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# 21 Apr 14 Fission, Fusion, Nuclear Energy, Nw With Brainstorming

## by Steve Koch, Professor at University of New Mexico on Apr 20, 2009

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Fission, Fusion, Nuclear Weapons, Nuclear energy

Fission, Fusion, Nuclear Weapons, Nuclear energy

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## 21 Apr 14 Fission, Fusion, Nuclear Energy, Nw With BrainstormingPresentation Transcript

• Today: E=mc 2 , Fission, Fusion, Nuclear Energy / Weapons
• Highly Recommended Reading Richard Rhodes’ “The Making of the Atomic Bomb” You will understand the physics very well after these past couple weeks of class
• What does E = mc 2 mean? The album name means &quot;(E) Emancipation (=) equals (MC) Mariah Carey (²) to the second power&quot; Hmmm In this case Einstein is more comprehensible and has higher credibility
• Einstein’s special theory of relativity (1905) said that mass and energy were “equivalent”.
• E=mc 2
• 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. Einstein’s comment about this formula in 1927
• E=mc 2 at the heart of nuclear energy
• About 20 megatons of TNT per kilogram mass
• 10 17 joules per kilogram (10 19 joules in electricity per year in U.S.)
• For nuclear reaction:
• Energy = ( change in mass) * c 2
• “ Binding energy” of isotopes High mass atoms can yield energy via nuclear fission Low mass atoms can yield energy via nuclear fusion Energy available from U-235
• Fission and Fusion Energy 1 kilogram of FUSION fuel Enough for 676 people per year 1 kilogram of FISSION fuel Enough for 176 people per year Hyperphysics @ Georgia State U.
• Clicker Question: The energy released from 1 kg of fission fuel could supply about how many people with a year’s worth of energy ?
• 0.01 people
• 1 person
• 100 people
• 1,000,000 people
• Clicker Question: The energy released from 1 kg of fission fuel could supply about how many people with a year’s worth of energy ?
• 0.01 people
• 1 person
• 100 people
• 1,000,000 people
• Uranium-235 change in mass during fission
• One possible fission reaction is:
• U-235 + neutron  Ba-141 + Kr-92 + 3 neutrons
Ending mass = 99.9% of starting mass 0.1% “missing” mass = kinetic energy of products (and some gamma rays)
• Fission of 1000 kilograms of Uranium-235 releases 1 kilogram of mass energy
• Equivalent to a few days of U.S. electrical needs
• (Can 100% of the heat energy be converted into electricity?)
• 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 absorbed
• Neutron absorbed by a non-fissile material (e.g. U-238)
Hyperphysics @ Georgia State U.
• Critical mass required for neutron efficiency In a small mass, nuetrons are more likely to escape from the surface of the material A 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 ratio… most efficient…used for critical mass definition
• Clicker question—critical mass
• Which shape would go “critical” at the lowest mass?
• Flat pancake
• A perfect cube
• A perfect sphere
• Clicker question—critical mass
• Which shape would go “critical” at the lowest mass?
• Flat pancake
• A perfect cube
• A perfect sphere
A sphere has the lowest surface area to volume ratio Much easier for neutrons to escape
• Critical Mass is the minimum mass required for a sustained chain reaction in a sphere of material
• Pure Uranium-235 50 kilogram sphere
• (10 inch diameter)
• Pure Plutonium-239 10 kilogram sphere
• (6 inch diameter)
• These values are for 100% pure samples… even moderately pure (say 20%) samples VERY hard to obtain
• >600 kg critical mass for 15% pure U-235!!!
• Nuclear fission weapons (as in WWII)
• In a small amount of time, create a super-critical mass of high-purity fissile material
Sub-critical mass Sub-critical mass Above critical mass, chain reaction!
• Nuclear fission weapons (as in WWII)
• In a small amount of time, create a super-critical mass of high-purity fissile material
“ Little Boy” Assemble two pieces of non-spherical uranium-235 into a super-critical sphere User: Dake on Wikipedia explosive Hollow bullet Cylindrical target
• Nuclear fission weapons (as in WWII)
• In a small amount of time, create a super-critical mass of high-purity fissile material
“ Fat Man” Compress a sub-critical mass of Plutonium-239 into a critical mass via implosion
• Nuclear fission weapons (as in WWII)
• You can see these designs are not very complex
High-purity or “weapons grade” uranium or plutonium are the limiting factors User: Dake on Wikipedia explosive Hollow bullet Cylindrical target
• Critical mass of pure U-235 about 10 inch dia. sphere 50kg (About the size of a bowling ball)
• Amount can be reduced by
• Implosion…critical mass only 10-15 kg
• Neutron reflectors (increase efficiency of neutron usage)
• About 6 kg of plutonium needed for implosion weapon About softball-sized Plutonium weapons much more difficult than with highly enriched Uranium But much less material is needed!
• Uranium-235 and Plutonium only practical NW fuels Highly difficult to purify Very easy to construct a nuclear weapon Emits little radiation while idle Easier to make / purify (from spent “peaceful” fuel) Somewhat difficult to construct a nuclear weapon Weapons-grade emits little radiation Uranium-235 Plutonium (mostly Pu-239)
• Nuclear Weapons in the media
• Enriched Uranium
• “ Enriched Uranium” >= 0.7% u-235
• “ Highly Enriched Uranium (HEU)” >= 20% u-235
• “ Weapon-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 effective Iran Concerns around Iran obtaining all the pieces of the “nuclear fuel cycle” = able to make HEU Complex politically and historically No real nuclear energy need North 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
• If smuggling enough plutonium for one weapon, which container would be just about the right size?
• Thimble
• Pop can (“soda can” for you non-Michigan people)
• Large beer cooler
• Regular sized freight container
• If smuggling enough plutonium for one weapon, which container would be just about the right size?
• Thimble
• Pop can (“soda can” for you non-Michigan people)
• Large beer cooler
• Regular sized freight container
• Nuclear weapons are out of control fission chain reactions
• Controlled 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”
• 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-235
• Graphite “moderating” blocks
• Strategy for controlling reaction by neutron absorption
Only 18 months from concept to successful construction in December 1942! More reactors constructed for production of plutonium for weapons
• Core of nuclear power reactors similar in principle “ CANDU” heavy water nuclear reactor Graphite replaced with “heavy water” as a neutron moderator Can operate with lower-grade uranium than light water reactor http://www.youtube.com/watch?v=fjgdgAhOzXQ&feature=related
• Brainstorming / small groups
• Discuss 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 oil
• produces NW material
• PRO: 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
• Some pros and cons of nuclear fission energy Pros: Only gaseous emission is water (steam) Large amount of energy from small amount of fuel Cons: Fuel is transformed into radioactive “waste” Careless operation can result in “nuclear accidents” Reactors can produce weapons-grade nuclear fuel
• Clicker question—Mass energy available
• Which reaction can potentially release more energy per reaction
• More energy can be released per fission event
• More energy can be released per fusion event
• Immediately, Nuclear Fusion was recognized as an even greater source of energy And of course, hydrogen is a very abundant element!
• Fusion power first harnessed for weapons
• Hyrdrogen fusion initiated by “conventional” fission bombs
• Fusion bombs or “H-bombs” 100’s of times more powerful than fission bombs
User:Boostin on wikipedia
• Fission energy took 18 months to harness We are still waiting for fusion power!!!
• Conceptually, fusion is as simple as fission
• E.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)
ITER Magnetic confinement fusion experiment The sun is a successful fusion reactor
•