UEET 603 Introduction to Energy Engineering Spring 2010 Nuclear Power
Nuclear Energy Nuclear energy is a way of creating heat through the fission process of atoms. Nuclear energy originates from the fission of atomic nucleus in a chain reaction. Nuclear fission reaction is controlled in a nuclear reactor to produce thermal energy.
The fission process takes place when the nucleus of a heavy atom like uranium or plutonium is split when struck by a neutron. The fission of the nucleus releases two or more new neutrons. It also releases energy in the form of heat. The released neutrons can then continue to split additional nucleus . - This releases even more neutrons and more nuclear energy. - The repeating of the process leads to a chain reaction.
Current Use of Nuclear Power All nuclear power plants convert heat into electricity using steam. The heat is created when atoms are split apart – called fission. The heat from fission reaction is used to produce steam, which is then used to turn a turbine and produce electricity using a generator. Power from nuclear fission accounts for about 19% of the nation’s total energy. Number of operating nuclear power plants is 120 ?? Mostly safe, successful and well regulated No new power plant has been built in last 30 years
Current Use of Nuclear power Nuclear power is generated primarily in stationary applications like in a nuclear power plants, propulsion of mobile system like naval vessels, especially submarines as well as several surface vessels. Since nuclear plants do not consume oxygen like conventional plants, it is quite attractive for use under sea. Also, ships powered by nuclear plants need to be refueled only after long period of operation. Nuclear power has also been developed for the propulsion of aircrafts and rockets.
Major Concern and Obstacles Safe operation of the plant and safe handling and disposal of nuclear fuels. Concern of environmentalists about the dangers of storing the radioactive byproducts of the process. Cost of building a new nuclear reactor is about $10 billion each – One of the main reason for a stagnant industry. Recent reduced cost of other fuels like coal, natural gas and oil, make the nuclear power production less competitive. Each project has to pass through a rigorous scrutiny and check through Nuclear Regulatory Commission (NRC) before construction can begin.
Needs economic guarantee from the government Without any support from the government it seems difficult for the utility companies to start any new projects. Government recently committed $8.33 billion in loan guarantees for the construction of two new reactors at the Alvin W. Vogtle Electric Generating Plant in Georgia - Expected to provide electricity to over a million people by next 5-6 years. - Expected to create new job opportunities.
Atomic and Nuclear Physics Fundamental Particles The physical world is composed of various subatomic or fundamental particles. There are variety of different fundamental particles, and scientists are still finding newer ones . However , only few of these are important in nuclear engineering: - Electrons - Proton - Neutron - Photon - Neutrino
Fundamental Particles Electrons: This particle has rest mass of and carries a charge Mass of a particle is a function of its speed relative to the observer In giving the mass of fundamental particle, it is necessary to specify the mass at rest with respect to the observer - termed as rest mass.
There are two types of electrons: Negatrons or negative electrons: Carries a negative charge Normal electrons encountered in this world. Positrons or positive electrons: Carries a positive charge Relatively rare in this world These two are identical except the sign of the charge
Electron Annihilation Process When under circumstances, a positron collide with a negatron, the electrons disappear and two (occasionally more) photon (particles of electromagnetic radiation) are emitted. Proton This particle has a rest mass of and carries a positive charge equal in magnitude to the charge on the electron. Protons with negative charge have also been discovered, but these particles are of no importance in nuclear engineering.
Neutron The mass of a neutron is which is slightly larger than the mass of the proton. It is electrically neutral. The neutron is not a stable particle, except when it is bound into an atomic nucleus. A free neutron decays to a proton with the emission of a negative electron (Known as )
Photon Particle equivalent of electromagnetic wave. This is a particle with zero rest mass and zero charge, which travels in a vacuum at only one speed, namely the speed of light Neutrino This also a particle with zero rest mass and no electrical charge. This appearsin the decay of certain nuclei. There are two types of Neutrinos: neutrinos and antineutrinos
Atomic and Nuclear Structure Atoms are the building blocks of all gross matter Atoms, in turn, consists of a small but massive nucleus surrounded by a cloud of rapidly moving (negative) electrons The nucleus is composed of protons and neutrons The total number of protons in the nucleus is called the atomic number (Z) of the atom.
