Blackout Halts Cooling System at Fukushima Plant

TOKYO — A partial power failure at the stricken Fukushima Daiichi nuclear plant in Japan has stopped the flow of cooling water to four fuel storage pools, but temperatures still remain well within safe levels, the plant’s operator said Tuesday.

The operator, Tokyo Electric Power Company, said its engineers were trying to repair a faulty switchboard that it blamed for the outage that began on Monday night. The failure also briefly cut off electrical power to the command center at the plant, which suffered a triple meltdown two years ago after a huge earthquake and tsunami destroyed reactor cooling systems.

The company, known as Tepco, said the current loss of cooling water was manageable because temperatures would remain at safe levels for at least four days, and the plant also has backup systems.

Still, the problems underscore the continuing vulnerability of the plant, which is beginning a complex cleanup of the three damaged reactors that is expected to take decades. Some experts have warned that the current cooling systems, some of which were hastily built by engineers frantically struggling to regain control of the overheating reactors, could be knocked out by another large earthquake.

Much of the ongoing concern has focused on the pools near the reactors that are used to store spent fuel rods. These contain far more radioactive material and have less shielding from the outside than the reactors, raising the specter of another massive release of contaminated particles.

The radioactive plume from the original accident in March 2011 forced the evacuation of some 160,000 residents in Fukushima, a region in northeastern Japan. Many of those evacuees still live in temporary shelters and may never be able to return home.

On Tuesday, Tepco said it was investigating the cause of the current blackout, which it believes it has traced to the switchboard and attached cables. The company said it was readying a replacement switchboard in case it cannot fix the original one.

In a statement, the operator said the temperatures in the pool near the No. 4 reactor had risen the highest since losing power, to 25.9 degrees Celsius, or 78.6 degrees Fahrenheit, still well below the safety threshold of 65 degrees Celsius. The company said it estimated that temperatures would remain at safe levels for another 109 hours, or four and a half days, if no new cooling water were added.

It said temperatures at the other three affected pools were lower.

The No. 4 pool has been a source of concern before, largely because the building in which it is housed was almost totally destroyed during the original accident by a hydrogen blast caused by melting atomic fuel. The explosion left the pool exposed to the outside air. Tepco has been building a new, more protected pool to which it plans to move the spent fuel rods.

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