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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

Discovery of Induced Nuclear Fission

* 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

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.

Meitner used barium ions, Ba2+, as a carrier and precipitated the radioactive products from the neutron bombardment.

Nuclear Fission

Discovery of Induced Nuclear Fission

Atomic weight of Ba2+ is 137

Neutron induced uranium nuclear fission reactions

Nuclear Fission

A simplified view of neutron induced fission:

n + 235U xxxEyy + uuuEww + 3 n

Nuclear Fission

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 + 4n

neutrons are releases

Nuclear Fission

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

If a 235U atom splits up into two nuclides with mass number 117 and 118, estimate the energy released in the process.

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

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

Nuclear Fission Energy – cont.

Assume the neutron induced fission reaction to be,

235U + n ® 142Cs55 + 90Rb35 + 4 n.

explain the results and estimate the energy released.

Solution – cont.

For the energy, consider the reaction and mass balance:

235U92142Nd60 + 90Zr40 + 3 n + Q235.04924 = 141.907719 + 89.904703 + 3x1.008665 + Q

Q = (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

Nuclear Fission

Estimate the energy released by the fission of 1.0 kg of 235U.

Solution

From 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).

1 mol235 g

6.023e231 mol

Discussion

This is a large amount of energy, and it is equivalent to the energy produced by burning tones of coal or oil.

Nuclear Fission

Energy (MeV) distribution in fission reactions

Kinetic energy of fission fragments

Prompt (< 10–6 s) gamma () ray energy

Kinetic energy of fission neutrons

Gamma () ray energy from fission products

Beta () decay energy of fission products

Energy as antineutrinos (ve)

167 MeV

8

8

7

7

7

Nuclear Fission

The Cyclotron and Fission Research

Particle acceleratorsmachines to speed up particles

Linear accelerators

Cyclotrons

Nuclear Fission

The Cyclotron and Fission Research

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

The Cyclotron and Fission Research

Threshold* Energy range (keV)

Reaction energy(keV) narrow-energy neutron

51V (p, n) 51Cr 2909 5.6-52

45Sc (p, n) 45Ti 1564 2.36-786

57Fe (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

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

Short notations238U (n, ) 239U ( , ) 239Np ( , ) 239Pu or238U (n, 2) 239Pu

Fast neutrons provided the reactions:

238U + n 239U + 239U 239Np + (t1/2 23.5 min) (t1/2 2.35 d) 239Np 239Pu +

Nuclear Fission

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

Uniting Political and Nuclear Power

F.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 seemto 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

Experiment: Neutron bombarded samples surrounded by water, wood, and paraffin are more radioactive - Fermi’s group discovered

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

Nuclear Fission

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

Thermal Neutrons Cross Sections

Uranium for Fission Fuel in Nuclear Reactor

113Cd 233U 235U 238U

c /b 19,820 46 98 2.7

f /b 530 580 2.7×10-6

t1/2/y 1.6×105 7×108 4.5×109

Nuclear Fission

Thermal Neutrons Cross Sections

Plutonium Isotopes

236Pu 237Pu 238Pu 239Pu 240Pu 241Pu 242Pu

f 150 2100 17 742 0.08 1010 0.2

t1/2 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

Nuclear Fission

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

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

Fission-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

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.

Nuclear Fission

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