Chapter 5: Chain Reactions, Nuclear Reactors, and Atomic Bombs
Bomb dropped on Nagasaki:
"Fat-Man": fission with Pu-239 -18/20 kilotons of TNT equivalent -sphere design: uranium and plutonium in center, conventional explosives force them together to start reaction, neutrons reflected back in for more of a chain reaction -used only 6 kg of plutonium (plutonium emits more neutrons in fission than uranium so need less of it; reaction goes faster, more complete chain reaction before plutonium blows apart) -less uranium than in "Little Boy" but bigger design due to sphere
Bomb that destroyed Hiroshima:
"Little Boy": gun-type bomb using fission of U-235 -first uranium bomb ever exploded -only 1.5% of material was fissioned -energy released was 13 kilotons of TNT equivalent
Fusion energy...
"is 50 years in the future and always has been"
Yucca Mountain, Nevada
-1/2 mile down -put in stainless steel -Senator Harry Reid didn't want Nevada to be known for gambling, prostitution, AND nuclear waste -problem: nuclear waste still piling up!
What happens when reactions start to run away?
-control rods made of boron or cadmium (typically) 'soak up' the extra, fast neutrons -control rods have to be released very quickly (driven by powerful springs)
Compound interest
-exponential growth bc growing at same rate -chain reaction bc doubling creates two amounts, each equal to original, and each of these will continue to double
Plutonium production
-for nuclear reactors only one neutron from uranium fission is used for another fission, the other is absorbed -control rods absorb neutrons without releasing energy 1) some neutrons are absorbed on U-238 2) U-238 becomes U-239 3) U-239 is radioactive and decays to a radioactive isotope of neptunium (Np-239) 4) decays into Pu-239
Oil application for growth time:
-if oil need doubled in 10 years, in one 10-year period would need to find more oil than had ever been drilled -peak oil idea BUT renewable energy, fracking changed predicted shortage
Population TED talk:
-if we continue present birth rate, world population will flatten out around 10 billion -2.1 children per woman required to maintain population for developed countries (replacement rate) -3.3 for poor countries
How does chain reaction work?
-more than one neutron comes out per uranium fission -if neutrons can be made to hit other uranium nuclei, doubling rule will lead to fission of almost every nucleus -only takes 80 doublings
Nuclear bomb chain reaction example:
-neutron splits U-235 nucleus through fission into 2 fission fragments; two neutrons released as well -neutrons hit other U-235 nuclei causing chain reaction -first generation: 1 atom split into 2; then 2 into 4, 8, etc. (modeled with ping-pong balls and mousetraps)
Describe nuclear power plants:
-turbine building: turbines & generator -containment building made of reinforced concrete: nuclear reactor (nuclear reactions take place, water heated up) (Chernobyl didn't have one) -cooling tower: hot water cooled -PWR: pressurized water reactor 1) in reactor pressure vessel, nuclear reaction takes place, thermal energy released 2) fuel assemblies found inside: fuel rods with pellets of uranium/plutonium fuel, rods strapped together, water runs through 3) water absorbs thermal energy to keep chain reaction going 4) water heated to over 500 degrees F, doesn't boil bc pressurizer keeps pressure constant 5) heated water pumped to heat exchanger/steam generator (shell and tube) -hot water flows through u tubes -heats up copper -water inside heat exchanger begins to boil to create steam (at atmospheric pressure) 6) steam fed through set of pipes to turbine building, drives turbine 7) turbines connected to electrical generator which produces AC electricity from rotational energy 8) condenser converts steam to liquid form, returns to steam generator -water comes form adjacent river, or cooled in cooling tower -water circulation systems kept separate (radioactive water never leaves containment building) -water in secondary system not radioactive, used to drive turbines -cooling circulation system provides cool water to condense steam in secondary circulation system
How much energy does 1 MW provide?
