Markets & Sectors

Drives in Nuclear Power Plants

8 Aug , 2019  

The influence of drive technology in generating the world’s energy is enormous, and nowhere is this more so than in the nuclear sector.

Nuclear power plants heat water to produce steam, which is used to spin turbines to generate electricity. The heat is produced by nuclear fission (the splitting of atoms into smaller atoms), and the process occurs inside a reactor containing uranium fuel (in the form of ceramic pellets) at its core.  

Approximately 450 power reactors in 50 countries generate 11% of the world’s energy. In the UK the figure is just under 20%, slightly ahead of the US and Russia but massively behind France where nearly three-quarters of its supply is from atomic energy. Twenty years ago nuclear power provided around 25% of the UK’s electricity, but decommissioning caused by safety, performance and political factors has seen this decline.

Future of nuclear power in the UK

It is difficult to accurately predict where nuclear power will be over the next 20 years, not least because the energy market is a complex business of public and private money operating within commercial and political constrains. We know that carbon emission targets will drastically reduce our reliance on fossil fuels, but we do not yet know precisely how this will be replaced.

Many within the environmental lobby favour renewable energy over nuclear. Yet treasury departments – and the UK has been no exception – tend towards nuclear as better placed to attract investment because it is more reliable and potentially more cost-effective. Yet the current direction of travel may favour renewables.

While the UK has big investors on board for atomic power, the walking away of Japanese giants Toshiba and Hitachi from nuclear projects at Wylfa and Oldbury has prompted the government to reassess its position. It seems the UK government’s eagerness to grant licences for nuclear power expansion is not always matched by those needed to see it through.

However, we can be sure – if not about its exact pecking order relative to others – that nuclear energy will be a vital component of our future energy supply. The matter will be influenced by the extent to which decommissioning of existing plants can be stalled by the granting of life extensions. Apart from safety, decommissions are largely triggered by a lack of competitiveness – often the result of subsidises given to promote renewables.

Challenges of nuclear power

Central to the production of nuclear power is a reactor: this is surrounded by a multifarious network of vessels, chambers, lines, pumps, rods, and turbines. Temperatures can reach as much as 1000C, and water is the lifeblood of the system. The speed, volume, pressure and temperature at which the water is transported is key.

variable frequency drives - vfd - in nuclear power plants

Achieving energy savings is nearly always a high requirement for granting life extensions. The main functions of electrical systems in a nuclear plant are to distribute power to and protect, the components as well as the power supply itself. Traditional technology consists mostly of motors acting asynchronously (performing a standalone function within a sequence of events) to drive the pump to power the plant. Additional pumps and a network of process control values vary the speed at which liquid passes through the plant.

How variable frequency drives save energy in nuclear plants?

When the fast passage of liquid is required, such as when large amounts of cooling agent are needed to cool down the reactor core, it is not unusual to have four big 6kV recirculation pumps and over 100 control values in action – a process that accounts for over 25% of the energy costs to run a nuclear power plant. Variable Frequency Drives (VFDs) have shown to save energy, improve performance, simplify the system and cut downtime.

The replacement of old motor technology with VFDs (combined with AV motors) at a Tennessee Valley nuclear plant allowed efficiency to jump from 70% to 95%, figures typical of similar upgrades worldwide. It demonstrated that the actual energy savings to be gained from controlling the speed of pumps was enormous because a small reduction in speed can make a big reduction in energy consumption. For example, a pump or a fan running at half speed may consume as little as one-eighth of the energy compared to one running at full speed.

The application of VFDs in this way can reduce electricity bills by as much as 50 percent, with the knock-on benefit of reduced CO2 emissions. They can also eliminate the need for oil inclusion in the mechanical coupling between motor and generator, thus removing the need for routine inspection and protection measures against oil fires.

Drive technology in nuclear power plants

Drive technology has further assisted with protecting nuclear fuel and fuel rods – both of which are vulnerable to turbulence – by ensuring water circulating around them flows smoothly, without disruption. Regulations are strict in this respect and when electricity supply is increased, its ascension should be measured and gradual: the precise control of VFDs allows this within desired limits.

Similarly, VFD features can automatically bypass a failed power cell in the event of a control system failure; it will switch to a duplicate control system and adjust its speed appropriately to allow the plant to continue its work. It accomplishes this by calculating the magnitude of the phase shift and voltage adjustment.

nuclear-power-plants

The role of drives in nuclear fission is as much about politics as performance. Whether a Plant’s life is extended or decommissioned depends in no small part on the ability of automation engineers to come up with solutions that can stand the test of time.

That EDF Energy is planning a wide range of life extensions suggests the engineers are at the top of their game. These life extensions include an average eight years for its Reactors; seven-years for Hinkley Point and Hunterston; and five years for Hartlepool. It also spent £150 million to prepare Dungeness for a 10-year licence extension, to 2028.

There can be no doubt that the deft, innovative mechanics of VFDs has – literally – given new life to the nuclear power industry in the UK and beyond. Nuclear power will always have its detractors, and those who favour renewables over all else will not relent in warning of its potential dangers. But it would be fair to say that drive technology has significantly contributed to alleviating many of the fears: and as a result has helped secure nuclear energy as a major world force for generations to come.

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Alex Byles

Alex Byles