Sediment from Hinkley Point C construction site is being disposed of at Cardiff Grounds

Pressure is increasing on the Labour-led Welsh government to halt the dumping of “nuclear mud” in the sea close to Cardiff after a campaign by an eclectic group of scientists, surfers and a pop star.

A motion calling on the government to suspend the licence allowing mud excavated from the construction site of the Hinkley Point C nuclear power station in Somerset to be disposed of just off the Welsh capital is to be debated next week in the national assembly for Wales.

The development prompted Cian Ciarán, the keyboard player from Super Furry Animals, to discontinue an attempt on Tuesday to obtain a court injunction to stop the dumping.

There is growing concern and anger that 300,000 tonnes of sediment from the Hinkley Point C site is being disposed of at the Cardiff Grounds sandbank.

Campaigners claim the mud has not been tested properly and could contain particles that may pose a health risk. They have described the sediment as “nuclear mud” and nicknamed the sea off Cardiff “Geiger Bay”, a play on Tiger Bay, the old slang name for the city’s docklands. One of their main concerns is that the sediment could be washed ashore in a storm.

Welcoming the assembly debate, Ciarán said: “This Labour government has taken the Welsh people for granted and has risked the health of the nation. For me the core message of the campaign remains unchanged: the absence of evidence is not the evidence of absence and therefore the precautionary principle should dictate a rethink. We will continue to seek the answers the Welsh public deserve.”

He called for the Welsh government’s patrol vessel, FPV Rhodri Morgan, to monitor the dumping that is taking place. “It is a duty for every Welsh assembly member to do right by its people and hold to account the government that granted the licence,” he said.

Neil McEvoy, an independent assembly member, has accused the Welsh government of being out of touch on the issue. If the dumping does not cease, he wants boat owners to form a “people’s flotilla” to take direct action and blockade Cardiff Grounds.

He said taking on EDF Energy, which is building Hinkley Point C on the English side of the Bristol Channel, had been a huge risk.

“This was always a David v Goliath case,” McEvoy said. “Cian took the brave decision to put his name to a legal challenge against a massive, billion-pound energy giant. He’s risked everything for the safety of Wales. Labour need to know that Wales is not a dumping ground for other people’s waste.”

The campaigners claim no proper environmental impact assessment has been carried out.

The motion for debate, put forward jointly by the Tories and Plaid Cymru, calls on the Welsh government to “publish more detailed evidence in response to concerns regarding risks to public health and the environment”, including allowing for further testing, and to “instruct Natural Resources Wales to suspend the marine licence that enables the disposal activity and undertake a wide-ranging programme of engagement and consultation with local communities and stakeholders across south Wales”.

EDF said Ciarán’s decision to drop the legal action was “good news for a project that is vital for the UK’s energy future and provides work for 25 Welsh companies and 1,000 Welsh workers”.

A spokesperson added: “EDF did everything that it was requested to do when it applied for a licence to dredge and deposit mud in the Severn estuary. The mud is no different to mud found anywhere else up and down the coast and it has been thoroughly tested by independent experts who confirmed it poses no threat to human health or the environment.”

EDF says it is dredging mud and sediment from the seabed before the drilling of six vertical shafts for the cooling water system. It says Cardiff Grounds is the only suitable site large enough to handle the amount of the type of sediment it is dredging.

The Welsh government has been asked to comment. It has previously insisted the mud is safe.