALIFICATION OF THE SYSTEM CODE AC² (SUBMODULE ATHLET) FOR THE SAFETY ASSESSMENT OF PASSIVE RESIDUAL HEAT REMOVAL SYSTEMS D. VON DER CRON, S. BUCHHOLZ, A. SCHAFFRATH Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) ggmbh Garching n. Munich, Germany Abstract GRS has been developing the thermal hydraulics system code ATHLET (now being a submodule of the AC² code, which is a substantial part of the GRS nuclear simulation chain) over many years. Since the code is widely used in nuclear supervisory and licensing procedures, its simulation capabilities have to represent the current state of science and technology. One notable innovative design safety feature of advanced light water reactors are passive systems. Unlike active systems which work at defined operating points, passive systems operate under conditions which are set on their own and may vary during the course of a transient dependent on the boundary conditions (e.g. pressure and temperature). Moreover, the driving forces of passive systems are usually relatively small compared to those of active systems so that uncertainties in the parameters can have a larger impact. Thus, the correct prediction of the behavior of these systems still proves to be a challenge for the AC² code. The present paper is focused on the recent efforts that have been made to qualify AC² for the simulation of slightly inclined horizontal heat exchangers like the emergency condensers used in the KERENA and CAREM reactor. 1. INTRODUCTION In order to simulate all relevant phenomena within a nuclear power plant, GRS uses various selfdeveloped and validated methods and computer codes. These codes are forming the so called nuclear simulation chain covering phenomena of neutron kinetics, thermal hydraulics within the cooling circuit and containment as well as structural mechanics [1]. Being part of this nuclear simulation chain, the code AC² covers the simulation of all operational states, incidents, accidents and severe accidents in a nuclear power plant with the GRS modules ATHLET, ATHLET-CD, COCOSYS and ATLAS. The submodule ATHLET (Analysis of Thermalhydraulics of Leaks and Transients) has been developed by GRS for many years. It is used for best estimate analyses of normal operation, incidents and accidents of the existing nuclear power plants. ATHLET has a substantial validation basis (e.g. LOCAs, transients, etc.) comprising OECD/CSNI validation matrices, start-up and operational transients of nuclear power plants, international standard problems (ISP) and benchmarks [2]. The currently latest release version of ATHLET within AC² is ATHLET 3.1A. The characteristics of passive systems, such as relatively weak and continuously changing driving forces relying on basic physical laws as well as independent onsets of such systems, pose a challenge for the correct simulation with system codes. The present paper addresses the efforts that have recently been undertaken by GRS to qualify ATHLET for the simulation of slightly inclined horizontal heat exchangers like the emergency condensers used in the KERENA and CAREM reactor. It should be noted that the paper concentrates only on pure steam condensation inside horizontal tubes; the presence of non-condensable gases is not treated here. The consideration of this very important, the heat transfer performance limiting factor in ATHLET will be investigated in further studies. Another factor the effect of condenser tube inclination is not discussed here because the heat transfer module of ATHLET treats all inclination angles smaller than approx as fully horizontal. In comparison, the inclination angle of the discussed NOKO test facility is 1.6 in the upper tube rows and 3.2 in the lower rows. 2. SIMULATIONS AND DEVELOPMENTS OF THE RECENT PAST This chapter provides a very brief summary of the ATHLET developments and simulations of the recent past with regards to emergency condensers. An extensive description of these topics was presented in [3]. 1

2 IAEA-CN-251-ID80 [Right hand page running head is the paper number in Times New Roman 8 point bold capitals, centred] 2.1. NOKO facility description and simulation results Starting point for the recent ATHLET developments related to horizontal passive residual heat exchangers were post-test simulations of experiments conducted at the NOKO test facility (Notkondensator test facility). The now dismantled NOKO test facility was constructed at the Forschungszentrum Jülich GmbH (KFA, now FZJ) for experimental investigations of the effectiveness of the emergency condenser of the BWR600/1000, the predecessor of the KERENA reactor. The facility had an operating pressure of 7.2 MPa and a maximum power of 4 MW for steam production. It comprised an emergency condenser consisting of eight tubes with original geometries and materials of the BWR600/1000 emergency condenser (EC); four of these eight tubes could be isolated by plugs, thus yielding a 1:13, resp. 1:26 scaling of the EC with respect to the original component. A thorough facility description can be found in [4]. An ATHLET nodalization of the main components of the facility is shown in Fig. 1. The pressure vessel represents the reactor pressure vessel and has a defined liquid level which can be regulated by the condensate drainage. Saturated steam is injected to the pressure vessel by a steam supply. The condenser tube bundle is connected with the pressure vessel by a feed line and a return line. It follows from this configuration that the liquid level in the pressure vessel has an influence on the liquid level inside the condenser tubes. As for the secondary side, the condenser tube bundle is located in the condenser vessel where it is completely immerged in water. While the steam and water on the primary side are saturated (except for the condensate which may be subcooled), the water on the secondary side is either subcooled or saturated, dependent on the performed experiment. The experiments conducted at the NOKO facility are described in detail in [5]. Their objective was to determine the heat transfer power of the EC under quasi-stationary conditions depending on various parameters such as primary side pressure, secondary side temperature and pressure, or liquid level in the pressure vessel. Many of these experiments have been simulated with ATHLET. For the sake of brevity, however, the paper concentrates on the simulation of a class of experiments in which the liquid level in the pressure vessel was low enough to ensure a complete primary side exposure of the condenser tubes and in which the secondary side water was in a saturated state. The conclusions that can be drawn from the comparison of these selected simulation results with the experimental data are representative for all simulated experiments. FIG. 1 ATHLET nodalization of the main components of the NOKO test facility (secondary side schematic) The first simulations of the NOKO facility were performed with ATHLET 3.0A. The nodalization scheme for the calculations was similar to the one shown in Fig. 1 with a modelling of the secondary side that allowed for circulation processes to occur. In order to make the results comparable, this nodalization scheme was used for all simulations with all code versions presented in this paper. As the simulation results deviated substantially from the experimental data (ATHLET 3.0A underestimated the EC power by a factor of approx. 2), condensation and boiling heat transfer models were modified and incorporated into the new code version ATHLET 3.1A.

