(Mapawatt Note: I worked in a nuclear power plant for 3 months as an intern when I was finishing up my degree in Mechanical Engineering in the Summer of 2005. I have some knowledge of nuclear power plant operations, I've touched a reactor core, and I've been on the turbine floor. I'm familiar with nuclear power generation, but I'm not an expert. If any experts in nuclear power are reading this and notice any mistakes, please comment below and help improve the accuracy of this article!)
***Update (3/15/11) - A third blast has hit the nuclear plant which has possibly damaged one of the reactors containment vessels. The explosion was not a nuclear explosion but was likely caused by hydrogen gas. Keep reading to learn more about how the containment dome can be affected.
***Update (3/16/11) - CNN has posted a pretty nice slide show explaining the Japan nuclear crisis as best they can with the information available.
Also, Popular Science has a great summary of nuclear power and what a nuclear meltdown means.
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Aside from the deaths and massive damage from the 8.9 magnitude earthquake and resulting tsunami that struck Japan on March 11, the earthquake's third disaster spawn is the severe damage to the Fukushima Daiichi nuclear power plant. From the Wikipedia page describing the cause of the problems at the nuclear plant:
Reactors 1, 2 and 3 were in operation at Tokyo Electric Power Company's (Tepco's) east coast Fukushima Daiichi nuclear power plant when the earthquake struck. Three other reactors were already shut for inspection but all three operating units underwent automatic shutdown as expected. Because plant power and grid power were unavailable during the earthquake, diesel generators started automatically to supply power for decay heat removal. This situation continued for one hour until the plant was hit by the tsunami wave, which stopped the generators and left the plant in black-out conditions.
The loss of power meant inevitable rises in temperature within the reactor system as well increases in pressure. Engineers fought for many hours to install mobile power units to replace the diesels and managed to stabilise conditions at units 2 and 3. However, there was not enough power to provide sufficient coolant to unit 1, which came under greater and greater strain from falling water levels and steady pressure rises. Tepco found it necessary to vent steam from the reactor containment. Next, the world saw a sharp hydrogen explosion destroy a portion of the reactor building roof. The government ordered the situation brought under control by the injection of seawater to the reactor vessel.
In a nuclear reactor, a coolant (usually a form of water) has to be continuously circulated through the reactor to keep it at a stable temperature (and also produce steam for power generation). If, for some reason, water can't be circulated, the reactor continues to increase in temperature, until it....well...melts. A question on "What is a meltdown?" from a CNN article on Q&A for the Japan's quake effects on the nuclear reactors:
Robert Alvarez, Senior Scholar at the U.S. Institute for Policy Studies, explained that a meltdown could happen when the water surrounding the core of the reactor boiled or leaked away, leaving the fuel rods exposed, allowing temperatures to rise to up to 5,000 degrees Fahrenheit.
"The radiation is so intense it's impossible to deal with it. The control room would be uninhabitable," he said. "Without cooling, cladding surrounding the fuel can ignite, and the fuel itself start to melt.
"Then you have a huge amount of radioactive gases and particles, and if the primary and secondary containment fails, you have a large amount of radioactive gases escaping into the environment."
Whether a meltdown happens in this case depends on whether the pumping and cooling system can be restored in time, and whether if a meltdown starts, the secondary containment is strong enough to stay intact, according to Alvarez.
There are two main types of nuclear reactors in use in the world for power generation: Boiling water reactor (BWR) and Pressurized water reactor (PWR). The nuclear reactors at Fukushima are boiling water reactors, which means the water that comes in contact with the nuclear fuel is the same water that turns into steam and drives the turbines. This is in contrast to the pressurized water reactor, which uses heat exchanges to separate the water that comes in contact with the nuclear reactor and the water that turns into steam and drives the turbine. The nuclear plant I worked at as an intern was a pressurized water reactor. This means that if there was ever a leak in the turbine loop, the steam that would have leaked out would not contain any radiation. In a BWR, any steam that leaks out would contain radiation. And while the BWR Wikipedia page has a list of disadvantages, I'm not certain if the fact that the Fukushima nuclear plant is a BWR increases the risk of radiation leaks.
The best description I've found on the internet describing what could be happening at the Fukushima power plant is on a comment left on a CNN article describing what happens when a BWR enters shutdown mode:
During an earthquake of this magnitude, the reactor would be expected to automatically shut down (called a reactor scram). Control rods are hydraulically driven into the core in less than 7 seconds. I do not know if this took place but if it did not, we’d probably hear about it because it would be such a big deal. Even with rods inserted, the reactor continues to produce heat equivalent to about 3% of its full power level. This is not the same as taking a pot off the stove and letting it cool. There are still some atoms splitting and fission products decaying that produce heat. This drops off slowly and is why there needs to be layers of redundant cooling with backup power. During such an earthquake, power from outside the plant would not be expected to be available.
