Policemen block an anti-nuclear protester at a rally in front of Tokyo Electric Power Company's headquarters building in Tokyo, April 23. Demonstrators criticized the operator of the tsunami-crippled Fukushima Daiichi nuclear plant, and demanded a halt to Japan's nuclear development plans. The sign reads, 'No Nuclear Power Plant.'

If that happened, it would be a major shift from the nuclear plant's current status, which the International Atomic Energy Agency still lists as "very serious." Cold shutdown means bringing the temperature inside the reactor down below the boiling point of water, so no steam pressure is being produced – a significant step if it happens, US experts said.

Before the temperature can be reduced, the building must be vented of radioactive air to allow operators to approach the reactor. In addition, American and British scientists have expressed concerns about the proposed step and newly discovered evidence of an ongoing radiation leak into the Pacific Ocean.

Bringing Unit 1 to cold shutdown

But important questions remain about how safe the cold shutdown process will be. To bring down the temperature in Unit 1, the company plans to inject water into the primary containment structure – a large, light-bulb-shaped container that surrounds the inner reactor vessel that holds the uranium fuel core.

Will the primary containment structure be able to hold the many tons of water that will fill it – or will it leak or split because it is damaged? Will a new quake or aftershock damage a containment structure that is far heavier because it is full of water?

"They're basically getting ready to run a big experiment," says Edwin Lyman, a nuclear reactor expert with the Union of Concerned Scientists, a nuclear industry watchdog group that is closely monitoring the situation in Japan. "It sounds like they're assuming that the structural issues [with the primary containment structure] aren't that serious – and there's debate over that."

Even so, filling up the containment structure "seems like a reasonable thing to do if they can't cover the cores in any other way," Dr. Lyman adds. "They're just stuck with doing whatever is going to work. The problem is, they're learning by experimentation – not by some well-thought through contingency plan."

The operator of the Fukushima Daiichi plant – Tokyo Electric Power Company (TEPCO) – last week revised estimates of the amount of damage sustained by the fuel cores of Units 1-3. In that estimate, the company revised downward the amount of core damage in Unit 1 from 70 percent to 55 percent. At the same time, it bumped up the damage level estimates for Unit 2 from 30 to 35 percent and for Unit 3 from 25 to 30 percent.

Preparing Unit 1

Before Unit 1 can be filled with water, though, workers need to remove radioactive air from the building in order to be able to get near it. TEPCO said Tuesday that eight workers would be ready to enter the heavily damaged building by Wednesday – the first time anyone would enter since the building was torn apart by a hydrogen explosion on March 12, the day after the plant was hit by an earthquake and tsunami.

To make the building safe to enter, the company began installing ventilation equipment at Unit 1’s turbine building Tuesday, the Yomiuri Shimbun newspaper reported on its website. Six such devices will extract highly radioactive air from the reactor building through hoses, the company said.

To prevent the radioactive air from leaking into the turbine building, a housing is being built that will cover the doors and provide "negative pressure," so air flows into the building, but not out of it. Hoses will be run to the reactor building so ventilation can start Thursday, the newspaper reported.

“While we have not seen or predicted any new significant challenges to safety at the site, we have only seen incremental improvements towards stabilizing the reactors and spent fuel pools,” said Gregory Jaczko, NRC chairman in his written remarks.

The NRC is reviewing the 104 nuclear plants currently operating in the US, to determine whether changes are needed to keep them safe. Their first report is expected next week.

New discovery: Seafloor radiation 100-1,000 times normal

Concern is also bubbling over radiation levels in the ocean near the plant – apparently due to an undiscovered leak from a damaged reactor. Ocean radiation readings taken Friday from the seabed near the plant were 100-1,000 times usual levels, TEPCO told the Mainichi Times.

In meetings Tuesday in London, Japan's Minister of Foreign Affairs Takeaki Matsumoto appeared to accept Britain's offer of a team of experts to help monitor sea radiation, the Mainichi Times reported.