1.1. BACKGROUND
At present, there are no direct and economical methods for storing electrical energy on a large scale; therefore, all electrical power systems currently require the generation of electrical energy to be adjusted continuously so that it closely matches the variable demand, in order to stabilize system frequency. If the electricity generated is insufficient to meet the demand, the system frequency will decrease; if the electricity generated exceeds demand, the frequency will increase. This balancing is achieved by increasing or decreasing the electrical output of individual generating units, so that the total electricity generated matches the variation in total electricity demand.
Some generating units need to be able to change electrical output rapidly (within a few seconds), and others may only need to change electrical output slowly (from a few minutes to a few hours). This adjustment of electrical output under the control of the grid system operator will depend on the variation in the mismatch between energy generation and demand. The adjustment may be required frequently (several times a day) or rarely (from once or twice a year to a few occasions in the lifetime of a power plant). It may also be necessary to change the electrical output of certain generating units to control power flows on the transmission system. In particular, in countries that have an electrical power transmission system that is connected to those of other countries, it is necessary to adjust the electrical power generated in order to control electrical power flows across the interconnections.
In balancing the generation and demand and controlling power flows, it is customary, in electrical power systems consisting of a spectrum of units utilizing different fuel sources for electrical generation, to operate the generating units in commercial/financial merit order, to minimize the overall operating costs. The generating units with the lowest marginal cost (generally the lowest fuel cost) are therefore operated at full power as much as possible. Generating units with higher marginal costs (higher fuel costs) are operated with reducing/ceasing electrical output at times of low electrical demand, or operated partially loaded, to increase or decrease generation when requested. Such units change power either automatically on signal or manually on instruction, as needed, thus providing ‘reserves’. In this balancing, some generating units in an electrical system are operated continuously at steady full rated power as much as possible, and are available 24 hours per day, 7 days per week. This mode of operation is termed ‘baseload operation’. In this publication, the term ‘flexible operation’ is used to describe any mode of operation that is not baseload, specifically focusing on changing electrical output to match generation with demand on the electrical grid system. In flexible operation, the electrical output of a generating unit can be adjusted, either automatically or manually, to meet the technical requirements and/or commercial agreements in order to control the frequency and power flows of the electrical system. In flexible operation mode, a nuclear unit may spend prolonged periods operating at less than full load. It is not necessary for all the generating units to operate ‘flexibly’, or to have the same capability for flexible operation, provided that there is sufficient aggregate
‘flexibility’ among all the generating units on the electrical system to allow electricity generation and demand to remain matched at all times.
Nuclear power plants have relatively low marginal costs, and so, in most Member States, nuclear generating units are operated in baseload mode. Consequently, the majority of existing operating plants do not have the need for or experience of operating flexibly, because they have been operated at steady and continuous full rated power, except when it is necessary to reduce power or to shut down for maintenance and refuelling, or for other operational or safety reasons. In the electrical systems where nuclear power plants are operated in baseload mode, other generating units (e.g. hydroelectric, coal, oil or gas fired) operate flexibly as required, to balance generation and demand.
In the early years of nuclear power, some owners/operators of nuclear units considered and requested designs with capabilities, and performed tests, for flexible operation. They also carried out a limited amount of load following operation, as reported in the literature [1–5]. Nevertheless, since that time, the majority of nuclear power plants have operated at baseload, and have optimized their plant and equipment for operation in that mode. However, there is a recent and increasing need in some Member States for nuclear power plants to operate flexibly. This need has arisen primarily either because nuclear generation has become (or will soon become) a large percentage of the total energy generation or because there has been the introduction and significant growth of other forms of energy generation that are not readily controllable (or have lower marginal costs), including the variable and intermittent forms of renewable energy such as wind turbines and solar photovoltaic panels.
Some Member States, such as France and Germany, have already needed to design or convert the majority of their nuclear power plants for flexible operation. Plants in those Member States have been designed or modified and have operated flexibly, and many reactor-years of experience and knowledge of flexible operation have been collected. This experience and knowledge is a beneficial resource for the nuclear industry in understanding the requirements, needs, challenges, solutions and lessons learned in order to make an informed decision on whether to build nuclear power plants for flexible operation or to convert existing plants from baseload to flexible operation.
1.2. OBJECTIVE
This publication aims to address all relevant aspects of flexible operation of nuclear power plants, specifically focusing on changing electrical output to match electrical demand and controlling the frequency of the electrical system, termed ‘load following’ and ‘frequency control’, respectively. It is intended to provide collective guidance, based on current knowledge and operational experience, for decision making in and implementation of flexible operation in Member States which are considering future flexible operation of their nuclear power plants.
1.3. SCOPE
This publication describes possible reasons why flexible operation of nuclear power plants may be required, and the various types of flexible operation that may be needed. These considerations would apply to any form of energy generation, including nuclear, independent of the technology involved. Further guidance is provided on what selection criteria to consider, what steps to take for feasibility studies and the decision making process, and what operational experience is available in the planning, design, licensing and operation phases. The technical issues that have been identified during flexible operation and any other foreseen plant challenges related to flexible operation of current reactor technologies are also discussed. Brief discussions of potential alternatives to flexible operation are included, as well as considerations of economic factors.
