An orphan Am-241 source inadvertently included in scrap metal was melted on the 23/2/2012 in a Finnish steel factory in Tornio. The first alarm of abnormal radiation level was given by detectors screening the slag pots. The activity of melted source can not be determined exactly, but it is estimated to fall in to Category 4 (0,6 – 60 GBq). The source is traced by the operator to stainless steel scrap delivered from Central Europe.
Due to its physical properties most of the Am-241 activity is partitioned in to the slag phase (aprox 99%) and dust phase (1%). Am-241 is captured in the dust suppression and filtration with other particulates in the dust phase and thus there were no environmental releases. Small percentage of the dust phase is released into the mill environment resulting in Am-241 suspension in the immediate aftermath of the incident and followed by surface contamination of the melt hall. In an Am-241 melt the steel remains uncontaminated.
Four workers attending to a separate non radiological issue in the vicinity of the electric arc furnace (EAF) are considered to have been exposed to Am-241 through inhalation. Dose estimates from whole body counting and 24 hour urine sample gamma spectrometry indicate that doses for the exposed EAF workers are below the detection limit for Am-241.
Workers used respirators and access to the melting plant was restricted until it was verified that no airborne contamination was present and the factory and contaminated surfaces were cleaned.
The factory has screening detectors of incoming scrap, including gate detectors and detectors on the loading machines etc. Despite of the state of art screening technology used, the source could pass through because of americium’s low gamma energy (59 keV) which is easily attenuated by other scrap.
Event date: Thu, 23-02-2012