Work to remove toxic water puddles in the reactor basements of the crippled Fukushima No. 1 nuclear power plant ground to a halt Sunday after its operator reported a huge spike in radioactivity — a spike that officials later said was inaccurate.

Tokyo Electric Power Co. initially said 2.9 billion becquerels of iodine-134 per cubic centimeter was detected from a puddle in the turbine building of reactor No. 2 on Saturday. The Nuclear and Industrial Safety Agency subsequently announced the figure Sunday morning based on Tepco’s report.

Later in the day, however, NISA questioned Tepco’s math, prompting the utility to say it had miscalculated.

Had the initial figure been correct, it would have indicated the level of radioactivity was 10 million times higher than what would typically be seen in a normal nuclear reactor.

Tepco and NISA, however, did not retract an earlier statement that radiation readings in the reactor Saturday surpassed an alarming 1,000 millisieverts per hour.

“The level of radiation is greater than 1,000 millisieverts. It is certain that it comes from atomic fission,” NISA’s Hidehiko Nishiyama told a news conference in the morning. “But we are not sure how it came from the reactor.”

Nishiyama said the vital components of the reactor — its core and containment vessel — are probably still intact, but that radioactive water may be seeping in from damaged pipes and valves.

The maximum allowable dose for a nuclear plant worker has been raised to 250 millisieverts for the crisis.

Tepco officials said levels this high indicate the radiation is either coming from the fuel rods in the reactor or from the spent fuel pool above.

Puddles had earlier been found in the basements of the turbine buildings in reactors 1, 3 and 4, with radiation levels at 60 millisieverts, 750 millisieverts and 0.5 millisieverts per hour, respectively.

Meanwhile, the level of radioactive iodine-131 that was detected in seawater near the Fukushima No. 1 plant has skyrocketed to 1,850 times the limit, compared with 1,250 times the previous day.

The cause of the seawater contamination hasn’t been traced yet, but experts say it’s possible that rain carried radioactive contaminants into the regular drainage system, which empties into the sea.

Coping with the contaminated water has sidetracked the workers, whose main focus is to restore power to the systems that cool the fuel rods in the reactors and the spent rods inside their pools.

According to Tepco, work has begun to remove the water in the No. 1 turbine building, but no plan has been set yet for removing water from the other buildings.

Meanwhile, Tepco belatedly said it will release information on the status of plutonium around the reactors — a major focus of international concern but a topic left untouched by most Japanese media.

But it will take several more days before the result is announced, Tepco said.

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