South Africa's state nuclear energy company is investigating links between workers' illness and deaths, and exposure to radiation, it says.
Several workers at the research facility have developed serious illnesses
The investigation comes after an environmental activist group obtained medical records suggesting high disease levels among nuclear plant employees.
The inquiry concerns the Pelindaba nuclear facility near Pretoria.
A government minister recently accused activists of "spreading panic" about illness among nuclear workers.
Nuclear Energy Corporation of South Africa (Necsa) spokeswoman Nomsa Sithole told the BBC News website that the inquiry had been launched in March, prompted by media reports of workers who had become seriously ill.
"We realise there is a need to dig deeper into these allegations," she said.
Ms Sithole said the first task had been to appoint someone to chair the inquiry. That person's name is be announced in the next two or three days, and the appointee will determine the investigation's methods and timeframe.
Mashile Phalane from Earthlife Africa, the group that first investigated the workers' illnesses, expressed frustration that his organisation had not been kept up to date with developments.
"They reported this investigation a long time ago, but we still don't even know who's chairing it," Mr Phalane told the BBC.
"They should talk to us - we know who the people are."
Mr Phalane said Earthlife Africa had identified 29 workers and former workers at Pelindaba who had become ill, and obtained medical records for 23 of them.
Five of these people had already died, he said. Thirteen had undergone medical examinations, and 10 had been found with diseases linked to radiation exposure, including skin cancers and eye diseases, according to Mr Phalane.
"More research is required," he said.
Earthlife Africa's announcements about the workers' illnesses drew a sharp response from the Minister of Minerals and Energy, Phumzile Mlambo-Ngcuka.
Department spokeswoman Yvonne Mfolo confirmed that Ms Mlambo-Ngcuka had said the government ought to pass legislation to stop people from spreading panic.
"What the minister was talking about was that organisations and individuals who make misinformed statements that spread panic," Ms Mfolo told the BBC.
Asked to comment on the minister's remarks, Mr Phalane of Earthlife Africa said: "I wish the minister could see those people. Some don't have a plate of food on their table - they can't work, some have no pension and they received no compensation."
Apart from the Pelindaba research reactor, South Africa has a nuclear power plant at Koeberg, near Cape Town.
South Africa's secret nuclear weapons project was halted and dismantled in 1989.