The nuclear nexus: Is flexible operation the way of the future?

December 8th, 2014, Published in Articles: Energize


The general view on nuclear power plant is that it is rigid and inflexible in output and is only suitable for baseload operation. This is not correct, and nuclear power plants can operate flexibly, within ranges and ramp rates comparable to other generation technologies. This makes nuclear a good candidate as a carbon free partner for variable renewable energy (RE) sources in future.

Nuclear power plants (NPPs) are usually operated as baseload units, which is the most efficient and least technically challenging mode of operation, and most existing NPPs have been optimised for continuous, full-power operation. Due to changes in the generation mix and licensing and operational requirements, utilities using NPPs are facing pressure to move to a state capable of varying power output to match varying regional electrical grid demands, i.e. transitioning from baseload operation to flexible power operation.

Flexible operation is possible with many of the NPPs in current use. Existing light water reactors (LWRs) can be operating in flexible mode with some modification, and advanced LWR designs include the capability to operate at levels well below full output, to increase and decrease output at ramp rates comparable to other conventional power plants, and to operate in a mode to provide system frequency control. Some operational nuclear power plants already operate in a flexible mode. The French system, which derives 75% of its electrical energy from nuclear, has successfully used flexible operation on its fleet of pressurised water reactors (PWR) to match demand for several decades.

Why consider flexible nuclear power generation?

The ability to vary power output is becoming a requirement for all sources of dispatchable generation, due to both mandatory and network requirements in world networks.

There is an emerging opinion that baseload generation will fall away in future and be replaced by a combination of “flexible generation and flexible demand”. Although the underlying logic of this opinion may be questionable and the statement may appear to be vague, it could be a future possibility. Flexible generation is seen at the moment to be a combination of variable renewable sources and storage, backed up by flexible conventional generation, such as gas turbines, which are seen as the optimum “dancing partner” for RE. Flexible nuclear, however, offers the added advantage of being “carbon free” and would lead to a completely carbon free generation mix, which is seen as the ideal situation for the future. If this is the goal which is being aimed for, then a combination of RE sources, electrical storage and flexible nuclear becomes the prime option [1].

Combining variable renewable generation with a flexible source of dispatchable generation is seen in some quarters as the future answer to climate change and greenhouse gas reduction. The current solution of preference is gas turbines (GTs), but GTs also emit CO2 and cycling reduces their effective lifetime. The ability to operate nuclear in a flexible mode offers new possibilities for the generating mix.

There is a slowly changing attitude towards nuclear among the climate change fraternity, which has presented some unexpected opinions, such as those expressed by a group of climate scientists in a letter circulated to various organisations. They proposed that “there is no credible path to climate stabilisation that does not include a substantial role for nuclear power” and as such “continued opposition to nuclear power threatens humanity’s ability to avoid dangerous climate change.” [2].

Current situation

A large number of utilities, government and international research and industry associations including the IEAE [5], the EU [3], EPRI [4] are investigating the possibility of flexible nuclear operation, and its impact on a possible future energy mix. Several have already produced study and guideline documents and standards on the issue, and research is ongoing. It seems likely that flexible operation will be included as a standard feature in new NPPs built using existing generation technologies, and next generation of NPPs.

About 75% of France’s total electricity comes from nuclear power. The large contribution of nuclear power capacity combined with normal demand variations means that some nuclear power plants are required to follow load and to provide frequency regulation in France. EDF’s approach to flexible nuclear power plant operation uses special “grey” control rods and a coordinated approach to flexible operation across the fleet. The Columbia Generating Station nuclear power plant, located near Richland, Washington, USA, with a large amount of hydroelectric generation, is operated flexibly to allow the regional system dispatchers to manage the large hydroelectric system. Some Canadian heavy water reactors are operated in a mode where output is reduced on a seasonal basis. According to EU experience the share of nuclear generated electricity has to be high before cycling is adopted for nuclear power plants. This is the option in France with a share close to 75% in the electricity production capacity. Other countries with a high share in nuclear generated electricity include Slovakia (56,3%) and Belgium (53,7%)which both partly operate in load following mode [2].

