Nuclear PhysicsNuclear Reaction Dr. Nidal Dwaikat An- Najah Nation University December, 2009
Nuclear Reaction In order nuclear reaction to occur two nuclei or particle And nucleus must approach each other. Two types of nuclear reactions : 1- fission reaction 2- fusion reaction Nuclear Reaction is denoted By X (Pi, Pf)Y Recoiling Nucleus Target Recoiling Nucleus Scattered Particle Incident Particle Example : 14N (a , p ) 17O
Fission Reaction : using fissionable material such as Uranium(235U) , 239Pu and 2333U. Thermal neutron : < 1 eV
The process of fission 1- A neutron collides with a uranium- 235 atom and creates a highly unstable uranium-236 atom. 2- As soon as the uranium-236 atom is created, it splinters into an isotope of barium (144BA), an isotope of krypton (89Kr), and three free neutrons are produced as well. This process can be summarized using the notation that was introduced earlier (the n in the equation stands for the word “neutron”):
The excitation energy added to the compound nucleus is equal to the sum of the binding energy of the incident neutron and its kinetic energy.
The criteria for choosing Nuclear fuel is : The criteria for choosing Fissionable material is : 1- Fissionable 2- Cross section for capturing neutron 3- Number of neutron emitted must be larger than one In order to Chain reaction continue
Thermal neutron at room temperature, E = KT= 300 K= 1/40 eV Low energy neutrons, electrically neutral, cant get much closer to the nuclei and interact with them • Capture of low energy was used to produce nuclei of higher A. It is, experimentally, observed that • The interaction of low energy neutron with odd-A nuclei such as 235U did not produce heavier nuclei. In stead the parent nucleus fragmented into two nuclei of smaller size. - The interaction of thermal neutrons with even-A such as 238U nuclei such as U238 does not produce fragmentation. Fission can take place in such nuclei when the neutrons have kinetic energies of the order of 2MeV.
-The percentage of 235U is 0.7% of natural Uranium - 238U is around 98.8% of natural Uranium.- 235U is fissionable - 238 U is non fissionable Uranium enrichment Is the process by which the concentration of 235U is increased over its natural value 4% for power generation 20% for Medical Application >90% for nuclear weapons
diagram represents the steps of Uranium enrichment by Gaseous diffusion
Other separation methods are also used 2- Electromagnetic method The earliest successful methods were electromagnetic isotope separation (EMIS), in which large magnets are used to separate ions of the two isotopes, and gaseous diffusion, in which the gas uranium hexafluoride (UF6 ) is passed through a porous barrier material; the lighter molecules containing 235U penetrate the barrier slightly more rapidly, and with enough stages significant separation can be accomplished. Both gaseous diffusion and EMIS require enormous amounts of electricity. More efficient methods have been developed.
3- centrifuge • The third method in widespread use is the gas centrifuge [Urenco (Netherlands, Germany, UK), Russia, Japan] in which UF6 gas is whirled inside complex rotor assemblies and centrifugal force pushes molecules containing the heavier isotope to the outside. Again, many stages are needed to produce the highly enriched uranium needed for a weapon, but centrifuge enrichment requires much less electricity than either of the older technologies.
4- Laser Isotopes Separation Atomic and molecular laser isotope separation (LIS) techniques use lasers to selectively excite atoms or molecules containing one isotope of uranium so that they can be preferentially extracted. Although LIS appears promising, the technology has proven to be extremely difficult to master and may be beyond the reach of even technically advanced states.
5- Thermal diffusion Thermal diffusion utilizes the transfer of heat across a thin liquid or gas to accomplish isotope separation. By cooling a vertical film on one side and heating it on the other side, the resultant convection currents will produce an upward flow along the hot surface and a downward flow along the cold surface. Under these conditions, the lighter 235U gas molecules will diffuse toward the hot surface, and the heavier 238U molecules will diffuse toward the cold surface.
