pdf Historical thorium contamination in a research facility (88 kB)
Description of the incident
For several years, a research centre used thorium for the synthesis of new materials; this work ceased over 10 years ago. The room where the thorium was handled was adjacent to a recently renovated office. A film dosimeter, stored in this office, twice registered a quarterly dose of 3.5 mSv.
A radiological survey was equested by the regulatory authorities, which determined the origin of this exposure. Thorium deposits had accumulated over the several years of operation in the laboratory’s sewage disposal pipes (the laboratory had no specific radioactive effluent pipeline) – and this was the main source of the recorded doses. A more thorough search revealed significant contamination of workbenches, fume cupboards, kilns and pipes.
The following actions were taken :
- The laboratory was immediately closed and sealed, with any remedial measures being subject to prior authorization.
- A decontamination operation was conducted by a specialized company.
- Information was distributed to laboratory personnel and to persons in the neighboring laboratories.
- The technical personnel working in the facilities received specific training regarding chemical and radiological risks.
The costs of decontamination were estimated to be €200,000 and the volume of waste produced by the decontamination work was twenty 200-litre drums.
Radiological consequences
- The main exposure route is thought to be from external radiation, ie as indicated on the film dosemeter. The office in question had only recently been occupied; prior to this it was a storage room.
- Bioassay tests performed on several staff members that potentially could have been contaminated were negative.
- During the decontamination, no airborne contamination was detected.
Lessons to be learned from the incident
Tight controls are required when working with dispersible radioactive materials, and in this case years of bad laboratory practice resulted in significant levels of radioactive contamination. Regular contamination monitoring is an essential control to avoid the accumulation of activity.
It is important to find and retain information on older facilities that may have changed use many years ago, but may still have a legacy of radiation risks. This is particluar relevant in cases where material such as thorium were hsitorically used (ie because it may have been regarded as a "natural" radioactive material, and not subject to the same controls applied to artificial radioactive materials).
A specific pipeline for contaminated liquid effluents should be used to collect all of this waste in a decay tank. This pipe should be checked regularly.
pdf Historical thorium contamination in a research facility (88 kB)