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Nuclear Physics

Phys 1020, Lecture 25:

Nuclear fusionNuclear radiation and you

FCQ – Finley.

Reminders:

Check your scores are correct on D2LReview session schedule on website

Grading schedule on website.

Final – Wed May 4th 1.30-4pm, G125Review lecture Thursday

• Structure of an atom and nucleus

• Nuclear forces and stored energy

• Nuclear fission- Alpha decay (spontaneous)

- Fission bomb (neutron induced fission)

• Nuclear Fusion

• Radioactivity and ionizing radiation

- Alpha, beta and gamma radiation- Why its bad for you

• Other interesting stuff that we won’t have time for- Nuclear power

- Nuclear medicine

Nuclear Physics

Sadly, (or perhaps much to your relief), demos will be limited……

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If don’t get swallowed or lost

the 3 free neutrons can goinduce more fission.

For a bomb we must have a CHAIN REACTION:

(Must release energy from many many fission reactions all at once)

- Neutron source to start things off

- Chunk of neutron fissionable material (235U , Pu)

- Each fission must produce more than one neutron that can fissionother nuclei

- Neutrons must be used efficiently to trigger more fission (not

escape through surface or be absorbed)- Small surface to volume ratio (sphere)

- Super-critical mass

- Pure fissionable material

Details of a fission bomb

Ur 235

Energy released during fusion reaction

Initial

nuclei

Product

nucleus

Energy

released

per

nucleon

Energy released because product nucleus are MORE tightly bound

(in bigger potential hole)

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Deuterium on

Tritium

Fusion bomb or “hydrogen bomb”

- Same process that powers the sun

- Stick small nuclei together to release energy

Which will release more energy during fusion?

a. Deuterium combining with deuteriumb. Deuterium combining with tritium

Deuterium on

Deuterium

PE

r r

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Stick hydrogen isotopes

together to make helium.

• Must supply energy to force H nuclei together – use a fission bomb.

• More energy per unit mass than fission. • No limit to size of bomb (unlike fission)

• End up with GIGANTIC bombs

- 1000 times bigger than first fission bombs

Fusion bomb

+ =2H 3H 4He

+

1n

+ ENERGY

push together

energy required

energy released if

push over the hump

activation energy of

100 million degrees

PE

r

2

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hydrogen isotopes:

tritium,deuterium

Pu

‘trigger’

chemical explosive

chemical explosive

Fusion bomb

3. Pu fission bomb

explodes

2. Chemical explosion

assembles super-critical mass of Pu

4. 3H, 2H forced

together and form 4He

1. Pu triggers shaped and

assembled at Rocky Flats

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Energy comparisons:

1 fission of Uranium 235 releases:

~10-11 Joules of energy

1 fusion event of 2 hydrogen atoms:

~10-12 Joules of energy

Burning 1 molecule of TNT releases:

~10-18 Joules of energy

1 green photon:

~10 -19 Joules of energy

Dropping 1 quart of water 4 inches ~ 1J of energy

Useful exercise… compare energy available in this volume of TNT, H2, and U235

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In the first plutonium bomb a 6.1 kg sphere of plutonium was used and the explosion produced the energy

equivalent of 22 ktons of TNT = 8.8 x 1013 J.

As the textbook says, 17% of the plutonium atomsunderwent fission.

How long would this power your house?

(assume you use 10 ×100W lightbulbs = 1000W)

More energy comparisons

a) 2days

b) 3 months

c) 6 years

d) 700 years

e) 3000 years

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US Nuclear weapons

US sizes = 170kTon-310kTon

Russian as large as 100MTon

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Immediate:

• Hideous amounts of energy -heat

-high energy particles (electrons, neutrons, x-rays, gamma-rays)

• Heat up air and blow everything in vicinity down• Exposure to massive amounts of ionizing radiation (photons and high

energy particles) causes death

Delayed:

• Radioactive daughter nuclei produced and dispersed.• These decay, producing ionizing radiation over a wide range of ½ lives

Destruction caused by a nuclear explosion

Ionizing radiation…

…the invisible ‘bad stuff’ released from nuclear reactions…

….but what exactly is it?

Ionizing radiation

In the early part of the 20th century, scientists observed many strange phenomena.

They did not know what these things really were (e.g. particles, light, other…) so

gave them random names.

X-rays

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X-ray’s to see how well the bones in your foot fit

into that new shoe.• Natural radioactive decay from an unstable nucleus:

• Discovered by Bequerel in 1896

• Uranium fogged a photographic plate despite being wrapped in black paper.

• Curies discovered other elements that did the same thing (radium and

polonium)

• But what was the ‘radiation’ that uranium was emitting made of?

• Curies recognized

• different types of radiation were being emitted

• that the radiation had some properties in common with X rays.

• Rutherford called these 3 new types of radiation alpha, beta, and gamma.

Alpha, beta and gamma radiation

Marie Curie discovered “radium” (a new element at that time, and it

was naturally radioactive or unstable).• All these strange phenomena are now recognized as ionizing radiation.

• Radiation: Energy that radiates from a source and travels through space.

• Ionizing radiation: Particle or photon of radiation must have enough

energy to break a chemical bond

Ionizing radiation

• Types of ionizing radiation:

- Helium nuclei (alpha particles)- Electrons (beta particles)

- Photons (Higher energy UV, X rays, Gamma rays)

- Neutrons

• All ionizing radiation damages cells, including DNA molecules, causing

rapid death (high dose) or potentially cancer (lower dose)- Some types more damaging than others

- Some types easier to stop than others

• Radioactive nuclei/atoms are chemically identical to stable isotopes.

