Nuclear Fission Reactors

From WolfWikis

Nuclear fission reactors are devices for maintaining controlled fission chain reactions. They are used primarily to produce electricity, but also for naval propulsion, research, and isotope production.

Contents 1 History of Nuclear Fission 2 History of Nuclear Fission Reactors 3 Uses of Nuclear Reactors 4 Science Behind the Fission Reactor 5 Future of Fission Reactors 6 Public View of Nuclear Fission Reactors 7 How the Scientific Community Views Nuclear Fission Reactors 8 References

History of Nuclear Fission

Pierre and Marie Curie discovered a new element, radium, in 1898. After months of refining pitchblende slag, they were left with a test tube of glowing salt, which they named radium. The glow from the radium indicated that atoms held a structure different from the long held solid ball model. Further investigation into emissions from the element revealed a structure composed of a nucleus with protons and neutrons surrounded by electrons. Nuclear fission reactions were discovered in 1938 by Otto Hahn and Lisa Meitner. They discovered that when a uranium nucleus was bombarded with a neutron, it resulted in elements about half of the weight of uranium. Leo Szilard discovered the release of neutrons from a fission reaction and realized that these could be used to initiate other fissions in a chain reaction. ^[1]

History of Nuclear Fission Reactors

Enrico Fermi built the first nuclear reactor in 1942, at the University of Chicago. It achieved criticality on December 7. Soon afterwards, a large number of reactors were built to produce plutonium and uranium for the Manhattan Project. The 1950's marked an expansion in the role of nuclear reactors beyond weapons production. Nuclear naval propulsion was shown to be a feasible option with the launch of the first nuclear submarine in 1954, the USS Nautilus. Also, nuclear reactors began to move from the military and government realm into the civilian realm when the first civilian nuclear reactor was built at North Carolina State College in 1953 for research.^[2] After World War II, nuclear reactors were investigated as a possible source of power. In 1951, the EBR-1 reactor in Idaho provided the first electricity produced by a nuclear reactor to a string of four light bulbs, and later to the entire reactor building.^[3] The first nuclear reactor that produced electrical power in commercial quantities was the gas cooled and graphite moderated reactor at Calder Hall in Great Britain, which began to produce 50 MW of electricity in 1953. The first commercial nuclear power plant in United States began operation in Shippingport, Pennsylvania, in 1957 with a power capacity of 60 MW. Civilian use of nuclear reactors grew rapidly in the 1960's and 1970's with the construction of more nuclear power plants. In 1979, a partial core meltdown took place at Three Mile Island in Pennsylvania. Although no one was injured and the accident was contained, the incident damped confidence in nuclear power and no new plants were constructed in America afterwards. In 1986, one of the reactors at the nuclear power plant in Chernobyl exploded after a poorly executed test. The disaster spread radioactivity all over Europe and rendered the immediate area around it uninhabitable. The accident further dampened public enthusiasm for nuclear power. Today, nuclear power accounts for 20 percent of electricity in the United States and 16 percent of the world's total electrical production.^[4]

Uses of Nuclear Reactors

In a nuclear power plant, electricity is produced in manner similar to that of oil or coal, where water is heated by the fuel and the resulting steam is driven across a turbine to generate electricity. However, the fission of one uranium atom releases around 200 MeV of energy, compared to around 55 eV from one combustion reaction in a fossil fuel. This is of interest to utility companies, because the high energy density of the fuel means that fuel has to be replaced much less frequently, and thus will lower operating costs. While a coal plant requires a trainload of coal for each day of operation, nuclear fuel is cycled about every 18 months. Nuclear power releases no greenhouse gases, which is of interest to environmentalists. However, it produces a minor quantity of highly radioactive waste which needs to be carefully stored. Long term storage is an issue that has not been resolved yet in the United States, as the promised government built repository at Yucca Mountain has not been completed yet.^[5]

