Nuclear Reactors
From Thermal-FluidsPedia
As of January 1, 2006, there are 441 nuclear power reactors in the world producing 368,000 MWe, or one-sixth of the world’s electricity (Table 1). The United States has the greatest number of nuclear reactors, but the fraction of electricity they generate is small compared to that in some European countries. For example, while nuclear energy accounts for 20% of US and 50% of Europe electricity. In total, there are 31 countries that use nuclear power to meet some of their electricity demands (1). Most nuclear reactors were constructed during the 1960s and early 1970s. Following the accidents at Three Mile Island (TMI) in 1978 and Chernobyl in1986 and the mounting volume of nuclear waste, public confidence in the use of nuclear reactors has declined. In the United States, the last nuclear plant went online in 1996, and in many other countries the number of new facilities being constructed has declined considerably. In contrast, in a rush to join the club of nuclear nations, many countries are building new nuclear reactors (Figure 1). In light of continued concerns regarding the storage and disposal of nuclear waste, even with the addition of these reactors, it is unlikely that overall nuclear power generation will increase in the near future, and some even predict that it may decline (Figure 2). However, the increasing dependence of the industrial world on imported oil, the volatility in prices of crude oil and natural gas, and the concern over environmental issues may tilt the balance in favor of nuclear energy sometime in the future.
A nuclear power plant produces electricity in almost exactly the same way that a conventional (fossil fuel) power plant does. We discussed the basic components of a power generating station in thermal energy. In a conventional thermal power plant, fuel (coal or oil) is burned to create heat, which boils water in a boiler (steam generator) to create steam. The steam is expanded in a turbine; the turbine runs a generator, which in turn generates electricity. A nuclear power plant uses the same basic principles, except that the boiler is replaced by a reactor core where nuclear fuel undergoes fission processes to produce heat (Figure 3).
Reactor Core
The reactor core is the heart of any nuclear power generation facility. Inside the reactor core, fission takes place and provides energy to produce the steam required by the power plant. A reactor vessel made of several inches of carbon steel surrounds and protects the reactor core. The pressure vessel itself is housed in a containment structure made of several feet of thick concrete reinforced with thick steel bars. The reactor core consists of four main components.
a. Fuel Assembly Nuclear fuel consists of pellets (usually about 1 cm in diameter and 1.5 cm long, about the size of your fingertip) of ceramic uranium oxide arranged in long tubes in the fuel rods, which are grouped into bundles (fuel assemblies) in the reactor core (Figure 3). The bundles are submerged in a coolant inside a pressure vessel. Depending on the type, nuclear reactors have a number of fuel assemblies that must be replaced occasionally as they deplete and fission products buildup.
b. Moderator A fast neutron is capable of causing fission in either U-235 or U-238. In lieu of the much larger concentration of U-238, it is unlikely that a fast neutron will hit a U-235 atom and cause fission. For a neutron to be captured by U-235, it must be sufficiently slowed down. Under these conditions neutrons have more times to be pulled by the nuclear forces imparted by the uranium nuclei and there is a higher probability to be captured. The moderator is the material that slows down the fast neutrons to thermal neutrons (neutrons with an average kinetic energy similar to molecules of gas of the same temperature). Without a moderator, the fission chain reaction cannot be sustained. With ordinary water as a moderator, uranium-235 must be enriched to about 3%. It happens that natural uranium (at 0.7% concentration) will also work if instead of ordinary water either graphite or heavy water (D2O) is used as a moderator. The Canadian deuterium reactor called (CANDU) is of this type.
c. Control Rods It was previously explained that a fission reaction splits a uranium atom into two smaller fragments and two or three neutrons. To sustain a steady and controlled chain reaction, one of these neutrons is needed to bombard the next uranium atom, causing the next fission and continuing the chain reaction. This means that in order to prevent a possible run away reaction, the concentration of neutrons in the reactor must be controlled. Control rods are responsible for removing the extra neutrons and control the power output from the reactor. To control the rate of reaction, the control rods, made of neutron-absorbing material alloys such as cadmium, hafnium, or boron, slide up and down between the fuel rods; these materials absorb neutrons, but do not cause fission. To produce more heat (power up the reactor), the rods are raised and more of the uranium bundles are exposed; to create less heat, the rods are lowered. To shut the reactor down in the case of an accident or to change the fuel, the rods are lowered all the way.
d. Coolant To carry away the heat of the reaction, coolants are needed. Water is usually the material of choice. Some gas-cooled reactors use carbon dioxide or helium gas, while breeder reactors use sodium, potassium, bismuth, or other liquid metals.