3 With these code developments, the simulation results improved significantly. However, for experiments with larger temperature differences between the primary and the secondary side and thus for larger EC powers, the agreement between the experiments and the calculations is not yet satisfying as can be seen below in Fig Other emergency condenser related simulations Beside the NOKO simulations, GRS conducted further simulations related to passive emergency condensers, notably post-test calculations of both stationary and transient experiments carried out at the INKA (Integral Teststand Karlstein) test facility of AREVA in Karlstein, Germany. The INKA facility is a representation of the KERENA reactor with all its volumes and passive safety systems, like the EC, the building condenser or the passive pressure pulse transmitter. The scaling in height is 1:1 while the volumetric scaling is about 1:24. The passive safety systems are in full scale, but their number is scaled down to 1:4 [6]. The concluding statement one can make related to these calculations is that ATHLET systematically under-predicted the EC power by approx. 30%. Additional temporary modifications of the ATHLET code gave rise to minor improvements of the INKA simulations, but a significant underestimation of the EC power still remained. 3. ONGOING WORK Since the simulation results of EC behaviour are not yet satisfying, further investigations have to be undertaken. For this purpose, GRS is currently participating in two joint research projects in the frames of which experimental work together with code development is envisaged: The first project, EASY (Integral experimental and analytical investigations regarding the controllability of design accidents with passive systems), is dedicated to the development of a coupled program system for the simulation of passive systems as well as their interactions, consisting of the AC² modules ATHLET and COCOSYS, and the validation of this program system by both single component tests and integral tests of design basis accidents, conducted at the abovementioned INKA facility. The objective of the project is the creation of a tool for verification and assessment of newly built nuclear power plants. The second project, PANAS (Passive Nachzerfallswärme-Abfuhrsysteme), is about the investigation of passive residual heat removal systems, with a focus on experimental analysis, model development and validation of both system codes and CFD codes. The GRS subtask is the development and validation of heat transfer models for evaporation and condensation at horizontal heat exchangers for ATHLET. Both projects are complementary in that PANAS concentrates on code development and validation by single effect tests while EASY is focused on the simulation of integral experiments. Hence, insights gained about particular physical phenomena within PANAS can be beneficially used for the integral calculations within the frame of EASY. The following subchapters describe the latest, within the project PANAS, implemented condensation models in ATHLET as well as related simulation results of the aforementioned NOKO experiments New heat transfer models In order to predict the condensation heat transfer of an emergency condenser accurately, the knowledge about the local flow pattern inside the condenser tubes is essential. So far, ATHLET lacks this information its condensation heat transfer models only roughly distinguish between two basic flow regimes. A literature review within the project PANAS came to the conclusion that the flow pattern map-based heat transfer models of Thome et al. and KONWAR are promising candidates for improving the capability of ATHLET to predict heat transfer coefficients for condensation in horizontal tubes KONWAR KONWAR (Kondensation in waagerechten Rohren, condensation in horizontal tubes) is a code module for the determination of heat transfer coefficients during the condensation in horizontal tubes. It is based on a 3

4 IAEA-CN-251-ID80 [Right hand page running head is the paper number in Times New Roman 8 point bold capitals, centred] modified flow regime map of Tandon et al. [7] and includes several empirical and semi-empirical correlations for the determination of the HTCs. KONWAR was developed in the 1990s as an extension module of ATHLET version 1.1 and it was validated against the NOKO experiments. The validation results showed a very good agreement between simulation and experimental data. A thorough model description as well as the validation results can be found in [5] Thome et al. The heat transfer model of Thome et al. [8] is based on the condensation flow pattern map of El Hajal et al. [9]. The facts that the model is based on mechanistic reasoning and that it can be extended for the prediction of heat transfer coefficients for condensation in the presence of non-condensable gases make it an appropriate choice for the usage in ATHLET NOKO simulations applying the new heat transfer models To prove the operability