Nuclear power provides clean electricity at a very low cost, and it does not produce greenhouse gases. However, the reaction at the heart of nuclear power produces radioactive wastes. The potential danger from escaped radiation from an accident has given nuclear power a fearsome reputation. Since the near-disaster in 1979 at Three Mile Island, in the United States, and the terrible accident in 1986 at Chernobyl, Ukraine, nuclear-energy producers have taken great strides to make their plants safer. In a world where polluting emissions have become a bigger and bigger concern, nuclear power will be a significant piece of the energy-production puzzle.

How does the cost of nuclear energy compare with that of energy generated by fossil fuels? A nuclear plant costs more to build than the typical coal-fired electric plant. Other expenses to be factored in include those for the disposal of contaminated wastes and the decommissioning of outdated plants. However, the fuel for a nuclear plant is much cheaper than fossil fuel. Accounting for all these factors, electricity from nuclear power plants costs the same or even less than energy from plants using fossil fuels.

Nuclear power provides about 16% of the world’s energy. There are more than 440 plants in 70 countries, with more in different stages of construction or planning. In the United States, nuclear power plants generate 20% of the electricity needed. Nuclear energy enables countries with limited supplies of fossil fuels to produce electricity independent of the world’s fossil-fuel market. Most of France’s electricity comes from nuclear power. Both Japan and France have growing nuclear-power programs.

	Click for animation. Nuclear power In a pressurized water reactor, there are two separate loops. The water heated by the nuclear reaction in the core is contained in pipes as it passed through the tank where the steam is created to power the turbine. The turbine is attached to a generator, which makes electricity.

Despite the increasing need for electricity, especially from nonpolluting sources, in many countries the investments in new nuclear plants are limited at best. The reason: fear of accidents. An accident at a nuclear plant can have widespread consequences for the general population around the plant and beyond. This is very different from fossil-fuel power sources. People working in the industry—for example, coal miners—face risks, but the people living near the mine or the plant generating electricity are not at risk during an accident.

The events at Chernobyl and Three Mile Island gave nuclear energy an image as a dangerous power source. This raises the costs involved in making plants safer. In addition, there are concerns about the storage of nuclear waste. But the industry has made great efforts to build safer plants, and thus far there have been no problems with waste storage.

Let’s take a closer look at nuclear power, how it produces energy, how a plant works, and the controversies that the industry faces.

How a Nuclear Reactor Works

Nuclear reactors create energy by nuclear fission, a process that splits the nucleus of an atom in two. An element with a heavy nucleus, most commonly uranium-235 (U-235), is hit by a free neutron. This divides the heavy nucleus into two lighter nuclei. The action of this split releases energy in the form of heat and radiation. Two to three other neutrons are released during this process. These neutrons then hit more U-235 nuclei and split them. This reaction continues on and on, forming what is called a chain reaction.  

Fission reactions fall into three categories, based on the speed of the reaction and the energy released. A reaction is subcritical, when on average, less than one of the free neutrons from each fission reaction hits another U-235 atom. This means that the fission reaction is below critical mass and will eventually die out. In a supercritical reaction, on average more than one of the neutrons hits another U-235 atom. Because of the heat produced by this rapid fission, the whole reaction explodes. An atomic bomb is a supercritical reaction.

For power generation, a critical reaction—the most stable type of reaction—must occur. This happens when on average exactly one of the neutrons from each fission reaction hits another U-235 atom. The fuel is at critical mass. It maintains a stable temperature, not explosively hot, and the reaction is not too slow to maintain fission in the fuel. The heat from the nuclear reaction converts water to steam. The steam turns turbines attached to generators, creating electricity. In a nuclear reactor, the reaction stays at critical mass through the use of devices to speed up or slow down the reaction as needed.

What makes a nuclear reaction so useful for generating power is that it releases much more energy than the typical combustion reaction. For example, a coal plant produces about 1 kilowatt of electricity by burning 0.45 kg (1 lb) of coal. The fission of 0.45 kg of uranium produces about 3 million kilowatts.

