In a mineral filler production plant, a powder product silo overflowed for a period of 45 minutes at the beginning of the morning; 15 t of product (ground calcium carbonate + 2.8% of auxiliary product) were released into the open air and scattered on top of the silo and on the roofs of the plant’s buildings. A proportion was blown by the wind over the docks along the canal, and onto the surface of the water for a distance of 300 m, between the plant and the lock. Fire-fighters set up 2 floating booms to prevent it from spreading any further and recovered the product using a cleaning company’s vacuum truck. Navigation on the canal was halted throughout this phase. At 3 pm, 95% of the product had been recovered, and cleaning continued for a further 3 days to recover the remainder. According to the operator, the overflow was caused by failure of the “silo full” detection system carried out by a caesium 137 detector. This detector had recently undergone regulatory radioactive emission inspections by an external company which had required temporary changes to the receiver setting. As the detector’s sensitivity had not been adjusted correctly, the sensor had not detected the product when the silo was full. The operator changed the operating procedure for this type of sensor in order to integrate the double checking of its setting by 2 different people. An information memo was sent to all staff.