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Nuclear Fission 1
Nuclear Fission energy for war and peace
Nuclear fission is a process, by which a heavy nuclide splits into two or more pieces
Nuclear fission reactions release a lot of energy.
Nuclear energy has been used for peace and for war.
Nuclear Fission 2
Discovery of Induced Nuclear Fission
Uranium
Neutrons
-radioactivity
??
?
* O. Hahn, L. Meitner, and F. Strassmann in Berlin * F. Joliot and I. Curie in Paris* Enrico Fermi in Rome
All three groups thought the reactions to be
238U (n, ) 239U92 (, ) 239E93 (, ) 239E94
Nuclear Fission 3
Discovery of Induced Nuclear Fission
Hahn (chemist), Meitner (physicist), and F. Strassmann (analytical chemist) used H2S to precipitate the radioactive products. The half-life measurements indicated to them that not one but
many elements were produced.
Uranium
Neutrons
-radioactivity
??
?
Meitner used barium ions, Ba2+, as a carrier and precipitated the radioactive products from the neutron bombardment.
Nuclear Fission 4
Discovery of Induced Nuclear Fission
Atomic weight of
Ba2+ is 137The Liquid Drop Model and Fission
Neutron induced uranium nuclear fission reactions
Nuclear Fission 5
Induced Nuclear Fission
The Liquid Drop Model and Fission
A simplified view of neutron induced fission:
n + 235U xxxEyy + uuuEww + 3 n
Nuclear Fission 6
Discovery of Induced Nuclear Fission
The Official History of the Manhattan Project:
Dr. Meitner brought the discovery of neutron induced fission to Copenhagen as she, a non-Aryan, exiled from Germany in 1938. She told Frisch, who told N. Bohr and Bohr told Fermi
Fermi fond out only 235U underwent fission, for example:
235U + n 142Cs55 + 90Rb37 + 4nneutrons are releases
Nuclear Fission 7
Nuclear Fission Energy
Variation of ME with Afor Some Stable Nuclides
ME amu
0.01
0.005
0.0
–0.005
A
H
3He
4He12C
FePb
U
n
Fission Energy
Nuclear Fission 8
Problem:
If a 235U atom splits up into two nuclides with mass number 117 and 118, estimate the energy released in the process.
Nuclear Fission Energy
Nuclear Fission 9
If a 235U atom splits up into two nuclides with mass number 117 and 118, estimate the energy released in the process.
Nuclear Fission Energy
From handbooks Stable nuclides with mass numbers 117 and 118 are 117Sn50, and 118Sn50 and masses are given below the symbols
235U 117Sn50 + 118Sn50
235.043924 = 116.902956 + 117.901609 + Qfe
Qfe = 0.2394 amu (931.5 MeV) / (1 amu)
= 223 MeV.
Discussion: The fission reaction equation is over simplified. Usually, neutrons are released too.
Nuclear Fission 10
Nuclear Fission Energy
Assume the neutron induced fission reaction to be, 235U + n 142Cs55 + 90Rb35 + 4 n. explain the results and estimate the energy released.
Solution:The neutron-rich fission products are beta emitters: 142Cs 142Ba + (~1 min) 90Rb 90Sr + (half-life, 15.4 min)142Ba 142La + (11 min) 90Sr 90Y + (27.7 y)142La 142Ce + (58 min) 90Y 90Zr (stable) + (64 h)142Ce 142Pr + (51015 y)142Pr 142Nd (stable) + (19 h)
Nuclear Fission 11
Solution – cont.For the energy, consider the reaction and mass balance:235U92 142Nd60 + 90Zr40 + 3 n + Q235.04924 = 141.907719 + 89.904703 + 3x1.008665 + QQ = (235.043924 - 141.907719 - 89.904703 - 3x1.008665)
= 0.205503 amu (931.4812 MeV/1 amu)
= 191.4 MeV per fission(1.6022e-13 J / 1 MeV)
= 3.15e-11 J
Assume the neutron induced fission reaction to be, 235U + n ® 142Cs55 + 90Rb35 + 4 n. explain the results and estimate the energy released.
Nuclear Fission Energy – cont.
Nuclear Fission 12
Nuclear Fission Energy
Estimate the energy released by the fission of 1.0 kg of 235U.
SolutionFrom the results of the previous two examples, energy released by 1.0 kg uranium-235 is estimated below:
(3.15e-11 J) 1000 g
= 8.06e13 J (per kg).
DiscussionThis is a large amount of energy, and it is equivalent to the energy produced by burning tones of coal or oil.
1 mol235 g
6.023e231 mol
Nuclear Fission 13
Nuclear Fission Energy
Kinetic energy of fission fragmentsPrompt (< 10–6 s) gamma () ray energy
Kinetic energy of fission neutronsGamma () ray energy from fission products
Beta () decay energy of fission productsEnergy as antineutrinos (ve)
167 MeV88777
Energy (MeV) distribution in fission reactions
Nuclear Fission 14
The Cyclotron and Fission Research
Particle accelerators machines to speed up particles
Linear accelerators
Cyclotrons
A Sketch of the Cyclotron
Ions, originated from the center of the cyclotron,accelerated by alternate voltages between the
Dee’s follow a spiral path acquiring high energyand exit from a window.
Highvoltage
Experimentstation
A Dee
Nuclear Fission 15
The Cyclotron and Fission ResearchA Sketch of the Cyclotron
Ions, originated from the center of the cyclotron,accelerated by alternate voltages between the
Dee’s follow a spiral path acquiring high energyand exit from a window.
