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Announcements •! Final exam: Monday 12/12, 2:30-4:20 pm
•! Same length/format as previous exams (but you can have 2 hrs)
•! Kyle Armour is away this week; see TAs in study center
•! JW will have extra office hours Thu-Fri this week:
•! 12:45-1:15pm before class, •! 2:30-3pm after class
(my office B303 PAB, or B305 conf room next door)
3
Lecture Schedule (to end of term)
Today
Uranium’s decay series
•! Natural uranium is abundant in many minerals
•! Relatively long lifetime but decays into other radioactive nuclei
Includes radium, studied by Marie Walenska-Curie
•! Notice how alpha decays go down 2 in Z and 4 in A, while beta decays go up 1 in Z and keep the same A
4
Radon (Rn)
Noble gas (like neon or helium;
Percolates out of concrete or
bedrock into
houses!
!"#$#!!% $%
!! Newer energy-efficient houses are tightly sealed to conserve heat so have poorer ventilation.
!! Older houses are typically “leaky” and have lower Rn levels.
!! Mitigation is mainly done by ensuring adequate air circulation to remove Rn and its decay products.
Granite and other common minerals
contain U, and emit Rn – concrete is made from local rocks, so Rn levels vary
according to location.
6
Fission reactions / chain reaction
•! Two isotopes of U (element 92) are involved: –! 99.3 % of natural U metal is U-238, only 0.7% is U-235
–! These are isotopes of the same element: chemically identical, cannot be separated by methods of chemistry
235U + neutron ! 2 or more neutrons + 200 MeV energy (+ debris)
How to get “fissionable material” from ordinary uranium metal?
Method 1: separate U-235 (=fissionable material) from natural U
Hard: have to use physics instead of chemistry! a)! vaporize uranium, ionize it, then bend ion paths in magnetic field
b)! Run U vapor through a series of filters: diffusion rate depends upon atomic mass, but only a 1% difference! Takes thousands of diffusion steps.
c)! Use a series of centrifuges to gradually separate isotopes via their small density difference*
Another idea: use U to make another element that is fissionable 238U + neutron ! 239U ! 239Pu (plutonium, new element not found in nature) 239Pu has good characteristics for fission too, so
Method 2: build a nuclear reactor and generate Pu-239 (which can then be extracted by chemical engineering methods)
Also hard: Pu is extremely poisonous (chemically), and mixed in with highly radioactive residues in reactor fuel rods
neutrons need to be slowed down to cause fission efficiently, so U fuel blocks were surrounded by carbon as a moderator in the U. Chicago experimental reactor
* In the news: Iran is doing this now
7
Nuclear power
•! First nuclear reactor was built in December, 1942
(under football stands at U. of Chicago!)
–! Pile of uranium and carbon blocks (obsolete term: “nuclear pile”)
–! Historical context
•! 1938: nuclear fission reaction is discovered in Germany
•! 1940: Enrico Fermi theorizes it may be possible to create a “self-sustaining fission chain reaction”
–! Each fission produces neutrons which trigger others: chain
reaction
–! Might be possible to get fast reaction = explosion producing 106 X energy released per atom in chemical reactions
•! 1941: Leo Szilard persuades Einstein (among others) to write President Roosevelt pointing out danger if Germany develops this first
•! 1942: Manhattan District of US Army Corps of Engineers is assigned to conduct R&D and if possible develop nuclear weapons (“Manhattan Project”)
–! Labs built at Los Alamos, NM (physics research), Oak Ridge, TN
and Hanford, WA (industrial-scale separation of U-238 from natural U)
–! These all still exist as “national laboratories” belonging to US Dept of Energy
Enrico Fermi
First reactor, 1943
8
Einstein’s famous letter to Franklin Roosevelt
•! Expresses concern about possible German effort in fission research:
9
Modern power reactors
•! Nuclear reactors now typically produce about 1000 megawatts of heat
energy ! steam ! turbines ! electric generators
•! Two varieties use ordinary (“light”) water:
–! Boiling water reactors (BWRs)
•! Reaction heat boils water moderator, steam is used to run turbines
–! Pressurized water (PWRs)
•! Like pressure cooker: water is superheated but does not boil
•! Heat exchanger transfers heat to external water supply to make steam
•! Others use “heavy water” as moderator
–! Deuterium = heavy hydrogen: p+n
–! D2O is like water but much more efficient as neutron moderator: can use natural U as fuel
–! CANDU reactor design (Canada) is cheaper to build and safer in many ways
•! But: requires D2O (Canadian product!)
