Discuss as:

Clearing up nuclear questions

NBC's Robert Bazell on the nuclear crisis.

Last updated 11:25 p.m. ET:

Three days after a catastrophic earthquake and tsunami hit Japan, the situation at the Fukushima Dai-ichi nuclear complex has turned into the biggest uncertainty of the crisis. Recovering from the seismic event will take tens of billions of dollars and years of work — but if the nuclear situation goes the wrong way, that would add dramatically to the disaster's cost.

How did all this happen, and how could it end? Different folks have different answers, depending on how they feel about nuclear power. Here's a roundup of the best answers I've been able to put together — accompanied by an invitation to add your own sources and perspectives as comments below:

Has there been a nuclear meltdown?

Authorities say partial meltdowns have probably occurred at three of the Fukushima Dai-ichi plants.

To understand what a "partial meltdown" means, we need to discuss how the reactors are constructed. Under normal conditions, the plants produce power by sustaining a controlled nuclear reaction inside a pressure vessel. Chain reactions in the nuclear core's uranium-filled fuel rods heat up water, generating steam that turns turbines to generate electricity. That steam is circulated through a cooling system and returned to the pressure vessel as water to keep the cycle going. The uranium oxide fuel is contained inside sheaths of zirconium metal that can withstand temperatures of 2,200 degrees Fahrenheit (1,200 degrees Celsius).

Control rods can be inserted between the fuel rods to shut down the main chain reaction in the uranium. But the water-circulating cooling system is needed as well to bring the temperature down while the radioactive decay subsides.

isotype.com / Reuters / Source: Deutsches Atomforum

The problem is that the power for the cooling system was cut off when the earthquake hit. Then the backup diesel generators were knocked out of commission by the tsunami. Backup batteries could keep the cooling system going for only about eight hours more. The plant's operator tried to bring in mobile generators to restore power, but the connections reportedly didn't match up.

Meanwhile, residual heat from radioactive decay continued to build up, and water continued to turn to steam. Eventually, the fuel rods became exposed. The temperatures apparently reached the melting point for the fuel rods' zirconium sheaths. That can result in uranium oxide fuel falling to the bottom of the pressure vessel — which is what some experts mean when they talk about a partial meltdown. Other experts, however, would reserve that term for a situation in which the nuclear fuel makes its way out of the pressure vessel but stays within a steel-and-concrete containment shell that surrounds the reactor. 

Is that why radioactive material escaped?

At the three Fukushima Dai-ichi reactors that are apparently experiencing partial meltdowns, the nuclear fuel is still contained within the pressure vessel. The radioactive material is not coming from the core itself. At reactors No. 1 and No. 3, the material is contained in steam that has been released from the vessels. Plant operators opened the steam valves to reduce the risk of a high-pressure explosion inside the vessels — in effect, letting off steam to keep the lid from blowing off a pressure cooker. The steam contains radioactive cesium-137 and iodine-131, which are byproducts of the uranium reaction. The authorities said the radioactivity in that steam is still below regulatory limits and should not pose any health risk.

Despite those reassurances, authorities ordered an evacuation of the area within a 12-mile (20-kilometer) radius of the Fukushima Dai-ichi plant, and have distributed stable iodine to evacuation centers as a precaution. If people are exposed to significant amounts of radioactive debris, taking doses of iodine can prevent the uptake of radioactive iodine and reduce the risk of thyroid cancer. 

Right now the radioactive plume is blowing out to sea, which means it's not wafting over Japanese population centers. It is wafting over the Pacific, however, and the U.S. Navy found that air crew members from the aircraft carrier USS Ronald Reagan were exposed to low-level contamination. The Navy says the crew members were decontaminated with soap and water, and all U.S. ships have been moved out of the downwind direction. Apparently, no harm was done.

Nevertheless, the contamination incident was worrisome to nuclear physicist Frank von Hippel, a former Clinton administration official who is now co-director of Princeton University's Program on Science and Global Security.

"I was surprised how high the radiation levels are," von Hippel told me. 

So what's being done?

Plant operators have been pumping cool seawater into the pressure vessels to replace the water that's being lost as steam, in an effort to keep the fuel rods from heating up further. They've added boric acid to the seawater, because boron suppresses the nuclear reaction and could accelerate the cooldown. Authorities were reluctant to turn to  this strategy because the seawater is so corrosive that it ruins the reactors for future power generation. But that's better than having the meltdown progress to an even worse stage.

