[PDF] - Nuclear Weapons Nuclear Weapons Yes Yes Y Y A. A. No No B. PDF Document

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Nuclear Weapons 1

Nuclear Weapons Nuclear Weapons

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Introductory Question Introductory Question

Is it possible to have 100 tons of plutonium

Is it possible to have 100 tons of plutonium and not have it explode? and not have it explode? Y Y

A. A.

Yes Yes

B. B.

No No

Nuclear Weapons 3 Observations about

Observations about Nuclear Weapons Nuclear Weapons

They release enormous amounts of energy

They produce incredible temperatures

They produce radioactive fallout

They are relatively difficult to make

They use chain reactions

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3 Questions about 3 Questions about Nuclear Weapons Nuclear Weapons

Where is nuclear energy stored in atoms?

Why are some atomic nuclei unstable?

How does a nuclear chain reaction work?

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Question 1 Question 1

Where is nuclear energy stored in atoms?

Where is nuclear energy stored in atoms?

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Atomic Nuclei Atomic Nuclei

Atoms are usually electrically neutral

They must have as many + charges as

They must have as many + charges as – – charges charges

Each electron must be matched by a + charge

A h f i i l A h f i i l

At the center of an atom is its nucleus

Extremely small (1/100,000th of atom’s diameter)

Contains most of the atom’s mass

Also contains most of the atom’s potential energy

Evidence is related to: E=mc

Evidence is related to: E=mc2

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Structure of Nucleus Structure of Nucleus

Nucleus contains two kinds of nucleons

Protons are positively charged

Neutrons are electrically neutral

T f i i l T f i i l

Two forces are active in a nucleus

Electrostatic repulsion between protons

Nuclear force attraction between touching nucleons

At short distances, the nuclear force dominates

At short distances, the nuclear force dominates

At long distances, the electric force dominates

At long distances, the electric force dominates

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Clicker Question Clicker Question

If you snipped the nucleus of a very massive

If you snipped the nucleus of a very massive atom in half, it would release energy stored in atom in half, it would release energy stored in h l i l i b h l i l i b

A.

A. the electrostatic repulsion between protons.

the electrostatic repulsion between protons.

B.

B. the nuclear attraction between nucleons.

the nuclear attraction between nucleons.

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Question 2 Question 2

Why are some atomic nuclei unstable?

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Nuclear Stability Nuclear Stability

The nucleons in a nucleus are in equilibrium

To be classically stable, that equilibrium must be stable

To be quantum

To be quantum-

mechanically stable, that equilibrium

mechanically stable, that equilibrium must also be the overall potential energy minimum must also be the overall potential energy minimum ust a so be t e ove a pote t a e e gy u ust a so be t e ove a pote t a e e gy u

Quantum mechanics and the Heisenberg uncertainty

Quantum mechanics and the Heisenberg uncertainty principle allow the nucleons to “try out” arrangements principle allow the nucleons to “try out” arrangements that are quite different from their equilibrium positions that are quite different from their equilibrium positions

If they find a path to a new, lower

If they find a path to a new, lower-

potential

potential-

energy

energy equilibrium, the nucleus may fall apart equilibrium, the nucleus may fall apart

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Radioactivity Radioactivity

Large nuclei have two possible problems:

Too many protons: too much electrostatic potential

Too many neutrons: isolated neutrons are unstable

B l b d i i k B l b d i i k

Balance between protons and neutrons is tricky

Large nuclei tend to fall apart spontaneously

These breakups are known as radioactive decay

and can include a splitting process called fission

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Question 3 Question 3

How does a nuclear chain reaction work?

