A blast furnace exploded at 17h13 in an integrated steelworks. Iron is made on site from basic raw materials in 2 blast furnaces; the furnace involved in the explosion was a 1950s design, which had been modified over the years. It consisted of a large 90 m-vessel supported on heavy steel columns. It was constructed in 2 sections, the top half having limited vertical movement on a lap joint with the bottom half. The furnace had a 3.5 cm thick outer shell and a refractory lining in which were inserted approximately 1,400 copper coolers. Water was pumped through the coolers to remove heat. It was capable of containing approximately 2,000 tonnes of burden (iron ore, coke and limestone) and had 24 tuyeres (nozzles for hot air) located at the base just above the hearth.

The accident event began with a rapid over-pressurisation of the furnace contents in the bosh area due to the interaction of water and hot molten materials. The immediate effect of this furnace over-pressure was to lift that part of the structure above the lap joint upwards, the furnace lintel rising up off the column heads normally supporting the furnace. It is estimated that the structure above the lap joint rose by some 0.75 m. This structure, with its associated burden, was estimated to weigh around 5 000 t The lifting of the upper part of the furnace left an opening of approximately 400-600 mm around the entire circumference at lap joint level. Gases and hot materials were ejected horizontally from this opening. During the period that this gap remained open, some 200 tonnes of liquid, solid, and semi-solid material were ejected onto the cast house floor. Gaseous material and dust rose into the cast house, exited through various openings in the building and into the atmosphere, where much of it ignited. A cloud of ignited dust and gaseous material was thrown several hundred feet into the air above the furnace. The furnace-top ‘bleeder’ valves also opened and discharged into the atmosphere. Gases from these valves also ignited.

The furnace then fell back down vertically as the pressure decreased. During the explosion it had twisted (anticlockwise) through approximately 20–50 mm and moved a distance off-centre of about 100 mm, leaving it supported on the lower lap joint plate, with the lintel mounting positions no longer in contact with the column top flanges but instead some 50–100 mm above the column tops. This meant that the weight of the furnace stack was resting, eccentrically, on the lower lap-joint plate.

There was little or no warning to personnel working on the cast house floor, although some witnesses later spoke of the event being preceded by ‘rumbles’ within the furnace moments before the actual explosion. Three employees that were on the furnace stack died and 17 employees and contractors in the cast house floor area were injured, 5 critically. Material damage amounted 2 M euros.

The blast furnace exploded due to significant internal overpressure brought about by the interaction of water and molten hot materials within the lower part of the furnace. The explosion occurred after a prolonged attempt – over two days – to recover the furnace from a chilled hearth situation caused by cooling water ingress.

The water had entered the furnace from its cooling system following a chain of events initiated by the failure of safety critical water cooling systems. At the time of the explosion, attempts were continuing to rectify the abnormal operating conditions that this had created and to recover the furnace.

The precursors to the explosion were a combination of significant failures in health and safety management extending over many years. These failures were not confined solely to the blast furnace plant; they extended elsewhere within the company, and in particular to the Energy Department which supported essential cooling water for the furnace.

A failure to carry out suitable risk assessments for blast furnace operations resulted in the failure to implement robust technical and procedural controls. There was insufficient redundancy and security of cooling water supplies, and overall cooling system reliability showed a downward and deteriorating trend over several months.