Alternatives to flexible operation of a nuclear power plant

In some circumstances, the need for flexible operation by a nuclear power plant could be avoided by using alternative technologies and means to provide balance or adjustment of the generation and demand. Application of these technologies and means depends on their economic and technical feasibility, and needs to be evaluated on a case by case basis. Some methods and facilities that may be alternatives to flexible operation of a plant, or reduce the need for its extent, may include the following:

8 For example, gas fired generating units may be unable to comply with nitrogen oxide emission limits when operating below 60% rated power.

— On an electricity network, where periods of lowest electrical demand are confined to a particular time of year, it is sensible to schedule the planned shutdown of the nuclear units for maintenance and refuelling for this time. This could avoid the need for flexible operation of nuclear generating units that would be necessary if they were in operation during these periods. A pumped storage power station will purposely increase electrical demand on the electricity system at low demand times, typically during the night, when it is in pumping (charging) mode. At other times, it provides additional generating capacity, and can operate flexibly to provide load following and frequency control. Hence, a pumped storage power plant may allow nuclear units to continue to operate at baseload, as it provides the capability for flexible operation that would otherwise have to be performed by nuclear units. This alternative has been implemented in some Member States where the specific geography is suitable. For example, it was utilized for the first nuclear units built in China [3]. Other forms of energy storage that could provide a similar function are under development (e.g.

flywheels or flow batteries). Figures 14 and 15 show the utilization of pumped storage for managing demand and maintaining baseload generation, respectively, in the Czech Republic on 5 March 2016.

— Where the electricity network in a country or region has suitable electrical interconnections with neighbouring countries or regions, it may be possible to develop commercial arrangements for the import/export of surplus generation during low demand periods (e.g. on weekends or at night), to reduce or avoid the need for flexible operation of nuclear units. The differences in the price of electricity among neighbouring countries or regions may provide a commercial incentive for building or increasing the capacity of such interconnections.

— It may also be possible to operate a nuclear unit at steady full thermal power while varying the electrical output, if other technologies could make use of the excess thermal power from the reactor (e.g. in forms of heat) that would otherwise be wasted [16]. This could include district heating, desalination of sea water or process heating (e.g. industrial steam, coal liquefaction/gasification or hydrogen production). As discussed in Ref. [17]⁹, a proper assessment of the technical and economic feasibility has to be performed to justify the alternatives being considered.

Some nuclear power plants in Member States are currently using part of their thermal power to provide heat for industrial or district usages. In this principle, as illustrated in Fig. 16, the heat transfer can be adjusted, allowing the electrical power to be varied while leaving the thermal power extracted from the nuclear core

9 Reference [17] discusses SMRs as a representative case; however, the basic economic principles apply to any technology.

FIG. 14. Increased demand by pumped storage in the Czech Republic on 5 March 2016 (reproduced from Ref. [15]).

unchanged. The provision of sufficient energy storage via hot water storage tanks would allow the varying heat demand to be met, independent of changes in the power supplied to the heating system by the reactor.

In this application, the economic models of heat and electricity generation have to be compatible. Use of the derived heat from reactors for district heating has been practised in several Member States, particularly in Northern, Central and Eastern Europe. In these plants, 5–15% of the thermal power is used for district heating [18, 19].

— Demand side management and load management, as well as other commercial arrangements, can be used to encourage electricity consumers to alter their electrical usage from periods of high electrical demand to periods of low electrical demand, including arrangements to increase night-time electricity demand. For

FIG. 15. Maintaining baseload generation with use of pumped storage in the Czech Republic on 5 March 2016 (reproduced from Ref. [15]). CCGT — combined cycle gas turbine, PP — power plant.

FIG. 16. District heating with constant reactor power and variable electrical output.

example, with the emergence of smart grid technology and electric vehicles, there may be more opportunities in the future for innovations in demand side management.

Commercial arrangements can also be used to persuade large industrial users of electricity to reduce demand on request when reserves are low, or to automatically disconnect demand in response to a low system frequency.

That is, large demand users may be willing to control their demand to provide ancillary services. One specific instance is in Indiana, United States of America, where services have been provided by an aluminium smelting plant [20, 21]. This would reduce the amount of frequency response and reserve that must be provided by generating units.