Raising Nuclear Reactor Safety to a Higher Level - The Supercritical CO2 Heat Removal System - "sCO2-HeRo"
Joerg Starflinger
University of Stuttgart, Institute of Nuclear Technology and Energy systems (IKE)
Pfaffenwaldring 31 70569 Stuttgart, Germany joerg.starflinger@ike.uni-stuttgart.de
Dieter Brillert
University Duisburg-Essen, Institute of Turbomachinery
Universitätsstr. 2 45141 Essen, Germany dieter.brillert@uni-due.de Otakar Frybort
Centrum Vyzkumu Rez S.R.O.
HUSINEC REZ 130 25068 Rez, Czech Republic
otakar.frybort@cvrez.cz
Petr Hajek UJV REZ, a.s.
Hlavni 130
25068 Rez, Czech Republic petr.hajek@ujv.cz Aldo Hennink
Delft University of Technology, Department of Radiation Science and Technology
Mekelweg 15
2629 JB, Delft, The Netherlands a.hennink@tudelft.nl
Thomas Freutel
Gesellschaft für Simulatorschulung mbH Deilbachtal 173
45257 Essen, Germany thomas.freutel@ksg-gfs.de
ABSTRACT
The sCO2-HeRo provides breakthrough options with scientific and practical maturity, which will be proven by means of numerical tools, like advanced CFD, and small-scale experiments to determine the performance of the components like a compact heat exchanger and a turbo-machine set. A demonstration unit of the sCO2-HeRo system will be installed in a unique glass model in order to demonstrate the maturity of the system. Finally, the potential of this system to deal with a range of different accident scenarios and beyond-design accidents will be shown with the support of the German nuclear code ATHLET.
1 INTRODUCTION
The “supercritical CO2 heat removal system”, sCO2-HeRo is a very innovative reactor heat removal concept as it improves the safety of both currently operating and future BWRs and PWRs through a self-propellant, self-sustaining and self-launching, highly compact cooling system powered by an integrated Brayton-cycle using supercritical carbon dioxide (sCO2) as its working fluid. Since this system is powered by the decay heat itself, it provides new ways to deal with beyond design accidents. The turbine of a Brayton-cycle provides more energy than necessary to drive the compressor, which means that the sCO2-HeRo system provides electricity in addition. Therefore, this system can be an excellent backup cooling system for the reactor core in case of a Fukushima-like scenario, with a combined station blackout (SBO), loss of ultimate heat sink (LUHS), or loss of emergency cooling. In addition, this system might also
expansion of the working fluid, it continues to an air-cooled heat exchanger (3-4). Air-fans, powered with electricity produced directly by the Brayton cycle, are intended to improve the heat transfer between the air-cooled heat exchanger and the air. The cooled sCO2 flows to a compressor (4-1), where it is compressed, before it re-enters the condenser.
Figure 1: sCO2-Heat Removal System [1]
Venker et al. [1-6] have studied the feasibility of this approach extensively. The result was that such a system attached to a BWR could possibly enlarge the grace time for interaction in case of an accident to more than 72 hrs.
3 THE PROJECT SCO2-HERO
Within the European project “sCO2-HeRo”, six partners from three European countries are working on the assessment of this cycle. The objective is to investigate the potential of the heat removal system and to assess its operation at laboratory scale (Technology Readiness Level (TRL) 3) [7]. This goal will be reached by integrating a small-scale sCO2-HeRo system into the PWR glass model at GfS, Essen, Germany. In total six work packages have been defined (see Figure 2), four of them related to technical aspects of the sCO2-HeRo system, which will be explained as follows, and two non-technical aspects (WP5: Exploitation and Dissemination and WP6: Management), which are not explained here.
Figure 2: Organisation of the sCO2-HeRo Project
3.1 Work Package 1: System Integration and Simulation
In work package 1 of this project, led by IKE, University of Stuttgart, Germany, cycle calculations are carried out to evaluate, how this heat removal system would work under different operating conditions. The main task is to define the boundary conditions given by the glass model and design a small-scale system meeting these boundary conditions.
Figure 3: Flow chart of the sCO2-HeRo attached to the Glass Model.
Figure 3 depicts the layout of the sCO2-HeRo system attached to one steam generator of the PWR glass model. A primary loop is connected to the glass model containing a motor driven valve and a compact heat exchanger (CHX). The steam is condensed in the CHX and flows back, gravity driven, into the steam generator. The heat is transferred to the sCO2 (secondary loop), where it drives the compressor and the turbine. The ultimate heat sink is ambient air. For more details, see Straetz et al. [8]. The main cycle parameters are given in the following Table 1. It shows that the system primarily fulfills the safety function by transporting the decay heat safely from the core to the environment (air). Although being low, as long as a positive thermal efficiency will be maintained under all operation conditions, the decay heat will be safely transferred to the ultimate heat sink.
ηtherm 4.5 % thermal efficiency
Q̇in,gm 6 kW heat input - glass model
3.2 Work Package 2: Heat Exchanger
In work package 2, led by Technical University of Delft, The Netherlands, the compact heat exchanger for the glass model is designed. The compactness shall be demonstrated having in mind the limited space inside a reactor containment. The heat exchanger consists of stacked plates (Figure 4) with straight channels of 2x1 mm for the condensing steam and Z-shaped channels of the same size for the sCO2. Several plates are diffusion-welded to form the heat exchanger.