What about these hydrogen explosions?

When the seawater hits the hot zirconium rods and uranium fuel, some of it is broken down into hydrogen and oxygen gas. Venting the steam allowed that hydrogen and oxygen to escape and build up between the pressure vessel and an outer structure that protects the reactor from the elements. At reactors No. 1 and No. 3, the hydrogen ignited, blowing the roof off the outer structure in each case. However, the pressure vessel and the steel containment shell remained intact. It's important to note that the hydrogen blast was not the result of any sort of atomic or "H-bomb" explosion, but was a purely chemical reaction.

Is the situation getting worse?

Yes. Authorities say another blast has been heard at reactor No. 2 at the Fukushima Dai-ichi plant. Details are sketchy, but the plant's owner, Tokyo Electric Power Co., said the explosion occurred near the reactor's suppression pool, a water reservoir that's part of the cooling system. A government spokesman said the pool was damaged, and there was concern that the No. 2 containment shell may have been breached. "A leak of nuclear material is feared," The Associated Press quoted Shinji Kinjo, a spokesman for Japan's nuclear safety agency, as saying.

Kinjo said radiation levels rose from 73 microsieverts before the blast to 11,900 microsieverts (11.9 millisieverts) three hours afterward. To put that figure in perspective, the U.S. Nuclear Regulatory Commission says that occupational exposure for adults working with radioactive material must be limited to 50 millisieverts per year.

In a nationally televised statement, Japanese Prime Minister Naoto Kan said "the level seems very high, and there is still a very high risk of more radiation coming out." Kan told people living within 19 miles (30 kilometers) of the Fukushima Dai-ichi complex to stay indoors to avoid radiation sickness.

AP also quoted Chief Cabinet Secretary Yukio Edano as saying that a fourth reactor at the complex was on fire and that more radiation had been released. "Now we are talking about levels that can damage human health. These are readings taken near the area where we believe the releases are happening. Far away, the levels should be lower," Edano said. Later, AP reported that the fire at reactor No. 4 was extinguished.

What about the nuclear fuel stored at the site?

The spent fuel rods at the Fukushima facility are stored in pools of water above the reactor. Plant operators have signaled that water levels were falling at reactor No. 1's storage pool, suggesting that the cooling system is failing. "It's on a slower fuse," von Hippel said, "but on the order of a week or so, it could boil down to the level of fuel."

What's the best-case scenario?

The seawater gambit keeps temperatures inside the pressure vessels under control for the next few days. During that time, the residual heat of radioactive decay dissipates, and operators no longer need to release steam from the vessels. Eventually, electrical power is restored to the cooling system, and each vessel's core can be removed. 

What's the worst-case scenario?

Authorities can't cool down the cores, and temperatures rise to the point that the uranium fuel melts into a mess on the bottom of the pressure vessel. The concrete-and-steel containment floor beneath the vessel has been built to contain a full core meltdown — but experts can't completely rule out the possibility of a breach that causes the highly radioactive material to escape into the environment.

Right now, the situation at Fukushima Dai-ichi is analogous to the Three Mile Island incident of 1979, which involved a partial core meltdown and a release of radioactive gases — but no breach in the reactor vessel. "It's at least as bad as Three Mile Island," von Hippel said. But if the nuclear fuel breaks out of the vessel, the situation could turn into something more like the 1986 Chernobyl nuclear accident in Ukraine, which sparked fatal cases of radiation sickness and spread contamination across a wide swath of Europe.

How long will this go on?

Even under the best-case scenario, it will take years to clean up the mess. "When you're dealing with spent fuel, you don't put it in cool, dry casks until three years after the reaction has stopped," von Hippel said.

More explanations of the nuclear situation:

Princeton physicist Frank von Hippel answers your questions about the nuclear situation in Japan during an online chat at 4:30 p.m. ET Tuesday, March 15. Click here to submit your questions in advance and participate in the chat.

Have you found some particularly good explanations for what's going on, or are there some burning questions yet to be addressed? I've purposely stayed away from discussing the potential health risks in depth, because my colleague JoNel Aleccia is handling that important angle. But if you'd like to shed more light (rather than heat) on the situation at Fukushima or Japan's other stricken reactors, please feel free to add your comments below.

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."