1 MW provides energy for about 600-800 homes
Types of nuclear waste:
1) Low-level waste: not very high level of radioactivity, anything that came in contact with radioactivity (e.g. clothes), anything after 10 half-lives 2) High-level waste: spent nuclear fuel (fuel rod contents) 3) Mixed wastes: mixture of low & high level 4) Transuranic wastes: any element higher than 92 which is Uranium (definitely radioactive, man-made) 5) Uranium mining & mill tailings: pulverized rock/solids remain after mining, alpha particles dangerous if inhaled
Assuming an average of 1270 kWh per month per home, how many homes could a 1 GW power plant supply?
1) convert power (GW) into energy (GWh) 30 x 24 x 1= 720 GWh/month 2) divide 720 x 10^9 by by 1270 x 10^3 kWh/month/home 3) = approx. 600,000 homes
Assuming an average of 10,766 kWh per year (for entire country) and 897 kWh per month, find the amount of homes that a 1 GW power plant could supply:
1) divide 720 x 10 ^9 by 897 x 10^3 2)=800,000 homes approximately
How do hydrogen bombs work?
1) explosion of a fission bomb creates an intense heat 2) heat causes the deuterium and tritium to reach energies that are sufficient to overcome their repulsion to each other (nuclei are both positively charged) and fuse 3) fusion releases energy and neutrons 4) high-energy neutrons cause fission in a uranium container (made of U-238) that surrounds everything 5) releases even more energy
How to separate U-235 from U-237:
1) gaseous diffusion -turn both uranium isotopes into gas -put through miles of tubing -barrier: tube with tiny holes slightly lighter ones go through barrier (enriched), heavier go on -not 100% separation -takes nearly 4000 stages, 40 acres for Manhattan project (Crystal campus is only 10 acres!) 2) centrifuge -stationary outer cylinder, inner rotary spinning very fast -lighter ones stay closer to middle, heavier are thrown out -typical plant may have 60,000 centrifuges to enrich uranium
How do power plants work:
1) heat 2) boil water to make steam 3) steam turns turbines 4) produces electricity using generator
How does gun-type bomb work?
1) piece of U-235 shot by cannon at another piece of U-235 2) combination above critical mass 3) fission chain reaction begins that releases nuclear energy which causes explosion
Public opposition to breeder reactors:
1) plutonium would be widespread: dangerous bc terrorists could get a hold of it and use it for nuclear bombs -but proponents say it is too difficult for terrorists to make implosion work 2) reactor explosion -most efficient breeder reactor would use fast neutrons (fast breeder) -but then safety aspect is lost & nuclear reactor could explode like an atomic bomb -proponents say safety systems
# of total kWh on my bill:
1,009 kWh
Babies per woman decrease when:
1. children survive 2. many children aren't needed for work 3. women get education & join labor force 4. family planning is accessible
Average kWh for both sections of class:
1270 kWh per month/home
TNT equivalent for Hiroshima=
13 kt (kilotons) of TNT dropped on Hiroshima -most people killed by blast/heat
How many neutrons released with fission of plutonium-239?
3 neutrons released per fission
What are nuclear power plants?
=thermal power stations bc creates heat to boil water to make steam to drive turbine (fan), inside turbine is generator to make electricity when turbine turns -all power plants same (make steam to turn generator, or wind turns generator!)
Compound interest formula:
A= P(1+ r/n)^ (nt) -for monthly use n=12 -for daily use n=365
Instanteous interest formula:
A=pe^(rt)
Fusion bomb theory:
Deuterium + tritium --> neutron + He (alpha particle) + energy -two bombs in one bc need high temp. to make D and T fuse bc positive charges repel, and energy to fuse comes from fission bomb -container of D and T at center, other elements surrounding, huge shockwave compresses other elements
Who achieved the first actual chain reaction?
Enrico Fermi in 1942
Moore's Law of Computers: exponential growth
In 1965, Gordon Moore noticed that the # of basic components that could be put on a chip had doubled every year; expected trend to continue -average doubling period is 18 months -Moore's Law continues (but circuit cannot be smaller than the size of an atom so can't work forever)
Who developed nuclear chain reaction?