Flexible operation limits

Flexible NPPs are capable of operating between 20 and 100% of maximum power, with ramp rates varying between 3 and 5% of maximum power/minute. This means that a system could move from the lower limit of 20% to maximum power output within 25 minutes, which compares with the ramp rates of coal fired and gas fired power plants.

Current NPPs operating in flexible mode can be used to contribute to regulation in three respects:

  • Primary power regulation for system stability (when frequency varies, power must be automatically adjusted by the turbine)
  • Secondary power regulation related to trading contracts
  • Adjusting power in response to demand (decrease from 100% during the day, down to 50% or less during the night, etc.)

Frequency control and system stability

This involves varying the output power of the NPP automatically within a range of 5% of the maximum power in response to system frequency variations.

Load following

Automatic load following is not used or allowed, and is done under operator control in response to requests. Load following does not appear to be continuous but in blocks, i.e. generation is increased or decreased by a fixed amount.

Fig. 1 shows the typical scheme for one yearly cycle for a reactor operating in load-following mode (variation from maximum (minus reserve) to minimum (first third part of the cycle), and then in primary and secondary regulation. When the fuel reactivity reserve is used (last part of the cycle) the reactor is no longer available for regulation (EU:JRC [2]).


Nuclear nexus-final-2-Fig_1

Fig. 1: The operation cycle of a flexible PWR NPP [2].

The amount of load following that a PWR can perform depends on where it is in the fuel cycle. PWR plants are very flexible at the beginning of their cycle, with fresh fuel and high reserve reactivity. But when the fuel cycle is around 65% through these reactors are less flexible, and they take a rapidly diminishing part in the third, load-following, aspect. When they are 90% through the fuel cycle, they only take part in frequency regulation, and essentially no power variation is allowed (unless necessary for safety). So at the very end of the cycle, they are run at steady power output and do not regulate or load-follow until the next refuelling [2]. This means that load following can only be effectively implemented in a large fleet of reactors where it is possible to stagger the fuel cycles to ensure that load following is continuously available.

Existing plant

New NPPs have the advantage of being designed with flexible operation in mind. An NPP that previously has operated in a baseload mode of operation only, and is now considering or is required to switch to, flexible operation, faces a more complicated situation. Depending on the NPP design, modifications to the plant, particularly to the control systems, may be required to support frequency control and load following operations. Also, adaptation of the safety analyses and licensing basis changes may be required. Similarly, existing operation and maintenance philosophies and procedures may need adjustment to support flexible operation.

Power output control mechanisms

Two basic power control mechanisms are used:

Turbine power control

In this system the power output of the NPP is controlled by diverting steam away from the turbine while maintaining the output of the reactor at a constant level. [2]. The steam may be re-circulated to the condenser or vented to the atmosphere. Re-circulating to the condenser affects the efficiency of the turbine, and venting may not be acceptable for environmental reasons. A unique solution where the NPP is operating as a CHP plant, providing district heating as well as electricity, diverts steam to the district heating system during flexible operation. This is a possible future application where CHP operation of new generation nuclear plants is considered as a basic requirement.

Reactor power control

In pressurised water reactors, control is normally implemented using control rods. Because the normal rods (black rods) are used for shut down operations, a less active control rod, known as a grey rod, has been developed for controlling output power. Partial or variable insertion of the black rods was found to result in an uneven power profile within the reactor core, which placed stress on the core. Variation of power consists of a sequence of rod insertion.

The French solution

EDF in France probably has the longest and most extensive experience of operating on flexible mode. All France’s nuclear capacity is from PWR units. There are two ways of varying the power output from a PWR: Control rods, and boron addition to the primary cooling water.

Using normal control rods to reduce power means that there is a portion of the core where neutrons are being absorbed rather than creating fission, and if this is maintained it creates an imbalance in the fuel, with the lower part of the fuel assemblies being more reactive that the upper parts. Adding boron to the water diminishes the reactivity uniformly, but to reverse the effect the water has to be treated to remove the boron, which is slow and costly, and it creates a radioactive waste [6].