Nuclear reactors can be used for ship propulsion. Much like in a commercial reactor, steam in a nuclear reactor is driven across a turbine, except that the turbine drives the propellers. This has not proven to be an economical method of propulsion. However, due to refueling times on the order of decades, these are common in military ships. Today, all United States aircraft carriers and submarines are nuclear powered.^[6] Nuclear reactors are common on Russian icebreakers due to the increased power they offer over diesel engines, which is useful in keeping northern shipping lanes open.^[7]

Nuclear reactors are used in research and isotope production. Research reactors are designed to produce large amounts of neutrons, which can be used in experiments to probe the structure of matter. Research reactors are also used in neutron activation analysis, where a sample is bombarded with radiation in order to make certain elements radioactive. The energy signatures of the decay of these isotopes can be detected, and the intensity indicates the amount of the material that is present. Components meant to be used in larger reactor can be tested or calibrated in research reactor. Isotope producing reactors are often similar to research reactors, except that the neutrons are directed onto blankets or targets composed of a material that will become the desired isotope through a series of nuclear reactions. Today, most isotopes are either for medical isotopes or for sources of radiation for industry.^[8]

Science Behind the Fission Reactor

The discovery of the relationship and equivalence of matter and energy in 1905 by Albert Einstein was the foundation necessary to utilize nuclear energy at the atomic level. The formula E = mc^2 states that an enormous amount of energy lies in a very small amount of matter. Harnessing the conversion of matter to energy is the core concept nuclear reactors are built upon.

All atomic nuclei beyond hydrogen-1 require a force to hold the constituent protons and neutrons together. At higher atomic numbers, the electrostatic repulsion of protons in close proximity is considerable. The strong nuclear force, acting only over an extremely short range, binds the nucleons together. A negative potential energy exists in the bound nucleus, the magnitude of which is known as the nuclear "binding energy." Since mass and energy are related by E = mc^2, the net mass of the nucleus is accordingly reduced by the mass-equivalence of this negative potential energy. The binding energy represents the amount of energy that would be required to be added to overcome the strong force and separate a nucleus into its component nucleons. Thus, a bound nucleus is in a lower energy state than the sum mass-energies of the separate free nucleons. The net reduction in mass of a bound nucleus compared to the free state of its nucleons is known as the "mass defect." ^[9]

A bound nucleus may be separated into two or more components if enough energy is added. The amount of energy added need not equal the entire binding energy (which would be required to completely separate all nucleons). A "threshold energy" exists which allows the nucleus to "fission," or split. Fission occurs only in nuclei of very high atomic number where the electrostatic repulsion from a large number of protons provides a positive potential energy counterpart to the negative potential energy present due to the strong force. Often, only a small amount of energy is required to overcome the threshold (in some cases, fission occurs spontaneously with no energy addition at all). When a large nucleus splits, it is always the case that the binding energies per nucleon of the resulting "daughter" nuclei are higher than for the "parent" nucleus.

Since the separate daughter nuclei have, in sum, greater binding energy than the parent nucleus, their net mass defect is greater. This reduction in net mass must be accompanied by the release of energy. The energy released in a particular nuclear reaction is known as the "Q-value." This Q-value is manifested in the kinetic energy gained in the reaction and is the energy equivalent of the difference in total mass defect before and after the reaction. The majority of the kinetic energy is transferred to the daughter nuclei. However, two to three neutrons are typically emitted in a fission reaction which account for a portion of the net kinetic energy gained in the reaction. These neutrons are "fast," or high-energy neutrons. Neutron generation by fission is the key to sustained energy production in a nuclear reactor. In addition to "prompt" neutrons generated directly from fission, some neutrons will be generated later from the subsequent decay of daughter nuclei. These "delayed" neutrons are essential to reactor control as they increase the mean neutron generation lifetime by several orders of magnitude. This mitigates the rate at which a chain reaction can propogate.^[10]

Uranium in various chemical and material forms is the main component of nuclear fuel. Natural uranium consists primarily of U-238 (99.3%) with a small proportion of U-235 (0.7%). U-238 will fission in most cases only if it absorbs fast neutrons upon impact and it has a much lower cross-section (probability of an interaction) for absorption and fission than U-235. The U-235 nucleus may fission upon absorption of neutrons of any energy and has an especially high cross-section for thermal fission. Thermal, in this case, refers to neutrons that have been "thermalized," or "slowed down" by kinetic impacts to an energy in thermal equilibrium with the surroundings. It is usually necessary to enrich (increase the proportion of U-235) uranium fuel for nuclear reactors to some extent to enable a sustained fission chain reaction to be possible.

