Concepts important for operation

3.4.1. Provisions for deep load following and fast runback

All grid systems require some generation to be able to operate flexibly and to allow generation to change to match variations in demand. Countries with a grid system interconnected to other countries will also need to operate flexibly to control power flows across the interconnectors to other networks as demand varies. Many SMR designers are required by potential customers to provide deep load following capabilities as part of the utility’s overall energy mix balancing strategy. One such strategy is presented in Fig. 5, which shows an SMR operating in a hybrid mode with renewables. This is particularly important for very small grids or remote grids where loads fluctuate rapidly. Some SMR concepts, in particular transportable nuclear power plants will need to operate for significant periods of time on very small local grids, which is approaching an operating state called ‘islanding mode’.

New nuclear units generally have greater load following capability than those found in the current fleet.

However, because of the effect of thermal transients during load changes (e.g. fuel and pressure vessels), they will have restrictions in their technical specifications that limit the magnitude or speed of load variation and the number of load cycles.

Fast runbacks are generally a reactor protective measure initiated in response to plant or grid transients. It is used to ensure reactor availability and to prevent reactor poison out. This protective strategy assists the operator in reducing stressful transient effect on the lifespans of plant SSCs.

Depending on the design characteristics of the reactor, a fast runback can be a measure used for deep load following, for example if a facility is generating in concert with local renewable power generation. To improve load following and the use of fast runback in SMR technologies, it is sometimes necessary to design alternative control strategies to adjust promptly the set point for the electrical energy provided to the grid, according to the load demands: for example, to limit the thermal transients on the plant during the load following operational mode, one measure might be to develop multiple input/output control strategies, in which several control actions could be performed on the plant to regulate not only the power production but the temperature field as well.

In some cases, it may not be possible to rely on a proportional integral derivative based strategy in which the different actuators tend to control the different output variables independently, providing set points which can also cause disturbance on the other system controlled variables. In those cases, the adoption of feedback single input/

output control loops might not yield acceptable performance.

3.4.2. Integrated supervisory control

Integrated, intelligent and autonomous supervisory control architecture has been investigated in many industries for some time [36]. Some SMR concepts explore an integration of control with SDP in a semi-autonomous fashion both to enhance the operation of the plant and to reduce the operation burden on plant staff. This approach has an overarching supervisory control strategy, with the SDP as one portion. The integrated architecture combines robust and resilient decision algorithms to enhance the operational safety and performance of SMRs through the detection, diagnosis and mitigation of anticipated faults and transients. As Jin et al. [37] report:

“The objective of robust control is to minimize the sensitivity of plant operations to exogenous disturbances and internal faults while achieving a guaranteed level of performance with a priori specified bounds of uncertainties. On the other hand, the role of resilient control is to enhance plant recovery from unanticipated adverse conditions and faults as well as from emergency situations by altering its operational envelope in real time. In this paper, the issues of real-time resilient control of nuclear power plants are addressed for fast response during emergency operations while the features of the existing robust control technology are retained during normal operations under both steady-state and transient conditions.”

Distributed Generation

Distribution Transmission

Centralized Power

Generation Distribution

Center Small Modular

Reactor

Turbine Building

FIG. 5. Distributed generation and the consequent possibility of reversal of the traditional power flow from the high tension to the medium voltage line.

The goal is to have an integrated fault tolerant and resilient system for control and for transient mitigation that incorporates the following technology features:

(a) Fault monitoring and diagnosis of instrumentation, devices, equipment and systems;

(b) Robust and resilient control strategies that mitigate faults and reconfigure the plant for safe and reliable continued operation.

Figure 6 is an example block diagram of a supervisory control system architecture for multiple modules with embedded fault diagnostics. As Ref. [38] reports:

“This approach provides the framework for autonomous control while supporting a high-level interface with operations staff who can act as plant supervisors. The final authority for decisions and goal setting remains with the human but the control system assumes expanded responsibilities for normal control action, abnormal event response, and system fault tolerance. The autonomous control framework allows integration of controllers and diagnostics at the subsystem level with command and decision modules at higher levels.”

Some current plants incorporate some of these design features; however, not at the degree expected for SMRs. For example, the first computer controlled (automated reactor regulating system) CANDU units were at the Pickering A station in 1971 (four 540 MW(e) units). Since then, the controls have gone through a series of upgrades, some of which incorporate additional functionality. The system currently has the ability to detect some abnormal conditions, such as heat transport pump shutdown or aberrant steam generator water levels. When abnormal specific conditions are identified, the control system automatically takes corrective or compensatory actions such as reducing power or shutting down the reactor. The setback (slow linear decrease) and step back (fast decrease) routines respond to abnormal condition signals and take action to restore the station to the normal operating domain.

Reactor Supervisory and Coordination Function

Source: Based on fig. 4 of Ref. [38].

FIG. 6. Supervisory control architecture for multimodular nuclear power plants.

Figure 7 presents an implementation scheme for a supervisory control algorithm with integrated SDP.

The integrated robust and resilient control architecture has been studied by several research groups in several countries. Jin et al. [37] present a solution developed for the IRIS reactor under the DOE grant Advanced Instrumentation and Control Methods for Small and Medium Reactors with IRIS Demonstration [39] and they report:

“[Figure 8] depicts the layout of an integrated robust and resilient control system that includes the robust controller, resilient controller, fault detector, finite state machine (FSM), a set of reference points, and a filter bank for bumpless transfer. The fault detector is extrinsic to both robust and resilient controllers.”

The integration of control with SDP will require detailed analysis of the system interactions to avoid unintended consequences. This is of utmost importance when developing a safety case for regulatory approval.

Perception

Feedback

Effectors

Internal observables State observables External observable

State changes

Low-level commands High-level

commands System

interface Diagnosis, prognosis, and

planning Control, sensing

Simulation -sensitivity analysis Simulator

(Simulink model)

Planner

& scheduler

Diagnosis &

prognosis

Executor &

controllers Sensor locations

Outputs

Inputs Current plan Revised plan World state World state changes

Knowledge World state Inferred world state updates

Replan

Expectations

Direct world state updates

Anomalies

IRIS system Models / knowledge base SMR

Source: Based on fig. 1.3 of Ref. [39].

FIG. 7. Integration of advanced instrumentation and controls modules for SMRs.