Total electrical charge of the nucleus is : +Ze In a neutral atom there are as many electrons as protons, i.e. Z-number of electrons moving around the nucleus. It is the number electrons that dictates the chemical behavior of atoms and gives identify of a element. Hydrogen (H) has one electron Helium (He) has two electrons Lithium (Li) has three electrons The number of neutrons in a nucleus is known as theneutron number (N) Atomic mass number: A = Z+N
The various species of atoms whose nuclei contain particular numbers of protons and neutrons are called nuclides. Each nuclides is denoted by the chemical symbol of the element (this specifies Z) with the atomic mass number as superscript. This determines the number of neutrons N = A-Z : Hydrogen nuclide with ( Z=1) a single proton as nucleus is the hydrogen nuclide with a neutron and as well as a proton in the nucleus. This called the deuterium of heavy hydrogen .
is the helium nuclides whose nucleus consists of two proton (two electron) and two neutrons. For better clarity, Z is also included in the symbol as a subscript. Atoms such as and whose nuclei contains same number of protons but different numbers of neutrons ( Same Z but different N and hence different A) are known asisotopes. Naturally occurring elements may exist in the nature with some stable isotopes and some unstable isotopes and expressed as percentage atoms of the element.
Stable and Unstable Isotopes Oxygen, for example, has three stable isotopes , , ( Z = 8, N = 8, 9 , 10 ) and five known unstable ( i.e. radioactive) isotopes , , , and (Z = 8, N=5, 6, 7, 11, 12). The stable isotopes ( and a few of the unstable isotopes) are found as naturally occurring elements in nature. However, they are not found in equal amounts; some isotopes of a given element are more abundant than others. For example: 99.8 % of naturally occurring oxygen atoms are the isotope . Rest are: 0.037% and 0.204%
Mass and Energy Einstein’s theory of relativity Mass and energy are equivalent and convertible, one into other. Complete annihilation of a particle or other body of rest mass releases an amount of energy which is given by Einstein’s formula c is the speed of light
For example, the annihilation of 1g of matter would lead to a release of - This is equivalent to 25 million kilowatt-hours Another unit of energy that is often used in nuclear engineering is the electronVolt(eV) This is defined as the increase in the kinetic energy of an electron when it falls through an electrical potential of one volts. This is in turn is equal to the charge of the electron multiplied by the potential drop
When a body is in motion, its mass increases relative to an observer according to the formula Total energy of a particle, that is, its rest mass plus its kinetic energy Kinetic energy is given as For v<< c Same as in Classical Mechanics
Energy of Atomic particles Neutron: Photon: Travels at the speed of light and has no rest mass, its total energy is given as Particle Wave Length For Neutron For Photon and all other particles of zero rest mass Where E is the kinetic energy in eV
Excited States and Radiation The z atomic electrons that cluster about the nucleus move in a well defined orbits However, some of these electrons are more tightly bound in the atom than other. For example only 7.38 eV is required to remove the outermost electron from a lead (Z=82), while 88 keV (or 88,000 eV) is required to remove the inner most or the k-electron. The process of removing an electron from an atom is call ionization and 7.38 eV and 88k3V are known as the ionization energies.
This leads to a excited state for the atom –has more energy than the ground state. It slow decays back to the ground state. When such transition occurs, a photon is emitted by the atom with an energy equal to the difference in the energies of the two states. Depending on the energy level of the excited state or the removed electron orbit level, radiation wavelength ( ultraviolet etc. ) can be determined.
Type of Power Reactor Light -Water Reactor (LWR) Gas Cooled Graphite moderated Reactor - High temperature gas cooled reactor (HTGR) Heavy -Water Reactor (HWR) Breeder Reactor (BR)
The first generation of reactor that is moderated and cooled by ordinary (light) water. • Water has excellent moderating properties as well as thermodynamic properties to produce steam. • Water also absorbs neutrons to such an extent that it is not possible to fuel a LWR with natural uranium – it simply would not become critical. • Uranium in LWR must always be enriched to some extent. There are two types of light - water reactors: Pressurized-water reactor (PWR) Boiling-water reactor (BWR) Light-Water reactor (LWR)
Pressurized Water Reactor (PWR) The water in a PWR is maintained at a high pressure in the range of 2000-2500 psi to prevent water from boiling. High pressure water is circulated through the reactor core to pick up heat without any boiling of water. Pressurize hot water is then circulated through the steam generator where heat is transferred to a secondary water stream that enters as liquid water and exits steam.
