You wouldn’t think that mud could ever be the subject of a campaign involving hundreds of people.

It is in Cardiff, where it has not only mobilised protesters but has also led to a High Court hearing and more than 100,000 people signing petitions. And now a sea battle looms over the issue.

French energy firm EDF is involved in construction of the new £19.6 billion Hinkley Point C nuclear power station in Somerset, on the English side of the Bristol Channel.

The station’s cooling system will involve driving three tunnels beneath the seabed more than 3km out into the channel.

EDF wants to dredge up 300,000 tons of mud from the seabed near Hinkley Point as part of the development, take the mud by barge over to the Welsh side of the channel and dump it in an area of the estuary known as Cardiff Grounds, a mile or so off Cardiff.

The first few thousand tons were dumped last week.

The problem is that the mud is being dredged from an area which is close to Hinkley A and B nuclear power stations. Hinkley A has been decommissioned. Hinkley B was due to be decommissioned in 2016, but EDF extended its life by seven years to 2023.

The issue is whether the mud’s proximity to the two power stations has led to any radioactive contamination. EDF says it’s safe. The campaigners say there’s no evidence of that.

The EDF dumping plan prompted the formation of the Campaign Against Hinkley Mud Dumping, whose protests have attracted hundreds of supporters. They argue that an Environmental Impact Assessment (EIA) of the mud did not sufficiently deal with the possibility of radioactive contamination. EDF says the assessment was carried out by the book.

This week the campaigners took their objections to the High Court seeking an injunction to halt the dumping.

The court adjourned for a week without awarding an injunction. So now, as the legal arguments continue, the protesters plan to gather a flotilla of small boats and mount a seaborne occupation of the Cardiff Grounds to physically prevent the dumping taking place.

Tim Deere-Jones is a freelance environment expert who runs a Marine Radioactivity Research Consultancy. He started a petition against the dumping which quickly gathered more than 7,000 signatures. He also founded the anti-dumping campaign.

He said: “We are deeply concerned that the dumping is now taking place. There have been three or four dredger trips and they are dumping 2,000 tons a time.

“They are doing it at night under cover of darkness, but you can see their lights from the Cardiff headland.

“There is support for a defensive flotilla. It’s a great idea. It has been floated and is being actively pursued. There are plenty of small boat owners in Cardiff and it’s only a mile and a half off Cardiff.”

Neil McEvoy is an independent Welsh Assembly member for South Wales Central.

He said that Hinkley Point A had produced weapons grade materials and that it was possible there were particles of plutonium and uranium in the mud.

“There have been a plethora of petitions with more than 100,000 signatures,” he said. “They are going to go on dumping.”

Supporting the flotilla plan, he said: “This can be done lawfully. If a number of boats decide to drop anchor around the perimeter (of the dumping area) and do a bit of fishing, there is nothing anyone can do about it.”