Hundreds of workers at Aberthaw power station in the Vale of Glamorgan have walked out over safety fears after the death of a 49-year-old man.
Workers want a full-time medic to be based at the power station
The man, who has not been named, died on Sunday, South Wales Police said. The ambulance service was told he had fallen from a platform into a pit.
Police have cordoned off the scene for their investigation and the Health and Safety Executive have been informed.
Workers will meet on Wednesday morning to discuss their concerns.
One contractor at the power station, which is near St Athan, outside Cardiff, said workers wanted a full-time medic to be based permanently at the site.
He claimed a van was used to take a man to hospital after he was injured there in a separate incident a few months before, because an ambulance took so long to arrive.
The workers who walked out had been employed on construction and maintenance work at the plant, a coal-fired power station which is currently not generating power because of servicing.
The site is owned by npower, but the workers who walked out are understood to have been working for contractors AMEC and Alstom.
A spokesman for AMEC said a registered medical nurse was due to start work on the flue gas desulphurisation project on Wednesday morning, following weeks of negotiations about first aid.
The Welsh Ambulance Service said it was called to the power station at 2237 BST on Sunday.
A paramedic and an ambulance arrived 19 minutes after the call and found the man was having a heart attack. He was taken to the University Hospital of Wales, Cardiff where he died.
South Wales Police said his next-of-kin had been informed.
The ambulance workers reported that medical staff were already on the scene when they arrived, and the ambulance workers assumed those were part of the workforce at Aberthaw, the service said.
A spokesperson from the union Unite confirmed there had been a walkout at Aberthaw over the lack of medical facilities there.
The union has told the workforce that its action is unofficial but it was waiting to hear from the power station's managers about whether they were prepared to respond to workers' demands for a registered nurse on site at all times.