The bulk of the following was originally published as several blog posts (see here, here, and here). However, given that this is a subject about which I'm sure most Americans will remain ignorant, even after the Fukushima incident is effectively forgotten, I decided to expand it and post it as a standing page.
AN INTRODUCTION TO NUCLEAR POWER
Composition and Construction of Commercial Nuclear Fuel
|A uranium fuel pellet|
Nuclear Reactor Operation
|Nuclear fuel rod assemblies|
|BWR and PWR schematics (click to enlarge)|
Water within the reactor cools the fuel assemblies, slows the emitted neutrons (slower neutrons are more effective at producing a fission reaction in uranium), and is used in conjunction with steam turbines to produce electricity. In a BWR, the fission reaction boils the water within the reactor vessel. In a PWR, the water in the vessel is maintained at a pressure that prevents it from boiling; the heat from the water in the pressurized "primary loop" is transferred to water in the "secondary loop", which is allowed to boil.
|Fuel assemblies in a nuclear reactor opened for servicing|
Reactor Shutdown and Defueling
The typical commercial reactor will operate for about 12 to 24 months before the fuel must be replaced. By this time, much of the uranium-235 within the assemblies has been changed into fission products (some of the most common being iodine-131, cesium-137, and strontium-90). Isotopes of plutonium will have also been produced when neutrons were absorbed by atoms of uranium-238. The reactor is shut down by fully inserting the control rods. At this point the reactor is "subcritical", meaning that, on average, each fission produces less than one additional fission. The fuel assemblies are then removed from the reactor and are placed in a water-filled pit or pool. Although the nuclear chain reaction has been stopped, the water is required to cool the assemblies since radioactive decay of the fission products produces a significant amount of heat. Eventually the rate of decay will fall sufficiently that the fuel assemblies can be removed from the pool and be placed in a dry cask.
If nuclear fuel assemblies aren't properly cooled, whether inside the reactor or outside in a cooling pool, they can become damaged due to decay heat. The zirconium cladding can bubble and burst open, releasing fission products into the air. Even worse, the overheated zirconium alloys can begin to oxidize rapidly, which can result in a fire. In the presence of steam, the oxidation process can produce explosive hydrogen gas.
|Spent fuel assemblies in a cooling pool|
RADIOACTIVITY VERSUS RADIATION
|Smoke rises from Fukushima|
Early on in the crisis, foxnews.com presented an article that said that "About 1,500 people had been scanned for radiation exposure". Another article suggested that iodine serves as a treatment against radiation exposure, stating that "virtually any increase in ambient radiation can raise long-term cancer rates, and authorities were planning to distribute iodine, which helps protect against thyroid cancer." Both these articles confused radiation and radioactivity.
What is Radiation?
Given that the reactors shut down shortly after the earthquake, the radiation the authorities were worried about was gamma radiation. Like visible light, gamma radiation is composed of photons, although they have a much higher energy than photons of light. Unless a person receives enough exposure to show signs of radiation sickness, there's no easy way to determine if someone has been exposed to radiation. That's why nuclear workers wear dosimeters, which measure how much radiation a person has received. Gamma radiation does not make things radioactive. A person could receive a lethal dose of radiation and yet their body wouldn't emit any radiation itself. Nor would an inanimate object become radioactive after being exposed to high levels of gamma rays. Neutron radiation, on the other hand, can cause a substance to become radioactive (this is called "activation"), but such high levels of neutron radiation are only encountered during an active fission or fusion reaction.
What is Radioactivity?
With regard to the general public, which was kept too far from the Fukushima plant to have received dangerous levels of radiation exposure, what Japanese authorities were worried about was radioactivity (this is often called "radioactive contamination" or simply "contamination"). Radioactivity consists of particles of matter that emit radiation. A good analogy of radioactivity is burning charcoal; the charcoal is the radioactivity and the heat given off is the radiation. Confusing radioactivity with radiation is like confusing hot coals with the heat being produced.
