The Iter project at Cadarache in Provence is receiving the first of about one million components for its experimental reactor.
Dogged by massive cost rises and long delays, building work is currently nearly two years behind schedule.
The construction of the key building has even been altered to allow for the late delivery of key components.
"We're not hiding anything - it's incredibly frustrating," David Campbell, a deputy director, told BBC News.
"Now we're doing everything we can to recover as much time as possible.
"The project is inspiring enough to give you the energy to carry on - we'd all like to see fusion energy as soon as possible."