Just five hours after Tokyo Electric Power Co. started normal operation of equipment to purify highly radioactive water at the Fukushima No. 1 nuclear power plant, the system had to be switched off at dawn on June 18 because of higher than expected radioactivity around the unit.
Just five hours after Tokyo Electric Power Co. started normal operation of equipment to purify highly radioactive water at the Fukushima No. 1 nuclear power plant, the system had to be switched off at dawn on June 18 because of higher than expected radioactivity around the unit.
The water decontamination process is important to the utility's plans to regain control at the stricken plant because it offers a way of dealing with the approximately 110,000 tons of radioactive water that has accumulated in turbine buildings and elsewhere on the site. Heavy rain is threatening to make that water overflow.
The failure of the system also compromises plans for "cyclic injection" of cooling water into the stricken nuclear reactors, which would allow fuel rods to be cooled on a sustainable basis by reusing contaminated water. That process was originally scheduled to start June 18 but will have to be put back, TEPCO said.
Company officials said they did not know what caused the high levels of radioactivity near the purification equipment and did not know when it would be back in operation.
The latest setback follows a series of problems with the system. It was scheduled to start full operation June 15, but leaks in water from dozens of pipe joints and glitches in computer programs operating the pumps plagued trials of the cleaning devices. Those problems eventually forced TEPCO to cut short the trials and go straight to full operation on 8 p.m. on June 17. Five hours later, the unit was again at a standstill.
The equipment combines a series of cylindrical containers (90 centimeters in diameter and 2.3 meters in height) filled with zeolite, a mineral that absorbs cesium. Radioactive water is carried through pipes linking the containers.
TEPCO planned to replace cartridges whenever radiation levels in excess of 4 millisieverts per hour were detected.
That threshold was set to prevent the exposure of workers to high radiation levels, but TEPCO said a radiation level of 4.7 millisieverts per hour was measured near the intake to one series of cylinders at around 0:54 a.m. on June 18.
A radiation level of 3.9 millisieverts per hour was found at the intake to another series of cylinders.
The containers were expected to be replaced once a month. TEPCO said it was surprised that the threshold had been exceeded in such a short time.
"We halted the operation because we could not decide what was taking place," an official at TEPCO's office in Fukushima said.
There were no signs of water leaks, but the process could only be resumed if the cause was found, the official said.
Slightly past 3 a.m. on June 18, TEPCO began the purifying operation by pumping in low-level radioactive water, which lowered the radiation levels to 1.76 and 0.95 millisieverts per hour, respectively.
TEPCO suspects that the high measurements may be attributable to the passage of highly radioactive water or to residues of sludge that could not be removed during an earlier stage of processing.
The purification equipment was expected to cut radioactive materials present in processed water to between one 1,000th to one 10,000th of original levels, TEPCO said.
But starting full operation of the system would make cleaning of the devices difficult. The interior of the pipes and containers would be contaminated by high levels of radioactivity, making repairs hazardous. Any overhaul would require full decontamination, further delaying the process.
Normal operation of the equipment would also produce 2,000 cubic meters of highly radioactive sludge by the end of the year, which would have to be stored in tanks on the site. No decision has been made on how to process it.
The cesium absorption equipment is made by U.S.-based Kurion Inc., while chemical elimination equipment used in the system is made by the French-based Areva SA. None of the devices have previously been used to purify radioactive salt water.
(This article was compiled from reports by Hiroshi Ishizuka and Naoya Kon.)