Technical issues that might arise with flexible operation of developing reactor technologies, such as fast breeder reactors, or novel small modular reactors (SMRs) are not within the scope of this publication. Flexible operation of SMRs will be discussed in a future document.
Nuclear reactors are used as the power source in many submarines and some large ships. These reactors necessarily need to change power output rapidly when required, and there are many reactor-years of experience worldwide of such flexible operation. However, the design of such reactors is different from those used for electrical power production, and that experience is not discussed here.
This publication is not intended to endorse or to invalidate the non-baseload mode of operation; rather, it is intended to provide Member States with an understanding of the reasons for operating nuclear units flexibly in a safe and efficient manner with quality and reliability — and of the challenges with, and solutions for, doing so. It is intended to lead to informed decisions on whether nuclear power plants need to operate flexibly, and to guide Member States in implementation of flexible operation in their plants, or in preparation of remedial actions for when the need for flexible operation arises, including the technical, operational and economic changes necessary.
The intended users of this publication are organizations involved in decision making regarding flexible operation of nuclear power plants or in implementing it in the Member States which are considering flexible operation. This includes Member States designing or building plants, and those considering flexible operation of existing plants that do not currently operate flexibly. Thus, the users of this publication may include the entities that are designing, providing, constructing, installing, maintaining, modifying and operating the nuclear power plants and the grid or are performing licensing and oversight of nuclear and grid governance. The following are foreseen as users:
— Utilities and plant owners/operating organizations;
— Architect–engineers/designers and technical support organizations;
— Regulatory bodies (nuclear and grid);
— Grid system operators;
— Energy planners (at governmental or utility levels).
This publication can be used as general guidance on understanding the characteristics of flexible operation and on determining whether nuclear power plants need to operate in non-baseload mode. It also can be used as a road map for deciding on and achieving necessary flexible operation in plants if that is deemed a necessity. This guidance and road map include considerations for new plant projects and also transition of existing baseload plants, and provide key aspects of plant design and configuration for flexible operation.
Users who are deciding whether to switch to flexible operation at their nuclear power plants could refer to Sections 2–4 to establish plant and grid needs for flexible operation. Section 5, for which the main intended audience is technical and operational staff, describes the technical and operational aspects of implementing a certain degree of flexibly operating plants.
As the implementation of flexible operation involves costs and benefits, the economic considerations in Section 6 may provide basic elements and models for financial and economic analysis. It is recommended that users refer to the sources of the information provided in this publication to expand on and utilize the topics discussed herein.
The aspects associated with the flexible operation of plants considered in this publication are not comprehensive of all needs, challenges and possible solutions, but are rather key concepts that could be taken into account, based on the current operational experience and technical and administrative fundamentals. Further, this publication is not intended as detailed and prescriptive implementation guidelines for achieving flexible operation in nuclear power plants. It is rather descriptive, covering major technical, managerial and economic topics, important milestones, and roles and responsibilities of stakeholder involvement in the process. The guidance provided is supplemented by specific examples of plant flexible operation from operational experience, as well as good practices and lessons learned. The IAEA, however, does not attest to the completeness and applicability of the examples of specific cases, which require validation and verification by users.
1.4. STRUCTURE
The main body of this publication is divided into seven sections. The annexes supplement the main text with examples, illustrations, case studies and specific discussions.
Section 2 defines and describes the various types of flexible operation that might be employed to meet electricity system needs. Section 3 explains why it may be necessary for some generating units to operate flexibly, why the majority of existing nuclear units presently do not operate flexibly and why they might need to do so in future. It also reviews some potential alternatives to flexible operation.
Section 4 provides a comprehensive discussion on whether to implement flexible operation in nuclear generating units and how to determine the feasible type and extent of flexibility. The roles of stakeholders in the decision making process are also discussed.
Section 5 discusses the main technical and operational issues that are important for flexible operation as they affect the common nuclear power plant design and technologies and operational programmes and procedures. It describes, from operational experience and fundamentals, aspects of flexible operation implementation in nuclear power plants.
Section 6 discusses the economic considerations related to flexible operation, focusing on associated costs and benefits at the plant, generation portfolio, grid and economy wide levels.
Annexes I and II provide details of practised methods and associated impacts, issues and possible solutions, with specific examples, for original design for flexible operation (German case study) and baseload to flexible operation conversion (French case study), respectively. These case studies illustrate the operational experience regarding various aspects of accomplishing specific levels of flexible operation or unique approaches to management of nuclear power plant flexibility. However, these do not endorse or suggest applicability to all cases, which require users to validate, verify and adapt them to their specific conditions. Annex III, therefore, gives some examples from the experiences of Member States concerning whether (or how) to operate nuclear units flexibly and the evolution of their decision making processes to illustrate methods of validation, verification and adaptation to accomplish or manage flexible operation of nuclear power plants. Finally, Annex IV provides a preview of the IAEA economic model to quantify the cost–revenue aspects for nuclear power plant operation in mixed energy systems, including flexible operation.
A glossary of specific terms and a list of abbreviations used are also provided in this publication.