The fundamental nature of the energy-dependence of cross-sections is extremely complex and only partially understood. It is related in part to the wave-particle duality of matter. Neutrons with very high energies will have shorter wavelengths that are on the order of the size of individual nucleons. Thermalized neutrons have larger wavelengths on the order of the size of nuclei. Thus, the speed of a neutron will affect whether its impact is "felt" locally in a nucleus or if its impact is registered by the nucleus as a whole. In any case, since nucleus energy states are quantized, only particular neutron kinetic energies (in the center of mass frame) will result in absorption at all. If a neutron is not absorbed, it may be scattered elastically or inelastically (the process of slowing down). Nuclei with different combinations of neutrons and protons will have different binding energies, nuclear structure and stability, accounting for the large range of possible cross-sections. Additionally, the neutron itself changes the binding energy of a nucleus if absorbed. Thus, even a neutron with zero kinetic energy may induce fission if absorbed. For very slow-moving neutrons, direct impact is not necessary. The neutron's large wave-length can allow quantum interaction if it is in close proximity to the nucleus.

In a critical reactor, per unit time, prompt neutrons generated from fission and delayed neutrons generated from daughter decays exactly balance neutron losses due to absorption and leakage out of the core. Departure from criticality is measured in units of "reactivity," or the fractional change in neutron population per generation. In a supercritical reactor, net reactivity is positive, more neutrons are generated than are lost per unit time, and reactor power increases exponentially. In a subcritical reactor, net reactivity is negative, more neutrons are lost than are generated per unit time, and reactor power decreases exponentially (approaching an asymptote). The primary means that the reactor operator has to control reactivity is by moving neutron-absorbing control rods in or out within the core. Other natural negative reactivity feedback mechanisms exist in addition to operator-controlled rods. The most important of these is known as "Doppler brodening." The Doppler effect results from quantized energy resonances within the absorption cross-section spectrum of U-238. At intermediate energies, neutrons are subject to "resonance absorption" in U-238 while kinetically slowing down by collision, removing them from the neutron population without inducing fission. As reactor power (and consequently temperature) rises, the U-238 atoms will vibrate more rapidly. Thus, in the center of mass frame, a neutron with any particular kinetic energy is more likely to fall into one of the resonance bands. The resonances are "spread out" and neutrons spend more time (while slowing down) within the resonances with rising temperature. The resulting loss of neutrons adds negative reactivity, causing power to decrease. A similar stabilizing effect is caused by the moderator (commonly the same as water coolant) heating up. As the moderator density lessens, neutrons are more likely to leak out of the core before being thermalized. This results in negative reactivity lowering power.^[11]

Many different general designs of nuclear reactors exist, including light and heavy water pressurized water reactors, boiling water reactors, liquid metal cooled reactors and graphite moderated reactors. Regardless of the design, the fundamentals of controlling the nuclear fission chain reaction are the same. Neutron losses must be balanced by neutron generation in a safe and controllable manner. With this accomplished, the heat generated through the transfer of fission reaction kinetic energy to surrounding material can be captured and used to produce steam. The steam is used to turn electric turbines which in turn produce electricity. None of this would be possible without the knowledge and understanding of the equivalence of matter and energy by E = mc^2.