Reactor Components

A reactor must be able to contain and control nuclear fission. To do this the reactor must control the release of the heat.

All reactors have the same basic parts: fuel, control rods, moderator, coolant, pressure vessel or tubes, steam generator, and containment structure. Most use uranium oxide pellets for fuel. The pellets are packed in tubes to make fuel rods, which are arranged in groups within the core of the reactor. The control rods adjust the rate of the reaction, speeding it up, slowing it down, or stopping it as needed. They are made of a material that absorbs neutrons, often cadmium, hafnium, or boron. The control rods can be moved in and out from between the fuel rods, absorbing more or fewer neutrons as needed to adjust the speed of fission. The fuel rods and control rods make up the core of the reactor.

The moderator and coolant surround the core. The moderator is a substance that keeps the neutrons produced by fission from moving too fast, so that they will be able to hit more nuclei and continue the chain reaction. Water, heavy water (deuterium oxide, or D2O), or graphite is the most common moderator. The coolant moves through the core to stabilize the temperature in the core. The coolant can be liquid or gas. In light-water reactors the moderator and the coolant are the same substance, water.

The core, moderator, and coolant are contained inside a pressure vessel or pressure tubes. The heat picked up by the cooling system makes steam, which runs a turbine. The turbine powers a generator, which makes electricity. A containment structure encloses this whole system, to prevent anything from coming in or radiation from leaking out if there is a malfunction. It is usually made of concrete and steel 1 m (3 ft) thick.

Types of Reactors

The earliest reactors put into operation, termed Generation I, are no longer in use, except for one in Great Britain. Generation II reactors produce most of the nuclear energy in the world today. By far the most common type of reactor is the pressurized water reactor. This reactor is based on the design used to build nuclear submarines for the U.S. Navy. It uses uranium oxide for fuel and water for the coolant and moderator. The coolant/moderator water moves in a separate loop from the water that becomes steam. This prevents all the water from becoming contaminated with radioactivity if there is a leak in the core.
http://www.uic.com.au/nip64.htm

Many nuclear plants feature a boiling water reactor, which has enriched uranium oxide as fuel and water as the moderator and coolant. All of the water in the plant forms one loop, from moderator to steam. It is less complex than the pressurized water reactor, but more vulnerable to contamination throughout the plant if a leak occurs. This is because the radioactive water is not separated from water used to create steam to drive the turbine. This type of reactor is found in the United States, Japan, France, Russia, and Sweden.

Schematic of boiling water reactor

Drawings courtesy of Australian Uranium Association.

The gas-cooled reactor, common in Great Britain, uses natural uranium or enriched uranium oxide for fuel, carbon dioxide for the coolant, and graphite for the moderator.

Schematic of gas cooled reactor

Drawings courtesy of Australian Uranium Association.

Canadian nuclear plants use the pressurized heavy-water reactor, nicknamed CANDU. This reactor uses natural uranium oxide for fuel and heavy water for the coolant and moderator.

CANDU reactor

Drawings courtesy of Australian Uranium Association.

Light-water graphite reactors, found only in Russia, use enriched uranium oxide fuel, a water coolant, and a graphite moderator. The newest types of reactors are the fast neutron reactors. There are four in operation, in Japan, France, and Russia. These use plutonium oxide and uranium oxide fuel, a liquid sodium coolant, and no moderator.

Most reactors currently under construction are Generation III reactors. Although the designs are not tremendously different from those of older reactors, Generation III reactors include a variety of improvements over older reactors. The first Generation III reactor began operation in Japan in 1996. Newer Generation III reactors have even more advanced features. They are more standardized, to reduce costs. They use simpler, more rugged designs, for ease of operation and greater safety. These reactors are projected to have a life span of about 60 years, longer than the 40 years of Generation II reactors. They are more fuel efficient, to use less fuel and produce less waste. Best of all, Generation III designs incorporate better safety features to prevent core meltdown, and to prevent environmental radiation leaks.