Fissile Isotopes Production It is possible to produce fissile isotopes from abundant non fissionable material, a process known as conversion The two most important fissile isotopes that can be produced by conversion are 233U and 239Pu 238U+n 239U Np Pu
Chain Reaction Neutron emitted by fission nuclei induce fissions in other fissionable nuclei ; the neutrons from these fissions induce fissions in still of the fissionable nuclei, and so on. Chain reaction
Multiplication factor (K) K=1 , critical (steady state ), chain reaction proceeds at constant rate . K > 1 ,supercritical , the number of fissions increase with time, therefore the energy released by the chain reaction increases with time. K<1 , subcritical, the number of fission decreases with time
Energy Released in Fission (E) n+ X Y1 + Y2 + 3n E = [ MX+Mn - (My1 + My2 + N Mn)]c2 Where MX is the mass of the fissile nucleus before fission, My1, My2 and Mn are Mass of fission fragments and fission neutron after fission. On average, some 200 MeV is released per thermal fission. This energy is distributed as shown
Nuclear reactor :is a device inside it the fission chain reaction can proceed in a controlled manner • The control is accomplished by varying the value of K, which can be done by the person operating the system • Neutron can disappear in two ways: 1- absorption in some type of nuclear reaction 2- by escaping from the surface of reactor (leakage) In case of critical state Neutron production rate = absorption rate + leakage rate
Moderation is a substance whose nuclei absorb energy from incident fast neutrons that collide with them without an excessive tendency to capture the neutrons - to accomplish the slow down of fission neutrons, the uranium in a reactor dispersed in a matrix of moderator - the energy transfer between collide objects is a maximum when the mass of these objects are equal
Type of moderators Three moderators in common use 1- light water (normal water) 2- heavy water (deuterium atom instead of ordinary hydrogen 3- graphite, a form of pure carbon. • Control of nuclear reaction rate • The rate of nuclear chain reaction can be controlled by the control rods. The rods made of a material such as cadmium or boron that absorbs slow neutrons. • As the control rods inserted further and further into the reactor , the reaction rate is progressively damped.
Type of nuclear reactor All nuclear reactors have the same principal components ,nuclear fuel, control rods, moderator and cooling system, but they differ in cooling system . 1- Pressurized- Water Reactor (PWR) use light water reactor as a coolant. Most of commercial reactor use light water both as a moderator and as a coolant. The water that circulates past the core is kept at a sufficiently high pressure, about 150 atom to prevent boiling The water enters the pressure vessel at a bout 280 co and leaves at about 320 c0, passing through a heat exchanger which produce steam that drives turbine. 2- Boiling – Water reactor (BWR) Low pressure, about 68 atom, inside the vessel allowed the steam to form and this steam is separated directly and sent to the turbine.
The BWR is simpler than PWR • Light water tend to capture thermal neutron and form deuterium 1H(n, g)2H. Therefore, natural uranium cannot be used as a nuclear fuel in light water reactor. Instead, enrichment uranium use • The fuel is in the form of UO2 sealed in long, thin zirconium-alloy tubes that are assembled together with movable control rods into a core that is enclosed a steel pressure vessel. • Heavy water Reactor (2H2O) is a better moderator than light water . Natural Uranium can be used as a nuclear fuel • High Temperature Gas -cooled reactor (HTGR). Graphite use her as a moderator. • Efficiency of HTGR (39%) is higher than PWR and BWR (32 to 33 %. • Graphite has a small cross section for neutron capture, so it is a good moderator.
