A sealed radioactive Cs-137 source contaminated the premises of waste management company and also the neighboring rooms used by Finnish Radiation and Nuclear Safety Authority (STUK). The event was found out on 7 March 2016.
The source, activity presently 360 MBq, had been used in a factory in Finland for level gauging and was delivered to waste management company as radioactive waste. When the gauge was received by the company, one of the staff tried to open the source shield in order to remove the actual cesium source for disposal. He did not have information or other indication that the source might not be intact. During the handling of the source contamination was released into the room where gauge was handled and also to adjacent rooms and air exhaust ventilation ducts.
The worker that was trying to open the gauge shielding was monitored by whole body counter, and no internal exposure was detected. Further spreading of contamination was eliminated after the event came into light, cleaning of contaminated areas started and are ongoing at the moment.
The event was classified as level 1 on the INES scale.
An abnormally high concentration of Cs-137 was detected in an air filter sample collected 3 - 4 March in sampler located in STUK’s premises in Helsinki. The concentration was 4000 micro Bq/m3, which is 1000 times higher than the usual concentration in the location. Cesium was evenly distributed in the sample and no other nuclides were detected.
Filters collected in the same period in other air samplers in Finland had no abnormal concentrations of cesium. Thus, considering the location of the samplers and atmospheric modelling, it was very unlikely that the cesium originated from outside Finland. After assessment of possible origins of the cesium, the origin was traced to rooms of a waste management company operating in the same building as STUK. The company receives disused radiation sources and prepares them for final disposal. Based on swipe samples, the contamination spread from these rooms to the roof of the building via ventilation system.
After assessment by STUK in co-operation with the company, a damaged cesium-137 source in the waste storage of the company was identified as the origin of the contamination. This source was immediately isolated. The source, activity presently approx 360 MBq, had been used in a factory as level gauge. The source was manufactured in 1970s and the cesium was in form of cesium chloride, which facilitated the spread of the contamination in the building. Thousands of similar types of gauges are in industrial use in Finland. When the gauge was received by the company, one of the staff tried to open the source shield in order to remove the actual cesium source for disposal. He did not have information or other indication that the source might not be intact. When trying to open the shield, contamination was released into the room and spread through ventilation to the air sampler located on the roof. The person who tried to open the source was measured for internal contamination by using STUK’s whole body counter, and no abnormal amounts of cesium was detected.
Some contamination was found inside the package used in the transport. As a precautionary measure, STUK sent its monitoring team to measure the areas where the gauge had been used and stored. No contamination was detected at these sites.
The detection of the cesium occurred four days after the source was opened due to collection, processing and measurement times in the air sampler that was used in the detection. Measurements and swipe samples from rooms near the origin of the contamination showed that garage located next to the rooms where the contamination originated had also been contaminated. Lower levels were also detected in the corridors next to these rooms. However, the contamination levels in all of these areas were so low that they have not been to health risk to any workers that have been in the area. Decontamination of those areas are ongoing and large part of areas are already released for use. STUK has also measured areas around the building and no abnormal levels have been detected.
STUK has required the company to provide a report of the event and a plan for decontamination of company’s premises. Pending these reports and the completion of the decontamination of the rooms used by the company, it’s operations have been temporarily suspended.
Event date: Mon, 07-03-2016