On the Subject of Nuclear Power

Introduction
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

Most commercial reactors use enriched uranium in the form of pellets for their fuel. Uranium consists mostly of the isotope uranium-238 along with a small amount of other isotopes. When uranium is enriched to be used as nuclear fuel, the isotope uranium-235 is increased above the 0.7% found in natural uranium (most power plants use fuel that's 3% to 5% uranium-235). As shown in the photo, these pellets are relatively safe to handle before undergoing nuclear fission. The pellets are sealed into tubes (often called "cladding"), which are usually composed of a zirconium alloy. Zirconium is the material of choice due to certain favorable properties, not the least of which is its near-transparency to neutrons (neutrons will have to be able to pass from one tube to the other in order to sustain a fission reaction). These tubes are then bundled together into fuel assemblies. In a boiling water reactor (BWR), like those used at Fukushima, the assemblies are encased in an additional thin-walled tube. A removable control rod is inserted into the fuel assembly to absorb neutrons and prevent a premature fission reaction. In a pressurized water reactor (PWR) (the most common reactor type in the United States), the control rods enter the fuel assembly from above; in a BWR the control rods enter from below. Notice that all elements of the fuel, the cladding, and the control rods are made of solid materials. I am always surprised to find that many people believe that nuclear fuel is some sort of glowing liquid.

Nuclear Reactor Operation

Nuclear fuel rod assemblies

The assemblies are inserted in a reactor vessel and the vessel is sealed. To start up the reactor, the control rods are withdrawn. Neutrons produced by the spontaneous fission of a relatively small number of uranium atoms strike other uranium atoms nearby, which causes still more fissions. A nuclear chain reaction (or "criticality") occurs when, on average, at least one neutron produced by each fission goes on to produce an additional fission. The control rods are partially inserted or removed as necessary to keep the reaction at the desired rate. The control rods contain materials like cadmium or boron that absorb neutrons and prevent an excessive rate of fission.

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
Media Misconceptions

Smoke rises from Fukushima

Throughout the Fukushima crisis, I was amazed at the misconceptions that the news media continued to spread. Among the most obvious was the difference between radioactivity and radiation.

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 combination of breached fuel assemblies, venting gas, damaged reactor components, and problems with keeping water in the Fukushima plant's cooling pools led to the spread of radioactivity into the atmosphere. People that were several miles away were safe from the radiation being emitted by the damaged reactors, but they were exposed to relatively small quantities of radioactive materials blown into the air by the explosions. Some contamination that drifted high enough into the atmosphere actually spread as far as North America, although reports indicated that the levels of contamination reaching the United States were "about a billion times beneath levels that would be health threatening". These reports were confirmed at the facility in Idaho where I work; trace quantities of radioactive iodine from the Fukushima incident were found using equipment that continuously samples the air for contamination. The amount of radioactivity found after several days of collection were minute and posed absolutely no threat to human health in the area.

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

It's because of the media's and the general population's misunderstanding of the difference between radiation and radioactivity that I keep having to explain to people that there's no such thing as an "anti-radiation suit". The yellow anti-contamination suits (sometimes called "anti-Cs") that people may be familiar with are designed to protect the wearer from radioactivity. The yellow fabric is an impermeable material that prevents radioactivity from getting onto the wearer's skin. Where contamination may be found in the air, a respirator is worn to prevent inhalation of the material. These suits do not protect the wearer from gamma radiation. People in anti-Cs are as concerned about radiation as someone in street clothes. Radiation can only be shielded by dense materials like steel or lead or by generous layers of water or concrete. A steel or lead suit that could reduce radiation levels reaching the wearer by 90% would need to be several inches thick and would probably look like something from Iron Man.

NUCLEAR POWER AND RELATIVE RISK
Natural Disasters
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.