Monju nuclear power plant Japan 3.08.2007
A breeder reactor Contains fissionable and non fissionable material one that can be made fissionable by absorbing a neutron. For example 235U and 238U suppose 3 neutrons per fission one is needed to induce A fission in another fuel atom and keep the chain reaction going. If the other two neutrons can be used to convert two non fisionable atoms (238U) into fissionable (239Pu), the two fuel atoms are produced where one is consumed . In the example , neutrons from 235U may be used to convert non fissionable 238U to fissionable 239Pu. 238U+ n 239U+ g239Np 239Pu A prerequisite to breeding is that, the number of neutrons produced per neutron absorbed in the fuel, should be larger than 2 (>2). In the example this achieved by the used of fast neutron and so no moderator is needed. b- b-
Example: Estimate the number of fission per second in a 100 MW reactor. Solution: Each fission of uranium releases about 200 MeV= 320x10-13 J. So the number of fissions per second in a 100-MW reactor is N=( 100x106)/(320x10-13) = 3x1018.
Fusion Reaction • -Two nuclei, below about A=56, combine to form a heavier nucleus • -The energy released in fusion per unit mass of material is comparable with that released in fission( ≈ 1MeV). • The advantages of Nuclear fusion • _ Light nuclei are more plentiful than fissile nuclei • Waste from fusion is less radioactive than waste from fission. Also the half-life of radionuclide from fusion is short and would decay away relatively rapidly. Therefore no need to store the waste for geological periods of time • -
- To fuse two nuclei wee need energy to overcome coulomb barrier. In fusion reactor, it is intended to generate this energy by heating the reactants. • When fusion is driven by heat energy , the process is called thermo nuclear fusion. It requires a very high temperature. However, thermonuclear fusion has been achieved in laboratory. • - when fusion driven by laser is called inerial confinement
Thermonuclear Reaction in the sun pp Chain P + P D + e+ + ue D + P 3He + g 3He + 3He 4He + 2P The resulting reaction being 4P+ 2D + 2P +23He 2D+ 2e+ + 2ue+23He + 4He + 2P Or 4P 4He + 2e+ + 2ue Q = [ 4 M (1H) – M (4He) ] c2= 26.9 MeV
The fusion reactions that appear most promising for a terrestrial fusion power reactor involve deuterium 2H1 and tritium3H2 Neutrons resulting from the reactions can be used to induce fission in a fission- fusion reaction or to take part in reactions like Li + n 4He2 + 3H2 (T) To release more energy.
Fusion by laser Fast ingnitor conception in inertial confinement
Nuclear Reactor Design Control rods Fuel rods
General types of proportional counter: • Gas flow proportional counter – with window ( alpha-beta) or windowless (tritium measurements) • Air proportional counter (Alpha counting only) • Sealed proportional counter (e.g. BF3, He-3 neutron detectors).
Filling gas The fill gas in the proportional counter (and a GM detector) is usually a noble gases because: • noble gases are not electronegative • don not react chemically with the detector components
Argon is the most widely used because of its low cost • Krypton and Xenon might be used if increased sensitivity to x- rays or gamma rays is required • Air ( alpha radiation ) • Hydrocarbon gases (e.g., methane, propane and ethylene) can also serve as a fill gas, but they have the disadvantage of being flammable • He-3 and BF3 are the most commonly employed gases in neutron detectors n + He-3 ے H-3 + p+ n + B-10 ے Li-7 + α+ • Tissue equivalent gas mixture such as : 64.4% methane, 32.4% carbon dioxide and 3.2% nitrogen might be used for certain application of dosimetry
In general, the proportional gas should not contain electronegative components such as (O2,H2O,CO2,CCl4,SF6)) electron heading towards the anode will combine with electronegative gas. Happens, a negative ion goes to the anode rather than an electron, and unlike the electron negative ion will fail to produce and avalanche
spurious pulses During the formation of an avalanche, some gas molecules/atoms are excited rather than ionized. When the electrons deexcite and return to their original energy levels, they emit photons of visible light or UV and the problem in these photons is: • can interact with the proportional gas and cause the avalanche to spread along the anode • can interact with the cathode wall
The solution is to add a small amount of a polyatomic quench gas such as methane. The quench gas preferentially absorbs the photons, but unlike the fill gas (e.g., argon), it does so without becoming ionized