Figure 4: Heat exchanger plates.
The two-plate configuration (Figure 4, right side) was tested at the SCARLETT test facility in Stuttgart [9], which provides supercritical carbon dioxide for experimental purposes.
The objective of this testing was to investigate the heat transfer capabilities of heat exchanger plates with small channels. The two-plate configuration was designed to remove 420W. In Figure 5, the heat removal QsCO2 is depicted over the heat input QH2O through the condensing H2O for three different pressures of sCO2 and an inlet temperature of about 40°C as an exemplary test case. Figure 5 clearly shows that the heat transfer capability of the two-plate configuration exceeds the required 420W quite well, providing a suitable heat transfer margin.
Additionally, taking the uncertainties of the measurements into account, the heat transferred from the H2O-side to the sCO2-side can be predicted very well with less than 10% error. Based upon these test results, a compact heat exchanger has been manufactured. In total 14 two-plate configurations are diffusion welded and equipped with inlet- and outlet plena and flanges.
Figure 5 depicts a CAD model of the heat exchanger on the left hand side and a photo of it on the right hand side. The heat exchanger has been tested for leak tightness up to 18 MPa and is ready for shipping to GfS where it will be installed and tested.
steam plate sCO2 plate stacked plates
Figure 5: Heat removal capability of downscaled heat exchanger plates.
The experiments are accompanied by the development of advanced computational fluid dynamics (CFD) models by TU Delft. Conventional CFD methods usually apply either to (almost) constant-density fluids or to compressible gasses, but sCO2 is neither. In combination with the strongly varying material properties, this means that numerical analysis of sCO2 flows has proved difficult. To address this problem, an innovative discontinuous Galerkin method is developed. The numerical scheme will be equipped with a Large Eddy Simulation (LES) model.
This will produce numerical heat transfer correlations, which can be used as input parameters by the German nuclear code ATHLET.
Figure 6: Compact Heat Exchanger (CHX) 3.3 Work Package 3: Turbo-Machine Set
In work package 3, led University Duisburg-Essen, Germany, the turbo compressor system (TCS) is designed [10]. Figure 7 shows the CAD drawing of the TCS on the left hand side, the real machine is shown on the photo on the right hand side. In the sketch, the compressor is depicted in green whereas the turbine is shown in red. Both components are quite small, their
Figure 7: Turbo-Compressor System.
3.4 Work Package 4: Integration into Glass Model
In work package 4, led by GfS, Essen, Germany, the components of the sCO2-cycle will integrated into the PWR glass model shown in Figure 8. The electrically heated model (scale 1:10) of a reactor pressure vessel is visible in the centre of the picture, made of glass.
Figure 8: PWR Glass model [11]
Its maximum power is 60kW. In the background two glass vessels are visible: The right one representing the pressurizer, the left one the pressure relief tank. On the left hand side and right hand side of Figure 8, the steam generators are visible. U-tubes made of glass transfer the heat to the secondary loop. The generated steam is condensed in a special condenser located on the rooftop. The sCO2-HeRo system will be attached to one of the pressurizers. The evaporating steam will be condensed in the compact heat exchanger. The condensate flows back gravity- driven into the steam generator. The heat drives the sCO2-turbine, which generates excess power. Two fan-driven air coolers will be installed outside of the building to transfer the heat to the ultimate heat sink. It is planned to execute the first commissioning tests at GfS, Essen in autumn 2017.
After the commissioning tests, the sCO2-HeRo will undergo a comprehensive test matrix:
Start-up and shut-down procedures will be tested, full load and part load conditions will be applied. Steady state and transient behaviour will be investigated. The sCO2-HeRo system will be equipped with many pressure transducers, thermocouples and mass flow meters. With these data, the cycle calculations and later ATHLET simulations will be validated. In addition, running these tests will help to gain operation experience of this innovative decay heat removal concept.
4 SUMMARY
The “supercritical CO2 heat removal system”, sCO2-HeRo is a very innovative reactor heat removal concept as it improves the safety of both currently operating and future BWRs and PWRs through a self-propellant, self-sustaining and self-launching, highly compact cooling system powered by an integrated Brayton-cycle using supercritical carbon dioxide as working fluid. ATHLET simulations of the system attached to a BWR demonstrated that the grace time for interaction in case of an accident possibly be enlarge to more than 72 hrs. Within the European project “sCO2-HeRo”, six partners from three European countries are working on the assessment of this cycle. The objective is to investigate the potential of this heat removal system and to assess its operation at laboratory scale (Technology Readiness Level (TRL) 3). To reach this objective, a small-scale sCO2-HeRo system shall be attached to the PWR glass model at GfS, Essen for demonstration purposes. The project is progressing very well. Cycle calculations were carried out (WP1) defining the target cycle parameters. A compact heat exchanger has been designed and manufactured (WP2). Downscaled steam condensation experiments in the SCARLETT facility at University of Stuttgart, Germany, demonstrated that the sCO2 is removing the latent heat easily. An innovative turbo-compressor system (WP3) has been built and is currently tested at the SUSEN sCO2-loop in Rez, Czech Republic. After testing, the components are shipped to Essen, where the cycle will be installed and used for gaining operation experience and for demonstration purposes.
ACKNOWLEDGMENTS
The project leading to this application has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 662116.
Conference, Warsaw, Poland, October 9-13, 2016.
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