Explosion during hot work

A news article mentions that a company was fined 20,000 pounds for a hot work accident.The article mentions the following:
"David Lightfoot, 58, was carrying out welding work on a large container at Indorama Polymers (Workington) Ltd’s site in Siddick. The container housed 380 tonnes of the explosive powder, Terephthalic acid.
The company was prosecuted by the Health and Safety Executive (HSE) following an investigation into the cause of the explosion. Workington Magistrates' Court heard that, on 29 October 2008, Mr Lightfoot was welding a metal component around a two and a half inch diameter hole that had been sealed with a temporary bung, when he was suddenly engulfed by a fireball.
Mr Lightfoot, from Cockermouth, had been welding for around 20 seconds before finding himself surrounded by flames that forced him down to his hands and knees. He and another worker jumped ten feet to the ground to escape the flames but Mr Lightfoot was left with severe burns to his head, face, arms and hands.
The HSE investigation concluded the bung had fallen out, allowing around 15 grams of the explosive powder to escape, which was then ignited by the welding flame, creating the fireball".

Read the full article in this link.