OPERATION AND SIMULATION OF A THREE-SHAFT, CLOSED-LOOP, BRAYTON CYCLE MODEL OF THE PBMR POWER PLANT

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1 OPERATION AND SIMULATION OF A THREE-SHAFT, CLOSED-LOOP, BRAYTON CYCLE MODEL OF THE PBMR POWER PLANT WMK van Niekerk, PG Rousseau & GP Greyvenstein School of Mechanical and Materials Engineering Potchefstroom University for CHE Private Bag X1, Potchefstroom, South Africa Tel: , Fax: , : mgiwmkvn@puknet.puk.ac.za Abstract The Pebble Bed Modular Reactor (PBMR) is currently being developed by the South African utility ESKOM, as a new generation nuclear power plant. This so-called high temperature gas-cooled reactor plant is based on a three-shaft, closed-loop, recuperative, intercooled Brayton cycle with Helium as the working fluid. In order to demonstrate the envisaged PBMR control methodologies, a model of the plant was built. The conceptual design of the plant was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system. This paper describes the results of the various tests performed on the plant to evaluate the different control methodologies as well as preliminary comparisons between measured and simulated results. I. INTRODUCTION The Pebble Bed Modular Reactor (PBMR) concept is currently being developed in South Africa as a new generation nuclear power plant. This so-called high temperature gas cooled reactor plant is based on a threeshaft, closed-loop, recuperative, inter-cooled Brayton cycle with Helium as coolant. In order to demonstrate the operation of this particular closed-loop configuration as well as to illustrate the envisaged PBMR control methodologies for start-up, load following, steady state full load and load rejection, a model of the plant was built by the Faculty of Engineering at the Potchefstroom University for CHE. This so-called Pebble Bed Micro Model (PBMM) was not intended to be an exact scaled-down version of the actual PBMR plant, but the plant layout has the same topology and representative major components. Also, the control system has the same topology and degrees of freedom as that of the PBMR plant. The design of the plant 1 was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system, making use of the performance characteristics of the individual components. The plant was designed, constructed and commissioned within nine-months from January to September. This paper describes the results of the various tests performed on the plant to evaluate the different control methodologies as well as preliminary comparisons between measured and simulated results. Detail comparisons will only be made once the instrumentation has been properly calibrated. That is part of a verification and validation process that is planned for 3. II. PLANT LAYOUT Some of the differences between the model and the actual PBMR plant are as follows: The turbo machines are off-the-shelf single-stage centrifugal turbochargers rather than purpose designed multi-stage axial flow machines. The plant was designed around these turbochargers because of obvious time and budgetary constraints. Although there are many differences in the detail design of axial and centrifugal flow machines, their overall performance characteristics within a system are essentially the same with regard to pressure ratio and isentropic efficiency versus non-dimensional mass flow rate. Therefore, it is sufficient to illustrate the overall operation of the cycle. The heat source is a high temperature electrical resistance heater of kw instead of the pebble bed nuclear reactor. The maximum design outlet temperature of the heater is 7 C. The working fluid of the model is nitrogen rather than helium. The main reason for this is to allow the use of off-the-shelf turbochargers that were developed for use in large internal combustion engines with air as working fluid. Nitrogen was chosen instead of air because it has essentially the same thermo-physical 1

2 properties but contains no oxygen. This is desirable since the presence of oxygen in the cycle may cause corrosion and flammability problems. The load on the power turbine shaft is an external load compressor rather than an electrical generator as is the case in the PBMR plant. The energy imparted to the fluid by the compressor is dissipated via an external load cooler. Figure 1 shows a schematic layout of the PBMM plant while Figure shows a solid model drawing and Figure 3 a photograph of the plant. All three turbochargers, the heater and the recuperator are situated inside a cylindrical pressure vessel while the pre-cooler, inter-cooler and external load cooler are situated outside of the vessel. The total length of the pressure vessel is 17m. 1 ELC 11 1 HS 3 PTCV PTC HPT 7 LPT 8 PT 9 31 HPC SBSOV LPC SIV SBS 91 RX 1 HPB LPB SBSIV GBP NEV 3 PV 1 IC PC 3 31 NIV 81 8 CT CWP Figure 1 Schematic layout of the PBMM plant. Turbo machinery Recuperator External load cooler Pre Cooler Electric Heater Inter Cooler Figure Solid model drawing of the physical layout of the PBMM plant.