Future of Fission Reactors

The future of fission reactors lies in the re-configuration and upkeep of the old types of fission reactors. Generation I reactors were developed in the 1960’s as prototypes and there are very few that are still running today. The majority of the reactors that are operating today are Generation II reactors and were developed based on the Generation I reactors. Advanced reactors known as Generation III reactors include European Pressurized Water Reactors (EPR’s), the AP1000 of Westinghouse and other advanced Boiling Water Reactors (BWR’s). The main idea behind Generation III reactors is to improve safety, economics and accident analysis and management. As of now there are more than a dozen Generation III reactors being designed and developed. A lot of these designs have evolved from the Generation II PWR, BWR and CANDU designs while other designs are completely new. One of these designs is the PBR or Pebble Bed Reactor (PBR) or High Temperature Reactor (HTR) which uses helium as coolant to directly drive a turbine ^[12]

Finally there are the Generation IV designs, none of which have been implemented into an actual reactor. These designs will not be on a commercial basis for at least 20-30 years. The Generation IV International Forum (GIF) which is representative of 10 countries was initiated in the year 2000. It is committed to the joint development of the next generation of nuclear technology. Currently there are six different systems that are being developed in the framework of the GIF. Most of the six systems support a closed nuclear fuel cycle in order to get the most out of the resources used and minimize the amount of waste produced.

Three of these systems are fast reactors that use sodium, lead or gas as a coolant. One is an advanced HTR, another is a supercritical water-cooled reactor and the last one is a molten-salt reactor concept. The latter three operate at low pressures and have significant safety advantages over the others. One of the latter three uses Uranium fuel that gets dissolved in the coolant. The temperatures range from 510°C to 1000°C and compared to the less than 330°C for today’s light water reactors, this is a significant increase. Because of this temperature increase, four of the new reactors can now be used for thermo-chemical hydrogen production ^[13].

Public View of Nuclear Fission Reactors

Since the events of the 1968 Chernobyl accident, recent polls have shown that there is growing support for advancement in the harnessing of nuclear energy. One major reason is that people are noticing the prices for energy are rising rapidly and people are starting to see a reason for nuclear power. Energy consumers are being hit by escalating oil and gas prices and are taking another look at nuclear energy. A survey conducted last year by the Nuclear Energy Institute found that out of 1000 people surveyed, 70% were in favor of nuclear energy and two thirds of those 1000 said that they wouldn’t mind a new reactor being built at an existing site. In another survey, another 1000 people who lived in a 10 mile radius of an already existing plant were asked their opinion about nuclear energy. Over 80% were in favor of nuclear energy and 76% said they wouldn’t mind adding a new reactor to an existing site. Overall, the public holds above all, the safety and waste management of this energy source. They don’t want another accident like Chernobyl. ^[14]

Conversely, there are obviously those who are drastically opposed to nuclear energy and nuclear reactors. A lot of those that are still against nuclear power believe that something like Chernobyl will happen again and it is not worth the risk to build more reactors. It has also been noted that most of those who are against nuclear power have in some way been affected by accidents like Chernobyl and Three Mile Island. Another interesting fact is that those who have never lived near a nuclear facility are more likely to be against building more reactors than those who have lived near a nuclear facility.

Surprisingly, the nuclear debate can be broken down into a battle between two distinct groups who seem to share a lot of the same ideas. Those that are for nuclear power believe that the future role of nuclear power is an important one and believe that we can easily solve our problems regarding nuclear waste. Those who are against nuclear power believe that there is no future for it and say that there is no way to solve the problem of nuclear waste. One issue that both sides agree on is that the public is not educated enough to understand the concepts behind nuclear power. Those who are for it say that the public is irrationally frightened of nuclear power and should be better educated in order to gain support for the nuclear industry. Those who are against it believe that the public is unworried about nuclear power and if they were better educated on the subject they would be able to grasp how dangerous nuclear power can be. It is interesting that each group argues that the government cannot be trusted to make wise decisions concerning the subject of nuclear power. The supporters think it’s because the government will listen to the anti-nuclear power group and the opposition believes that the government will listen to those who are in favor of nuclear power. ^[15]

How the Scientific Community Views Nuclear Fission Reactors

Since it was scientists in the first place who came up with how to harness the power of the atom and use it to create nuclear power, most scientists will be strong supporters of nuclear power and energy. What most scientists today are trying to do is to work with the public to help them better understand the concepts behind nuclear power. These scientists realize that the main goal is the welfare of the people, and the more supporters they can gain for this rise in technology, the better off the human race will be. Dr. Jan Dodderlein looked to preserve public interest by emphasizing the fact that the public deserved to be presented with correct information regarding what was going on in the nuclear industry.

