NTPC (National Thermal Power Corporation) officials have said that "extremely high pressure" built up in the boiler furnace because of ash collecting in its outlet. The unit, the newest of six in the plant at Raebareli in Uttar Pradesh, hadn't been cleaned in two-three days. A fireball leapt up and pressurized steam gushed out just where dozens of workers were standing, killing or burning them.
Flames and steam hiss around the boiler in dramatic visuals shot on mobile phones just seconds after the explosion. Engineers can be heard shouting in fear on the video.
Around 100 workers were scalded by the steam that burst out like a pressure cooker. Some survivors have spoken of a rattling in the unit and then intense heat "that could melt human flesh". Why there were so many workers standing so close to the boiler is being investigated.
NTPC has admitted that engineers were aware of the ash deposition and were trying to clean it without shutting the plant.
But the unit should have been shut because it was new, says Shailendra Dubey, who heads the All India Power Engineers Federation. "It was commissioned in March hurriedly...it seems work was actually not completed," he said.
RS Rathee, the regional executive director of NTPC said on Thursday: "It is normal procedure that we carry out repair work when plants are still working . Shutting down a plant is a decision we take later. All our officers are experienced. The decisions were taken by them . It's not that we shut down a plant at the first problem."
The 1,550-megawatt plant supplies power to six states. If one of its units had been shut for a few days, it wouldn't have made a big difference to the power supply, said Mr Dubey.
Officials are investigating whether alarm systems were working, to give enough time for workers to be evacuated.