The owner of the Fukushima nuclear power plant, Tokyo Electric Power Company (Tepco) began work on Friday to remove a cover placed over the building housing reactor No.1, a major step towards dismantling the plant.
This will allow workers to remove debris and then spent fuel from the building's interior, Efe news agency reported.
A polyester lining was installed three years ago over the site of a concentrated hydrogen explosion during the 2011 disaster, to reduce radioactive emissions.
Work began Friday at 6.45 am as workers inserted a tube suspended from a crane into a hole drilled in the roof of the cover, to spray a chemical to inhibit radioactive dust from spreading into the air.
This process will be repeated at 48 locations in the polyester lining over the next week, before its removal commences, a process which will take more than a year, said Tepco.
The company carried out a preliminary test of the process in October 2014.
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Tepco had initially announced the removal process would begin in July 2014, but then delayed it after radioactive material was detected in rice paddies near the plant.
The spread of material was apparently caused by dust that rose during the removal of rubble surrounding reactor No.3.
Tepco then devised this complex process to avoid a repetition of that incident.
The Fukushima disaster, caused by the earthquake and tsunami of March 2011, is the world's worst nuclear accident since the Chernobyl tragedy in the Ukraine in 1986.
Harmful emissions and radioactive discharges have precipitated the evacuation of thousands of people from and seriously affected agricultural and fishing activities in areas around the plant.