Leo Szilard in 1932
MIRV
Multiple Independently targetable Reentry Vehicle -multiple missiles contained which can go in different directions -as many as 24 separate rockets -used for submarines (don't need to resurface)
What is a critical mass?
NECESSARY FOR EXPLOSION in order for chain reaction to work, there must be enough uranium so that the emitted neutron hits another uranium nucleus instead of escaping between the nuclei and out of the bomb
first test of hydrogen bomb:
Nov. 1, 1952 at 7:15 at Eniwetok Atoll -Ivy Mike -produced a yield of 10.4 megatons of TNT -however 77% of final yield came from fast fission of U-235
3 isotopes of hydrogen:
Protium (1H1): 1 proton Deuterium (2H1): 1 proton, 1 neutron, positive charge, found in seawater Tritium (3H1): 1 proton, 2 neutrons, positive charge, radioactive
Difficulty with plutonium bombs:
Pu-239 from nuclear reactors normally has a high fraction of Pu-240 which is highly radioactive -tends to predetonate bomb -implosion design has to be used which is extremely difficult to design and build (probably requires resources of a whole country)
Doubling time formula:
T2= 70/(%growth per year) -if something goes up 7%/year, means it will double in 10 years -growth in any doubling time> total of all preceding growth (e.g. rice on chessboard)
The Population Bomb
Thomas Rohr Malthus argued population growth obeys rules of chain reaction Paul Ehrlich -if average parent has 2 children, then doubling every generation -food supply grows more slowly so only way to stop population bomb is disease and famine (note: issue is transporting food, not growing it) -nowadays birth control is alternative -current estimation that world population will top out at 10 billion
Present stockpile of nuclear weapons:
US and Russia both have around 7000 nuclear warheads
Terrorist nukes:
US fears terrorists could make a simple nuclear bomb -problem in making a uranium bomb would be obtaining the purified U-235; requires expensive program which could be easily destroyed unless well-hidden (fear is that N. Korea sells the purified U-235) -problem with plutonium bomb would be mastering the technique of implosion
Air bursts:
airbursts produce shockwaves which amplify blast effect -ground bursts spread radioactivity!
How many nuclear reactors in US?
around 100
How is plutonium often arranged in bombs (implosion)?
arranged as a spherical hollow shell with explosives on the outside -explosives drive shell into a little blob and compress it -compression pushes atoms close enough together that neutrons released are unlikely to leak out so compressed plutonium has a smaller critical mass than un-compressed plutonium
Why is energy released when nucleus splits?
because bonds tighten in 2 new elements that are formed (fission doesn't release energy, the two new elements tightening does!)
Why doesn't water over 500 degrees F boil?
boiling pt. is 212 degrees F but the liquid water is under high pressure
Hydrogen bomb:
bomb based on fusion of hydrogen also called "THERMONUCLEAR BOMB" or NUCLEAR BOMB (bc fusion takes places bc of high temp.)
Plutonium fission bombs:
bomb tested at Alamogordo and one dropped on Nagasaki used Pu-239 -easy to get, produced in most nuclear reactors and can be separated out
Nuclear bomb:
bomb that uses the energy of the nucleus to release energy
Folding paper demo:
can only fold in half so many times (exponential growth) -each time you fold paper, width is halved and thickness doubles -got to 7 folds
Challenge in building plutonium bomb:
challenge during WW2 was getting plutonium so compressed that chain reaction would "go to completion"
How is the plutonium extracted?
chemically separated from uranium through uranium reprocessing
How much uranium did the first nuclear bomb (one that destroyed Hiroshima) contain?
contained 60 kg of U-235 -but only 600 g actually underwent fission -explosion so great, neutrons can't reach parts of uranium, didn't split -10 kg can ensure devastating destruction but packed more bc they knew would spread out
Cask
contains mix of kinds of waste (very safe according to video)
Nuclear reactors
devices in which a sustained chain reaction takes place using U-235 -doesn't involve doubling; instead from each fission only one of the emitted neutrons hits another nucleus to cause another fission -power comes out in heat, heat boils water into steam, steam turns a turbine (fan)
Why is it hard to locate hidden centrifuge plants?
