Physics 116 Session 39 Nuclear physics Dec 5, 2011 Email: - - PowerPoint PPT Presentation
Physics 116 Session 39 Nuclear physics Dec 5, 2011 Email: ph116@u.washington.edu 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
(my office B303 PAB, or B305 conf room next door)
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Today
Includes radium, studied by Marie Walenska-Curie
decays go up 1 in Z and keep the same A
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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.
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– 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
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(under football stands at U. of Chicago!)
– Pile of uranium and carbon blocks (obsolete term: “nuclear pile”) – Historical context
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
write President Roosevelt pointing out danger if Germany develops this first
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
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energy steam turbines electric generators
– Boiling water reactors (BWRs)
used to run turbines – Pressurized water (PWRs)
does not boil
water supply to make steam
– 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
Cherenkov light from electrons emitted into water surrounding a reactor (Daya Bay, China)
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Most common type of power reactor in use today
generates steam for turbines from clean water)
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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!
waste now stored in water tanks
leakage of WW-II era waste containers at Hanford, WA –
– USA just put off settling this, problem, again…
– 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
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– 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
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!)
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
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potentially dangerous exposures
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 SFP , , spent fuel pool ar spent fuel pool area; ea; RPV, Reactor Pressure Vessel; SCSW, Secondary Concrete Shield Wall.
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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)
Industrial-political alliances intrude:
selection, and safety standards
T2K neutrino experiment
accelerator lab in Tokai, near Tokyo
Davis was working there on 3/11/11 !
JW’s personal note:
5)647.89%% ):;47<7"):;% :4)=7':"% "0*47>&="7$% Tokyo Kamioka Tokai
N
!5% 67%8+9(/):%;8%<*=)+0)%
Scott’s photos of scene at J-PARC (Japan proton accelerator research complex)
3-,-I/4%.2-3L-=):% 'C3/.I.2'C3%'M9+M/)%
3-,-I/%12%9-P):%2Q0/):%2. % /G'+R,/C1%
S-.%)2'1*%TL-U/%*/+I*1% L-)%2C9=%!,%-1%D2(-+V%
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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
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
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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
– 1 a.m.u. (atomic mass unit) is 1.660510-27 kg – tiny difference of 4.5810-29 kg, but... – multiply by c 2 to get 4.1210-12 J per nucleus formed – nuclear fusion is ~20 million times more potent than mere chemical bond breaking (eg, TNT)!
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– Mentioned in Chinese records c. 30 BC – Telescopic observations by Galileo in 1611
allow predictions +1!
http://www.ngdc.noaa.gov/stp/SOLAR/SSN/ssn.html
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Revised prediction! This solar cycle will be lower
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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
themselves vary in intensity
– Several historical episodes of few or no sunspots for many years
Maunder Minimum Dalton Minimum
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– 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
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happens....
– "Scattering" experiments
– 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!
assuming they are combinations of 6 quarks proton beam of electrons
scattering
Going deeper into structure of matter
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are produced when atoms are smashed (over 200 identified)
– "elementary" because thought to be fundamental in 1950s
– Truly no known substructure (as of today!)
(Greek: hadros = strong)
– protons, neutrons; plus pions, kaons, lambda particles...etc – now known to be made of fundamental particles: quarks
(Greek: leptos = weak)
– responsible for radioactive decays – electrons, plus muons, taus and associated neutrinos
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elementary particles, and properties of fundamental particles
– Electric charge of electron and proton are exactly equal and
– 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