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 19

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.