The Fukushima No. 1 nuclear power plant is running out of storage space for slurry, a mud-like waste product containing radioactive materials removed from contaminated water that is still accumulating at the site.

If the slurry tanks reach full capacity, Tokyo Electric Power Co. may have to review its operations on treating contaminated water. The slurry problem could also destabilize the overall premise of the company’s decommissioning work.

TEPCO said it has come up with measures to deal with the problem, but hurdles remain and some past mistakes must still be addressed.

Slurry is removed from contaminated water through the Advanced Liquid Processing System (ALPS).

The radioactive sludge is then placed in High Integrity Containers (HICs) at a temporary storage facility on an elevated platform on the south side of the No. 1 to No. 4 reactors at the Fukushima plant.

The storage facility is part of the spent cesium absorption tower.

Over the past year, TEPCO has added about one HIC every two days on average.

As of March 2, the facility contained 4,143 HICs, or 98 percent of the storage capacity of 4,192 tanks.

TEPCO had feared full capacity would be reached in spring, but it said it has secured enough space for an additional 192 HICs by the end of April.

The utility also said it will continue to add HICs, and that up to 4,720 containers can be stored in already-secured spaces at the plant.

The company said it can prevent the HICs from becoming totally full until around April 2026 at the earliest. If all countermeasures work, full capacity can be staved off until around June 2027.

However, TEPCO’s estimates do not take into consideration a possible unexpected increase in slurry volume from retreating water that is not meeting the standards for release into the sea.

Moreover, it is unclear whether the utility can secure more storage room if additional HICs are required.

Seen through a fence and surrounded by concrete walls, the storage facility looks like a dam measuring 7.7 meters high, nearly 30 meters wide and at least 100 meters deep.

“Inside this box are HICs, which contain slurry,” a TEPCO worker told reporters at the site in late January. “It will remain stored here for a while until a disposal method is determined.”

That brings up another problem.

TEPCO’s slurry dewatering facility has yet to become operational.

The company in 2021 submitted an application to build the facility, hoping it would reduce the volume of slurry by about 70 percent.

But TEPCO was forced to revise the design after the Nuclear Regulation Authority pointed out inadequate measures to protect workers from radiation exposure.

Radiation readings of 10 millisieverts per hour have been recorded on the surface of some HICs. At that level, a worker remaining in the area for about five hours would be exposed to the annual limit for radiation dosages.

For that reason, HICs must be stored in concrete boxes that can block radiation.

TEPCO plans to start operations of the dewatering facility in March 2027, four years behind schedule.

“We were a little naive (about the situation),” Akira Ono, president of TEPCO’s Fukushima Daiichi Decontamination & Decommissioning Engineering Co., said at a news conference in October last year. “I think we should have taken what the NRA pointed out more seriously.”

TEPCO also failed to take into account the shortened lifespan of HICs caused by the high radiation levels.

The NRA said 56 HICs will reach the end of their lifespans this year, and they could be damaged.

Since the triple meltdown in 2011, the plant’s premises have become covered with storage tanks and other facilities for radioactive waste.

There is no more extra space on the grounds of the Fukushima plant.

An area on the south side of the plant is crammed with tanks containing water processed by ALPS.

On the north side are various facilities, including temporary storage structures containing highly radioactive debris and other waste covered in soil.

The government and TEPCO aim to complete the decommissioning of the plant between 2041 and 2051.

But they have yet to come up with a plan on how to store or treat the slurry and most of the other radioactive waste.