Today: E=mc2, Fission, Fusion, Nuclear Energy / Weapons

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Highly Recommended ReadingRichard Rhodes The Making of the Atomic BombYou will understand the physics very well after these past couple weeks of class

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What does E = mc2 mean?The album name means "(E) Emancipation (=) equals (MC) Mariah Carey () to the second power"HmmmIn this case Einstein is more comprehensible and has higher credibility

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Einsteins special theory of relativity (1905) said that mass and energy were equivalent.E=mc2

Mass is a form of energy

Conservation of Energy holds when mass is recognized as another form of energy Under this theory mass is not an unalterable magnitude, but a magnitude dependent on (and, indeed, identical with) the amount of energy. Einsteins comment about this formula in 1927

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E=mc2 at the heart of nuclear energyAbout 20 megatons of TNT per kilogram mass

1017 joules per kilogram (1019 joules in electricity per year in U.S.)

For nuclear reaction:

Energy = (change in mass) * c2

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Binding energy of isotopesHigh mass atoms can yieldenergy via nuclear fissionLow mass atoms can yieldenergy via nuclear fusion

Energy available from U-235

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Fission and Fusion Energy1 kilogram of FUSION fuelEnough for 676 people per year1 kilogram of FISSION fuelEnough for 176 people per yearHyperphysics @ Georgia State U.

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Clicker Question: The energy released from 1 kg of fission fuel could supply about how many people with a years worth of energy? 0.01 people1 person100 people1,000,000 people

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Clicker Question: The energy released from 1 kg of fission fuel could supply about how many people with a years worth of energy? 0.01 people1 person100 people1,000,000 people

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Uranium-235 change in mass during fissionOne possible fission reaction is:

U-235 + neutron Ba-141 + Kr-92 + 3 neutronsEnding mass = 99.9% of starting mass0.1% missing mass = kinetic energy of products (and some gamma rays)

Fission of 1000 kilograms of Uranium-235 releases 1 kilogram of mass energyEquivalent to a few days of U.S. electrical needs(Can 100% of the heat energy be converted into electricity?)

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Need controlled chain reaction for useful fission energy Efficient use of neutrons is required for chain reaction!Some key sources of inefficiency:

Neutron leaves sample without being absorbedNeutron absorbed by a non-fissile material (e.g. U-238)Hyperphysics @ Georgia State U.

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Critical mass required for neutron efficiencyIn a small mass, nuetrons are more likely to escape from the surface of the materialA larger mass will have more interior volume, and more chances to absorb neutrons

Eventually, the size of the sphere is large enough to sustain a chain reaction

More surface area!A sphere has the highest volume to surface area ratiomost efficientused for critical mass definition

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Clicker questioncritical massWhich shape would go critical at the lowest mass?

Flat pancakeA perfect cubeA perfect sphere

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Clicker questioncritical massWhich shape would go critical at the lowest mass?

Flat pancakeA perfect cubeA perfect sphere

A sphere has the lowest surface area to volume ratio

Much easier for neutronsto escape

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Critical Mass is the minimum mass required for a sustained chain reaction in a sphere of materialPure Uranium-23550 kilogram sphere(10 inch diameter)

Pure Plutonium-23910 kilogram sphere(6 inch diameter)

These values are for 100% pure sampleseven moderately pure (say 20%) samples VERY hard to obtain >600 kg critical mass for 15% pure U-235!!!

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Nuclear fission weapons (as in WWII)In a small amount of time, create a super-critical mass of high-purity fissile materialSub-critical massSub-critical mass

Above critical mass, chain reaction!

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Nuclear fission weapons (as in WWII)In a small amount of time, create a super-critical mass of high-purity fissile materialLittle Boy Assemble two pieces of non-spherical uranium-235 into a super-critical sphere

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Nuclear fission weapons (as in WWII)In a small amount of time, create a super-critical mass of high-purity fissile materialFat Man Compress a sub-critical mass of Plutonium-239 into a critical mass via implosion

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Nuclear fission weapons (as in WWII)You can see these designs are not very complexHigh-purity or weapons grade uranium or plutonium are the limiting factors

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Critical mass of pure U-235 about 10 inch dia. sphere50kg (About the size of a bowling ball)Amount can be reduced byImplosioncritical mass only 10-15 kgNeutron reflectors (increase efficiency of neutron usage)

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About 6 kg of plutonium needed for implosion weaponAbout softball-sizedPlutonium weapons much more difficult than with highly enriched Uranium

But much less material is needed!