The Media's Incorrect Terminology
When the media refers to a "radiation leak", they don't actually mean that radiation is being leaked. And nobody can ever 'get radiation on them'. In a nuclear accident what is being leaked is radioactive particles, which usually takes the form of a microscopic dust. Sometimes this dusting of radioactivity is called "fallout". Above-ground nuclear weapons tests like those performed in the Pacific and in Nevada during the '40s and '50s can produce an enormous amount of fallout.
Release of Radioactivity at Fukushima
As I mentioned in the section about nuclear fuel above, overheated fuel assemblies can produce hydrogen gas. This process occurred at the Fukushima plant when hydrogen was created by the oxidation of the cladding of the under-cooled fuel assemblies inside the reactors. To reduce the likelihood of damage to the reactor vessels, operators vented the hydrogen out of the reactors and into the reactor buildings, which caused several explosions. The decision to vent the gas was definitely the right one since an explosion within the reactors would have been even more disastrous.
|An anti-contamination suit|
The specific radioactive materials released during the accident consisted of various fission products such as radioactive isotopes of cesium, strontium, and iodine. The 1,500 people mentioned in the foxnews.com article were being scanned for radioactivity (particles of fission products) on their clothing, skin, or inside their bodies. This contamination is detectable through the radiation emitted by the radioactive particles that are on the victims or inside them. Since strontium is chemically similar to calcium and is absorbed by the bones, and iodine is absorbed by the thyroid, these substances can spend a long time inside the human body. Long term internal exposure to radiation can cause serious health problems. The reason why Japanese authorities were planning on distributing iodine (the non-radioactive form, of course) is because it saturates the thyroid and prevents it from absorbing the radioactive version that may have contaminated the environment.
What About Anti-Radiation Suits?
|An "anti-radiation suit"|
would look more like this
NUCLEAR POWER AND RELATIVE RISK
Just looking at the statistics, I think the most obvious lesson of the Tōhoku earthquake and the resultant tsunami (i.e., the disasters that caused the Fukushima accident) is that it's more dangerous to live by the beach than it is to live by a nuclear plant. The earthquake and the tsunami (which caused most of the casualties) left a death-toll of around 10,000 people, while the nuclear accident had not officially killed anybody at the time this page was written (although two workers were missing who may have been killed in an explosion). Yet the lesson that so many anti-nuclear activists and ordinary Americans will take away from the disaster is not that beachfront property may be a poor investment, but that nuclear power is dangerous and bad. In fact, even in light of the 230,000+ deaths caused by the 2004 Indian Ocean tsunami, I think most people would still like to own a home by the ocean.
|Three Mile Island|
To prove my point about relative risk, let's begin by approximating how many people may have been killed by nuclear power. Since I couldn't possibly guess how effective Soviet medicine was in the 1980s, we'll just count all 4,000 potential cancer cases caused by Chernobyl as fatalities. Thus, an upper limit of 4,063 people may have been killed by nuclear plant accidents since 1961, with the vast majority of that number being questionable. If we divide that number by 57 years (the number of years that have passed between now and 1954, when the first nuclear reactor to provide power for an electrical grid came online) we end up with an average number of approximately 71 deaths per year due to nuclear power plants. Remember, this number was boosted by an estimated number of cancer cases that might have been incurred by the Chernobyl disaster. If we only use confirmed numbers then the average number of deaths per year drops to about 1.
|A lot more deadly than any nuclear reactor|
|Electricity: Threat or menace?|
|Windmills of DEATH|
Air Pollution and Power Production
|Don't breathe too deep|
The point I'm trying to make is that human life is full of risks but that many people seem to be confused about what risks they should worry about. Even if the Fukushima accident turns out as bad as the most extreme estimates suggest (estimates which are almost invariably developed by anti-nuclear organizations), the number of deaths that could be caused by radiation sickness and radiation-induced cancer would be far exceeded by the number of auto deaths that occur in the United States in a single month. Yet most of the Americans who are stocking up on iodine tablets probably don't think twice about getting into a car (and I'd bet a good percentage of them don't even bother to put on a seatbelt). Although I spend every work day in close proximity with nuclear materials and energy, I know that the most dangerous part of my day is the drive to and from work.
Questions or Comments?
If you have any questions or comments about nuclear power, go ahead and email The Atomic Spud.