don't require large amounts of power AND are very quiet (due to fine balancing of cylinders to prevent them from spinning themselves apart)
Energy released by atomic bomb
each fission of a uranium nucleus releases about 30 million times as much energy as a molecule of TNT (so 10 kg of U-235=300 kilotons of TNT) so 10 kg of U-235 enough for nuclear weapon
How much energy did first nuclear bomb release?
energy of about 20 kilotons of TNT (less than 300, so showing that not every atom fissioned before bomb blew itself apart)
Short form for breeder reactors:
extra neutrons interact with uranium to produce plutonium
Atomic bomb:
fission
What is released when U-235 nucleus fissions?
fission fragments (most dangerous radiation) two neutrons (allow for chain reaction, hit other U-235 nuclei)
What is the most modern/efficient method for enriching uranium?
gas centrifuge: 1) uranium combined with fluorine to make a gas (uranium hexafluoride) 2) placed in rapidly rotating cylinders 3) gas with heavier U-238 concentrates on outer part of cylinder, leaving lighter U-235 near center 4) these are piped out separately enrichment in one centrifuge is small; gas must be pumped through thousands of centrifuges to get enough enrichment for nuclear power plants or weapons
Mushroom cloud
happens with any large explosion if big enough -hot gas from explosion rises, cooler part of cloud spreads out when reaches the tropopause, really hot center continues to rise
How much does U-235 have to be enriched for a bomb?
has to be enriched to about 80%
How much does U-235 have to be enriched for a nuclear reactors?
has to be enriched to only about 3% for 2 reasons: -don't require that both neutrons hit U-235, only one; if one is absorbed, okay for reactor -nuclear reactor uses a moderator (chemical mixed in with fuel that slows down neutrons without absorbing them; usually water, deuterium oxide, and graphite)
Doubling time:
how long it takes for something to double if you know what % increase is
Breeder reactors
if you put Pu-239 in a nuclear reactor you get 3 neutrons per fission, but only need 1 neutron -with extra neutrons, put U-238 in reactor and make more plutonium -called a BREEDER REACTOR bc it makes more Pu-239 fuel than it consumes
What are a large fraction of fission fragments?
large fraction of fission fragments are isotope strontium-90 -highly radioactive half-life of 29 years -gets into food supply (falls on grass, eaten by cattle, passed through milk to kids and is concentrated in bones as similar chemical makeup to calcium)
If bomb exploded on ground:
lot of energy into ground, more building/earth sent into atmosphere so radioactivity spread farther (wanted to maximize damage w blast/heat, if exploded too high would go into space)
How are modern centrifuges made efficient and relatively small?
maraging steel (used for uranium centrifuges, rocket bodies, golf clubs): must be made of very strong materials to keep from breaking apart because they spin so fast
How do moderators work?
moderators have light nuclei that don't absorb neutrons -neutrons hit moderator and bounce off, but lose energy -in nuclear reactors, fast neutrons from fission bounce off moderator and become slow neutrons which are more readily absorbed on other U-235 nuclei so enrichment only needs to be 3%
How are neutrons slowed?
moderators in the reactor vessel e.g. H2O, D2O (deuterium oxide), or graphite
Nuclear fallout:
most danger from large nuclear weapons comes from nuclear fallout: consists of fission fragments from the uranium or plutonium -fallout worse if bomb exploded near ground bc dirt and other materials are caught up in fireball, rises in air, heavy dirt makes fission fragments fall, bringing radioactivity to ground(Hiroshima and Nagasaki exploded high to maximize blast)
Why are power plants usually near water source?
need lots of water to make steam
Important feature of nuclear reactors:
need slow (thermal) neutrons to work and not cause a runaway reaction
How do you make a critical mass?
needs to be enough material so that after each fission, more than one of the neutrons will hit another nucleus -requires sphere of uranium weighing 200 kg -US added neutron reflector to reduce mass required to 15 kg
Can reactors explode like atomic bombs?
no because they use slow neutrons/ not sufficiently enriched/not critical mass BUT they can explode bc heat, heats up water, pressure builds, leads to tremendous explosion (e.g. with Chernobyl and Fukushima)
How much uranium did bomb that was dropped on Nagasaki contain?
no uranium! 6.4 kg of plutonium
Do we have fusion reactors?
no; "50 years in the future..."
