Nuclear reactors have one job: to split atoms in a controlled reaction and use the released energy to generate electrical power. Over the years, reactors have been viewed as both a miracle and a menace.
When the first U.S. commercial reactor went on line in Shippingport, Pa., in 1956, the technology was hailed as the energy source of the future, one that some believed eventually would make electricity too cheap to meter. Countries around the world built 442 nuclear reactors, and about a quarter of those reactors were built in the United States [source: Euronuclear.org]. The world has come to depend upon nuclear reactors for 14 percent of its electricity [source: Nuclear Energy Institute]. In fact, futurists fantasized about having nuclear-powered automobiles [source: Ford].
Then, 23 years later, when Unit 2 at the Three Mile Island power plant in Pennsylvania suffered a cooling malfunction and a partial meltdown of its radioactive fuel, feelings about reactors changed radically. Even though the stricken reactor's containment held and there was no major radiation release, many people began to see reactors as overly complicated and vulnerable to human and equipment failures, with potentially catastrophic consequences. They also worried about the radioactive waste from reactors. Worse yet, many wondered if government regulators and the nuclear power industry were leveling with the public. As a result, the construction of new nuclear plants stopped in the United States. When a more serious accident occurred at the Soviet Union's Chernobyl nuclear plant in 1986, nuclear power seemed doomed to obsolescence [source: Union of Concerned Scientists].
But in the early 2000s, nuclear reactors began making a comeback, thanks to rising energy demand, diminishing fossil fuel supplies, and the growing concern about climate change due to carbon dioxide emissions. Late in the decade, the U.S. Nuclear Regulatory Commission began to approve permits for new plants, and President Barack Obama included nuclear power as a key part of his energy plan.
But then, in March 2011, yet another crisis hit -- this time at the earthquake-stricken Fukushima Daiichi nuclear power plant in Japan -- raising worries again.
In this article, we'll explain how nuclear reactors work, what happens when they malfunction, and the risks they pose to our health and the environment compared to other energy sources. We'll also take a look at what technological advances could make the nuclear reactors of the future safer.
But first, let's look at how nuclear fission, the process that actually produces the energy, actually works.
Harnessing a Nuclear Reaction
Put simply, a nuclear reactor splits atoms and releases the energy that holds their parts together.If it's been a while since you took high school physics, we'll remind you how nuclear fission works: Atoms are like tiny solar systems, with the nucleus where the sun would be, and electrons orbiting around it. The nucleus is made up of particles called protons and neutrons, which are bound together by something called strong force. Perhaps it was named "strong force" because it's almost too powerful for us to imagine -- many, many billions of times stronger than gravity, in fact [source: Bryson]. Despite the strength of strong force, it's possible to split a nucleus -- by shooting neutrons at it. When that's done, a whole lot of energy is released. When atoms split, their particles smash into nearby atoms, splitting those as well in a chain reaction. (Think a multi-car crash on the freeway.)
Uranium, an element with really big atoms, is perfect for atom splitting because its strong force, though powerful, is relatively weak compared to other elements. Nuclear reactors use a particular isotope called uranium-235 [source: Union of Concerned Scientists]. Uranium-235 is rare in nature; the ore from uranium mines only contains about 0.7 percent uranium-235. That's why reactors use enriched uranium, which is created by separating out and concentrating the uranium-235 through a gas diffusion process [source: NRC].
This process is what gives an atomic bomb, like the ones that were dropped on Hiroshima and Nagasaki, Japan, during World War II, such terrible power. But in a nuclear reactor, the chain reaction is controlled by inserting control rods made of a material like cadmium, hafnium or boron, which absorb some of the neutrons [source: World Nuclear Association]. That still allows the fission process to give off enough energy to heat water to a temperature of about 520 degrees Fahrenheit (271 degrees Celsius) and turn it into steam, which is used to turn turbines and generate electricity [source: Union of Concerned Scientists]. Basically, a nuke plant works like a coal-powered electrical plant, except that the energy to boil water comes from splitting atoms instead of burning carbon [source: Nuclear Regulatory Commission].
In the next section, we'll talk about the different types of reactors and how their key parts work.
Nuclear Reactor Components
There are several different types of nuclear reactors, but they all have some common characteristics. All of them have a supply of radioactive fuel pellets -- usually uranium oxide, which are arranged in tubes to form fuel rods in the reactor core [source: World Nuclear Association].The reactor also has the previously mentioned control rods -- made of neutron-absorbing material such as cadmium, hafnium or boron -- which are inserted into the core to control or halt the reaction [source: World Nuclear Association].
A reactor also has a moderator, a substance that slows the neutrons and helps control the fission process. Most reactors in the United States use ordinary water, but reactors in other countries sometimes use graphite, or heavy water, in which the hydrogen has been replaced with deuterium, an isotope of hydrogen with one proton and one neutron [source: World Nuclear Association, Federation of American Scientists]. Another important part of the system is a coolant -- again, usually ordinary water-- which absorbs and transmits heat from the reactor to create steam for turning the turbines and cools the reactor core so that it doesn't reach the temperature at which uranium melts (about 6,900 degrees Fahrenheit, or 3,815 degrees Celsius) [source: World Nuclear Association]. (We'll explain why a meltdown is a very bad thing later in this article.)
Finally, a reactor is encased in a containment, a big, heavy structure, typically several feet thick and made of steel and concrete, that keeps radioactive gases and liquids inside, where they can't hurt anyone [source: World Nuclear Association].
There are a number of different reactor designs in use, but in the United States, about two-thirds of the reactors are pressurized water reactors (PWRs). In a pressurized water reactor, the water is pumped into contact with the core and then kept under pressure, so that it can't turn into steam. That pressurized water then is brought into contact with a second supply of unpressurized water, which is what turns to steam to turn the turbines. The remaining third of reactors in the United States are boiling water reactors (BWRs). With BWRs, the water that comes directly into contact with the reactor core is allowed to become steam for generating electricity [source: World Nuclear Association].
In the next section, we'll look at the potential risks nuclear reactors pose, and how to evaluate them.
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