Week 1 Day 3 Nuclear Reactions

8/3/2019 Week 1 Day 3 Nuclear Reactions

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At the end of the 19th century, many scientists did

not realize they were on the edge of a revolution

in physics

The most importantfundamental laws and factsof physical science have allbeen discovered, and theseare now so firmly

established that thepossibility of their ever beingsupplanted in consequenceof new discoveries isexceedingly remote Our

future discoveries must belooked for in the sixth placeof the decimals.

-- Albert Michelson, 1894


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http://en.wikipedia.org/wiki/File:Einstein1921_by_F_Schmutzer_4.jpg


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ION SOURCE

ANALYSER

DETECTOR

MASS SPECTROMETER

19 20 21 22

23

20Ne 90.92%

21Ne 0.26%

22Ne 8.82%


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Discovery of the neutron

James Chadwick 1932


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Notation


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H H H

http://education.jlab.org/glossary/isotope.html
http://education.jlab.org/glossary/isotope.htmlhttp://education.jlab.org/glossary/isotope.html


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Chart of nuclides with half-lives (note those with t over 1 hour)


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XA

Z YA - 4

Z - 2 + He4

2

XA

ZY

A

Z + 1+ b

0

-1

XA

ZY

A

Z - 1+ b

0

+1

Alpha-decay

(alpha-

particle)

(electron)

(positron)

Beta-decay


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Rn22286

He42

+Po21884

He42

Th234

90

Pa234

91

+ b0

-1

Eaxmples


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X

A

Z X

A+1

Zn +

Neutron capture


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The neutron to proton ratio is

critical in determining the stabilityof a nucleus.

Elements above the belt of

stability undergo b-decay and

elements below the belt of stability

undergo positron emission or

electron capture.


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Aston (1922) If hydrogen is transformed into helium a

certain quantity of mass must be annihilated in the process.

The cosmical importance of this conclusion is profound and

the possibilities it opens for the future very remarkable,

greater in fact than any suggested before by science in the

whole history of the human race.

We know from Einsteins Theory of Relativity that mass andenergy are interchangeable and that in c.g.s. units a mass m at

rest may be expressed as a quantity of energy mc, where c is

the velocity of light.Even in the case of the smallest mass this

energy is enormous.


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proton: 1.007276 amu

neutron: 1.008665 amu

electron : 0.000549 amu

Masses of atomic constituents as free particles

unit 1 amu = 1/12 (mass of12C)

= (approx.) 1.660538782 1024 g


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He4

2

2 protons: (2 x 1.007276 amu) = 2.014552 amu

2 neutrons: (2 x 1.008665 amu) = 2.017330 amu

2 electrons: (2 x 0.000549 amu) = 0.001097 amu

Total combined mass: 4.032979 amu

The atomic mass of He atom is 4.002602 amu.This is 0.030366 amu less than the combined mass.

This difference between the mass of an atom andthe sum of the masses of its protons, neutrons,

and electrons is called the mass defect.

= 4.002602 amu

Strange arithmetic of atomic masses -> mass defect


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What causes the loss in mass?

According to Einsteins equation E = mc2

Convert mass defect to energy units

The energy equivalent can be calculated

E = m c2

E = (5.0441 x 10-29kg) (3.00 x 108m/s)2

= (4.54 x 10-12kg m2/s2) = 4.54 x 10-12 Jper He nucleus

This is the NUCLEAR BINDING ENERGY, the energy released

when a nucleus is formed from nucleons.


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Semi-Empirical Binding Energy

Surface Area

Electrostatic

Symmetry

PairingVolume

oddodd

oddeven

eveneven

0Paring:


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Binding energy per nucleon

http://hyperphysics.phy-astr.gsu.edu/

630 726 P d i t SZILARD L J 28 1934 N 19157 d 19721 [Cl 39


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630,726. Producing neutrons. SZILARD, L. June 28, 1934, Nos. 19157 and 19721. [Class 39

(i)] A neutron chain reaction generates power and produces radio-active isotopes. Thereaction takes place in a mass 3, Fig. 1, comprising indium and beryllium, bromine or uranium.