Gun-type for atomic bomb:
not all critical mass in one chunk bc could go off when you assemble: 40% at front and 60% at back, pressure mechanism triggered 60% to fire into 40%
nMRI
nuclear magnetic resonance imaging
Peaceful use of fission:
nuclear power plants!
Waste from nuclear plants v. coal:
nuclear: 2000 tons/yr but radioactive coal: 400,000,000 tons/yr but some radioactive as well!
How many nuclear reactors in CA?
only 1: Diablo Canyon near San Luis Obispo -2 reactors (can tell from pic bc 2 containment vessels) -will be closed down by 2025 -saltwater fine but kelp can clog pipes
If chain reaction proceeded by doubling how many generations would it take to split 10 kg of U-235?
only 84 generations (plutonium takes fewer since each fission releases 3 neutrons instead of 2)
Secret of hydrogen bomb:
plutonium fission bomb emits enough x-rays that they can be used to compress and ignite the tritium-deuterium combination ALSO instead of using tritium, bomb can contain a stable isotope of lithium called Li-6: -solid so must be stored at high density -neutrons from fission break up Li-6 to produce tritium, so fuel created in same microsecond that bomb explodes -fusion fuel=lithium combined with deuterium (lithium deuteride)
How many centrifuges does a typical centrifuge plant have?
several thousand but entire collection could fit inside a large lecture hall (could produce enough enriched uranium for several nuclear bombs a year)
Why is replacement rate 2.1?
sex ratio at birth is 105 boys/100 girls and for replacement level every woman needs to be replaced by a woman 210 kids have to be born to 100 women to get 100 girls (so 2.1)
Why are uranium bombs more popular than plutonium bombs (e.g. for terrorists)?
simpler design, BUT requires highly enriched U-235 which is hard to make
Fission bombs:
splitting heavy nuclei (only bombs dropped were fission)
What is enriched uranium used for?
to make bomb OR for power plants (Saddam Hussein ordered centrifuges, US designed them to be slightly faulty)
Dangers of plutonium:
toxic due to chemical effects and radioactivity (main reason) Pu-239 has half-life of 24,000 years; alpha particle radiation (only harmful if ingested/inhaled) -lethal dose is 500 mg -primary danger is inducing cancer (80 micrograms to cause cancer with high probability) -inhalation: death within a month for 20 mg
Hydrogen bombs/thermonuclear weapons:
uses heat from a plutonium or uranium fission bomb to fuse molecules of deuterium and tritium
WIPP
waste isolation pilot plant -defense-related transuranic (TRU) waste disposed of in desert in New Mexico (e.g. clothing, tools) -entombed by salt Solution to challenges: encase in glass so water can't dissolve
DNA "Fingerprinting": The Polymerase Chain Reaction (PCR)
way to multiple samples of DNA
weapons grade U-235 percentage versus for nuclear reactors
weapons grade=80% nuclear reactors= 3%
Obtaining enough uranium-235:
when mining uranium, get mix of U-235 (only 0.7%) and U-238 isotopes; have to enrich uranium to get more than 0.7% -both isotopes have identical chemical properties but U-235 weights a bit less so they move at slightly different speeds
Fusion bombs:
when you fuse together light elements (really only isotopes of hydrogen), energy is released SO: fusing together deuterium & tritium
Is circuling radioactive water ever replaced?
yes and plant is closed down, water stuck in barrels
