"The NNR regards any nuclear-related event seriously and in this instance has instituted its own independent investigation," NNR chief executive Boyce Mkhize said on Friday.
Responding to a letter sent to him by Greenpeace Africa, Mkhize said preliminary information indicated the doses the workers were exposed to on September 12 did not warrant regulatory intervention.
However the NNR would continue to follow up and monitor the situation closely.
He said the leak occurred when cobalt dust particles from inside primary system pipes were dislodged when the pipes were opened during a reactor refuelling outage.
The cobalt had been carried out of the piping by air.
Mkhize said a radiation alarm did not go off because, according to the information available to the NNR, the "quantity and magnitude of radioactivity released was not enough to reach the alarm thresholds set on the radiation monitors in the area".
The workers were exposed to radiation levels considerably below the allowable dose determined by the NNR.
The allowable dose was 20 000 microsieverts, and the highest dose measured in the group was 500 microsieverts.
Greenpeace energy campaigner Rianne Teule said in the organisation's letter that her main concern was that alarms were not activated in the area where the leak occurred.
She said Eskom did not follow the international nuclear industry practice of setting it as low as reasonably possible.
The release of the radionuclides was only detected when the workers were monitored at the end of their shift.
Teule said it remained unclear how seriously affected the 91 employees were by the leak.