The incident happened in June, but was only revealed in a report last week by the UK Government's Office for Nuclear Regulation (ONR).
"The incident in question occurred during a standard procedure and involved a radiographer approaching the source while it was still exposed," said an ONR spokeswoman. "On realising this, the radiographer returned to the control device and ensured the source was retracted. The dose accrued was below any notifiable limit."
The Ministry of Defence stressed that the safety of workers was its priority. "During a radiographic procedure similar to an X-ray, a trained member of staff was briefly exposed to 0.037 milliSieverts of radioactive materials," said a spokewoman. "This is well below the safety limit of 0.3 milliSieverts. The worker was in no way harmed."
But John Ainslie of the Scottish Campaign for Nuclear Disarmament, said: "If Faslane is not able to properly handle the radiography sources that are used for routine work, what confidence can we have in its ability to keep the public safe from an accident involving a nuclear weapon or a nuclear reactor?"