At 9.15 a.m. on 26 January a fire broke out when part of a particle detector was being dismantled. This apparatus comprised 80 plates of depleted uranium which is used for its strong particle absorption power. Depleted uranium is only very slightly radioactive. The work was being done by two people from a specialist company inside a sealed tent with an entrance lock. The tent itself was located in the large North Experimental Hall on the Prévessin site.
As the first plate was being removed some uranium dust fell onto the floor and spontaneously burst into flames. This is a phenomenon which can occur with fine particles of metal such as magnesium. The plates themselves cannot catch fire.
The two technicians immediately alerted the CERN Fire and Rescue Service, which arrived on the scene within five minutes. The fire, which remained confined within the closed area, was rapidly extinguished.
Once the fire had been put out, the measures implemented by CERN's Radiation Protection Group revealed that there had been no radioactive or chemical contamination outside the protected area. The two technicians doing the work had been wearing specialised protective clothing and equipment. They were not contaminated. The personnel present in the large North Hall have also been checked. The closed area where the dismantling work is taking place will be cleaned by a specialist company.
Depleted uranium, a by-product of the uranium enrichment process, is very heavy and is sometimes used in place of lead as a ballast and counterweight in aeroplanes and boats.
When depleted uranium dust burns, uranium oxides are produced, which are toxic. In the case of the fire at CERN, this toxic contamination was contained within the special area.