Fast deuterons from a canalray tube 1 bombard a deuterium target 28 to produce initiating

neutrons which react with In to produce In and " tetra neutrons " of mass about

4.014. These tetra neutrons react with the Be, Br or U to produce double the number of simpleneutrons, thereby providing a chain reaction. Emerging neutrons transmute a layer 9 to produce

radio-active substances. Alternatively, Fig. 3, the initiating neutrons may be produced by passing

cathode-rays through a sheet 402 of Pb or U to generate hard X-rays which react with beryllium

in the mass 3 (or an inner mass 407) to yield neutrons. The critical thickness of the layer 3 for a

self-sustaining chain reaction is stated to be of the order of 50 cms. Tetra neutrons are stated to be

produced when neutrons of 100,000 e.v. to 8 m.e.v. energy react with the In. Power isobtained by heat exchange from water or mercury passing through cooling tubes 107, 110, 111.


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Fission discovered as a result of

a wrong hypothesis

Enrico Fermi was bombarding large nuclei

with neutrons and observing their reaction

products. Typical ReactionNucleus gains a proton:

N ZX + n N+1

ZX* N+1

Z+1Y + e-

Chemical Differences expected by increasingthe atomic number.


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Enrico Fermi

Bombarded almost every

element in the Periodic

Table with neutrons. Thought he had obtained

transuranic elements

through bombardment of

U, missed possibility offission.


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Uranium was the largest stable nucleus

Fermi thought that he had created a transuranicelement (element beyond uranium)

But chemical analysis of the products confusing ! Work repeated in Berlin by

Lise Meitner

Fritz Strassmann

Otto Hahn


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Big differences in behavior of uranium isotopes

U-235 and U-238

Natural uranium = 99.3 % U-238 + 0.7% U-235

U-235 undergoes fission very easily for slow

neutrons

U-238 undergoes fission (essentially) only for fastneutrons, otherwise captures neutrons, making a

chain reaction in natural uranium difficult to

achieve.
http://en.wikipedia.org/wiki/File:U235_Fission_cross_section.png


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http://en.wikipedia.org/wiki/File:U235_Fission_cross_section.png


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How a fission reactorworks. Physics in Action

36Nat. U

f) b (i) b (r) b238U 0.6 2.9 ~0

235U 1.3 2.3 ~0

NatU 0.61 2.9 ~0

Fast neutrons (2MeV)fission scatt. capture

Thermal neutrons (kT ~0.02 eV)fission scatt. capture

f) b (i) b (r) b238U 0 0 2.72

235U 579 0 101

NatU 4.13 0 3.42

capture

Strategy:Use a moderator to slow down fission

neutrons to thermal energies -> more fission by U-235


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Neutron-induced chain reaction


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How a fission reactor works. Physicsin Action Lecture 39

LOSSES FROM CHAIN

REACTION

Fast neutrons from fission

Some fission in 235U + some

capture in 235/238U

Some fast fission in 238U

n-flux increases by factor eSlowing down in fuel and

moderator

Fraction not leaking out PNLf

Leakage of fast neutrons from coreSlowed from MeV to eV

Fraction not captured =p

Radiative capture in 238U resonances Thermal neutrons

Fraction not leaking out PNLth

Leakage of thermal neutrons from coreThermal neutrons absorbed in

core

Fraction absorbed in fuel =f

Thermal neutrons captured in

non-fuel material (e.g.

moderator)

Number of neutrons emitted per

neutron absorbed in fuel = h

For stable chain

reaction, k=

PNLFPNLthepfh = 1

Neutron induced chain reaction


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http://www.cfo.doe.gov/me70/manhattan/images/CP1DrawingLarge.gif


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Glenn Seaborg

Discovered

plutonium at U.C.

Berkeley, Feb. 23,1941.

239Pu also undergoes

fission and is madefrom 238U.


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Plutonium Production

23892U +

10n

23992U

23992U

23993Np +

0-1 t1/2= 23.5 min

239

93Np 239

94Pu + 0

-1 t

1/2= 2.35 days


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Hanford

Secret City on the Columbia River in

Washington State. A series of nuclear reactors designed to

produce plutonium.

A chemical plant to purify plutonium.


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Interior of 221 B plant showing


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Interior of 221-B plant showing

workers on the canyon deck


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221-T Chemicalseparation plant


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http://www.nrc.gov/NRC/STUDENTS/pwr.gif

Week 1 Day 3 Nuclear Reactions

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