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 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)
Lecture Schedule (to end of term) Today 3
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 Radon (Rn) Noble gas (like neon or helium; Percolates out of concrete or bedrock into houses! 4
� � 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 Granite and other common minerals decay products. contain U, and emit Rn – concrete is made from local rocks, so Rn levels vary according to location. !"#$#!!% $%
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 235 U + 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* * In the news: Iran is doing this now Another idea: use U to make another element that is fissionable 238 U + neutron � 239 U � 239 Pu (plutonium, new element not found in nature) 239 Pu 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 6
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 Enrico Fermi • � 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 First reactor, 1943 – � Might be possible to get fast reaction = explosion producing 10 6 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 7
Einstein’s famous letter to Franklin Roosevelt • � Expresses concern about possible German effort in fission research: 8
Modern power reactors • � Nuclear reactors now typically produce about 1000 megawatts of heat energy � steam � turbines � electric generators • � Two varieties use ordinary (“light”) water: Cherenkov light from – � Boiling water reactors (BWRs) electrons emitted into water surrounding a reactor • � Reaction heat boils water moderator, steam is (Daya Bay, China) 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 – � D 2 O 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 D 2 O (Canadian product!) 9
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) 10
Safety issues: waste disposal Radioactivity from spent reactor fuel • � Radioactive waste is the really big 1 becquerel (Bq)=1 decay/sec problem for fission reactors 1 TBq=10 12 Bq – � 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 (10 4 yr!) – � Fuel rods can be chemically processed to extract Pu and other isotopes: security issues! 11
Safety issues • � 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 – � 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!) Chernobyl reactor after fire 12
3. Fukushima reactor disaster, 2011 • � Complex of 6 power reactors on shoreline – started operation 1971-1976 • � 4.4 Gigawatts total power • � 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 • � No radiation deaths; 2 cleanup workers later received potentially dangerous exposures DW, dry well enclosing reactor pressure • � Massive national consequences: electric power vessel; WW, wet well water pool; SFP SFP , , spent fuel pool ar spent fuel pool area; ea; RPV, Reactor shortage, fisheries and farmland ruined, fallout caused Pressure Vessel; SCSW, Secondary abnormal radiation levels as far away as Tokyo Concrete Shield Wall. 13
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 14
• � &'(')*+,-% ()#)*+',&%-&'.'/+'% ./-012.%3+)-)1/.4%% JW’s personal note: • � Our project in Japan: T2K neutrino experiment 12%,'34*% • � Based at particle accelerator lab in Tokai, near Tokyo !"#&'% • � My PhD student Scott Davis was working there on 3/11/11 ! !*)#)0&% !"#$%#& '#(")& !"#$"%
5)647.89%% ):;47<7"):;% :4)=7':"% "0*47>&="7$% Kamioka Tokyo N Tokai 67%8+9(/):%;8%<*=)+0)% !5%
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