The disaster wiped out practically the entire shift of 17 workers who were on site at the time, according to a list of names posted on social media.
A video showed the moment of the explosion, with a huge yellow fireball rising into the air, and photos of the aftermath showed a scene of total destruction, with smoke still rising from the rubble.
The exact cause was unclear, with TASS news agency quoting a source as saying it had been caused by a failure to observe proper technical procedures.