High pressure and high temperature steam is then turns a turbine to produce electric power. Large PWR system uses as many as four steam generators, which produce steam at about 560 F and 900 psi. This gives an overall efficiency of 32-33 % for a PWR plant. The condenser is cooled by a cooling water loop using pumps and cooling tower that rejects heat to environment
Boiling Water Reactor (BWR) Referred to as the direct cycle. Water is boiled directly in the reactor vessel and produces steam to turn the turbine. Steam is produced directly inside the reactor and there is no need of a separate steam generator. Steam from the rector goes directly to the turbine to produce power. More effective in removing heat from the fission reaction using latent heat rather than sensible heat.
Less water is pumped through the reactor than a PWR for the same net power output However, the water becomes radioactive in passing through the reactor core. Since this radioactive water is utilized in the electricity producing side of the plant, all of the components like the turbines, condensers, reheaters, pumps, piping be shielded in a BWR Plant. The pressure in a BWR is approximate 1000 psi, about half the pressure in a PWR.
As a results the wall of the pressure vessel for a BWR need not be as thick as it is for a PWR. However the power density (Watt/cm^2) is smaller in a BWR than a PWR, and so overall dimension of a pressure vessel for BWR must be larger than for PWR.
Heavy-Water ( )Reactor Heavy-water moderated and cooled reactor (HWR) has been under development in several countries, especially in Canada. Heavy-water moderated reactor is suitable for use with natural uranium. Canada has large resources of natural uranium. Removes the need for expansive uranium enrichment plant.
Such a reactor can operate on natural Uranium because the absorption cross section of deuterium ( D = ) for thermal neutron is very small, much smaller for example than the cross section of ordinary hydrogen ( H = ). However, deuterium in is twice as heavy as hydrogen in , so that is not as effective in moderating neutrons as . They require more collisions and travel greater distances before reaching thermal energies than
The core of an HWR is therefore considerably larger than that of an LWR, but much smaller than a natural uranium, gas cooled graphite moderated reactor. In order to avoid the use of large and expensive pressure vessel, Canadian design uses pressure tube concept that encapsulated fuel within a hollow tubes. The coolant passes through the tubes and coolant do not come in direct contact with the heavy water moderator. In Canadian HWR design, heavy water is also used as the coolant.
One important thing about the HWR is that the reaction is not inherently stable. Thus an accidental increase in power leads automatically to further increase in power and rapid external intervention is required to bring reactor under control
Breeder Reactor The world reserve of are not adequate to meet the indefinite needs of the growing nuclear power industry ( may be for 100 years) Only the advent of the breeder reactor can achieve the full potential of the world’s uranium and thorium supply. It is possible to manufacture certain fissile isotopes from abundant non-fissile materials by a process know an conversion.
The two most important fissile isotopes produced from conversion are and . The fissile isotope is obtained from fissile isotope of thorium by the absorption of neutrons. is obtained from nonfissile , which is one of the major component of natural resource of uranium. has to be irradiated in a reactor, which normally occurs in most the reactors. So most of the reactor are fueled with uranium which is only slightly enriched in . Practically all of the fuel in these reactors is therefore and conversion of into takes place during the normal operation of the rector.
The conversion process is described in terms of conversion ratio or breeding ratio - Defined as the average number of fissile atoms produced in a reactor per fissile fuel atom consumed. In a breeder reactor every effort is made to prevent fission neutrons to slow down. So, light-water is excluded from the core. There is no moderator in the core and the core contains only fuel rods and coolant. In these reactors, different other types of coolants such as sodium are used.
Types of Breeder Rectors: Liquid-metal Cooled fast Breeder Rector (LMFBR) Gas cooled fast breeder Reactor (GCFR) Molten salt breeder reactor (MSBR) Light water breeder reactor