AC power is now available at Units 1, 2, and 4 of the six-reactor quake- and tsunami-hit Fukushima Daiichi nuclear plant in Japan, according to recent updates; however, the International Atomic Energy Agency (IAEA) still believes “the overall situation remains of serious concern.”

As periodic water spraying of Units 2, 3, and 4 and the common spent fuel pool has continued, crews have restored power to some instrumentation in all units—with the exception of Unit 3. That unit’s main control room has lighting but no power to its equipment or instruments.

Among positive developments are that the pressure in the reactor pressure vessel and drywell of Unit 3 is stable, Graham Andrew, a special adviser to the IAEA director told the media today. However, pressure has increased in both the reactor pressure vessel and the drywell of Unit 1, where seawater injection is being increased. “Until heat can be removed from Unit 1, pressure tends to increase as water is injected,” he explained. “The reactor feed water system is being used, in addition to water injection through fire extinguisher lines.”

Temperature readings in both Units 1 and 3 are still high and of “some concern” —particularly temperatures of water in the spent fuel pool at Unit 3, Andrew said. With the help of the U.S. Armed Forces and Japan’s Self Defense Force, crews have sprayed tonnes of water on the pool by helicopter and water cannon. Temperatures at Unit 2 are stable, however, he said.

But in Unit 2—where a blast on March 15 was heard near the suppression chamber, leading to speculation that the containment vessel has been damaged—pressure readings appear to be less reliable. The IAEA said with concern that “only limited data is available concerning the reactor pressure vessel and reactor containment vessels’ integrity of Unit 2.”

Workers have successfully connected Units 5 and 6 to offsite power sources, and both reactors are safely in cold shutdown, the IAEA said.

Hydrogen explosions shook Unit 1 (on March 12), Unit 2 (on March 14), and Unit 3 (also on March 14) in the aftermath of the 9.0-magnitude earthquake and what the plant operator Tokyo Electric Power Co. (TEPCO) now describes as a 14-meter tsunami wave. A fire subsequently broke out at Unit 4, which was not in service at the time of the quake.

By regulation, the Daiichi plant was fully prepared for a tsunami of up to 5.7 meters.

On Monday, all workers at the plant were forced to evacuate after light gray smoke was sighted rising from the southeast side of the 5th floor roof of Unit 3. The following day, TEPCO said, “the color of smoke changed to somewhat white” and it slowly dissipated.

Andrew said that an IAEA radiation monitoring team took additional measurements at distances from 30 to 73 km from the Fukushima nuclear power plant. Results from gamma dose-rate measurements in the air ranged from 0.2 to 6.9 microsieverts per hour. The beta-gamma contamination measurements ranged from 0.02 to 0.6 megabecquerel per square meter.

“The measurements indicate that the radiation dose rates at the Daiichi site are decreasing,” he said. “Absent further releases from the site, this is to be expected as relatively short lived radionuclides such as Iodine-131 decay away. At the Daiini site, small spikes have been observed in gamma dose rate measurements; these are most likely to be the result of releases carried by the wind from the nearby Daiichi site.”

The deposition of iodine-131 and cesium-137 varies across 10 prefectures from day to day, but the trend is “generally upward,” Andrew added. In contrast, environmental radiation monitoring data in the Fukushima Prefecture outside the 20-km evacuation zone, “shows mostly decreasing values.”

So, far, no significant risk to human health has been identified, he said.

Sources: POWERnews, TEPCO, IAEA