Classification of Nuclear Reactors
In addition to commercial reactors used for the generation of electricity, nuclear reactors are used in research, in ships, and in submarines. Depending on the type of coolants used, commercial fission power reactors can be divided into water, gas, or sodium cooled reactors. Depending on whether ordinary or heavy water is used, water reactors can be classified as light water or heavy water reactors. Table 2 gives summary of all nuclear reactors currently in operation throughout the world.
Besides their intended use for generating electricity, nuclear plants have been used for the propulsion of large surface naval vessels and submarines. The United States, the United Kingdom, Russia, and France all have nuclear-powered submarines. The Soviet government built and operated the first nuclear-powered icebreaker in the Arctic. Other countries have constructed nuclear cargo ships, but high operating costs and restrictive port regulations have prevented their commercialization. In addition to the stated applications, a variety of small nuclear reactors at power levels around one megawatt have been built for education, training, and research.
Light Water Reactors (LWR)
Light water reactors (LWR) typically use enriched uranium as nuclear fuel, ordinary water as the moderator, and may be of either pressurized or boiling water types. A pressurized-water reactor (PWR) operates at a pressure high enough so that water never boils. Boiling-water reactors (BWR), operate at a somewhat lower pressure, allowing the water to boil within the core. The steam produced in the reactor is piped directly to the turbine before it is condensed and pumped back to the reactor. Since the steam becomes radioactive, it is necessary to shield the entire steam loop, including the turbine, from the outside by placing it inside the containment building. All US commercial nuclear plants (including the power reactors at Three Mile Island) are light-water reactors, using water as both the coolant and the moderator. About two-thirds of these are pressurized water reactors; the remaining third are boiling water reactors.
Heavy Water Reactors (HWR)
Heavy water reactors use heavy water (D2O) instead of ordinary (light) water (H2O) as the moderator. The major advantage of heavy water reactors is that they can use natural uranium instead of enriched uranium to operate. Because deuterium is a more stable isotope of hydrogen, it has less affinity to absorb neutrons (a), and there is a higher probability of collision with uranium-236 and a higher rate of conversion to plutonium. Since no enrichment facilities are needed, these reactors are considered dangerous from a proliferation perspective. Worldwide, there are 40 reactors of this type, mainly in Europe and Russia. There are no commercial heavy water reactors in the United States.
High Temperature Gas-Cooled Reactors (HTGR)
These reactors operate with natural or enriched uranium, use graphite as the moderator, and employ helium as the coolant. Helium is much less corrosive than steam, and is thus less likely to cause a leak in the pipes. The hot helium gas can directly drive a gas turbine or be used through a heat exchanger to heat water to steam that drives a steam turbine. Because of the high operating temperature (750oC) common with this type of reactor, efficiency is higher - around 40% compared to the 33% achieved by the water-cooled reactors.
Fast Breeder Reactors (FBR)
Like fossil fuel, uranium reserves are limited (2, b). It is possible to extend the supply of nuclear fuel if non-fissionable component of nuclear fuel is converted into fissile materials. By doing so, more fissile plutonium nuclei is produced than consume, hence the term “breeding”. The breeder system uses uranium-238 as its fuel. It can however, transform into fissile plutonium-239 through collision with fast neutrons (Figure 4). The sequence of reactions is:
238U + n -> 239U -> 239N -> 239Pu
Uranium-238 + Neutron -> Neptunium-239 -> Plutonium-239
Because neutrons are not slowed down, no moderator must be used, which rules out water as a coolant. A suitable coolant is a liquid metal like sodium which has excellent heat transfer properties and a very high boiling point, which provide significantly better cooling and allow operation at essentially atmospheric pressure eliminating need for a heavy pressure vessel (3). The main concern with these reactors is that, in the presence of water or even moisture in the air, liquid metals become highly explosive. Therefore, breeder reactors require special care to assure the safety of reactor operation.