3 Figure 3 Photograph of the actual PBMM plant. The major components in the cycle are (see Figure 1): LPC IC HPC PV RX HS HPT LPT PT PTC ELC CT CWP SBS PV Low pressure compressor. Inter-cooler. High pressure compressor. Pressure vessel. Recuperator. Heater. High pressure turbine. Low pressure turbine. Power turbine. Power turbine compressor. External load cooler. Cooling tower. Cooling water pump. Start-up blower system. Pressure vessel. The following major valves also form part of the system: GBP LBP HBP NIV NEV SIV SBSIV SBSOV PTCV Gas cycle by-pass valve. LP compressor by-pass valve. HP compressor by-pass valve. NICS injection valve. NICS extraction valve. System in-line valve. Start-up blower system inlet valve. Start-up blower system outlet valve. Power turbine compressor valve. III. RESULTS rejection and load following as well as the results that were obtained. III.A. Start-up During start-up the cold plant must be brought to self sustained operation. The description of the preparation and conditioning of the plant for start-up falls outside the scope of this document. The SIV is closed and the SBS is used to circulate the nitrogen through the cycle. The SBS is a positive displacement device and the flow-rate remains essentially constant. Heat is then added to the nitrogen in the heater. This energy is converted in the turbines into shaft work to power the compressors. For start-up, the power to the heater is kept constant at about kw. As the system heats up, the inlet temperature of the nitrogen flowing to the heater rise which causes the outlet temperature of the heater and the shaft work from the turbine to increase. This causes the pressure increase across the SIV to drop. The cycle spirals towards self sustained circulation and the SBS is disengaged when the pressure increase over the SIV becomes negative. The cycle is said to have bootstrapped. The results for a test run are shown in Figure. From the graph it can be seen how the power consumption of the SBS decreases as the compressors starts to contribute to the power required to circulate the gas. Initially there is a high pressure increase across the (closed) SIV. This decreases slowly and the moment it reaches -.kpa, the SIV opens and the SBS is disengaged as can be seen by the sudden drop to zero of the SBS power consumption curve. This section describes the tests that were conducted on the model namely start-up, inventory control, load 3

4 Pressure Incease over SIV [kpa] Time [minutes] Figure Pressure increase over the SIV and the SBS power consumption as function of time The exit temperature of the heater is probably the most important determinant of the bootstrap point as the energy that the turbines can deliver, depends on the gas inlet temperature. An estimation of the bootstrap temperature was made by calculating (with Flownet), the steady-state pressure increase over the SIV as function of heater outlet temperature. In Figure a comparison between the calculated and measured variables can be seen. The actual bootstrap temperature is 98 C while the calculated temperature is 8 C. It should be kept in mind that it is a comparison between a calculation done at steady-state while start-up is a transient condition. A more valid comparison will only be possible once the integrity of the measurements has been confirmed during the verification and validation study that will be conducted in 3. Pressure increase over SIV [kpa] HPT inlet temperature [ o C] Measured Predicted Figure Measured and predicted pressure difference across the SIV as function of heater outlet temperature SBS power consumption [kw] III.B. Inventory control The output of the power turbine can be increased by increasing the mass of nitrogen in the cycle. Therefore, after the system has bootstrapped and the outlet temperature of the heater is controlled stably at C, nitrogen is injected into the cycle to increase the power output. The suction pressure of the LPC is taken as an indication of the mass of nitrogen in the cycle. In Figure the output of the power turbine and the suction pressure of the LPC are shown as function of time. The immediate effect of injecting nitrogen into the system is to reduce the power output of the power turbine. Injection takes place via the NIV at point 1 in Figure 1. Injection causes a pressure increase in the line downstream of the Power Turbine (PT) which decreases the pressure differential over the PT and reduces its power output. The injected nitrogen also increases the load on the compressors. It is clear from the graph that the power output drops the moment injection commences and the suction pressure of the LPC increases. It takes the system a while to overcome this effect and for the output of the power turbine to rise again above its original value. LPC inlet pressure [kpa] Time [minutes] Figure Increasing the power output by increasing the mass of nitrogen inside the pressure vessel. The dips in suction pressure at 8, and 7 minutes are caused by the nitrogen injection system. The nitrogen in stored in the Nitrogen Inventory and Control System (NICS). The NICS consists of four tanks connected to a header. The header is connected to the pressure vessel via an injection control valve (NIV) and an extraction control valve (NEV). Isolation valves separate each tank from the header. The NICS must supply nitrogen for injection but also store extracted nitrogen. Before start-up, the tanks are filled with nitrogen. The pressure in the tanks is initially about 8kPa. Injection takes place by feeding nitrogen from one tank at a time to the cycle. If the pressure in a tank drops too low for the injection flow rate to be maintained, its isolation valve closes and the isolation valve of a full tank is opened. The change over from one 3 1

