Load Following of Small Modular Reactors: Comparison
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Please note this is a comparison between Version 2 by Catherine Yang and Version 1 by Harold Chisano Oyando.

Small modular reactors (SMRs) are generally defined as nuclear reactors with power outputs between 10 megawatts electric (MWe) and 300 MWe. In order to solve such a problem, a load-following operation that adjusts the output of the existing large-capacity thermal power plant or nuclear power plant according to the increase or decrease in the load is required. While this is not technically impossible, it is technically and economically undesirable. The next alternative that comes to mind is a small modular reactor (SMR). This is because, by operating multiple SMRs of less than 300 MW together, as much as necessary, it is easy to carry out a load-following operation, and the safety of an SMR is much higher than that of existing large scale nuclear power plants.

  • small modular reactor (SMR)
  • load following
  • power grid
  • variable renewable energy (VRE)

1. Requirements for the Load-Following Operation of SMRs

Since an SMR has a smaller unit capacity compared to a large nuclear reactor, thermal power control is relatively easier and less risky in terms of nuclear safety. In addition, because start up and shut down are faster, bulk output power control is possible through unit control.
Load following means changing the power generation as closely as possible to the expected power demand. Load-following generation can match demand by the output changes in a planned manner or in response to instructions or signals from the grid control center or transmission system operator (TSO). Changes in output can be large or small and frequent or infrequent [9][1]. 

1.1. Power Change Dependent on Grid Plans

(1)
Planned Operation
This refers to the planned control of the power plant output between 20% and 80% based on the power supply and demand plan, and the output control timescale is hours or days. In some cases, the reactor output is adjusted to the level of 20% to 80% for the repair or recovery of the reactor, and the timescale in this case is hours. The power output maneuvering range is a function of time. During the first 65% of the fuel cycle, output power is controllable between 100% of nominal power and around 25% of nominal power. Then, the power control range is gradually reduced from 25% to 80% of nominal power because of excess reactivity and low boron concentrations. Nuclear power plants can operate at a minimum power level of 10%. However, the minimum output is around 20%, as for many conventional power plants [16][2].
(2)
Unplanned Operation
This refers to the case where the power plant output is unplanned and adjusted according to the power grid conditions, and the load-following operation is performed by controlling the output of the nuclear reactor between 20% and 80%. The timescale for TSO instruction operation is minutes.

1.2. Power Change Dependent on Frequency

(1)
Automatic Generation Control (AGC)
This refers to automatically reducing or increasing plant output within a limited range according to signals from the transmission system operator (TSO). This type of operation is also referred to as the ‘Automatic Generation Control’ (AGC). The power change is typically within 20~40% of the RTP [10][3] or ±10% of the rated thermal power (RTP) [12][4], and the timescale is minutes.
(2)
Governor-Free (GF) Control
This refers to controlling frequencies outside a specified frequency range, either by reducing the generator output by the turbine governor when the system frequency exceeds the maximum limit or increasing the output when the system frequency falls below the minimum limit. The plant operator, in response to the frequency deviation, can initiate frequency control automatically or manually. The power change is typically within 20~40% of the RTP [10][3] or ±10% of the RTP [12][4], and the timescale is seconds.

1.3. Coordinated Rapid Load Following

A coordinated control approach accomplishes a rapid load-following operation by wisely combining the ‘reactor following turbine mode’ and ‘turbine following reactor mode’ along with the satisfaction of the reactivity constraints. The coordinated mode of power variation can be explained as follows:
The reactor is assumed to operate normally in the ‘reactor following turbine mode’ and at a certain power output level of Pth = Ptha, much lower than 100% full power (FP). In that condition, the output is increased to a high value of Pthb within a short time interval “T” and held to that value for the rest of the time. The coordinated mode may be due to the nature of the daily load curve. For example, where the reactor’s power has to match a growing demand, a predefined instant consumption suddenly increases [10][3].

1.4. Multi-Module Unit Operation

If the plant has multiple SMRs, the entire power output of the plant can be adjusted by the disconnection of some reactor modules during periods of low demand, for scheduled maintenance, or when significant, high-priority energy becomes available from intermittent renewable energy system (RES) [17][5]. The performance data of the SMRs are not yet verified by commercial operation, but for German nuclear power plants, at least 3 h of downtime and at least 1 h of operation have been established, taking into account the start and end times [18][6].

1.5. Cogeneration with Non-Electric Applications

Due to SMRs’ ability to provide CO2-free energy, applications in the district heating field are mainly being discussed. Although smaller than traditional nuclear power plants (NPPs), SMR plant designs can provide increased safety through passive systems, reduce costs, and increase quality through factory-based manufacturing and other advantages [19][7]. The primary side circuits of the SMR can be operated at rated capacity, and only a fraction of the heat can be converted for other purposes, such as district heating, desalination, or hydrogen production [20][8].
The requirements that the cogeneration SMR must have in order to perform a load-following operation are as follows: First, the distance between the SMR and the heat utilization plant should be reasonable. This is because heat is not easier to transmit than electricity. Second, the steam extraction amount and control speed suitable for SMR to perform load-following operations should be technically acceptable from the perspective of coproduction plants and be economically feasible. Third, the temperature of the steam must be adequate to produce a coproduct.
The higher the temperature, the more types of cogeneration facilities are available, so this is a key parameter. Most light water reactor (LWR) coolant outlet temperatures are around 300 °C [21][9]. Future high-temperature reactors can operate at higher temperatures. For example, the coolant outlet temperature of a sodium-cooled high-speed reactor is approximately 500~550 °C compared to 850~950 °C for a high-temperature gas reactor (HTGR) [22,23][10][11].