While most scientists agree that nuclear power should be promoted, there are still some disagreements on certain aspects of it. Two scientists in particular, Prof. Hans A. Bethe and Prof. Hannes Alfven, both Nobel Prize winners, had different viewpoints on the need for nuclear power. Prof. Hans A. Bethe, believed that nuclear power was needed to avoid the energy crisis while Prof. Hannes Alfven argued that nuclear power was not the solution to the energy crisis and the problem should be solved by non-nuclear means. The main problem, the scientists agreed was that the public did not have the proper education on the subject of nuclear power and reactors, therefore what they didn’t know/understand scared them. In the end there were several things agreed on by the scientific community:

1.) It is important to keep the public currently and accurately informed on the consequences of operating nuclear power plants as well as other means of energy production.

2.) International organizations such as IAEA and WHO should play a better role in the broadcasting of information on nuclear power and energy and should contribute to the general awareness and confidence of the public.

3.) If the world focuses more on using nuclear energy for power production, the world will be less likely to look towards nuclear weapons. ^[16]

References

1. ↑ Larsen, E. (1958). Atomic energy: A layman's guide to the nuclear age. Great Britain: Hennel Locke, Limited.
2. ↑ North Carolina State College. First temple of the atom. Retrieved 4/26, 2009, from http://www.lib.ncsu.edu/specialcollections/eresources/text/engineering/reactor/EPbroch.xml
3. ↑ Idaho National Laboratory. Experimental breeder reactor 1. Retrieved 4/26, 2009, from http://www.inl.gov/factsheets/ebr-1.pdf
4. ↑ Sweet, W. (1988). The nuclear age Congressional Quarterley, Inc.
5. ↑ Shultis, J. K., & Faw, R. E. (2008). Fundamentals of nuclear science and engineering (2nd ed.) CRC Press.
6. ↑ Shultis, J. K., & Faw, R. E. (2008). Fundamentals of nuclear science and engineering (2nd ed.) CRC Press.
7. ↑ Bellona. (1997). Nuclear icebreakers. Retrieved 4/26, 2009, from http://bellona.org/english_import_area/international/russia/civilian_nuclear_vessels/icebreakers/30107
8. ↑ Shultis, J. K., & Faw, R. E. (2008). Fundamentals of nuclear science and engineering (2nd ed.) CRC Press.
9. ↑ Shultis, J. K., & Faw, R. E. (2008). Fundamentals of nuclear science and engineering (2nd ed.) CRC Press.
10. ↑ Lewis, E. E. (2008). Fundamentals of Nuclear Reactor Physics, Elsevier, Inc.
11. ↑ Lewis, E. E. (2008). Fundamentals of Nuclear Reactor Physics, Elsevier, Inc.
12. ↑ European Commission. (1995-2009). Energy research: Various types of fission reactors. Retrieved Apr. 25, 2009, from http://ec.europa.eu/research/energy/fi/fi_bs/article_1175_en.htm.
13. ↑ Hore-Lacy, I. (2008). Generation IV nuclear reactors. Retrieved Apr. 25, 2009, from http://www.eoearth.org/article/Generation_IV_nuclear_reactors
14. ↑ (1) International Atomic Energy Agency. (2006). The Shifting Sands of Nuclear Public Opinion Nuclear Communicators Wrap Up Meeting at the IAEA in Vienna. Retrieved Apr. 20, 2009, from http://www.iaea.org/NewsCenter/News/2006/pime.html
15. ↑ Grimston, M. C. (2002). Nuclear Energy: Public Perceptions and Decision-making. Retrieved Apr. 20, 2009, from http://www.world-nuclear.org/sym/2002/grimston.htm
16. ↑ International Atomic Energy Agency. (1977). Nuclear Power and Public Opinion. Retrieved Apr. 23, 2009, from http://www.iaea.org/Publications/Magazines/Bulletin/Bull193/19304703440.pdf