The operator of the Fukushima No. 1 nuclear power plant, which suffered core meltdowns in 2011, has decided to delay the removal of nuclear debris by about one year from 2021 due to the coronavirus pandemic, sources said Wednesday.

The process of removing the melted fuel, the most difficult part of cleaning up the facility, was to begin at the No. 2 reactor in 2021, but the virus spread has stalled tests in the U.K. of a robot arm that is to be used for the removal, the sources said.

Of the Nos. 1 to 3 reactors that experienced meltdowns following a massive earthquake and tsunami, the removal procedure was to start at the No. 2 unit because the operator, Tokyo Electric Power Company Holdings Inc., had the best grasp of its internal condition, they said.

Tepco had planned to insert a robot arm into the unit’s containment vessel, from which it would initially extract around 1 gram of the debris at a time, then gradually expand the amount as it works toward removing several kilograms a day.

The company was originally scheduled to verify in August the viability of the robot arm in the U.K. and transfer the equipment to Japan in February 2021 so that workers could start training with it.

However, the spread of COVID-19 in the U.K. has caused delays, as precautionary measures were taken to limit the number of workers at factories, according to the sources.

Tepco is considering doing part of the testing in Japan.

The removal process is expected to take several years for the No. 2 unit, which according to one estimate contains 237 tons of debris. Together, the three units are estimated to house around 880 tons of debris.

The government and Tepco have said they expect the plant’s decommissioning to be completed between 2041 and 2051.