The incident is sufficiently serious to have been reported directly to Government ministers, as part of the Nuclear Installations Inspectorate (NII) "statement of nuclear incidents".
But investigators are unable to understand how the man, who has not been named, came to have swallowed or inhaled the plutonium.
The plutonium was detected in routine urine samples analysed during June and July last year. Further samples were analysed in August, from which experts deduced that the man must have been exposed to the plutonium some time between January and June 1995. Faecal samples, however, failed to show any plutonium.
A spokesman for BNFL said the company was still investigating the incident but that "we don't expect full and detailed results until April or May". An interim report will go to the NII by the end of the month, he said, which "will tell us whether he has breached the dose limits or not".
Plutonium is very difficult to detect once it is inside the body, even when enough is present to give the individual an excessive dose. Experts point out that if someone were to inhale plutonium dust, then a quantity totalling "something less than the size of a grain of sugar" would be sufficient to cause of breach of safety limits.
The worker is described by BNFL as "an experienced individual" and has been re- assigned to non-radioactive duties, pending the final outcome of the investigation into his case. He had been working in the plutonium finishing plant at the end of the reprocessing line which dealt with spent nuclear fuel from Britain's first generation Magnox reactors.
But a Sellafield spokesman said that he had spent most of his time in his office, in the control room, or at the operating face, which is isolated from sources of plutonium. No one else in the plant had shown signs of internal contamination and there had been no incidents triggering plutonium-release alarms which could be related to the worker's exposure.
A spokesman for the NII said that plutonium existed in several forms which would affect the degree of risk.
If the plutonium were in an insoluble form, then "it can pass out easily and the prognosis would be good".Reuse content