Just after the explosion, flames leapt up and pressurized steam gushed out just where dozens of workers were standing, perhaps killing or at least blinding them instantly. Thick fumes filled the unit, making it difficult to see. On the video, engineers can be heard shouting in fear.
Around 100 workers suffered burns and 20 of them are said to be in a very critical condition. Some survivors have spoken of a rattling in the unit and then intense heat "that could melt human flesh".
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, producing a disaster.
The 1,550-megawatt NTPC power plant in Raebareli, with six power generating units, supplies power to six states.
Questions have been raised about whether, having been aware of a problem, NTPC should have shut down the unit to prevent the deadly explosion.
Mr Rathee clarified: "We only shut down a plant or a unit when we feel we cannot repair it during ongoing operations. We have a central control room in Delhi and we were sending out all parameters there too."
The 1,550-megawatt plant, with six power generating units, supplies power to six states.
Reports that a safety valve failed to work have not been confirmed so far. It is also being investigated whether alarm systems were working, to give enough time for workers to be evacuated.
Speaking to reporters, NTPC chairman Gurdeep Singh called the explosion "the rarest kind of incident". He said the people maintaining the unit were the most experienced.
"It will take nearly three to six months to make the unit functional again," Mr Singh told reporters.
