The reactor towers at a former nuclear power station in Snowdonia are set to be partly demolished using ground-breaking technology.
The process to downsize the two reactors will take two years
The two 170ft towers at Trawsfynydd will be downsized following calls from a public inquiry in 2002.
The aim is to lessen the visual impact of the power station while the decommissioning process continues.
The location is set to return to being a green-field site by 2098.
The nuclear reactors at Trawsfynydd were shut down in 1991 and the power station closed two years later.
Since then, the decommissioning process has seen all the highly radioactive nuclear fuel removed and sent to Sellafield for reprocessing.
The most dangerous material now left on site - so-called intermediate waste - includes radioactive metal that surrounded the fuel rods in the reactors.
This is due to be moved into "safe stores" which are being built on the site, until it too can be taken away.
Following a public inquiry into the future of the site in 2002, the two 170ft reactor buildings are being reduced in height by about a third to lessen to visual impact of the site.
The two reactors each house six boilers, each weighing 1,000 tons.
They are made of solid steel and are 130ft high, 18ft in diameter, and up to 7ins thick in places.
Before the reactors can be cut the boilers need to be broken down and moved to a lower level of the building.
A massive lifting rig has been designed specifically for the process of cutting the boilers into smaller 100 ton pieces.
The waste will be stored on site until it is safe to be moved
Simon Parsons, the site manager at Trawsfynydd, said the project was "a monumental challenge".
"It's an historic day for us. It's the culmination of four years of effort and preparation," he said.
Many of the 500 workers at the site, almost half of whom are from the local area, have been trained specifically for this project.
Mr Parsons said he hoped the technology, and the skills of the workforce would be used at other sites around the world.
Gwynedd councillor Tom Ellis also welcomed the move.
"I wholeheartedly welcome it because when you have a lump sum in the region of £1m its bound to some good in the area."
The remaining radioactive material on the site is set to be stored on site until 2088 when it will finally be removed.
The station is set to return to a green-field site and re-open for public use in 2098 at a total cost of around £1bn.