Nuclear Energy and radiation Proton ( -mass = 1.66 x 10 ... · PDF fileNuclear Energy and...

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Nuclear Energy and radiation

Phys 1010, Day 14:

Questions?

Nuclear Weapons 16.1

Nuclear Power 16.2

"I am become Death, the destroyer of worlds.“

–J. R. Oppenheimer (quoting Bhagavad Gita)

Reminders:

HW Today

Review next Tues

Exam Next Thursday 7:30 PM

hydrogen

1 p deuterium

1 p, 1n helium

2 p, 2 n Uranium 238

92 p, 146 n

Proton (positive charge) – charge = 1.6 x 10-19 Coulombs mass = 1.66 x 10-27 kg.

Neutron (no charge) – no charge

mass = 1.66 x 10-27 kg.

Electron (negative charge) – charge = -1.6 x 10-19 Coulombs mass = 9.10 x 10-31 kg.

Atom ingredients:

Each element has different number of protons.

Oxygen has 8 protons, 8 neutrons:

Consider a nuclei with 7 protons, 7 neutrons (nitrogen atom)… what if we want to

add another proton to make oxygen (8 protons):

What will we need to do to get proton stuck to nucleus:

a. just give it a little push so it will hit nucleus dead on and it will drift

towards nucleus and stick.

b. the closer it gets, the harder you have to push, will take lots of work

c. you’ll need to push really hard at first and then less as you get closer

d. you’ll have to push the proton towards the nucleus with a fixed amount

of force (constant force).

Oxygen has 8 protons, 8 neutrons

Consider a nuclei with 7 protons, 7 neutrons… what if we want to add another

proton to make oxygen:

What will we need to do to get proton stuck to nucleus:

b. the closer it gets, the harder you have to push, will take lots of work ... Lots

of energy

Force on proton given by Coulomb’s law (k = Coulomb’s constant):

In general: Force = k x charge of object #1 x charge of object #2 (distance between objects)2

F = k x charge of nucleus in coulombs x charge of proton in coulombs (distancenucleus-proton)2

The smaller the distance the larger the force!

Like charges repel: 7 positives One positive

Big repelling force

Bigger repelling force

5

What if threw proton so starts out going towards nitrogen nucleus with a

lot of speed (lots of kinetic energy)? Starts with lots of kinetic energy Repelling force from nucleus slows down proton Proton’s kinetic energy converted into electrostatic potential energy, as it gets closer to nucleus

r, separation distance

separation distance, r

Potential energy

Potential energy curves-

represent energy to bring

particles together.

Kinetic energy

if at center, want to roll down hill/fly apart … lots of electrostatic potential energy

Gravity energy analogy.

Potential energy curves- represent energy to bring particles

together. Gravity energy analogy. r, separation distance

separation

distance, r

Potential energy

Kinetic energy

Potential energy = k x charge of 7 proton nucl. x charge of single proton

at separation (separation distance)

distance of r

Charge of 1 proton = 1.6 x 10-19 Coulombs; Charge of 7 protons=11.2 x 10-19 C

So at 10-15 m away (~ radius of nucleus),

Potential energy = 8.99 x 109 N m2/C2 x 11.2 x 10-19 C x 1.6 x 10-19 C

(10-15 m)

= 1.61 x 10-12 Joules = 10 million electron Volts. (1 electron volt = energy gained 1 chemical reacction. So this is 10 millions times

bigger/atom)

2

7

Potential energy curve (Energy vs. separation distance)

for bringing electron in to proton

a. This curve would be flat, not going up or down

b. look like bringing a proton into a proton except upside down so going

down instead of up.

c. would look same as proton on proton

r

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potential energy curve for bringing electron in to proton

b. look like proton on proton except upside down so going

down.

Electron and proton get pulled together,

curve tells us that must add energy to separate electron and proton.

electron wants to roll into center as close as possible.

r

Lots of EPE

when separated

What about a neutron? No electrostatic repulsion!

How can a bunch of repulsive positively charged protons stick together?

Like hopper toy- really strong nuclear force between protons

and neutrons overwhelms electrical force if really close together.

Electrical force is like gravity, pushes over distance.

like nuclear force, only strong when very close (protons and

neutrons only feel this if they can hold hands).

like double sticky tape… only works if in close contact.

Neutrons help with this force (more neutrons further up

the table

suction cup or spring-

Nuclear force- Why like this?

That is the way nature is!

Stabilizing effect of neutrons in the nucleus …

why do you need them?

