Reactor Schematic Diagram Two Feed and Two Outputs
Decoupling control of both turbine power and reactor power in a marine use multi-reactor and multi-turbine nuclear power plant
Abstract
In multi-reactor and multi-turbine nuclear power plants (MMNPP), several reactor units working in parallel provide steam to turbines that generate power for different purposes. Reactor units can be classified as load following reactors (LFRs) and fixed-power reactors (FPRs) to provide more flexible, adaptable and secure power sources to the loads. Due to a common steam header that all reactors and turbines are connected to, power change in one turbine can interfere with the steady operation of other turbines (and their power output) when operating in load following scenarios. For the same reason, changes in the power output of LFRs can influence that of FPRs. These may threaten the stable and safe operation of the plant, especially in the harsh environments of the sea and ocean. To reduce the coupling effect between each turbine and that between each reactor, this paper propose three kinds of decoupling control methods. One of them is based on feedforward compensation decoupling control method, aimed at decoupling the turbine power outputs only. Another one is composed of a feedforward feedwater controller and a feedback header steam pressure controller, designed to reduce the mutual interference between reactor power outputs. The third control strategy is the combination of above two methods, decoupling turbine power outputs as well as reactor power simultaneously. Simulation results indicate that among the proposed strategies, the second and third method provide better performance than the first one, by reducing both of the coupling effects to the minimum with faster power tracking capability and strong robustness.
Introduction
In a MMNPP, several small modular reactor units operate in parallel to supply steam to a common steam header. The steam then is distributed to several steam consuming units with certain shares (Waltrip, 1989; Nuerlan et al., 2020). The reactor units can be classified as load following reactors (LFRs) and fixed-power reactors (FPRs) to provide more flexible, adaptable and secure power for the external demand. The energy produced by the MMNPPs can be simultaneously provided for multiple purposes. One example is that the MMNPPs could be used in submarines and aircraft carriers to provide the driving force and electricity for ships (Reistad and Olgaard, 2006; Hirdaris et al., 2014). The conventional control system of MMNPPs will consist of reactor power controllers, a steam pressure controller for the header, steam generator feedwater flowrate controllers for and turbine governors (Nuerlan et al., 2020). Any imbalance between the total power outputs of reactor units and the total load demands of steam consuming units can be reflected in a change in header steam pressure. This imbalance can be controlled by a collaborated operation of header steam pressure controller, feedwater flowrate controllers, reactor power controllers and turbine governors. However, the mediating role of the header steam pressure may couple the dynamics of header inlet flows as well as header outlet flows, and may further lead to undesirable mutual interference between power outputs of reactor units and that of turbines. This coupling effect in some cases may induce steady and safe operation problems of the MMNPPs.
The load following characteristics analysis of modular reactors and coordination control of the reactor units are important for the safe and stable operation of the plant. There are several researches focusing on the control of reactor units of MMNPPs. Dong et al. (2016(a); 2016(b)) designed a reactor coordination controller based on fluid flow network for a two-module HTR-PM plant. Kim and Bernard (1994) pointed out the interdependency between reactor modules and provided operating strategies. The study of Perillo et al. (2011) showed that MMNPPs have the advantage of providing continuous power supply when one of the units is shut down for maintenance, which also demonstrated the load following capability of the MMNPPs when the reactor units are operating at different power levels.
Header steam pressure control is another issue that needs to be addressed. Li (2007) offered a feedforward-feedback cascade steam pressure control strategy for the steam header of a two-reactor and four-load NPP. The author's previous works (Nuerlan et al., 2020) focused on the coupling effect among header outlet flows, and designed a decoupling controller based on feedforward compensation decoupling method. However, this work should be extended to the decoupling control of the turbine power outputs. It can be seen that previous studies have not dealt with the coupling effect between reactor power outputs in a MMNPP.
Coupling among various parts of a system is a physical process and cannot always be artificially shut off. However, undesirable coupling among components of a system can be reduced by adding an appropriate pre-compensation controller in the system, which is the basic idea of the decoupling control of multivariable systems (Wang et al., 2018). The interference between reactor power outputs can be minimized designing a feedforward signal to the LFRs. Moreover, the turbine power output coupling can be reduced applying valve opening compensators to scale down the effect of the header steam pressure change to the turbines based on feedforward compensation decoupling method (Garrido et al., 2012).
In this work, nonlinear mathematical models of the header and turbines for a MMNPP with m reactor modules and n steam turbines is developed first. A two-reactor and five-load MMNPP for a marine ship is used for numerical simulations. Then, the coupling effects are analyzed between each reactor unit and between each turbine unit. This will be followed by the feedforward compensation decoupling controller design for the turbine throttle valve openings. Following is the development of the set point feedforward-feedback controller for the feedwater valve of reactor units. Dynamic simulations are carried out to evaluate the control effect of the designed decoupling controller in Section 5. Conclusions are reported in Section 6.
Section snippets
Dynamic modeling of the header and steam turbines
A schematic diagram of a MMNPP consisting of m small integrated reactor units, a single steam header, n steam turbine units, a condenser, a feed water pump and necessary pipelines connecting them is shown in Fig. 1. It is shown that q units of the turbines are propulsion turbines (p-turbines, Turbine-1 to Turbine-q), which provide driving force for the ship. The remaining are electricity generating turbines (e-turbines), supplying essential electric power for the ship. Mathematic models of the
mPower control program
BWX has picked an alternate strategy for reactor control which is based on a constant hot leg temperature, independent of thermal power than the conventional strategy which controls the average coolant temperature as a constant at 320 °C. Keeping hot leg temperature as constant means that the average temperature of the coolant must change. However, in mPower, the reactor power control is still achieved by controlling the average coolant temperature. Variation of the set point for coolant
Turbine power decoupling
Deduction of transfer functions between input vector and output vector of header can be found in the previous work of the author (Nuerlan et al., 2020). The OTSG steam enthalpy is not included since its effect is little compared to OTSG steam pressure (Nuerlan et al., 2020). The decoupling objects here are the power outputs of p-turbine and e-turbine, so the transfer function between turbine power and corresponding
Result analysis and discussion
In order to investigate the control performances of the two ingredients of the proposed control system and the combined version of them, decoupling effects of the three decoupling controllers, turbine power decoupling control (TPDC), reactor feedforward-feedback control (RFFC) and turbine power decoupling and reactor feedforward-feedback control (TPD-RFFC) are investigated by applying them into the two cases reported in Section 3. These are also compared to the control results of the
Summary and conclusions
Focusing on the coupling effect between power outputs of two turbines and that of two reactors in the two-reactor and five-load MMNPP for a marine ship, this paper presents three decoupling control methods. First one is based on feedforward compensation decoupling control, aiming at minimizing the mutual interference between propulsion turbine and electricity generating one. Second method uses feedforward-feedback feedwater flowrate and header steam pressure control to decouple the reactor
Credit author statement
Enclosed is the copy of manuscript by Areai Nuerlan, Jiashuang Wan, Pengfei Wang, Fuyu Zhao titled "Decoupling control of both turbine power and reactor power in a marine use multi-reactor and multi-turbine nuclear power plant.", which we wish to be considered for publication in Progress in Nuclear Energy Journal. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors acknowledge the support provided by the China Scholarship Council (201906280380), China Postdoctoral Science Foundation (Grant No. 2018T111068), and the Fundamental Research Funds for the Central Universities (xjj2018072).
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Source: https://www.sciencedirect.com/science/article/abs/pii/S0149197020303449
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