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Induced Fission Induced Fission

A large nucleus can break when struck hard

Collision knocks its nucleons

ut of stable equilibrium
ut of stable equilibrium

Collision

Collision-altered nucleus altered nucleus

Collision

Collision altered nucleus altered nucleus may undergo induced fission may undergo induced fission

Since a neutron isn’t repelled

Since a neutron isn’t repelled by nucleus, it makes an ideal by nucleus, it makes an ideal projectile for inducing fission projectile for inducing fission

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Clicker Question Clicker Question

Fissioning

Fissioning nuclei may also release protons, but nuclei may also release protons, but those fission those fission-

produced protons don’t cause

produced protons don’t cause subsequent fissions because they subsequent fissions because they

A.

A. don’t have enough mass to cause fissions.

don’t have enough mass to cause fissions.

B.

B. collide too softly with nuclei to cause fissions.

collide too softly with nuclei to cause fissions.

C.

C. don’t have enough momentum to cause fissions.

don’t have enough momentum to cause fissions.

D.

D. don’t collide with nuclei at all.

don’t collide with nuclei at all.

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Chain Reaction Chain Reaction

Since neutrons can induce fission

Since neutrons can induce fission

and induced fission releases neutrons,

this cycle can repeat,

this cycle can repeat, a chain reaction! a chain reaction! a chain reaction! a chain reaction!

Each fission releases energy

Many fissions release

Many fissions release prodigious amounts of energy prodigious amounts of energy

Sudden energy release

Sudden energy release produces immense explosion produces immense explosion

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Requirement for a Bomb Requirement for a Bomb

A fission bomb requires 4 things:

An initial neutron source

a fissionable material (undergoes induced fission)

h fi i n m

t r l dditi n l n tr n h fi i n m t r l dditi n l n tr n

each fission must release additional neutrons

material must use fissions efficiently (critical mass)

235 235U and

U and 239

239Pu are both fissionable materials,

Pu are both fissionable materials,

but

but 235

235U is rare and must be separated from

U is rare and must be separated from 238

238U and

and 239

239Pu is made by exposing

Pu is made by exposing 238

238U to neutrons.

U to neutrons.

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The Gadget & Fat Man The Gadget & Fat Man

Each of these fission bombs started as a

Each of these fission bombs started as a 239

239Pu

Pu sphere below critical mass (6 kg) sphere below critical mass (6 kg)

It was crushed explosively to supercritical mass

It was crushed explosively to supercritical mass d l d d l d

and promptly underwent

and promptly underwent an explosive chain reaction. an explosive chain reaction.

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Introductory Question Introductory Question (revisited)

(revisited)

Is it possible to have 100 tons of plutonium

Is it possible to have 100 tons of plutonium and not have it explode? and not have it explode? Y Y

A. A.

Yes Yes

B. B.

No No

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Little Boy Little Boy

This bomb started as a

This bomb started as a 235

235U hollow sphere

U hollow sphere below critical mass (60 kg) below critical mass (60 kg)

A cannon fired a

A cannon fired a 235

235U plug through that sphere

U plug through that sphere so that it exceeded critical mass so that it exceeded critical mass so that it exceeded critical mass so that it exceeded critical mass

and it promptly

and it promptly underwent an underwent an explosive chain explosive chain reaction. reaction.

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Clicker Question Clicker Question

A

A 235

235U plug that is shot through a hollow

U plug that is shot through a hollow 235

235U

U sphere can initiate an explosive chain reaction sphere can initiate an explosive chain reaction even though it doesn’t hit anything because even though it doesn’t hit anything because

A.

A. it only has to assemble a supercritical mass.

it only has to assemble a supercritical mass.

B.

B. nuclei can collide even when the plug doesn’t.

nuclei can collide even when the plug doesn’t.

C.

C. atoms can collide even when the plug doesn’t.

atoms can collide even when the plug doesn’t.

D.

D. a chain reaction doesn’t involve any collisions.

a chain reaction doesn’t involve any collisions.

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Summary about Summary about Nuclear Weapons Nuclear Weapons

Nuclear energy is stored in atomic nuclei

Nuclear fission released electrostatic potential

Each fission releases an astonishing energy

Induced fission permits a chain reaction

Fission bombs explode via a chain reaction