An explosion occurred in a building in which nitrocellulose formulations were prepared, and recovered single-base powders crushed, for subsequent use in the process of manufacturing double-base spherical powders. (The process involves the manufacture in an aqueous medium of small spherical granules from fresh or recycled nitrocellulose prior to adding nitroglycerine.) Although the recovered powder crusher is not implicated in the accident, it was damaged as a result of the fire in the building. The formulations are made from granules recycled after sieving, crushed single-base recovered powders and fresh nitrocellulose in the form of nitrocotton fibres. To make the formulations, nitrocellulose granules of various origins, stored under water in tanks in the building where the accident occurred, are transferred between tanks in the neighbouring buildings and the tanks in this building via a network of piping fitted with centrifugal pumps. The nitrocellulose granules in the building in question are first pumped into a pycnometer tank to be weighed and then transferred to other decanting and formulation tanks located in the same building. The formulations produced in this way are then transferred, by pumping, to granulation tanks at a later stage in the production line, where they are dissolved in ethyl acetate. An explosion occurred in the transfer pump and the piping linking storage tank B5 with the pycnometer tank during the transfer of nitrocellulose granules in the medium of water between these tanks. The investigations showed that the explosion of wet single-base powder had created detonation conditions in the pump and the vertical pipes downstream of the pump. A quantity of powder estimated at between 10 kg and 30 kg was detonated in the accident. The detonation sheared open tanks B4 and B5 in the vicinity of the pump in question. Tank B5, contained granules recycled after sieving and was almost empty, spilled into a pit and the contents of tank B4, consisting of recovered powder in the form of monotubular grains, were partially discharged onto the building’s concrete floor and were not involved in the explosive reaction. It is estimated that there were 6 tonnes of powder in tank B4 at the time of the explosion. The other tanks in the building and the pipe network were seriously damaged by the primary debris from the detonation of the pump and piping and by the blast wave. The explosion caused major damage to the building and threw debris out to a radius of 50 to 70 metres. Windows were broken within a radius of 100-150 metres. In the building affected, the explosion set off a fire in a store containing dry single-basis powder ready for feeding into the crusher.

The internal emergency teams intervened very quickly and started to fight the fire which broke out after the explosion. The external emergency services arrived very quickly on the scene. Set up of a post-accident crisis unit comprising the fire brigade, the police, the municipality, the Governor, civil protection, a communication unit and the competent Seveso inspection services (FPS [Federal Public Service] Economy, RAM [Major Accident Risk] cell of SPW [Service Public de Wallonie]). Clean-up work and site security operations were organised in cooperation with a French company specialised in decommissioning and dismantling pyrotechnic facilities. The situation immediately after the accident may be described as follows. – According to estimates, there were still 6 tonnes of nitrocellulose granules in the building, partially spread on the ground, plus 18 tonnes in the vertical tanks, in the pit or trapped in the piping. – On the ground floor of the building, parts of the floor were covered with nitrocellulose granules mixed with a great deal of debris and rubble. – The building had been severely damaged: only the metal structure remained and this was in very poor condition. – In the event of the remaining nitrocellulose exploding, the effects could extend beyond the site and affect people living in the neighbourhood. The following urgent action was taken: – Saturating with water (drenching) was recommended, considering the fire risk – and even explosion risk – on account of the powder remaining on the ground and in the atmosphere. To that end, an internal fire team attended continuously for 8 weeks to ensure watering. – In agreement with the authorities, a site clearance procedure has been established: Stage 1: reducing the quantity of powder spilt and hence limit the likelihood of its reacting by: – continuous daily hosing of the powder on the ground; – filling the vertical tanks with water; – monitoring of water levels; – isolation of all networks into and out of the building (overhead and underground). Stage 2: Organise the manual collection and packing of the powder spread on the ground. This has been a very gradual operation, organised per areas of a few square metres following risk assessments based on photos of the areas in question and subject to work permits. The powder is recovered using an aluminium shovel and water-filled containers. Stage 3: Securing of walls and building framework at risk of falling before further work on site. Stage 4: Drainage of tanks. Stage 5: Treatment of pipes containing powder. Stage 6: Collection of remaining waste on the ground. Stage 7: Dismantling of the building. After obtaining a technical opinion from external experts, and in agreement with the authorities, a 350 m security perimeter was set up. Local residents were evacuated during the clearing and cleaning of the industrial site commencing at noon on 13 March, 6 days after the explosion. After one week, the security perimeter could be reduced as the risk of explosion could be ruled out thanks to the safety measures put in place. Site clearing and remediation works are still ongoing three months after the explosion.