Can be absorbed by body/plants and then decay, causing damage.

Less damaging

Harder to stop

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1. Alpha particles: high – speed helium nuclei

- Most of radiation is this type

- Easy to stop – clothes, paper, skin, couple of cm of air- Slows down, grabs electrons and become a He atom

- Only (but very) dangerous if ingested or inhaled (Litvinenko)

- Most common alpha emitter is radon- Dangerous because it’s a gas

- Gets into lungs, decays, a damages DNA of sensitive lung cells.

+ +

as don’t get far - because they hit things

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2. Beta particles: High speed electrons

• Emitted when a neutron in a nucleus turns into a proton• Smaller and lighter than alpha particles

• Less damaging but more penetrating

• Stopped by a few mm of aluminum

3. Gamma rays: High energy photons

• Emitted when a nucleus drops to a lower energy state• No mass or charge – more penetrating, less damaging

• Depending on energy, requires cm or m of heavy material

(lead, packed soil, concrete) to stop rays. Travels long way in air• Most pass straight through body, but lots of damage if absorbed

• Can cure as well as cause cancer………

.

En

erg

y

4

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4. Neutrons

• Because they lack any charge, exceptionally penetrating

• But because they have mass, very damaging….nasty combo….

• Neutron activation can make other materials radioactive

• Shielding from neutrons requires many ft of H containing materials (water, also

concrete.)

• Damage to tissue depends on energy but always more damaging per J than β or γ.

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An odd world…You find yourself in some diabolical plot where you are given an alpha (a)

source, beta (b) source, and gamma (g) source. You must eat one, put one in

your pocket and hold one in your hand. You …

a) a hand, b pocket, g eat

b) b hand, g pocket, a eat

c) g hand, a pocket, b eat

d) b hand, a pocket, g eat

e) a hand, g pocket, b eat

Have you ever been exposed to ionizing

radiation from nuclear processes?

A) Not as far as I'm aware

B) Only medical X-rays

C) Yes, once in a while from multiple sources

D) Yes, a number of times from multiple sources

E) I'm exposed all the time, continuously.

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Millirem, is a measure of damage done to you by radiation, so it takes into account

- Amount of energy absorbed by your body

- How damaging the specific type of radiation is to biological tissue.

- Sievert is the SI unit measuring the same thing. 1 mSv = 100 mrem

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Results of radiationdose in rem = dose in rad x RBE factor (relative biological effectiveness)

RBE = 1 for g , 1.6 for b, and 20 for a.A rad is the amount of radiation which deposits 0.01 J of energy into 1 kg of absorbing material.

+ primarily due to atmospheric testing of nuclear weapons by US and USSR in the 50’s and early

60’s, prior to the nuclear test-ban treaty which forbid above-ground testing.

Nuclear power generation

A hugely important issue for this and future generations

• Current status

• Safety facts and fears

• Pros and cons

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United States of America

104 reactors with 99,000 MegaWatts

20% of electricity

In 1990, the Public

Service Company of

Colorado initiated

steps to

decommission the Fort

St. Vrain high

temperature gas

cooled reactor and to

convert the site to a

peaking plant, fired by

natural gas.Test reactor operated by the US Geological survey in Denver

Test reactors are used in almost every field of science: physics,

chemistry, biology, medicine, geology, archeology, and

environmental sciences

USA has 104 reactors with 99,000 MegaWatts

France alone has 59 reactors with 63,000 MegaWatts, 77% of

their electricity generation !

World has 443 reactors, 367,000 MegaWatts capacity.

USA electricity from nuclear ~ 20%

France electricity from

nuclear ~ 77%

Germany and Japan electricity from

nuclear ~ 25%

South Korea electricity from nuclear

~ 40%

The fraction of US electricity coming from nuclear plants over

the past 10 years has been:

A) steadily increasing

B) stayed about the same

C) steadily decreasing

D) not sure/other

How many people were killed in the

Three Mile Island nuclear accident in

1979.

A) 0

B) 10

C) 100

D) 1,000

E) 10,000

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Fukushima accident

In 2011 a tsunami hit the coast of Japan and damaged the

cooling systems at the Fukushima nuclear power plant.

Over 19,000 people died that day.

What percentage of those deaths was related to the accident at

the nuclear power plant?

a. 100%

b. 75%

c. 50%

d. 5%

e. 0%

Advantages:

• No CO2 emissions Small impact on global warming

• No SO2, CO, NOx, particulates, etc.

• Basically (air) pollution free.

•Less radiation leakage in normal operation than coal burning.

•Safety record in US better than fossil fuels (despite 3 Mile Island).

• Uranium relatively abundant, relatively inexpensive.

• Quantity needed to mine is relatively small (compared to coal).

Recall “R/P” = resource amount / current production rate

~ “simple time to end”

It is about R/P ~ 150 years for Uranium fuel.

“Breeder” reactor technology would extend that to 1000’s of years.

How long will our (US) Uranium reserves last if we ramp up nuclear

energy to cover all our electrical needs, using present reactor

technology?

A) 30 years

B) 300 years

C) 1000 years

D) 5000 years

E) Even longer than that.

Disadvantages:

• Public fears

• “New” technology, hard to

understand, unknown risks

• Catastrophic failure can have severe consequences

• Radioactive waste disposal (how and where?)

• Nuclear weapon proliferation (fuel and waste can be reprocessed

for bombs).

• Reactor construction costs more than coal burning plant (in the

USA currently). Decommissioning is expensive.