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Uranium-235 and Plutonium only practical NW fuelsHighly difficult to purify

Very easy to construct a nuclear weapon

Emits little radiation while idleEasier to make / purify (from spent peaceful fuel)

Somewhat difficult to construct a nuclear weapon

Weapons-grade emits little radiationUranium-235Plutonium (mostly Pu-239)

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Nuclear Weapons in the mediaEnriched Uranium

Enriched Uranium >= 0.7% u-235Highly Enriched Uranium (HEU) >= 20% u-235Weapon-grade Uranium >= 90%

Weapon-grade U-235 = simple nuclear weapon(depleted uranium is leftover U-238)Dirty Bomb

Only effective for panic

Ordinary bomb as effectiveIran

Concerns around Iran obtaining all the pieces of the nuclear fuel cycle = able to make HEU

Complex politically and historically

No real nuclear energy needNorth Korea

Generating plutonium with existing nuclear reactors (currently dismantling for food)

Already has tested plutonium weapon

Desperate energy need

Concerns about selling weapons-grade plutonium and missiles

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Clicker QuestionWeapons-grade PlutoniumIf smuggling enough plutonium for one weapon, which container would be just about the right size?

ThimblePop can (soda can for you non-Michigan people)Large beer coolerRegular sized freight container

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Clicker QuestionWeapons-grade PlutoniumIf smuggling enough plutonium for one weapon, which container would be just about the right size?

ThimblePop can (soda can for you non-Michigan people)Large beer coolerRegular sized freight container

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Nuclear weapons are out of control fission chain reactionsControlled chain reactions can be used in nuclear reactors for peaceful purposes

In fact, first man-made chain reaction was a nuclear reactor, Chicago Pile-1

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Chicago Pile-1 used natural uranium (0.7% U-235)Critical mass very high compared to pure U-235

Strategy for slowing neutrons to increase chance of absorption by U-235Graphite moderating blocks

Strategy for controlling reaction by neutron absorptionCadmium control rods

Only 18 months from concept to successful construction in December 1942!

More reactors constructed for production of plutonium for weapons

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Core of nuclear power reactors similar in principleCANDU heavy water nuclear reactorGraphite replaced with heavy water as a neutron moderatorCan operate with lower-grade uranium than light water reactor http://www.youtube.com/watch?v=fjgdgAhOzXQ&feature=related

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Brainstorming / small groupsDiscuss with each other and come up with pros and cons of energy from nuclear fission reactors

Pros Clean energy (no CO2 emission, only steam or water released); creates jobs (employs average joes); long-lasting resource; reduced dependency on foreign oilproduces NW materialPRO: Better designs can dramatically improve safety and waste problems

Cons destroys jobs for the average joe; not really sure what to do with waste (spent fuel)-if poorly managed can contaminate and harm local people; produces NW material

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Some pros and cons of nuclear fission energyPros:

Only gaseous emission is water (steam)

Large amount of energy fromsmall amount of fuelCons:

Fuel is transformed into radioactive waste

Careless operation can result in nuclear accidents

Reactors can produce weapons-grade nuclear fuel

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Clicker questionMass energy availableWhich reaction can potentially release more energy per reaction

More energy can be released per fission eventMore energy can be released per fusion event

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Immediately, Nuclear Fusion was recognized as an even greater source of energyAnd of course, hydrogen is a very abundant element!

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Fusion power first harnessed for weaponsHyrdrogen fusion initiated by conventional fission bombs

Fusion bombs or H-bombs 100s of times more powerful than fission bombsUser:Boostin on wikipedia

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Fission energy took 18 months to harnessWe are still waiting for fusion power!!! Conceptually, fusion is as simple as fissionE.g. the sun shown above

In practice, fusion is way way way way way way way way harder to control in a machine(fusion weapons work well, but not good for a reactor)ITERMagnetic confinementfusion experimentThe sun is a successfulfusion reactor

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