Nuclear Disasters

Three Mile Island

I am always amazed by how risk-averse Americans are with regards to some things while having absolutely no trepidation about other things that are much more likely to kill or injure them. For example, nuclear power is one of those touchy subjects that causes panic or fear in millions of Americans. Yet the total number of Americans that have been demonstrably killed by nuclear power (not counting a handful of early incidents involving experimental bomb cores) is three. That's right; three people, all of whom were military personnel killed in the explosion of the experimental SL-1 reactor in 1961. One study suggests that one or two cancer deaths may have occurred after the Three Mile Island accident, but this number was determined statistically and cannot be confirmed. Worldwide there have only been 63 confirmed fatalities directly associated with nuclear power (this number omits weapons and military related incidents as well as the Fukushima crisis, which isn't quite finished yet).

Chernobyl

The Chernobyl disaster, which was caused by a poorly designed reactor and an ill-advised experiment that disabled essential safety systems, accounts for the bulk of the fatalities (53 out of the 63). However, it must be admitted that the official figure may be misleading since the Soviet Union was never known for releasing honest casualty figures. Shortly after the accident, the United Nations Scientific Committee of the Effects of Atomic Radiation (UNSCEAR) estimated that 4,000 people would suffer from cancer due to the accident (later reports suggested that this was an overestimate). Not all of these 4,000 people would have died prematurely since some types of cancer associated with radiation exposure (e.g., thyroid cancer) are relatively treatable and have a decent survival rate.

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.

Automobile Accidents

A lot more deadly than any nuclear reactor

Using the exaggerated number of 71 deaths per year, it becomes horribly ironic that most anti-nuclear activists protesting at a nuclear power plant must have driven there. Every year around 30,000 to 40,000 Americans are killed in auto accidents. It is estimated that a worldwide total of 1.2 million people are killed annually in auto accidents. This means that, on average, nearly 17,000 times more people die annually in car accidents than in nuclear plant accidents. Clearly we're in the middle of an automobile-spawned apocalypse. So where are the anti-automobile activists?

Electrical Accidents

Electricity: Threat or menace?

How about an energy source that we interact with daily? In 1993 550 people were killed by electricity in the United States. Given that US electrical safety standards are significantly higher than they are in parts of the developing world, the number of electrical fatalities worldwide is undoubtedly much higher. Either way, this number is almost eight times higher than our average annual rate of nuclear power-related deaths (this assumes that 550 is a fairly representative fatality rate for more recent years). Honestly, how can we even allow electricity into our homes when it's clearly so dangerous?

Windmill Accidents

Windmills of DEATH

While we're throwing numbers around, it turns out that accidents involving wind power have killed 73 people between 1975 and 2010. Note that this number of fatalities (which were incurred over a period of 35 years) exceeds the 63 officially confirmed deaths caused by nuclear plant accidents (after 57 years of commercial nuclear power). The most common cause of windmill related accidents was blade failure. When a turbine blade fails, the blade or pieces of the blade can be thrown at a lethal velocity. Now that wind turbines are being built in ever greater numbers and closer to inhabited areas, we can only expect the number of accidents to go up. Too bad all those activists are too busy protesting nuclear plants to care about the windmill farms being built near residential areas.

Air Pollution and Power Production

Don't breathe too deep

Now that we're back on the subject of electricity production, let's talk about coal-produced power. In 2006 coal plants generated about 49% of America's electricity and 68.7% of China's electricity (nuclear power accounts for about 20% of US electricity). Ironically, because coal contains a number of naturally radioactive isotopes, it is estimated that US coal burning in 1982 released 155 times more radioactivity into the air than the Three Mile Island accident. Ignoring the effect of radioactivity, we find reports suggesting that 750,000 Chinese die prematurely each year due to air pollution. The World Health Organization has claimed that 2.4 million die each year due to air pollution. Much of this pollution comes from burning coal for electricity. Now, I know that there are anti-coal plant activists, but many of them are the kind that conveniently forget that coal power is a major source of the energy used to make solar cells, manufacture wind turbines, or to charge their "environmentally friendly" Chevy Volts. Modern life (which is more than 20 years longer and healthier than life in the early 20th century) would not be possible without the electricity produced by burning coal.

Everyday Risk
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.