The first large-scale plant of this type, called Super Phoenix, went into operation in France in 1984. From the beginning, the plant was the focus of anti-nuclear and environmental groups, who eventually succeeded in closing down the plant in 1998. Currently, the only reactor of this type, BN-600, is operating in Russia producing 600 MWe. India has started construction of a 500 MWe prototype reactor, scheduled to be operational by 2010. Many factors including the availability of sufficient uranium fuel for the near future, safety concerns, the fear of terrorism, and nuclear proliferations have greatly dampened the enthusiasm for these reactors and many countries have stopped work on fast breeder technology all together.
Pebble Bed Modular Reactors (PBMR)
The reactor core is made of thousands of billiard-ball sized pebbles, each consisting of several thousand enriched uranium (up to 10%) microspheres (kernels) the size of small sand grains (0.9-mm). Each microsphere is coated with several layers of graphite that act as moderator and a silicon carbide outer shell. Unlike today’s nuclear reactors that boil water to drive steam turbines, pebble bed reactors use an inert gas like helium to drive gas turbines directly. Helium coolant enters the reactor and heated to about 900oC before it enters the turbines to generate electricity. After the fission is completed, the coating acts as a casket isolating and retaining the radiation long enough for the uranium to decay to reasonably safe limits (Figure 5). Shutdown will be done similar to other reactors, by inserting the control rods. These reactor are small, modular, flexible in design, and can be competitive with fossil fuels.
Proponents of this type of reactor claim that this design is intrinsically safe and that there is no possibility of catastrophic nuclear accidents or fire; even if all helium coolants were lost, the temperature would never rise above 1,500°C (well below the 3,000°C needed to melt the uranium). Furthermore, the silicon carbide and graphite coating holds up better than the zirconium-alloy jackets of conventional fuel rods. So even in the worst case scenario where all cooling water has stopped, all contaminations stay within the pebbles walls and reactor automatically shuts off. The opponents agree that, although the reactor is inherently safer than the current water reactors, it may still pose a danger similar to the Chernobyl nuclear accident (to be discussed later in this chapter). A crack in the reactor could allow air into the reactor core and cause the graphite to burn.
No commercial reactors of this type are operating anywhere yet. China operates an experimental PBMR with a larger 200 MWe “demonstration” reactor planned. South Africa is planning to construct several commercial plants to eventually produce 4,000 - 5,000 MWe of electric power, but because of substantial financial risks, its fate is uncertain (4). No PBMR is operating in the United States.
Retiring of Nuclear Reactor
Nuclear reactors are usually designed to last about 40 years, after which embrittlement and corrosion of the reactor vessels could pose a danger. By the year 2033, all 103 nuclear reactors currently operating in the United States will have reached the end of their original 40-year license period and must be shut down or must be upgraded for new licenses (5). Nuclear reactors cannot be simply shut down and demolished as most non-nuclear facilities are. The building, equipment, and the leftover fuel are radioactive and will continue to generate heat and emit radiation long after the reactor has stopped operation. Most often following the reactor close down, the plant enters a storage mode during which it is basically left alone but guarded for many years or decades to allow the intense short-lived radioactive material to decay and become less active. Another approach is to entomb the reactor under heavy protective layers of lead and concrete and leave it there until they are permanently disposed. The final phase of retirement is the dismantling or decommissioning of the plant by tearing it up using special robots and moving and storing its critical parts in a permanent storage site. Nuclear waste disposal will be discussed in nuclear radiation in more detail.
Reactor Safety
One of the most frightening aspects of nuclear power is the possibility of something going wrong with disastrous consequences. In one scenario, a malfunction of the control rods or a loss of coolant results in a “runaway” reaction and an uncontrolled release of energy that melts the fuel rods and causes them to fuse. The fused rods would then burn through the reactor vessel and the containment floor. Such a meltdown would cause leakage of dangerous radioactive materials into the environment. Two of the most famous (or infamous) nuclear accidents occurred at the Three Mile Island (TMI) reactor no. 2 in the United States and the Chernobyl reactor no. 4 in Ukraine.
