RESUMEN
This paper discusses an accident, which occurred in one of the radiation application centres in Iran, follow-up investigations as well as lessons learnt. In January 2004 the Regulatory Authority was informed through a university radiation protection officer of an accident regarding orphan sources. Investigations revealed that one Am-Be and three (137)Cs sources in the container were subject to extensive heat due to burning of the container and melting of the paraffin content of the container; consequently, sources were stuck to the side wall of the container, but they were still undamaged and no radioactive leaking had occurred. Further investigations showed that the container had been given to the above mentioned centre a long time before by a foreign well-logging company without notifying the Regulatory Authority. Follow-up measurements and assessments indicated that the collective effective dose due to the accident is unlikely to be more than 21 mSv; consequently, no severe deterministic effect to individuals was expected. The findings showed that the main reasons for the accident were as follows: (1) violation of obligation under radiation protection regulations by the owner of the sources; (2) leaving the sources in an improper storage condition; (3) unauthorised access to the radiation sources at the owner centre; and (4) lack of an integrated national registration system in the Regulatory Authority.