On April 2nd 2008, during periodic radiological surveillance outside the controlled area, several solid radioactive particles were detected in different places outside plant buildings and inside NPP site: terraces of fuel, auxiliary, control and turbine buildings, mechanical penetrations and different places at ground level.

According to the information available, the origin would be at the release of radioactive particles through the chimney of the fuel building ventilation system. On November 26th 2007, this system was contaminated during cleaning operations on the fuel transfer channel, at the end of the refuelling outage. The final decontaminating fluid was thrown into the fuel pool, and unexpectedly suctioned by the surface ventilation screens. Three days later, the emergency ventilation system was stopped, bypassing HEPA filters and normal ventilation started, and consequently spreading the contamination outside.

Approximately 5.000 MBq were suctioned by the ventilation system, 3.975 MBq of them were collected in successive decontamination tasks, 20 MBq are pending to collect, and other 1.044 MBq have been retained by emergency ventilation filters.

Based on preliminary licensee estimations, the release outside the buildings was set at 235.000 Bq. Present estimations have increased the amount of radioactive release to 85 MBq. The particles analysis shows an isotopic composition of Co-60 (53,6%), Co-58 (11,42%), Mn-54 (21,50%), Cr-51 (7,12%), Nb-95 (3,63%), Zr-95 (2,08), Fe-59 (0,65%).

The licensee initially took the following actions: withdrawal of the particles as they were located; additional surveillance within the site; scanning of closed areas to the site following dominant winds (NW).

CSN sent an inspection team to search for evidence and possible causes, and to make independent radiological verifications. Preliminary radioactive scanning in the public neighbourhood of the site has found no radioactive contamination, though more measures are planned for the coming weeks. CSN has ordered a deeper investigation including root cause analysis and a radiological review of exposed people.

The event on November 26th was not notified to CSN, in spite of a reporting criterion. Preliminary information was not even distributed within the licensee to the operation staff. Three days later, the ventilation system was set to normal conditions, bypassing HEPA filters, without checking the contamination level inside the ventilation conducts. Neither was the radiological surveillance program modified after the incident nor the first finding of particles. New radiological data were retained unnecessarily by licensee until April 14th.

Location: ASCO-1
Event date: Fri, 04-04-2008
Nuclear event report
Legenda & explanation