Protons

Repel from electrostatic force (all distances)

Protons also attract some from Strong force

(very short distances <nuclear radius)

Quickly the Strong force can’t win over the

size of the nucleus

Like putting a bunch of people who find each

other repulsive in same room. They can only

keep from falling apart if they can clasp

hands… The crowd quickly becomes an

UNSTABLE SITUATION

Stabilizing effect of neutrons in the nucleus …

why do you need them?

Add neutrons

They don’t repel anyone.

They still have the Strong force

The bind protons and neutrons

They let the protons stay a bit further apart

without adding + charge

Like putting a bunch of neutral people

between that can hold hands but don’t

cause anyone to run away… MORE

STABLE SITUATION…

(more or less)


electrostatic

repulsion

attractive

nuclear


combination--real nucleus

Energy scale gigantic compared

to chemical energy.

Why? Simple coulomb’s law.

F= k (charge of #1)(charge of #2)

r2

Chemistry- forces between

electrons and protons on distance

scale of atomic size (> 10-10 m).

Nuclear forces- forces between

protons on distance scale

10-100,000 times smaller.

10,000 times closer means forces

have to be 100,000,000 times

bigger to hold nucleus together

(have to beat 1/r2). Lots more

potential energy stored!!!

Potential energy curve for proton approaching nucleus

3

helium

Gets deeper until iron

(26 P, 30 N) less deep if

bigger.

Really big nucleus, >100 P, >100 N,

like Uranium or plutonium

Atomic nuclei

beryllium

harder to push together,

but bigger drop when do.

Really stable:

Have to add a whole bunch of

energy to break up!

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2P-2N

If alpha particle, finds

itself outside, lots of PE,

zooms away PEKE

Nuclear decay- one kind of nucleus changes into another. alpha decay, (beta decay), induced fission

A. alpha decay- alpha particle = 2p and 2n. They escape together.

Most radioactivity is this type (e.g. radon).

“tunnel” out.

Not real tunnel it doesn’t lose

energy.

Quantum physics says

Tiny objects are spread out

Very small fraction of time

appear outside-

when happens-- run for it!!

2protons-2neutrons stuck

together.

(chained together prisoners

escaping!)

nucleus1 3

tunneling difficulty = width x depth of tunnel

hard- takes long time,

billions of years!

2

medium easy!,

happens in

millionths of

a second! How much energy released?

a. 1 most, 2 second, 3 least

b. 2 most, 1, 3 least

c. 3 most, 2, 1 least

d. 3 most, 1, 2 least

energy is difference from

bottom of crater to outside.

3 is most, 2 second, 1 is least

E

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Neutron Induced fission- key to atomic bombs

•two smaller nuclei

•few extra free neutrons

•LOTS OF ENERGY!!

•(+sometimes other bad stuff)

“parent” nucleus

“daughter” nuclei

Neutron

B. Radioactive decay-fission

“daughter” nuclei – come out in excited nuclear energy state

…. Give off gamma rays as drop to lower energy.

Jumps down in energy …

Gives off gamma ray…

VERY HIGH ENERGY PHOTON

Neutron Induced fission- key to atomic bombs



•LOTS OF ENERGY!!




Neutron

B. Radioactive decay-fission

Found in strange results from Enrico Fermi’s lab

Otto Hahn saw similar results and called to his

former student for help

Lise Meitner came to his aide and explained what

was happening.

Neutron Induced fission- key to atomic bombs •two smaller nuclei

•LOTS OF ENERGY!!

•It is a messy process with lots of

•few extra free neutrons “parent” nucleus


N

Uranium 235

92 p, 143 n

N

Neutron absorbed

Excites U235 nucleus up above potential

barrier

Splits into two smaller nuclei…

which zoom apart due to electrostatic

repulsion! (there is even more energy

from other sources)

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neutron induced fission Fermi first did it, it was very hard to figure out how this could be.

Chain reaction: neutrons making fission that makes more neutrons, that makes more

fission that makes more neutrons, that makes more fission that makes

more neutrons, that makes more fission that makes more

neutrons, that makes more fission that makes more

neutrons, that makes more fission that

makes more neutrons, that makes more

If don’t get swallowed

the 3 free neutrons can go

induce more fission.

Ur 235

1st generation

2nd generation

3rd generation

100 generations

per microsecond!

2 x 2 x 2 .…x2 fissions

each with LOTS of

energy.

BOOM!

Fission Nuclear explosion

“atomic bomb”

Uranium 235

or plutonium

100 generations

per microsecond!

