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The nuclear crisis at Fukushima threatens to expose large numbers of people to radioactive contaminants, harming public health. Radiation has already escaped the reactor complex. The severity and size of the leaks is unclear at this time. The government has evacuated everyone within about 13 miles of the plant, and told everyone within 20 miles to stay indoors. The New York Times reports that authorities are making "a series of rapid and at times confusing pronouncements on the crisis," making reliable and timely information hard to come by.

However, on March 15, the The Institute for Science and International Security in the United States ranked the disaster a 6 on a 7-point scale of severity -- making it the worst nuclear emergency since Chernobyl in 1986.

There are two primary sources of potential radiation leaks; the reactor cores involved in power generation, and the storage pools where the reactor operator stores used nuclear fuel rods.

Spent Fuel Storage Pools

There are 6 reactor units at the stricken Fukushima Daiichi Nuclear Power  Station. Each unit stores used fuel rods in deep pools located above ground in the secondary containment building of the reactor. Importantly, these pools do not have the same level of protective shielding as the fuel rods within the reactor core itself. And some of the buildings have suffered damage from hydrogen explosions, which may have left pools open to the atmosphere.

In order to keep the spent nuclear fuel from overheating, the pools have to be constantly cooled. If the rods overheat, they could boil off the water in the pool. Alternately, if damage to the reactor building penetrated the pool itself, water could leak out. Should the fuel rods become exposed to open air, they could catch fire, spreading radioactive smoke into the atmosphere.

That would be "worse than a meltdown," David Lochbaum of the Union of Concerned Scientists told reporters at the New York Times. If a fire occurred, the smoke would loft radioactive material higher into the atmosphere, causing contamination over a potentially wide area, depending on prevailing winds.

According to analysis by Robert Alvarez, a senior scholar at the Institute for Policy Studies and former Senior Policy Advisor to the Secretary and Deputy Assistant Secretary for National Security and the Environment at the U.S. Department of Energy during the Clinton administration, a single spent fuel pool can contain 3 to 8 times more radioactive material than the reactor core at Chernobyl.

Alvarez reports:

A 1997 report for the Nuclear Regulatory Commission (NRC) by Brookhaven National Laboratory also found that a severe pool fire could render about 188 square miles uninhabitable, cause as many as 28,000 cancer fatalities, and cost $59 billion in damage. A single spent fuel pond holds more cesium-137 [a radioactive element that stays dangerous for hundreds of years] than was deposited by all atmospheric nuclear weapons tests in the Northern Hemisphere combined.

In 2003, my colleagues and I published a study that indicated if a spent fuel pool were drained in the United States, a major release of cesium-137 from a pool fire could render an area uninhabitable greater than created by the Chernobyl accident.

As of 11 AM Pacific, March 16 2011, the New York Times is reporting that the spent fuel pool at the Number 4 reactor is dangerously overheating. The paper is reporting that "there were concerns about the pools at 5 and 6 as well, and possibly those at the other reactors."

Apparently the military had developed a plan to spray water on the overheating pool via helicopter, but the possibility of flying through unsafe radiation levels caused officials to postpone the plan.

Reactor Cores

At Fukushima Daiichi reactors 1, 2 and 3, nuclear reactions were underway, generating power, when the earthquake and tsunami struck on March 11. The reactors shut down automatically during the quake.

However, even after a reactor shuts down, the cooling systems must remain active in order to circulate fresh water and keep the reactor core from overheating, which could cause fuel to melt and potentially release radioactive material into the atmosphere. (For a more detailed explanation, see these articles from the New York Times.)

Apparently the combination of loss of power, a tsunami flooding the backup generators, and human error caused the primary and backup cooling systems to fail at each reactor.

Pressure levels inside several of the reactors began to rise beyond safe levels. Plant operators declared an emergency situation and began to vent the extra pressure.

In a last-ditch effort to cool the three reactors at Fukushima Daiichi, operators began a plan to pump seawater containing boron into the reaction chamber to cool the fuel rods and quench any fission reactions. But the effort was fraught with technical difficulties. Water levels have dropped to dangerous levels at times, and fuel rods began to melt down – increasing the risk of a breach of reactor containment and an uncontrolled release of radioactive material.

Somehow, hydrogen gas built up in two buildings housing reactor vessels. The gas exploded, blowing the roofs off of the buildings and causing unknown damage inside. Two other explosions have apparently damaged the primary containment structure housing the reactor vessel inside reactors 2 and 3.

As of 11 PM pacific time on March 16, the New York Times reported that:

Japan’s nuclear crisis intensified again Wednesday, with Japanese authorities announcing that a containment vessel in a second reactor unit at the stricken Fukushima Daiichi plant in northeastern Japan may have ruptured and appeared to be releasing radioactive steam. That would be the second vessel to be compromised in two days.

The vessel had appeared to be the last fully intact line of defense against large-scale releases of radioactive materials from that reactor, but it was not clear how serious the possible breach might be.

The paper is reporting that on Tuesday, 750 workers were evacuated from the plant, leaving a skeleton crew of 50 to battle the crisis at increasingly grave risk to their health. On Wednesday, the reactor's operator reported that staffing had been increased to 100 people, an increasing proportion of whom were soldiers in the Japanese Self Defense Force.

This crisis is not likely to end anytime soon. Even if the workers manage to extinguish the fires, cool the spent fool storage pools and successfully keep the reactor cores from melting with seawater injections, the required venting of steam from the reactor cores will lead to ongoing releases of an unknown quantity of radioactive material into the atmosphere, potentially for months into the future.

The Earthquake and Tsunami

On March 11, 2011, Japan suffered an earthquake off the northeastern coast of the main island, Honshu. The earthquake was a 9.0 on the Richter scale, the strongest ever recorded in Japan, and one of the strongest earthquakes recorded on the planet in the last century. Following the quake, a tsunami flooded nearby coastal areas, causing widespread destruction.

The damage affected several nuclear power plants in the country.  Eleven separate reactors shut down automatically in the incident. Fukushima Daiichi was not the only facility to suffer problems. Backup cooling systems at Fukushima Daini Nuclear Power Station, 10 miles away, also failed at 3 out of 4 reactors -- although workers appear to have kept the crisis there from spiraling out of control.

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