History

The first country to harness atomic energy was the United States, which dropped atomic bombs on Hiroshima and Nagasaki, Japan, at the end of World War II. The concept of using nuclear reactors to provide power for other purposes began with the nuclear-powered submarine, developed by the U.S. Navy in the late 1940s and early 1950s. The first nuclear submarine was launched in 1954.

During the early 1950s, U.S. president Dwight Eisenhower declassified much of the information related to building nuclear reactors. The United States Atomic Energy Commission (now the Nuclear Regulatory Commission) sponsored research into different types of nuclear reactors at this time. Many countries quickly learned how to build nuclear reactors to generate electricity. The Soviet Union opened its first nuclear power plant in 1954; Great Britain’s first went into operation in 1956. The reactor used for the submarine provided a model for the first American nuclear power plant, built in Shippingport, Pennsylvania, in 1957. France came online in 1957, and by the early 1960s the nuclear-power industry was well established. The industry expanded greatly in the 1970s, when oil prices climbed.

Safety

The biggest concern about nuclear power is safety, more specifically radiation leaks. Fission produces radiation as a by-product. This radiation contaminates the coolant and moderator, leading to radioactive water and steam. Leaks of either of these are of great concern.

Serious accidents at nuclear power plants are not that likely, but if they happen the consequences may be severe. Thus far, there have been only two major accidents. In 1979, a technical malfunction at the Three Mile Island plant in Pennsylvania nearly led to a release of radioactive material. About half of the core melted, but safety mechanisms contained the radiation. And no one was hurt or killed.

A much larger accident occurred at Chernobyl, Ukraine, in 1986. A reactor exploded, releasing a significant amount of radioactive material high into the atmosphere. People in Russia and parts of Europe were exposed to radioactive fallout. The accident killed 31 people, and the exposure to radiation sickened many more. In the region around Chernobyl, unhealthy levels of radiation contaminated up to 233,000 sq km (90,000 sq mi) of land. This includes parts of Belarus and Russia as well as Ukraine. More than 130,000 people were forced to move after the explosion. The consequences are still being felt. Around Chernobyl, a zone with a radius of 30 km (19 mi) has been deemed uninhabitable, although some elderly people have returned to the only homes they know. Thyroid cancer, usually pretty rare, occurs more frequently, especially in children. Researchers estimate that all told, about 4,000 cases of thyroid cancer now and in the future can be attributed to the effects of Chernobyl.

After each of these accidents, safety procedures were upgraded. Since then, there have been no major accidents at nuclear power plants.

Once the fuel at a nuclear power plant is used up, what’s left (the spent fuel) remains radioactive and requires careful disposal. This includes breaking down the remaining product by reprocessing, which recovers reusable uranium through a combination of manual and chemical techniques. What is not reused is then transported to the disposal site. The fuel, reprocessing by-products, and chemicals used in reprocessing are all radioactive.

Spent fuel remains radioactive for many, many years, although the radioactivity diminishes over time. Therefore it must be disposed of in a tightly sealed manner. The technique used is to bury the spent fuel deep in the Earth in geologically inactive areas. Nuclear waste disposals are usually remote, unsettled areas.

On September 30, 1999, a serious accident at a uranium reprocessing plant occurred in Japan, northeast of Tokyo. Three workers used too much uranium to make fuel and set off an uncontrolled reaction. The three were exposed to excess doses of radiation (two died), and 116 more were exposed to some radiation. An investigation after the accident concluded that the workers had not been trained properly, which led to the accident.

Transportation of waste is also an issue. There has never been an accident involving the transportation of nuclear waste. The containers that hold the waste are very secure and sturdy.

The last safety concern for the nuclear-power industry is its vulnerability to terrorist activities. Countries that are under terrorist threats have increased security at nuclear facilities.

Future

Nuclear power remains popular in countries with limited land and limited fossil-fuel resources. It is here to stay as an important alternative to fossil fuels.

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