Cherenkov light from
electrons emitted into water
surrounding a reactor
(Daya Bay, China)
10
Pressurized Water Reactors
Most common type of power reactor in use today
•! Reactor core is contained in a steel vessel
•! Reactor vessel is inside a concrete building
•! Water from reactor never leaves the reactor building (heat exchanger generates steam for turbines from clean water)
11
Safety issues: waste disposal
•! Radioactive waste is the really big
problem for fission reactors
–! Used-up fuel elements from fission reactors are highly dangerous: all sorts of chemically-separable (ie
easy to do) isotopes in them
–! No plan in place for storing them long-term in USA!
•! Most (40,000 tons) high-level waste now stored in water tanks on reactor sites
•! Significant problem from leakage of WW-II era waste containers at Hanford, WA –
–! USA just put off settling this, problem, again…
•! Long-term storage (104 yr!)
–! Fuel rods can be chemically processed to extract Pu and other isotopes: security issues!
1 becquerel (Bq)=1 decay/sec
1 TBq=1012 Bq
Radioactivity from spent reactor fuel
12
Safety issues
–! Core starts to melt down!
–! Steam and hydrogen (with radioactive contaminants) were vented to the atmosphere
–! No loss of life can be directly attributed to accident, but it killed the nuclear power industry in the USA
»! The accident occurred just a few days after release of the movie The China Syndrome !
2.! Chernobyl reactor, near Kiev, Ukraine
•! April 26, 1986: workers conducting tests on an obsolete and decrepit Soviet-era power reactor violate safety rules, cause meltdown
–! Unsafe carbon+water moderator design with no containment building.
–! About 5% of reactor core was vented to the atmosphere; kept secret!
»! First news came from a reactor plant in Sweden, where workers noted rising
background radioactivity from their own monitors
–! 31 workers and firefighters killed, 10 deaths directly attributed, huge area
contaminated, thousands exposed to radioactive debris; several thousand excess thyroid cancers (but - no other statistically significant public health problems!)
•! Three names come up when we discuss nuclear power:
1.! Three Mile Island reactor, near Harrisburg, PA
•! March 28, 1979: pumps in water line fail; heat is not removed and core temp rises; workers make errors (fail to read gauges, fail to take proper actions) due to poor system design
Chernobyl reactor after fire
13
3. Fukushima reactor disaster, 2011
•! March 11, 2011: magnitude 9 earthquake 70km offshore
•! Reactors survive earthquake without damage, shut down
•! Tsunami wave 14m tall hits Fukushima
•! Wave destroys power lines and emergency power system
•! Cooling-water flow stops – cores melt down
•! Hydrogen gas created by chemical reactions with seawater
•! Hydrogen explosions blow open reactor buildings
•! Fires spread radioactive contaminants
•! Complex of 6 power reactors
on shoreline – started operation 1971-1976
•! 4.4 Gigawatts total power
•! No radiation deaths; 2 cleanup workers later received
potentially dangerous exposures
•! Massive national consequences: electric power shortage, fisheries and farmland ruined, fallout caused
abnormal radiation levels as far away as Tokyo
DW, dry well enclosing reactor pressure
vessel; WW, wet well water pool; SFP,
spent fuel pool area; RPV, Reactor Pressure Vessel; SCSW, Secondary
Concrete Shield Wall.
14
Fukushima: in the news again today!
JW Editorial:
Problems with fission power are not technical but societal: we know how to
build safe reactors, and operate them safely - but they are “too expensive”!
(as they said about “safe cars”, in 1970s)
•! Need for cheap electric power
•! Failure to deal with waste storage
Industrial-political alliances intrude:
•! Political interference with site
selection, and safety standards
•! Inadequate regulation and monitoring
of compliance
•! &'(')*+,-%
./-012.%3+)-)1/.4%%
!"#$"%
!"#&'%
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•! Our project in Japan:
T2K neutrino experiment
•! Based at particle
accelerator lab in Tokai, near Tokyo
•! My PhD student Scott
Davis was working there on 3/11/11 !
!"#$%#&
'#(")&
JW’s personal note:
5)647.89%%
):;47<7"):;%
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Tokyo
Kamioka
Tokai N
!5%67%8+9(/):%;8%<*=)+0)%
>?@AB%/C1.-C0/%
Scott’s photos of
scene at J-PARC (Japan proton
accelerator research complex)
on 3/12/11
D*/%E-.0*%!!%
F-.1*G'-(/%•! H+IC+J0-C1%/K1/.+2.%
3-,-I/4%.2-3L-=):%
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•! @N%+CO'.+/):%C2%)/.+2')%
3-,-I/%12%9-P):%2Q0/):%2.%
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•! @2%1)'C-,+%3-,-I/%1*+)%
S-.%)2'1*%TL-U/%*/+I*1%
L-)%2C9=%!,%-1%D2(-+V%
18
Fusion: Where does the sun get its energy?
•! Thermonuclear fusion reactions in the sun’s center
What do those words mean?