2 x 2 x 2 .…x2 reactions

with a lot of energy

each.

Each one is a millionth

of a fission reaction.

BOOM!

This is very similar to a

conventional explosion

Methane + 2 Oxygen

mainly stable

combining will release energy (heat)

Heat+CH4+2 O2 they combine immediately

releasing more heat

1st generation

2nd generation

3rd generation

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Recipe for fission bomb. 1. Find neutron induced fissionable material that produces bunch of extra

free neutrons when fissions.

2. Sift it well to remove all the other material that will harmlessly

swallow up the extra neutrons. (THE HARDEST STEP.)

3. Assemble “supercritical mass”, really fast!. Need enough stuff that the

neutrons run into other nuclei rather than just harmlessly leaving

sample. If your mass tends to melt with a small

fizzle you are not assembling fast enough

to be supercritical. Put together faster.

4. Let sit for 1 millionth of a second- will bake itself!

hydrogen isotopes

tritium,deuterium

Plutonium “trigger”

chemical explosive

chemical explosive Fusion bomb (simple picture)

Shaped plutonium and assembled bombs at Rocky Flats.

Plutonium reaches

supercritical, explodes.

(fission bomb)

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Al Bartlett

CU professor 1950-2013

Photographed atomic explosions at Los Alamos with nano second resolution 1944-1945

Was a CU professor from 1950-2013 Had a lot of interesting ideas about population, sustainability, fuel use, etc

Energy:

1 fission of Uranium 235 releases:

~10-11 Joules of energy

1 fusion event of 2 hydrogen atoms:


Burning 1 molecule of TNT releases:


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

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

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

88,000,000,000,000 J

How does 6.1 kg relate to 22 ktons?

(the 22 k tons is 10 million times more

massive)

Would have to drop that quart of water from

1 billion kilometers (our model of gravity

fails first).

The hardest part of getting a nuclear bomb is the

material “Front End”

- obtain 235U (HEU=at least 80%) by the

same methods used to make Low Enriched

Uranium (LEU), 3-4%.

Have to use tiny mass difference very hard

Bohr told his American colleagues

that a bomb couldn't be made without

turning the entire country into a giant

factory. When he saw Los Alamos he

said, “I told you so.”

Gas centrifuge (use inertial difference between

masses).

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The hardest part of getting a nuclear bomb is the

material

“Back End”

-obtain 239Pu from Spent Nuclear Fuel: chemistry 238U loves neutrons: grabs them and converts to

239Pu (another process called beta decay)

239Pu is very much like 235U

Chemical separation of Pu is much easier.

Deuterium on

Deuterium

Deuterium on

Tritium

Fusion bomb or “hydrogen bomb” Basic process like in sun. Stick small nuclei together.

Which will release more energy during fusion?

a. Deuterium combining with deuterium

b. Deuterium combining with tritium

Answer is b. Incoming deuterium particle has comparatively more potential

energy!

Fusion bomb or “hydrogen bomb” Basic process like in sun. Stick small nuclei together.

Stick hydrogen isotopes

together to make helium.

push together

energy required

energy released if

push over the hump

+ =

Simple if can push hard enough- just use sun or fission bomb.

More energy per atom than fission. Can use LOTS of hydrogen.

End up with GIGANTIC bombs 1000 times bigger than first fission bombs

activation energy of 100 million degrees

helium Plutonium reaches

supercritical, explodes.

Tritium etc. nuclei pushed together

and combine to form helium.

Fission bomb- chain reaction, hideous amounts of energy

comes off as heat and high energy particles (electrons,

neutrons, x-rays, gamma rays) “Radiation”. Heats up air that

blows things down.

ans. c. Makes and spreads around lots of weird radioactive “daughter”

nuclei (iodine etc.) that can be absorbed by people and plants and decay

slowly giving off damaging radiation.

Lots of free neutrons directly from explosion can also induce radioactivity in

some other nuclei.

In atomic bomb, roughly 20% of Pl or Ur decays by induced fission

This means that after explosion there are

a. about 20% fewer atomic nuclei than before with correspondingly fewer

total neutrons and protons,

b. 20% fewer at. nucl. but about same total neut. and protons.

c. about same total neutrons and protons and more atomic nuclei,

d. almost no atomic nuclei left, just whole bunch of isolated Neut.s and

prot.s.,

e. almost nothing of Ur or Pl left, all went into energy.

