Use of natural radionuclides to determine the time range of the accidental melting of an orphan radioactive source in a steel recycling plant.

In the rare event that an orphan radioactive source is melted in an Electric Arc Furnace steel recycling plant, the radionuclides present are partitioned in the different products, by-products and waste. As a consequence of an unforeseen melting of a radiocesium source, cesium radioisotopes can be found in the dust, together with many natural radionuclides from the decay of radon and thoron, which are present in the atmosphere, picked up from the off-gas evacuation system and associated with the dust of the air filtration system ("baghouse"). In this work we verified that the activity concentration of ²¹²Pb in this dust is essentially constant in a specific factory so that it is possible to use it to date back to the time of the accidental melting of the orphan radioactive source. The main features of this method are described below, together with the application to a particular case in which this method was used for dating the moment in which the dust was contaminated with ¹³⁷Cs.