–! Sun is 16 million degrees Celsius in center: thermo!
–! Enough thermal motion to jam protons together (fusion) despite their mutual repulsion, and make deuterium (heavy hydrogen, nucleus has neutron + proton), then helium (2p+2n): nuclear!
–! Nuclear fusion releases 10~20 million times more energy per atom than chemical reactions, in general
•! Chemical bonds release about 1 eV (electron volt) when broken
•! Nuclear binding energy is about 10 million eV (MeV) per nucleus
4 protons:
mass = 4.029 amu
4He nucleus:
mass = 4.0015
2 neutrinos, photons (light quanta)
Difference: about 0.028 of a proton mass amu = atomic mass unit ~ proton mass
1 amu = 1/12 mass of a carbon atom
1 gram = 6x1023 amu
19
E = mc 2 in Sun
First picture of the Sun in neutrinos!
(Super-Kamiokande, 1998) Picture of the Sun in UV light
(today! see http://umbra.nascom.nasa.gov/images/)
Closeup of a sunspot
•! Helium nucleus is lighter than the four protons!
•! Mass difference is 4.029 - 4.0015 = 0.0276 a.m.u.
–! 1 a.m.u. (atomic mass unit) is 1.6605!10-27 kg
–! tiny difference of 4.58!10-29 kg, but...
–! multiply by c 2 to get 4.12!10-12 J per nucleus formed
–! nuclear fusion is ~20 million times more potent than mere chemical bond breaking (eg, TNT)!
20
Sunspot predictions (NYT, 3/7/06)
•! Sunspots = cooler spots of surface turbulence on sun
–! Mentioned in Chinese records c. 30 BC
–! Telescopic observations by Galileo in 1611
•! Sun activity influences Earth environment •! New observations and computational methods
allow predictions
+1!!
http://www.ngdc.noaa.gov/stp/SOLAR/SSN/ssn.html
21
Revised prediction! This solar cycle will be lower
22
Change happens!
•! Sunspots are known to affect Earth weather, although the exact mechanism is not understood yet
–! Sun pours out protons (MeV energy range) which cause auroras around magnetic poles;
intensity is correlated with sunspot numbers
–! Earth’s magnetic field sometimes links up with Sun’s to form magnetic pathways which help
solar protons reach Earth, or deflect them
•! The number of sunspots observed varies in an 11-year cycle, but the cycles themselves vary in intensity
–! Several historical episodes of few or no sunspots for many years
•! Typically correlated with colder temperatures: “Little Ice Age”
•! 1816 = “the year with no summer”! see http://www.pbs.org/saf/1505/features/lia3.htm
Maunder
Minimum
Dalton
Minimum
23
There are many variables related to our Sun (and climate)
•! This plot shows variations in factors like
–! Precession (wobbling of earth’s rotation axis)
–! Obliquity (tilt of Earth’s axis relative to orbit plane)
–! Eccentricity (elliptical-ness) of Earth’s orbit
Are these connected to glacial recession/advance?
Global warming/cooling?
http://en.wikipedia.org/wiki/Solar_variation
24
...even protons and neutrons have constituents
•! Whack protons and neutrons with electrons and see what happens....
–! "Scattering" experiments
•! Protons do not act like smooth blobs, but rather like a bag full of tiny, hard particles
–! direct evidence for point-like objects making up protons
–! Also: properties of p, n (and other "elementary particles" observed in experiments) have regularities - reminiscent of periodic table!
•! Can "explain" properties of all known subatomic particles by assuming they are combinations of 6 quarks
proton
beam of electrons
scattering
Going deeper into structure of matter
25
Some terminology...
•! "Elementary particles" = objects that make up atoms (n,p,e) or are produced when atoms are smashed (over 200 identified) –! "elementary" because thought to be fundamental in 1950s
•! "Fundamental" particles or constituents of matter –! Truly no known substructure (as of today!)
•! Hadrons = elementary particles subject to strong nuclear force (Greek: hadros = strong) –! protons, neutrons; plus pions, kaons, lambda particles...etc
–! now known to be made of fundamental particles: quarks
•! Leptons = elementary particles subject to weak nuclear force (Greek: leptos = weak)
–! responsible for radioactive decays
–! electrons, plus muons, taus and associated neutrinos
•! All leptons are considered fundamental (as of today!)
26
The Elementary Particles are related
•! Symmetries and connections allow us to deduce structure of elementary particles, and properties of fundamental particles
–! Electric charge of electron and proton are exactly equal and opposite, to remarkable accuracy (13 decimals)
–! Neutrons left alone for about 15 minutes on average will “Beta-decay” (old term for radioactive decay process) into e, p, and
neutrino (very light, chargeless lepton)
neutron
proton
neutrino
electron Poof!
neutron