When there is a catastrophic incident at a nuclear facility, control rods are inserted to absorb the result of the nuclear fission process in order to slow it and reduce the heat generated. However, the control rods to not immediate stop the nuclear reaction in the fuel rods (and in reality, the nuclear reaction continues in the fuel rods for years after they are removed from the reactor. These are stored in the spent fuel pools). The comment left on the CNN article questions:
- Were the control rods inserted all the way to slow the reaction sufficiently?
- Are the pressure relief valves operating correctly?
- How has the backup cooling system - that relies on electricity - been affected?
To prevent disaster, they are pumping sea water into the reactors to cool them, but doing so has its own costs:
The use of seawater shows that authorities are giving up future use of the Daiichi plant and are focusing solely on protecting people and the environment, experts said.
"If they are (using seawater), it's because they have no other choice," said James Walsh, a research associate at the security studies program at MIT. "The last thing you want to do is pump seawater and boron into a reactor."
The salt and boron will corrode the reactor, he said.
You've also probably heard about the explosions at the power plant. These explosions seem to be the result of auxiliary systems that have been damaged, and not actual nuclear explosions. But it seems that the third explosion has damaged the nuclear containment building. When most people think of a nuclear power plant, they probably picture the cooling towers and overlook the containment building. This is probably because in the Simpsons, at Homer's nuclear plant the cooling towers play a more dominant role in the illustration (see below):
The containment buildings are actually represented as the orange domes and in reality they have a shape similar to this.
Most people thin From Wikipidedia on the topic of containment building:
A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation. The containment is the final barrier to radioactive release (part of a nuclear reactor's defence in depth strategy), the first being the fuel ceramic itself, the second being the metal fuel cladding tubes, the third being the reactor vessel and coolant system.
The worst nuclear power accident in history did not have a containment building. Aside from limiting damage if a melt down occurs in the reactor, containment buildings are meant to prevent external damage from reaching the reactor. In the U.S., containment buildings are meant to withstand the impact of missiles and commercial planes. Damage to the containment building increases the risk of nuclear radiation escaping into the surrounding environment.
So we know that the earthquake had a catastrophic effect on the power plant and at least one of the reactors is dead, but what does this mean for the future of nuclear power in Japan, and nuclear power policy in the rest of the world now that old fears will be ignited?
Let's put nuclear safety in perspective....
From AmericanHeart.org on the damages of air pollution:
During the last decade, however, epidemiological studies conducted worldwide have shown a consistent, increased risk for cardiovascular events, including heart and stroke deaths, in relation to short- and long-term exposure to present-day concentrations of pollution, especially particulate matter.
and...
A 1994 report on the adverse effects of particulate air pollution, published in the Annual Reviews of Public Health, noted a 1 percent increase in total mortality for each 10 mg/m3 increase in particulate matter. Respiratory mortality increased 3.4 percent and cardiovascular mortality increased 1.4 percent. More recent research suggests that one possible link between acute exposure to particulate matter and sudden death may be related to sudden increases in heart rate or changes in heart rate variability.
The Environmental Protection Agency (EPA) has declared that "tens of thousands of people die each year from breathing tiny particles in the environment." A recent report released by the nonprofit Health Effects Institute in Cambridge, Mass., agrees with the EPA assessment. This study was reviewed by Science magazine and clearly shows that death rates in the 90 largest U.S. cities rise by 0.5 percent with only a tiny increase – 10 micrograms (mcg) per cubic meter -- in particles less than 10 micrometers in diameter.
and finally....
Some research has estimated that people living in the most polluted U.S. cities could lose between 1.8 and 3.1 years because of exposure to chronic air pollution. This has led some scientists to conclude that
Nuclear power plants produce zero air pollution.
Because the earthquake in Japan has damaged the Fukushima nuclear plant, there is going to be a lot of discussion related to the safety of nuclear plants in the U.S. and around the world. This is an important and necessary discussion, as nuclear power plants are complicated machines (and there is still the issue of dealing with nuclear waste). But let's ensure the discussion takes into account the realities of all types of power generation. Nuclear power has flaws, but pollution from fossil fuel emissions is responsible for the deaths of thousands of people each year. From a page dedicated to risks of nuclear power: "Since air pollution from coal burning is estimated to be causing 10,000 deaths per year, there would have to be 25 melt-downs each year for nuclear power to be as dangerous as coal burning."
I'd love it if we could power the world with solar and wind turbines, but at the current moment, this isn't practical. As Japan has showed, nuclear power isn't risk free, but it is causing much less deaths that fossil fuel emissions.
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