5 tank to another interrupts the injection flow and lead to a sudden drop in the LPC suction pressure. As the LPC suction pressure drops, the output from the power turbine momentarily increase for the same reasons the power output drops the moment the LPC suction pressure increases. In the same way, extracting nitrogen from the system decreases the inventory and leads to a drop in the output of the power turbine. III.C. Load rejection In case of a loss of load scenario for the actual Pebble Bed Modular Reactor (PBMR), it must be possible to quickly reduce the power output of the power turbine to prevent the generator form over speeding. This is done by opening the GCBV. This connects the points of highest and lowest pressure within the system and reduces the overall system pressure ratio and therefore also the power output. Special care must be taken in this case to maintain self-sustained operation after the load rejection, albeit at a much lower power level. In the actual PBMR the GCBV is a quick acting, open-close valve. After the power output has been reduced, the GCBV is closed again and the output of the cycle at the reduced level is controlled by the slower compressor bypass valves. In the PBMM an ordinary (segmented ball) control valve is installed to make load rejection at different flow rates possible. In this test it is demonstrated that the output can be quickly reduced by opening the GCBV. The installed compressor bypass valves were not used as they were designed for load following - that will be described later. In Figure 7 it can clearly be seen how the output of the Power Turbine is reduced by opening the GCBV. The influence of the valve opening on the rate of decrease has not been investigated but it is reasonable to expect that opening the valve more will lead to a quicker reduction in the power output. This was not attempted for fear of jeopardizing self sustained operation as it was clear that the present setting was close to the point where self sustained operation will not be possible any more Time [sec] Output GCBV Figure 7 Reducing the power output by opening the GCBV III.D. Load following: Increasing output It is necessary for the plant to be able follow gradual changes in the power set-point. As explained earlier, if nitrogen is injected into the system to increase the output of the power turbine, the outlet pressure of the power turbine momentarily rises and the power output falls before it starts to increase. To prevent this unwanted transient, bypass valves are installed across the compressors. These valves have to be slightly open before the power output can be ramped up. As nitrogen is injected, these valves are closed gradually to decrease the load on the compressors and prevent the dip in power output. In Figure 8 the power output set-point is ramped up. It can be seen how the HPC bypass valve closes the moment injection starts to prevent a drop in the power output. (The bypass valve around the LPC is not shown but does the same.) A smooth increase in power output is achieved. A tank change at the end of the injection phase causes the sudden variation in the power output Valve position [%] Valve opening [%] Figure 8 Increasing the power output by closing the compressor bypass valve.

6 In Figure 9 the injection flow rate and the LPC suction pressure is shown. As nitrogen is injected the LPC suction pressure rises but the closing bypass valve prevents a sudden increase that will cause a drop in power output. The effect of the tank change is visible in the injection flow rate. Injection flowrate [kg/s] Figure 9 Injection flow rate and LPC suction pressure during an increase of power output. III.E. Load following: Decreasing output The plant must also be able to gradually decrease its power output without the unwanted transients described earlier. This is done by extracting nitrogen while opening the compressor bypass valves to prevent the sudden increase in power output caused by the extraction of nitrogen. Extraction takes place from the point of highest pressure the outlet of the HPC, point 3 in Figure 1. The extracted nitrogen is stored in the NICS tanks. One tank at a time is used. If the pressure in the tank increase to such an extent that the extraction flow rate cannot be maintained, its isolation valve closes and the valve of an empty tank is opened. The output of the power turbine for a decreasing output is shown in Figure 1. It can be seen how the LPC suction pressure drops as nitrogen is extracted. At approximately 7, 9. and 13 minutes, NICS tank change over took place. Although it influenced the LPC suction pressure the power controller maintained a smooth ramp down of the power output. The change in suction pressure of the LPC is much more than in the case of the rising output. Figure 11 shows the compressor bypass valve opening and the extraction flow rate. Note again the effect of tank changes on the extraction flow rate. Note also that the opening of the bypass valve does not vary nearly as much as in the case of the rising output. The tank changes cause the bypass valve to open for a while and then close again as soon as the tank is on line. LPC suction pressure [kpa] LPC suction pressure [kpa] Figure 1 Decreasing the power output by lowering the LPC suction pressure. Valve position [%] Figure 11 Compressor bypass valve opening and extraction flow rate during a decreasing power output. IV. CONCLUSIONS As the plant has been started up eighteen times and has been running self sustained for a total of almost thirty hours, a good indication of the behaviour of the plant was obtained. However, more operating experience is necessary to fully understand the plant. The following conclusions can be made at this stage: a) A fully functional, properly controllable, robust plant based on a three-shaft, intercooled Brayton cycle was designed, built and commissioned. This was made possible to a large extent by the availability of the Flownet software that could simulate the integrated plant. b) The tests indicate that the control methodologies designed using the simulation software are indeed effective in controlling the plant. During operation the plant is stable and easily controllable. The plant seems to be robust and the turbo compressors are running well within their design specifications. Extraction flow rate [kg/s]

7 c) The load rejection test involved only the opening of the GCBV. This proved that it is possible to reduce the output of the power turbine quickly. It should be determined if the drop in the power turbine output can be made faster by opening the valve more or whether this will make the cycle unstable. d) The agreement between the first measured values and the simulated values are good, but detailed comparisons will only be possible during the validation and verification phase the will take place during 3. REFERENCES 1. G.P. GREYVENSTEIN and P.G. ROUSSEAU, Design of a physical model of the PBMR with the aid of Flownet, Nuclear Engineering and Design, Accepted for publication, (3). G.P GREYVENSTEIN, An implicit method for the analysis of transient flows in pipe networks, Int. J. Numer. Meth. Engrng., 3, (). 7