The hypothesis that nitrocellulose in a water medium could burn and then detonate as a transition fromcombustion without being primed by another detonation was initially considered but then ruled out after testing. The detonation of the wet nitrocellulose granules must have been triggered by an initial detonation of dry material. It was concluded that the cause was a technical issue associated with the type of pump which had led to a possible temperature increase in the nitrocellulose granules after loss of prime and drainage of the pump chamber. The creation of a vortex in the tank upstream of the pump caused the pump to lose its prime, enabling the nitrocellulose granules in the pump to come back into contact with air and, in local areas, dry out. The hypothesis is that the dried powder trapped behind the pump impeller then became hot and detonated inside the pump chamber, transferring the detonation to the column of wet nitrocellulose located above the pump. The fact that the pipework upstream of the pump found after the explosion was not destroyed by an internal detonation supports the hypothesis that a vortex caused the pipes to be drained prior to the explosion. Hypotheses and deductions: 1) Vortex effect: It seems that the level of tank B5, to which the centrifugal pump was connected, was low at the time of the accident. In identical circumstances on earlier occasions operators had noticed a vortex effect at the bottom of the tank which could be the cause of air ingress into the pump, causing it to lose prime. During tests to try to recreate the circumstances of the accident, the operator noted: – a 50°C increase in the (surface) temperature of the pump within 15 minutes; – that the liquid column downstream of the pump failed to empty. Thus, since there is no longer a flow of liquid to the vertical column but only an oscillating phenomenon caused by intermittent discharge of the unprimed pump, the solid particles tend to drop to the bottom of the column, near the pump, where they form a blockage that stops liquid flowing back into the pump and keeping the product residues in the pump wet. 2) Sequence of events: It is hypothesised that the following sequence of events led to the accident: – Air ingress caused by a vortex led to the pump losing prime, obstructing the movement of the product through the pipes and a blockage forming in the column downstream, preventing liquid flowing back. The lack of liquid flow combined with the presence of air caused the pump to heat up and, in local areas, dry out the nitrocellulose. At the same time, nitrocellulose particles dropped into the column downstream of the pump. – The nitrocellulose particles inside the pump became drier in local areas, caught fire and detonated, causing a second detonation of the wet powder in the downstream piping.

The substance used in the process takes the form of nitrocellulose granules in a water medium and wet nitrocellulose (dampened with no less than 25 % water (by mass)).Wet nitrocellulose – i.e. dampened with no less than 25 % water by mass – is [ADR] classified as 4.1D: flammable solids, self-reactive substances and solid desensitised explosives. When combustion is initiated, wet nitrocellulose does not make the transition to detonation. In dry form, however, nitrocellulose is an explosive substance classified 1.1. Although classified 4.1, nitrocellulose detonation is nonetheless possible up to water concentrations of 80 % if there is considerable energy input in the form of a shock wave generated by another explosive detonating in the vicinity and triggering a detonation reaction. In that case detonation is possible even with water present. The transition from combustion to detonation is not, however, possible with the wet product. The spontaneous inflammation temperature of nitrocellulose is 160°C. It is estimated that 10 to 30 kg of powder detonated. At the time of the explosion it is estimated that there was 6 tonnes of powder in water in the neighbouring tank B4. Some of this powder was discharged onto the floor but was not involved in the detonation reaction.

The investigation has not yet been finalised and the causes have not been fully identified. Lessons and immediate steps: Replacement of the company’s electrical centrifugal pumps (40 in total) with Venturitype pumps. Venturi pumps operate on the principle of aspiration of the fluid to be conveyed by means of negative pressure brought about by the movement of a motive fluid (water). This type of pump means that friction can be reduced and the product conveyed can be kept wet continuously since the motive fluid is water.