Three Mile Island
Although various safety features make serious nuclear accidents unlikely, an accident did occur on March 28, 1979 at the Three Mile Island Nuclear Power Station near Harrisburg, Pennsylvania (Figure 6). The incident was triggered by a sudden pressure drop in the core because a relief valve failed to close. Failing to notice the valve malfunction, operators turned off the emergency cooling pumps that went into operation automatically when the water started draining. This and a number of additional operator errors eventually led to the loss of some coolant, which ultimately resulted in severe core damage and the release of large volume of fission products into the containment structure. The reactor suffered a partial meltdown of the uranium fuel rods. To prevent overpressure, some of the radioactive gases was vented out. Fortunately, majority of the radioactive release stayed within the containment, and very little radiation escaped outside the reactor. Although human exposure to radiation was relatively minor, the economic cost and the psychological stress on the public was significant.
The TMI accident is considered the worst nuclear disaster in US history. Under mounting pressure from the public, Congress enacted legislations for more stringent regulations for design, construction, and operation of nuclear reactors. In addition, the nuclear industry formed its own watchdog, the Institute of Nuclear Power Operation (INPO) that oversees safety standards and assures that power plants follow these standards and maintain active and continuous training of nuclear operators. Following the Chernobyl accident, the World Association of Nuclear Operators (WANO) was formed to fulfill a similar role to that of INPO on a global basis.
Chernobyl
On April 26, 1986, about 7 years after the TMI incident, the controversy was set ablaze again, this time at the Chernobyl nuclear power plant located about 130 km north of Kiev, the capital of Ukraine. One of the four nuclear reactors exploded and released more than 50 tons of radioactive material - 10 times that of bomb dropped on Hiroshima - into the environment. The scientific consensus is that the accident was the product of a flawed reactor design coupled with inadequate training and serious mistakes made by the operators of the plant (c). Unlike the TMI, the Chernobyl accident was not a result of loss of coolant, but followed a sequence of events that led to a fire to the carbon moderator.
The effects of the disaster at Chernobyl were widespread. The immediate casualties were among the clean-up crew, firefighters, and pilots who died from acute radiation exposure. Environmental damages included the destruction of vast areas of agricultural and farmland and poisoning of major surface and underground water reservoirs. Large areas of Ukraine, Byelorussia, and Russia were contaminated, resulting in the relocation of roughly 200,000 people from within 30 kilometers of the plant. Inhabitants of surrounding areas could not drink water or eat food produced locally. In Europe, many plants and animals were also contaminated and had to be destroyed. The economic damage from the accident is estimated at more than 13 billion dollars (Figure 7).
Initially, prevailing winds carried the radioactivity northwest from the plant across Byelorussia and into Poland, Hungary, and Sweden. Radiation levels in many parts of Europe rose well above normal before wind carried and scattered the radionuclide a long way away from the reactor site. Long-term effects on public health have been difficult to determine and are subject to considerable controversy, but many believe that the effects will be felt for many years in terms of widespread birth defects and various forms of cancer. Some believe that in Russia alone, 10,000 people will die as a direct result of radiation exposure from the Chernobyl accident over a 70 year period, the average lifespan of humans at the present. Worldwide the number is estimated at 25,000. Most, however, argue that the health problems caused directly by radiation are not easy to measure; many are caused by poor nutrition, lower health care conditions, and the anxiety and stress produced by fear of radiation exposure. What is clear, however, is that the rate of cancer and other deadly diseases among the more than half a million workers who participated in the Chernobyl cleanup has been significantly higher. Environmental and health consequences of radiation exposure are discussed in nuclear radiation.
Question: A common fear among many ordinary people is that a reactor meltdown may result in an explosion similar to those of nuclear bombs. Could nuclear reactors detonate in this fashion?
Answer: No! Explosions of this type are mostly a myth. The fuels used in the construction of nuclear weapons are plutonium and highly enriched uranium (90% or higher). Commercial nuclear reactors use low-grade fuels containing only 3-5% fissionable uranium-235. The remainder is uranium-238, which acts to absorb the neutrons and prevent chain reactions of the type necessary for nuclear bombs.
