The reactors of nuclear power stations across Britain have been shut down 20 times a year because of faults - prompting fears over safety and the UK's energy supply.

An analysis for local authorities reveals that 15 reactors have had 62 unplanned shutdowns in the last three years. They have been hit by electrical, boiler and valve defects, fires, storms, vibrations and the discovery of tiny cracks.

The build-up of large amounts of seaweed has twice forced reactors at Torness in East Lothian to close, in May and November 2013. The seaweed clogs filters that are part of the plant's vital seawater cooling system.

The analysis was carried out by the Edinburgh-based nuclear consultant Pete Roche for a 50-strong group of nuclear-free local authorities.

The councils are calling for an urgent review of whether nuclear power can be relied on to provide enough power over the winter, while raising concerns about safety.

The high rate of unplanned shutdowns "has to be of great concern to the public for our energy security over this winter and onwards," said the group's chair, Manchester Labour councillor Mark Hackett.

"I call upon the UK Government, the National Grid and the nuclear regulators to urgently review the safety issues around such a large number of unplanned shutdowns.

"The Government also has to prioritise alternatives over the next 12 months to ensure the unreliability of nuclear power does not lead to the lights going off around the country."

According to Roche, unplanned shutdowns cause "major headaches" for planners trying to ensure a consistent and reliable supply of electricity to homes and offices. He pointed out that on November 20, seven of the UK's 15 reactors were offline while five were operating with reduced outputs, meaning that nuclear power was supplying only 43% of the electricity it should.

This contrasted with wind power, which varied in a much more predictable manner, he said. In the first half of 2014, renewable energy from wind, hydro and other sources produced more power in Scotland than nuclear stations.

The worst record belongs to Dungeness nuclear power station in Kent, which had to be shut down 21 times between 2012 and 2014. The two Torness reactors were shut down 10 times over the same period (see table top left).

One of the reactors at Hunterston in north Ayrshire had to be shut down within a few days of restarting in October because of turbine vibrations. Two hairline cracks were also discovered in the reactor's graphite bricks, prompting safety fears.

A major cause of shutdowns is the age of reactors, which are more than 30 years old and "past their sell-by dates", argued Roche. Nine of the UK's 15 reactors have been operating longer than originally planned, he added.

Roche also accused the French state company that runs the reactors, EDF Energy, of failing to report their performance to nuclear industry publications. This was because their operating record was poor compared to other countries, he alleged.

Dr Richard Dixon, director of the environmental group Friends Of The Earth Scotland, welcomed Roche's analysis. "Just like the big lie that nuclear electricity would be cheap, we now find that it's not reliable either," he said.

"From storms and seaweed problems to major fires and reactor cracks, this is an impressive catalogue of vulnerabilities that can knock out more than half the UK's reactors all at once. This unreliability can only get worse as these ageing reactors limp into extra time."

The future was about green energy, Dixon argued. "We are now very good at predicting how much power we'll get from our mix of wind, tidal, hydro and solar at any given moment, but nuclear reactors can fall off the grid at a moment's notice, leaving a big hole to be filled."

EDF Energy declined to comment directly on the rate of nuclear shutdowns. However, a company spokeswoman said: "In 2013, the output from EDF Energy's nuclear power stations totalled 60.5 terawatt hours of electricity, which was the best performance in eight years.

"EDF Energy is one of the UK's largest energy companies and the largest producer of low-carbon electricity, producing around one-fifth of the nation's electricity from its nuclear power stations, wind farms, coal and gas power stations and combined heat and power plants."