2. Other Considerations for Load Following

2.1. Regulatory Requirements

(1)
Safety Regulations
The reactor will continue to generate significant heat from the decay of fission products that persist on a logarithmic timescale, even when the chain reaction is completely stopped. The principle of providing “defense in depth” against scenarios where the NPP is unable to provide long-term core decay heat removal shall be provided [24][12]. US nuclear regulatory commission (US NRC) safety and licensing criteria related to electric power are contained in general design criteria (GDC) 17 [25][13]. The design criteria of preferred power supply (PPS) and its interface with the class 1E power system, switchyard, transmission system, and alternate ac (AAC) source are described in IEEE Std 765, ’IEEE Standard for Preferred Power Supply (PPS) for Nuclear Power Generating Stations’ [26][14]. IAEA safety standard series No. SSG-34, ’Design of Electrical Power Systems for Nuclear Power Plants’ [27][15], provides the safety guide on the necessary characteristics of electrical power systems for nuclear power plants and of the process for developing these systems.
If the safety-related systems actuate by passive means and their continued operation relies on natural cooling principles, a safety-related electrical system is not required. For this reason, NuScale requested in the license document that it be excluded from GDC 17 [28][16].

2.2. Technical Considerations

(1)
Physical aspects of power regulation
In terms of load following by fuel rod control, the following factors affect the maneuverability. By the moderator effect and Doppler effect, if the temperature of the primary coolant is increased, reactivity is decreased. When the reactor power increases, the power distribution is pushed to a lower part of the fuel [11][17]. If power variation is made by control rods, they deform the axial distribution of power and 135Xe. Thus, it is an additional challenge for the load following with large magnitudes of power variations. At the end of the fuel cycle, the margins for the maneuverability decrease because the boron concentration is almost zero and the control rods are in the upper position [11][17]. The use of the control rod alone for power control has negative consequences, such as flux distribution disturbance, component material fatigue, mechanical wear, and adverse impacts on the burn-up balance in the core [29][18].
(2)
Influence of the load following on the lifetime of components
Operating the NPP in load-following mode introduces technical disadvantages, as the plant components are exposed to numerous thermal stress cycles. This results in faster aging and requires a more sophisticated system for reactor monitoring and control [29][18]. Load cycling results in variations in the coolant temperature and thus in the temperatures of different components. Repeated temperature changes can create cyclic changes in the mechanical load of a part of the equipment and cause local structural damage (fatigue) to these elements. As a result, the maximal number of load-following operation cycles during the whole operational lifetime of the plant should be considered in the equipment qualification of the safety-related components [11][17].

2.3. Interaction of Grid Characteristics with Nuclear Power Plants

(1)
Effects of Grid Frequency Change on NPP
Changes in frequency affect NPP operation through the speed governor of the turbine generator and through the speed change in the pump that delivers the flow to the reactor and the secondary coolant circuit. If the frequency drops, the turbine/generator examines the load based on the governor droop setting and frequency deviation. The mismatch between the reactor output and the produced electric power requires intervention from the control system. As the frequency rises, the turbine speed governor closes the throttle valve on the turbine to reduce power. When the reactor output has not changed, the reactor output is greater than the power drawn by the turbine. This mismatch causes transient overtemperature and overpressure in pressurized water reactors. Modern turbines for the grids of developed countries can only operate for a few minutes at a frequency below their rated frequency. These adverse operations have a cumulative effect and are only allowed for a certain total period over the lifetime of the turbines [30][19].
(2)
Effect of Grid Voltage Change on NPP
A rapid voltage drop is mainly caused by electric fault on a transmission line. The magnitude of the voltage dip depends upon the distance from the fault, the type of the fault, and upon the performance of the automatic voltage regulator (AVR) of the generators connected to the grid [31][20]. During sharp voltage dip conditions, all motors connected to the auxiliary power system of the NPP will be retarded. The magnitude of the retardation is determined by the voltage dip and its duration, the characteristics of the motor, and the mass moment of inertia of the motor pump assembly [30][19].
If the grid has a light load and the NPP remains connected with long lines at the remote end, the grid voltage may be higher than generator voltage. If high grid voltage is continued for a long period, then the generator connected to the grid may be unstable because the generator must consume a large value of reactive power (Mvar). On the other hand, if the grid voltage remains low, the large motors of the NPP cannot start or can be retarded.

References

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  14. IEEE Std 765-1983; IEEE Standard for Preferred Power Supply (PPS) for NUclear Power Generating Stations. IEEE: New York, NY, USA, 1995.
  15. IAEA. Design of Electrical Power Systems for Nuclear Power Plants; Specific Safety Guide, No. SSG-34; IAEA: Vienna, Austria, 2012; pp. 1–144.
  16. NuScale Power. Nuscale SMR Technology: An Ideal Solution for Repurposing U.S. Coal Plant Infrastructure and Revitalizing Communities; NuScale Power: Corvallis, OR, USA, 2021.
  17. Nuclear Energy Agency. Technical and Economic Aspects of Load Following with Nuclear Power Plants; NEA, OECD: Paris, France, 2011; pp. 1–51.
  18. Wenisch, A.; Becker, O. NPP Output Flexibility Expectations in the Light of Reality; Österreichisches Ökologie-Institut: Vienna, Austria, 2010.
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  20. Chowdhury, A.H.; Rabby, M.K.M. A study on low grid voltage problem near Rooppur nuclear power plant. In Proceedings of the 8th International Conference on Electrical and Computer Engineering, ICECE, Dhaka, Bangladesh, 20–22 December 2014; pp. 289–292.
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