of the coupled heat transfer packages together with ATHLET and to get a first impression of the predicted heat transfer, the simulations of the NOKO experiments described in chapter 2 were repeated with coupled versions of both ATHLET 3.1A/KONWAR and ATHLET 3.1A/Thome. The results of some of these simulations can be seen in Fig. 2. Compared to ATHLET 3.1A alone, the coupled versions of ATHLET with KONWAR and Thome yield slightly higher heat transfer coefficients and thus predict a better performance of the EC. While all models predict the EC power accurately for experiments with small differences between primary and secondary side saturation temperatures, the EC powers for intermediate temperature differences are predicted best by ATHLET/Thome, and the experiments with the largest ΔT are approximated closest by ATHLET/KONWAR. This rough description applies not only for the shown simulation cases, but also for the other calculated NOKO experiments. FIG. 2 Comparison of experimental data and simulation results, using different versions of ATHLET. 4. CONCLUSIONS AND OUTLOOK It can be concluded that the code developments accompanying the version upgrade from ATHLET 3.0A to 3.1A led to significant improvements of the calculation of both condensation and boiling heat transfer at the condenser pipes. The newer developments which comprise the introduction of more detailed flow pattern map-

5 based heat transfer models lead to slightly improved simulation results of the NOKO experiments; however, these results are still not yet satisfying and for the test cases with large condenser powers differ clearly from the experimental data. Although the NOKO test facility was a single component test stand, it was not a single effect test stand because several physical processes acted in parallel: condensation inside the tubes, boiling and circulation outside the tube bundle and heat conduction through the tube walls (the conductivity of which remains unclear). For this reason, within the project PANAS, GRS engages in ATHLET simulations of the COSMEA facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) [10]. The COSMEA facility is a single effect stand designed to study condensation heat transfer and flow structure. The test section mainly consists of a single, slightly inclined tube which is cooled by forced convection. The instrumentation includes among other things thermocouples for a detailed measurement of the temperature distribution and an X-ray tomography system for the investigation of local flow patterns. It is expected that the simulations of the COSMEA single effect experiments will help to localize the major deficits of ATHLET s heat transfer models with regard to emergency condenser simulations. ACKNOWLEDGEMENTS The project EASY (reference number RS1535A) is funded by the German Federal Ministry of Economic Affairs and Energy (BMWi). The project PANAS (reference number 02NUK041), where GRS is a subcontractor of Technische Universität Dresden, Chair of Hydrogen and Nuclear Energy, is funded by the German Federal Ministry of Education and Research (BMBF). The work presented under the headline Simulations and developments of the recent past was funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) within the projects RS1507 and RS1519. REFERENCES [1] SCHAFFRATH, A. et al., The nuclear simulation chain of GRS, International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear power Plants, Vienna, Austria (2017). [2] LERCHL, G. et al., ATHLET Validation, GRS P 1 / Vol. 3 Rev. 4, GRS ggmbh, Garching, March [3] BUCHHOLZ, S., VON DER CRON, D., SCHAFFRATH, A., System code improvements for modelling passive safety systems and their validation, KERNTECHNIK 81 5 (2016), 1 8. [4] SCHAFFRATH, A., JAEGERS, H., Allgemeine Beschreibung des NOKO-Versuchsstandes, Jül-3167, Forschungszentrum Jülich, Jülich, [5] SCHAFFRATH, A., Experimentelle und analytische Untersuchungen zur Wirksamkeit des Notkondensators des SWR600/1000, Jül-3326, Forschungszentrum Jülich, Jülich, [6] WAGNER, T., LEYER, S., Large Scale BWR Containment LOCA Response Test at the INKA Test Facility, 16 th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURET H-16), Chicago, IL, USA (2015). [7] TANDON, T.N., VARMA, H.K., GUPTA, C.P., A new flow regimes map for condensation inside horizontal tubes, Journal of Heat Transfer Vol. 104 (1982) [8] THOME, J.R., EL HAJAL, J., CAVALLINI, A., Condensation in horizontal tubes, part 2: new heat transfer model based on flow regimes, International Journal of Heat and Mass Transfer 46 (2003) [9] EL HAJAL, J., THOME, J.R., CAVALLINI, A., Condensation in horizontal tubes, part 1: two-phase flow pattern map, International Journal of Heat and Mass Transfer 46 (2003) [10] GEISSLER; T. et al., Experimental and numerical investigation of flow structure and heat transfer during high pressure condensation in a declined pipe at COSMEA facility, 16 th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURET H-16), Chicago, IL, USA (2015). 5