Didcot power station collapse: Operation 'most challenging' for fire chief
The chief fire officer working at a collapsed Didcot power station site said it was the "most technically challenging" operation he had seen.
Oxfordshire Fire and Rescue chief Dave Etheridge said the Didcot site "reminds me of Chernobyl, the sheer extent of what we're dealing with".
One person is known to have died in the collapse on Tuesday.
But Mr Etheridge said it was "highly unlikely" three missing people would be found alive.
The families of the three missing men visited the site on Wednesday.
Mr Etheridge told BBC Radio Oxford: "I hope that they felt that was a way of understanding the enormity of the challenge that we are facing, but we won't step back from that challenge.
"I have been in the fire service for 30 years and undoubtedly the incident we are facing at the moment is the most technically challenging incident I have ever come across."
He said the area being searched was 55m (180ft) long, 30m (98ft) wide and 25m (82ft) high, and "unstable".
Heavy engineering cranes and lifting equipment are needed and would arrive at the site on Thursday, said Oxfordshire County Council.
Thermal imaging cameras, drones with audio sensing equipment, sniffer dogs and military remote-controlled vehicles are already being used in the search.
The mayor of Didcot, Des Healy, said a book of condolence was to be set up at the town's Civic Hall.
He said: "We've got to keep the families at the forefront and so I'm glad they were brought down last night and able to see it first hand."
The demolition of the decommissioned Didcot A power plant was being carried out by Coleman and Company.
About Didcot A Power Station
Oxfordshire's coal-fired Didcot A Power Station was turned off in 2013, after 43 years in service.
The station included six cooling towers, measuring 375ft (114m) in height, which dominated the skyline of the town.
Hundreds of people gathered to watch when three of the towers were demolished in the early hours of 27 July 2014.
RWE Npower expects complete deconstruction of the site by the end of this year.
A gas-burning power station - known as Didcot B - opened in 1997 on the site and continues to operate.
A major fire was declared at Didcot B in October 2014, with 20 fire crews sent to tackle the blaze, which was caused by an electrical fault.