Highvoltage
Experimentstation
A Dee
7Li (p, n) 7Be3T (p, n) 3He1H (t, n) 3He2D (d, n) 3He2D (t, n) 4He3T (d, n) 4He
Fusion reactions studied using the cyclotron
Nuclear Fission 16
The Cyclotron and Fission Research
Threshold* Energy range (keV)Reaction energy(keV) narrow-energy neutron
51V (p, n) 51Cr2909 5.6-5245Sc (p, n) 45Ti1564 2.36-78657Fe (p, n) 57Co 1648 2-1425__________________________________* The threshold energy is the minimum energy of proton required for the reaction.
Neutrons of desirable energy is required for fission research.
Nuclear Fission 17
The Cyclotron and Fission Research
For neutron sources from the cyclotron, energy can be varied.
Energy dependence of neutron induced fission studied.
The cross section data enabled nuclear reactor design.
fast neutrons - 10 MeV to 10 KeV)
slow neutrons - 0.03 to 0.001 eV for neutron induced fission
Nuclear Fission 18
The Synthesis of Plutonium
Fast neutrons provided the reactions:
238U + n 239U + 239U 239Np + (t1/2 23.5 min) (t1/2 2.35 d) 239Np 239Pu +
Pluto
Neptune
Uranus
Short notations238U (n, ) 239U ( , ) 239Np ( , ) 239Pu or238U (n, 2) 239Pu
Nuclear Fission 20
Uniting Political and Nuclear Power
Neutron induced fission reactions release energy and neutrons, thus it is possible to start a chain reaction for nuclear power.
Dictator Hitler (political power in 1933) made many scientists in Austria, Hungary, Italy and Germany uncomfortable and they came to the U.S.A.
Hitler invaded Poland, Hungary, Slovak and other European countries. Nuclear fission was discovered in Germany, and nuclear power threatens the world.
Leo Szilard, Eugene Wigner, and Edward Teller drafted a letter and Einstein signed the letter for president Roosevelt of U.S. to use political power for nuclear power.
Nuclear Fission 21
Uniting Political and Nuclear PowerF.D. Roosevelt (Einstein’s address omitted). . . . . . . (address omitted)Sir:Some recent work by E.Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the partof the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations: . . . . (middle part omitted) . . . . I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.
Yours very truly,
Nuclear Fission 22
Thermal Neutrons
Conclusion:Slow neutrons (0.03 to 0.001 eV) are more effective for inducing fission of 235U
Fast neutrons (10 MeV to 10 KeV) favours neutron capture reaction of 238U
Light atoms are effective moderators
Experiment: Neutron bombarded samples surrounded by water, wood, and paraffin are more radioactive - Fermi’s group discovered
Nuclear Fission 23
Thermal Neutrons - ModeratorsA Maxwellian Distribution of Kinetic
Energy (Ek) of Molecules in a Material.
Ek
No. of molecules withkinetic energy Ek
298 K 373 K
Light atoms are effective moderators
Nuclear Fission 24
Thermal Neutrons Cross Sections
Cross section () a measure of reaction probabilityThermal neutron cross sections (c)Thermal neutron cross section for fission (f)
1H 2H 12C 14N 16O 113Cd c /b 0.33 0.00052 0.0034 1.82 0.0002 19,820
Moderators: H2O vs. D2O vs. C
Fermi’s avoided N2 in his first nuclear reactor and used Cd for emergency
Nuclear Fission 25
Thermal Neutrons Cross Sections
Uranium for Fission Fuel in Nuclear Reactor
113Cd 233U 235U 238U c /b 19,820 46 98 2.7f /b 530 580 2.7×10-6
t1/2/y 1.6×105 7×108 4.5×109
Nuclear Fission 26
Plutonium Isotopes
236Pu 237Pu 238Pu 239Pu 240Pu 241Pu 242Puf 150 2100 17 742 0.08 1010 0.2t1/2 2.9y 45 d 88 y 24131y 6570 y 14y 3.8×105y
Neutrons Capture Cross Sections of Cadmium Isotopes
106Cd 108Cd 110Cd 111Cd 112Cd 113Cd 114Cd c / b 1 1 0.1 24 2.2 19,820 0.3
Abundance/% 1.25 0.89 12.45 12.80 24.13 12.22 28.37
Thermal Neutrons Cross Sections
Nuclear Fission 27
Fission Productsnuclides produced in nuclear fission
Data on fission products are required for reactor design, operation, and accident responses.
The study of fission products requires the separation, identification, and quantitative determination of various elements and isotopes.
Fission products emit particles until they are stable.
Mass number range: 40 - 170
Elements range: all the elements in the 4th, 5th, and 6th periods. including the lanthanides.
Nuclear Fission 28
Fission Products
Fission yield is the relative amounts of nuclides formed in fission reactions. The fission yield curve shown here shows most fission reactions split fission atoms into two unequal fragments.
Nuclear Fission 29
Nuclear Fission ProductsFission-product and their decay data are needed for social and environmental concerns, and for the management of used fuel.
Fission nuclides usually have very short half lives.
Typical medium-life fission products: 85K 10.7 y, 90Sr 29 y, 137Cs 30 y,Typical long-life fission products: 126Sn 1.0e5 y, 126Tc 2.1e5 y, 91Tc 1.9e6 y, 135Cs 3.0e6 y, 107Pd 6.5e6 y, and 129Tc 1.6e7 y.
Xenon poisoning: 115Xe, c = 2,640,000 b, and t1/2 = 9.2 h
Nuclear Fission 30
First Fission Nuclear Reactor
Fermi’s group assembled natural uranium into an atomic pile to test the feasibility of a sustained chain fission reaction.
Key elements: fuel, neutron moderator, control rod, neutron detector, and radioactivity detector
Dec. 2, 1942, Fermi achieved sustained chain reaction, and the first fission reactor provided data for future design of nuclear reactors.
Today, more than 400 power nuclear reactors provided energy world wide, more than 100 of them in the US.