So to minimise these impacts for the last 25 years EDF has used less absorptive “grey” control rods, which weigh less from a neutron point of view than ordinary control rods, in each PWR reactor. This allows sustained variation in power output [6].

Boiling water reactors

In boiling water reactors (BWR), power output is controlled by the circulation rate of water into the core rods for slowing the circulation rate, typically between 100 and 70% of the rated output, and insertion of control rods in addition for a range from 100 to 40% of rated output [8]. Recirculation pump flow control is used in BWRs to control core reactivity and change core power. One advantage of recirculation pump flow control is that it has a more even affect on the core flux, which minimises power peaking and thermal limit concerns. Reactivity changes performed using recirculation pump flow control are limited by core dynamics and cannot be used alone for large changes that reduce thermal power below 70 to 80%.

Control rod position changes along with recirculation flow changes are typically required to adjust thermal power below 70 to 80% to ensure operation outside the core instability regions of core flow. Two disadvantages of variable recirculation flow control include potential increased chance of seal leakage and possible introduction of secondary harmonic effects (harmonics associated with the vane passing frequency of the reactor recirculation pumps) with longer operation below nominal design conditions. These two disadvantages could be mitigated by increased monitoring of the system to determine when increased seal leakage and harmonic effects occur [5].



Studies of systems operating in flexible mode show a slight increase in maintenance costs and downtime. The main area affected are [2]:

  • Control rods and drive mechanisms in PWR systems. Constant adjustment of control rods adds to wear on the mechanism. Use of control rods leads to early deterioration of control material (consumption ) in control rods. This could result in additional downtime to replace worn rods and components.
  • Load cycling causes temperature cycling. This places additional stress on components.
  • Variation of water circulation in BWR. This results in additional stress on pump components and valves.

Operator training

Operators of existing NPPs are trained in start-up and shut down operations but generally not frequency control or load following. Retraining has to be undertaken if flexible operation is being considered. Experience has shown that operator skills and understanding of the process are an essential part of the overall success [6].


Operation has shown no increase in safety related incidents or “scrams” in NPPs operating under flexible regime.

Fuel usage

Flexible operation results in a slight increase in fuel usage [7].


The impacts are considered to be insignificant compared to the advantages of using a flexible mode of operation [2]. South Africa seems firmly committed to inclusion of new nuclear capacity in a future generation mix. However, in both the IRP 2010 and its proposed amendment, nuclear is regarded as a baseload source only, and flexible operation has been ignored. All of the Nuclear systems being considered can be operated in a flexible mode, and it is surprising that this has not been taken into account in the projections.


[1] S Waldman: “Examining nuclear as a climate option”, The energy collective: 11 November 2014,

[2] K Caldeira, et al: “To those influencing environmental policy but opposed to nuclear power”, published in the New York Times, 3 November 2013,

[3] C Bruynooghe, A Eriksson, and G Fulli: “Load-following operating mode at nuclear power plants (NPPs) and incidence on operation and maintenance (O&M) costs. Compatibility with wind power variability”, JRC scientific and technical reports: EUR 24583 EN – 2010,

[4] S Bernhoft: “Programme on technology innovation: Approach to transition nuclear power plants to flexible power operations”, EPRI, November 2104.

[5] IAEA: “Non baseload operations in nuclear power plants: Load following and frequency control flexible operations”, IAEA Nuclear energy series No. NP-T-3.23.

[6] World Nuclear Association: “Nuclear power in France”,

[7] T Franch: “Flexible operations keep American nuclear facilities competitive”,

[8] Areva: “Kerena: The 1250 MWe boiling water reactor”,

Send your comments to:


Related Tags

Related Articles

  • South African Government COVID-19 Corona Virus Resource Portal
  • Ministerial determinations propose 13813 MW of new-build by IPPs, none by Eskom
  • Crunch time for South Africa’s national nuclear company, Necsa
  • Dealing with the elephant in the room that is Eskom…
  • Interview with Minerals & Energy Minister Gwede Mantashe