All six units at Japan's troubled Fukushima Daiichi plant have been hooked back up to the country's electrical grid, but officials have yet to flick on the power switch.
"They've connected the wires to the pumps but they haven't turned the pumps on for fear of an explosion," Dr. Michio Kaku, a theoretical physicist said. "There could be hydrogen gas there and if they hook up the power and then they turn it on and there's a spark, that could set off a hydrogen gas explosion."
Workers were seen smiling this morning after turning on the lights in the control room of unit 3, NHK reported. The step forward comes after a turbulent few hours where a boiling spent fuel pond at Unit 2 alarmed experts.
The fuel pond appears to have stabilized after emergency crews dumped 18 tons of seawater into the storage pool. Workers continue to douse units three and four with water at the disaster-stricken facility.
"The last three days we've had reassuring words, we've turned the corner, things are stable but it's on knife's edge, any small earthquake any spent fuel pond boiling incident could cause the workers to evacuate," Kaku said.
This morning a 6.6 magnitude earthquake rattled the country. Yesterday, smoke billowing from unit 2 temporarily halted work at the plant.
Japan's Nuclear Safety Agency confirmed today that more tests have been ordered for radiation in seawater as fear spreads that Japan's seafood supply might be contaminated.
Prime Minister Naoto Kan said that no seafood from Fukushima prefecture has reached fish markets since the 9.0 magnitude earthquake and tsunami struck March 11, NHK reported.
TEPCO announced that the level of iodine in the seawater surrounding the Fukushima plant is 27.1 times higher than the normal level. The level of cesium is 2.5 times higher than normal levels.
Contaminated spinach and milk has already been pulled from store shelves.
U.S. NRC Optimistic About Stabilization of Fukushima
Japanese workers made significant progress over the weekend at the power plant, connecting crippled reactor cooling systems to power lines.
Restoring power to the water pumps means workers will be able to cool the cores and prevent a meltdown.
"The fact that offsite power is close to being available for use by plant equipment is the first optimistic sign that things could be turning around," said Bill Borchardt, executive director of the Nuclear Regulatory Commission Operations on Monday.
Borchardt said that while U.S. officials believe several reactors have experienced some sort of core damage, the containment structure around the radioactive core is largely intact and water is flowing to cool the radioactive rods.
"The radiation releases and dose rates that we see on site are mostly influenced by units 3 and 4 spent fuel pool," said Borchardt. "And TEPCO and the Government of Japan have been making a concerted effort to address those issues."
The effort, which has progressed sporadically over the past week, stalled temporarily today at Unit 3, which lost its roof in an explosion last week, after smoke from an unknown source began rising from it.
"Cables might have caught fire," Tokyo University professor Nato Sekimura said. "It's understandable that people evacuated to study what happened in the area to secure their safety."
Meanwhile, traces of radiation from the damaged nuclear plant were still detected miles from the plant, including in some vegetables and water supplies, raising alarm by Japanese residents and spurring U.S. officials to continue urging precautionary steps.
The State Department distributed potassium iodide tablets to U.S. government personnel and their families in Japan "out of an abundance of caution" but instructed them not to consume the pills yet. Potassium iodide helps the body block absorption of some radioactive materials.
The Pentagon also ordered the aircraft carrier USS George Washington and destroyer USS Lassen to move farther out to sea off the coast of Japan out of radiation concerns.
ABC News' Luis Martinez, Kirit Radia and Barbara Garcia and the Associated Press contributed to this report.