Amid a full-blown humanitarian crisis from a massive earthquake and tsunami, Japan is racing against time to avert a nuclear catastrophe.

Plant workers at Fukushima Daiichi are struggling to cool the reactors and spent fuel held in pools also on site. Because of explosions caused by the buildup of hydrogen, it is believed that two of the containment structures that hold the reactors have been breached, greatly increasing risk of a release of a large amount or radioactive material.

Although it's still an unstable situation, it's clear the damage at the Fukushima Daiichi plant is worse than the Three Mile Island accident in 1979 and will be studied for years to come. Based on comments from experts and published reports, this FAQ attempts to shed some light on the current situation, with an emphasis on understanding the health implications from radiation.

What is the latest on the attempts to cool the reactors Plant workers are using the improvised technique of pumping seawater with added boron (which slows down nuclear fission) into the reactors at Fukushima Daiichi. As of Thursday in Japan, officials said that a hydrogen explosion has occurred in a second building which contains the hot nuclear reactor core. The containment vessel, in which the reactor is held, may have been breached at the No. 2 and 3 reactors, according to a New York Times report today. The explosions that have occurred are likely hydrogen explosions when hydrogen from the water mixes with the metal cladding over the uranium rods.

What is the picture on radiation releases In the first two days of the crisis, plant operators vented steam in an effort to control the buildup of pressure created by steam from cooling. Even though the reactor shut down during the earthquake, there is still "afterheat" created by residual radioactive material. In releasing the steam, radioactive cesium and iodine were released in the environment, but the levels were considered relatively modest. However, the apparent explosions in the reactor cores are far more serious, which led to the evacuation of people within 12 miles and an order to stay indoors within an 18-mile zone.

"It's unknown how great that containment breach (in reactor No. 2) is but it's a very bad thing because it means a much greater opportunity for the escape of radioactive material," said Ian Hutchinson, a professor of nuclear science and engineering at the Massachusetts Institute of Technology. MIT organized nuclear experts to discuss the situation yesterday afternoon.

The most significant risk is to plant workers, where a fire at a spent fuel pool led to the evacuation of all workers except a core crew. Radioactive material has been carried hundreds of miles away, but outside the evacuation zone, the radiation exposure is not enough to affect health significantly, the panel of MIT experts said.

In Tokyo yesterday, there were levels of radiation detected that were as high as 30 to 40 times above the background dose of the ambient environment. To put that in perspective, you would have to experience those dose rates for a few days to have the equivalent dose of a chest X-ray, they said.

The New York Times has a chart showing the recorded radiation releases and how they compare to other radiation exposures from natural sources or a whole-body CT scan. The Health section of the Times also noted that a number of precautions can be taken to avoid risk to the general population.

In terms of health damage, what should we be watching for There are two areas of great concern right now in terms of further radiation releases. The spent fuel pools remain hot and, since the cooling system at the plant has been compromised, they have overheated, which could lead to a situation similar to a meltdown, according to the Union of Concerned Scientists. Unlike the reactor core, the spent fuel does not have the thick steel and concrete to keep nuclear material from spreading in the case of a release.

The situation at the reactor core also poses great risks. If they cannot be cooled, then the nuclear fuel, which is shaped as long rods, will melt. In addition to concerns over breaches in the containment structures, there isn't full confidence that the building itself can effectively prevent melted fuel from entering the environment.

During a press briefing organized by the Union of Concerned Scientists, global security program director Edwin Lyman said yesterday in the case of a full meltdown, melted fuel would collect at the bottom of the steel reactor. "Modeling shows and tests that have been done at Sandia National Laboratories have confirmed that this is a condition that could lead to vessel melt-through, where the molten core will actually corrode through the steel liner and...then drop to the concrete floor of the containment building. Once that happens, the ability to contain the accident is greatly reduced, because the core is liquefied, and it then flows out--it spreads across the floor."

Lyman said there is a "known vulnerability" in this Mark 1 containment building which means that the melted fuel could break through the corners of the structure. The reactor building itself can also act as a way to cool and contain the melted fuel, but since there's damage to that from the explosions, it's could be a "large radiological release to the environment," he said.

If there isn't a leak into the environment out of the building, people should be watching for gas releases from the reactor buildings. The prevailing winds are westerly which means that air-borne radioactive material will be carried out to sea.

"This is important because the radioactive material that comes out of the plant basically will be carried by the weather, whatever it is. As long as it's blowing out to sea, the health effects can be expected to be limited regardless of how large the radiation release is. The effects on the plant, the workers are a different story, but for the general population that's what you'd pay attention to," said Michael Golay, professor of nuclear science and engineering at MIT, said yesterday.

What happens if there is a large-scale release of radioactive material During a press briefing today, the Union of Concerned Scientists' Lyman said the biggest concern is the spent fuel pools because of the lack of a containment barrier. If the radioactive cesium and iodine were released at a high rate, as happened at the Chernobyl accident, it would be a major problem. "If there were a large release of cesium, it could potentially create a zone that would be uninhabitable without extensive clean up," Lyman said. Those fragments are energetic enough that they can be absorbed by the skin, not only by breathing, he said.

Radioactive iodine is a health concern because it is absorbed readily by the thyroid gland, posing a risk of cancer, particularly to children. Taking potassium iodine tablets is a way to prevent the uptake of radioactive iodine. There are other radionuclides, which each have a different pace of decay, with Cesium-137 being the longest. Experts say that even if there is a large-scale leak in Japan, it highly unlikely to affect people living in the U.S.

Do U.S. plants have the same design as Fukushima Daiichi Wouldn't newer designs be safer Yes, six of the 104 nuclear reactors use the same Mark 1 design, according to the Nuclear Energy Institute, an industry group. There are new designs with as many as four backup cooling systems and some that rely on gravity rather than pumps. In a press briefing today, Lyman said that all designs have limits when you reach a situation of severe catastrophe as is happening now in Japan.

What effect will this have on the "nuclear renaissance" The Fukushima Daiichi crisis is already seen as far worse than the Three Mile Island accident in 1979, where there was a partial meltdown in the core and relatively modest amounts of radioactivity were released.

But Three Mile Island and Chernobyl, considered the worst in history, had a major impact on public opinion, so it's clear this crisis is a major blow to the advancement of nuclear power. Already, Germany and Switzerland have put in place reviews of their programs. China, which is making several new nuclear plants, is reported to have suspended renewal of new plants.

Countries, including the U.S. and France, already deeply committed to nuclear power will likely remain that way in the immediate future. At a Congressional hearing today, Energy Secretary Steven Chu recommended $36 billion in loan guarantees for construction of nuclear plants. Still, there could be reviews of earthquake readiness and potentially more stringent safety standards.