The workers were decommissioning the Trawsfynydd station in Gwynedd last June.

Trawsfynydd station manager Alun Ellis: "A certain amount of internal exposure is inevitable."

They were using breathing apparatus to clean the cooling pond where irradiated fuel used to be stored at the plant.

According to BNFL, no long-term damage to their health occurred.

But two workers suffered a significant dose equivalent a quarter of the annual dosage permitted. Four others had much lower traces of radiation.

Station manager Alun Ellis said the level of radiation for all workers undertaking that kind of work was tested regularly and samples sent for examination to Sellafield.

Station manager Alun Ellis said radiation levels are checked regularly

It was during one of those tests that the contamination was discovered.

The two workers involved were part of a group of eight employed by a firm of German subcontractors working at the site.

The firm, Hertel, is one of the world's leading decontamination companies.

It is thought that dust from the cleaning process was responsible for the contamination and possibly a faulty seal on the breathing apparatus of the two men led to the incident.

BNFL immediately closed the area and called in the nuclear installations inspectorate to supervise an internal investigation.

It is not the only lapse in safety according to the HSE Nuclear Installations Inspectorate

The investigation has recommended better ventilation be installed and has agreed that decommissioning in that area can recommence from the second week of December.

The cause of the initial contamination has still not been discovered.

Decommissioning began at the Trawsfynydd plant in 1993.