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U235 and U238 atoms are placed into a container, which are likely to result in a

chain reaction (resulting in explosion) when a free neutron triggers fission of one of

the U235:

#1 #2 #3 #4

#5

a. #2 only

b. #1, #2, and #5

c. #2 and #4

d. #2, #3, and #4

e. #2, #4, and #5.

Correct answer is c. (#2 and #4) Analysis:

#1 is too sparse .. most neutrons will leave box before hitting another U235.

#2 is good.. Pure U235, densely packed, large package

#3 has too many U238’s… more U238’s than 235’s. Free neutrons more likely to be

absorbed by 238’s than to hit and fission another 235.

#4 is OK … More U235’s than 238’s, still densely packed, large package

#5 is too small of package … neutrons likely to escape package before hitting another

U235.

Lots of uranium in the ground… why not just

blow up or heat up until it melts?

U235 is less stable than U238. So 1.8 billion years ago natural

Uranium ore had about 3% U235.

This is how much we enrich Uranium for a reactor

Why didn’t it happen earlier? Uranium too diffuse.

For 2 BY after earth’s formation the Uranium

remained diffuse.

Then life released Oxygen.

Oxygen reacted with the Uranium making it mobile

and in Gabon it concentrated.

There was water present which acted to slow down

neutrons that were too fast.

There were no other elements that soaked up the

neutrons and…?

It didn’t explode. Instead it slowly burned for millions

of years.

Now 4 BY ago

U235 and U238 Lots of uranium in the ground… why not just

blow up or heat up until it melts?

It did once (at least once).

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Radioactive materials and “radiation”

Daughters often have too few neutrons

to stick together, so radioactive, divide more.

Other bad/energetic stuff that comes out.

Neutrons

electrons (“beta particles”)

photons (“gamma rays”)

helium nuclei (“alpha particles”)

Why radiation bad? Jocell

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Alpha particles: helium nuclei - most of radiation is this type

- common is Radon (comes from natural decay process of U238), only really

bad because Radon is a gas .. Gets into lungs, if decays there bad for cell.

+ +

In air: Travels ~2 cm ionizing air molecules and slowing down …

eventually turns into He atom with electrons

If decays in lung, hits cell and busts up DNA and other molecules:

Beta particles:

energetic electrons … behavior similar to alpha particles, but

smaller and higher energy

Usually doesn’t get far -- because it hits things

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•LOTS OF ENERGY!!




Neutron

Sources of Gamma Radiation

“daughter” nuclei – come out in excited nuclear energy state …. Give off gamma rays as drop to lower energy.

Jumps down in energy …

Gives off gamma ray…

VERY HIGH ENERGY PHOTON

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gamma rays: high-energy photons - So high energy can pass through things (walls, your body) without being

absorbed, but if absorbed really bad!

In air: Can travel long distances until absorbed

+ +

In body, if absorbed by DNA or other molecule in cell …

damages cell… can lead to cancer.

Most likely

If pass through without interacting with

anything in cell then no damage.

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+ + Also break DNA cancer

But also can cure cancer-

Concentrate radiation on

cancer cells to kill them.

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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

a - very bad, but easy to stop -- your skin / clothes stop it

b - quite bad, hard to stop -- pass into your body -- keep far away

g- bad, but really hard to stop--- rarely rarely gets absorbed

Me--- I pick (d)---

Results of radiation dose 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.

~4,000 counts/min =

.002 Rem/hr How much energy do we use?

on what?

domestic

foreign

Renew + nuclear.

How do you get “power” out of

Uranium. The same way to get useful power from carbon based

fuels

Carbon things can

explode

Or you can burn

them slowly

How do you get “power” out of

Uranium. The same way to get useful power from carbon based

fuels

Nuclei can explode

Or you can burn

them slowly

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What about US oil?

'03 DOE-EIA

2.1E9 bbl/yr

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U.S. Oil Curves

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What does this mean?

Of course this is an estimate (and perhaps worst case); what is best

case? 57

What is inevitable?

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1. A chain reaction as it occurs in atomic bombs is

a. the chemical reaction used to form the metal used in making chains.

b. a series of chemical reactions where each one triggers the next.

c. the sequence of one nucleus splitting up and causing another to split up followed by

another etc.

d. the set of steps required to assemble an atomic bomb.

1 2

3 4

2. To the left are energy profiles for

various atoms. Which decays most

rapidly and which never decays?

a. 3 most rapid, 4 never

b. 1 most rapid, 2 never

c. 2 most rapid, 1 never

d. 4 most rapid, 3 never

e. 4 most rapid, 1 never

“Paying Attention Quiz”

1. c, 2.c

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