The China Syndrome
In 1979, the release of the movie “The China Syndrome” had, and continues to have, a stalwart effect on the public view of nuclear power plants (Figure 8). The themes discussed in the film remain relevant, as the issues it raised over the safety of nuclear power are still a source of debate. The title refers to a worst-case scenario of a US reactor core meltdown where the contents of the reactor would have enough heat to melt a hole through the earth, all the way to China (d). Although scientists agree that this scenario is highly unlikely, the idea of a nuclear accident does give nuclear power an ominous image that is difficult to refute (6).
Perhaps the most effective aspect in the film’s power of persuasion was its unintentional release date. The film debuted in March 1979, less than two weeks before the Three-Mile Island accident. The real-life headlines were so similar to the movie’s plot, says China Syndrome executive producer Bruce Gilbert, that he thought “someone had seen the picture and sabotaged the plant.”
Together, the film and the parallel crisis sparked a move to pull the plug on the nuclear-power industry. In the following months, several power plants were shut down as safety precautions while plans to open others were scrapped. The latest global public opinion poll conducted by IAEA shows that only 28% of the population consider nuclear power safe. 25% consider it to be dangerous and all existing plants should be shut down (7).
References
(1) International Atomic Energy Agency (http://www.iaea.org/programmes/a2).
(2) World Nuclear Association website (http://www.world-nuclear.org).
(3) Matzke, H., “Development status of metallic, dispersion and non-oxide advanced and alternative fuels for power and research reactors,” IAEA-TECDOC-1374, September 2003
(4) Pebble Bed Modularized Reactor, Centurion, Republic of South Africa (http://www.pbmr.co.za/).
(5) Farber, D., and Weeks, J., “A Graceful Exit? Decommissioning Nuclear Power Reactors,” Environment, 43, No. 6, July/August 2001.
(6) Sauter, M., Entertainment Weekly, March 20 1998 Issue.
(7) “Global Public Opinion on Nuclear Issues and the IAEA,” Nuclear Technology Review 2006, pp.7, IAEA GC(50)/INF/3.
(8) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) In other words, the neutron cross-section of heavy water is significantly smaller than in light water.
(b) The estimate is of course based on low cost availability of uranium fuel and may be extended if the market can bear higher electricity prices.
(c) One fundamental difference between the Chernobyl reactor and the water-cooled reactors operated in the United States and elsewhere is the role that steam plays during an accidental loss of coolant. In water-cooled reactors, steam may accumulate to form pockets known as voids. With excess steam, more voids are created and water becomes less effective as a moderator; as a result, the chain reaction is not sustained and less power is produced which tends to shut down the reactor. When moderator and coolant are kept separate (as was the case in Chernobyl), any loss of coolant reduces the cooling capacity without affecting the moderator and the rate that neutrons are released. As the reactor heated, the rate of the fission chain reaction increased, until the cooling water turned to steam and exploded. This in turn, accelerated the chain reactions and increased power output. More power means additional steam, less cooling, less neutron absorption, and still more power. With a lack of proper safety precautions the process may continue to dangerous levels. Water-cooled nuclear reactors are therefore inherently safer because they do not face this risk. In addition, the lack of containment structures in the Chernobyl plant, similar to those present in the American, European, and Japanese nuclear plants, resulted in substantial releases of radionuclide and added to the severity of the accident.
(d) A hole initiated in the United States extended all the way through the earth would not end up in China, but somewhere in the Atlantic Ocean.
Further Reading
Bodansky, Nuclear Energy Principles, Practices, and Prospects, Second Ed., Springer, 2004.
Seaborg, G., T., Peaceful Uses of Nuclear Energy, University Press of the Pacific, 2005.
International Journal of Nuclear Engineering and Design, Direct Science Elsevier Publishing Company, devoted to the Thermal, Mechanical, Material and Structural Aspects of Nuclear Fission.
Journal of Fusion Energy, Springer Netherlands. It features articles pertinent to development of thermonuclear fusion.
External Links
Federation of American Scientists (http://www.fas.org/nuke/intro/nuke/index.html).
International Atomic Energy Agency (http://www.iaea.org).
DoE Office of Nuclear Energy, Science & Technology (http://www.ne.doe.gov).
American Nuclear Society, (http://www.ans.org).
World Association of Nuclear Operator (WANO) (http://www.wano.org.uk).