A fire started at 10:15 am on a packing and storing swimming pool and water treatment chemicals. The fire started in the production area at the back of the factory unit. A 1 tonne bulk bag of sodium dichloroisocyanurate dihydrate was being emptied by closed screw conveyor from ground level to holding hoppers at mezzanine floor level. The chemical was then used to fill small plastic containers, under gravity, for retail. The electric motor driving the screw conveyor was located above the holding hoppers. The equipment had been running for about an hour, and was left running while the operators went on a break, as it had automatic level switches which would switch off the auger when the hoppers were full. Witnesses reported smoke seen rising from the auger tube of the screw conveyor. The fire alarm was raised and the factory unit was evacuated.
The Fire Service arrived on site following an automatic alarm and were about to investigate the source of white smoke when the colour of smoke changed, followed by a 20 m fireball.
The fire was contained within the factory building. At its height 100 firefighters and 20 fire engines tackled the blaze wich produced a large smoke plume. Residents were asked to stay indoors and keep their doors and windows shut.
Due to the speed of development of the fire, chemicals (with a ph of 1), following the rupture of IBC containers, entered the River Colne before emergency bunds were in place. Various outfalls from the drainage system were identified as the incident progressed, and these were blocked by the Environment Agency emergency response team. After the installation of bunds and drain bungs, the chemicals were pumped out.
The river is polluted : more than 2500 fish were killed over a 6 km stretch of the river. It is estimated that the river will take 4 to 7 years to return to pre-incident condition. No significant human injury was reported, although there were some reports of minor respiratory irritation. The factory unit involved in the incident was completely destroyed, but there was only minor material loss to neighbouring premises. Various parked vehicles were damaged and one commercial mini bus was destroyed. Businesses were disrupted at other units on the industrial estate, with access being unavailable for approximately 48 hours. The A40 trunk road was closed for 24 hours due to the smoke plume blowing directly across the road. Access to Andoversford village was also disrupted for 24 hours. The Company appointed decontamination specialists to clear the contamination held within the bunded area, and to remove the risk of rain run off adding to the pollution risk.
A decomposition, initiated by the heating of the sodium dichloroisocyanurate dihydrate inside the polypropylene tube of the conveyor, is the supposed cause of the fire.
The decomposition temperature might have been significantly lower than the 240° C temperature stated on the safety data sheet, as this is for a <5mg sample and other experimental work has identified 45 Kg can decompose at 650 C. Once at decomposition temperature, the chemical, which is an oxidiser, is self-reactive and generates heat which in turn led to plastic combustion.
The accident showed that the mitigation measures to prevent the run off into watercourses were inadequate. The company was unaware of the full layout of the drainage system and how their inventory could get into the watercourse. It was fined 66,000 pounds for failing to take the correct safety measures for the storage of chemicals at the site. Finally, the sole UN classification of the sodium dichloroisocyanurate dihydrate does not give an accurate description of the reactive nature of the chemical and possible hazardous conditions.