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Explain fast and slow fission Know how fast and slow fission are appliedDemonstrate critical massDemonstrate power production by nuclear fissionDemonstrate the difference between fission and fusion reactionsRecognize that a mass greater than the critical mass is needed to produce an uncontrollable chain reaction.
Explain fast and slow fission Know how fast and slow fission are appliedDemonstrate critical massDemonstrate power production by nuclear fissionDemonstrate the difference between fission and fusion reactionsRecognize that a mass greater than the critical mass is needed to produce an uncontrollable chain reaction.
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All you need to know is..1. Nuclear Physics is about probabilities (interaction).2. Particle interactions can be understood in terms of simple billiard balls.3. You have to think like a neutron
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MODULE 2 , CHAPTER 2• Actinides fission and some isotopes can sustain a chain reaction• 2 types of fission – spontaneous and induced fission• Can look up isotope characteristics• A sustained chain reaction can be used for peaceful and non-peaceful purposes
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Uncontrolled Nuclear Chain Reaction 10-8 s Fission =10 ns Number = Shake Gen # neutrons 1 20=1 2 31=3 3 32=3x3=9 4 33=3x3x3=27 5 24=16 6 25=32 64 263=9x1018 80 279=6x1023 81 280=1.2x1024In U-235 Metal / Pu-239 Metal 82 281=2.4x1024This happens very fast!
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Energy/Fission 0.94 kg Of U-235= 200 MeV= 3.2e-11 J 82 generationsKnee Bend = 100 J = 2^81 = 2.4x1024 fissions < 1x10-6 s = 1 millionth of a secondUNCONTROLLED Total Energy = (2.4e24 fis)(3.2x10-11 J/fis)CHAIN REACTION = 7.68e13 J = 18.3,000 t of TNT! = 18.3 kT TNT = 1.2 X Little Boy Bomb!
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Fissile Materials: Can sustain an explosive ﬁssion chain reaction – notably plutonium of almost any isotopic composition and highly-enriched uranium (Def’n, IPFM). NeutronsU-235 Density HIGH Mouse Trap Unclamping FAST Simulates fission U-235 35
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Critical Mass=Sustain Chain Reaction GUN TYPE WEAPON SELF SUSTAINING = CRITICAL MASS ESCAPE ABSORBED FISSIONS STOPS AFTER SECOND FISSIONS SELF WITH REFLECTOR GENERATION SUSTAINING (CONFINING) INCREASING FISSILE MATERIAL MASS Prevent neutrons from escaping is the goal! 38Figure: Courtesy Wikipedia
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Crowded Room Analogy: 2 Ways to Sustain a Chain REACHING CM BY Reaction INCREASING SIZE OF ROOMINCREASING DENSITY Fissile Material Neutron IMPLOSION WEAPON GOAL: INCREASE Bring nuclei PROBABILITY OFcloser together GUN-TYPE WEAPON FISSION TO OCCUR
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Theodore Taylor (1925-2004) 1 Significant Quantity =25 kg U-235 HEU HEU= >20% U-235 See comment: http://goo.gl/b04i0“Amounts of U-235 as small as 1 kg are significant quantities. He did not state that anyone can build a bomb with 1 kg of U-235, but did suggest that this is roughly the amount that good designer would need” Consistent with 82 gen’ns NRC 2004 Testimony, “Nuclear Arms Race”, Craig & Jungerman
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Because of the highdensity of uraniummetal a SQ willoccupy a small volume!
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1) Prevent neutrons from escaping 2) Increase the probability of fissions 3) Prevent neutron absorptionGUN TYPE BOMB Two pieces of subcritical HEU brought together for an instant! RECALL: Crowded room analogy make the room bigger so the probability of fission increases before neutrons escape!
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With modern weapons-grade uranium, thebackground neutron rate is so low thatterrorists, if they had such material, wouldhave a good chance of setting off a high-yield explosion simply by dropping one halfof the material onto the other half.Mostpeople seem unaware that if separate HEUis at hand its a trivial job to set off anuclear explosion . . . even a high schoolkid could make a bomb in short order.Luis Alvarez, Adventures of a Physicist (Basic Books, 1987), p. 125.
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• Not very safe – 2 pieces that are subcritical but if inadvertently combined could cause an explosion.• Must keep away from moderators• Not very efficient – not very high yield• Weapon of choice for non-state actors (terrorists)• Barrier is getting the HEU in the first place!
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• Uranium is mined but is only 0.711% U-235 and 99.289% U-238.• Need to remove the U-238 to increase the proportion of U-235 to U-238.• Start with 7 U-235 and 993 U-238 marbles
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• Start with 7 U-235 and 993 U-238 marbles• To get to 5% HEU it means that 7/140=0.05• I have to go from 993 to 133!• I need to pull out 860 U-238 marbles!• A large part of the work is done to get from 0.711% to 5% HEU!
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• 7 U-235 /133 U-238 = 5%• How do I increase to 90%?• 90% ~ 7 U-235/ (7 U-235 + 1 U-238)• I go from 133 to 1! Need to subtract 132 more which is a big improvement from 853!• BOTTOM LINE: Enriching from 0.711% to 5% is a large part of the work.
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It has to do with “pre-detonation” or triggering the bombto explode before the optimum conditions have beenEstablished.Nuclide SF /kg per Neutrons/fiss CM (kg) SF/CM 100 μsec ion per 100 μsecU-235 5.627e-7 2.637 45 0.0000 25U-238 6.776e-4 2.1Pu-239 6.916e-4 3.172 17 0.0011Pu-240 48.33 2.257 82299 % Pu-239 + 1% Pu-240 8 BOTTOM LINE: Gun Type Will Not Work for Plutonium!
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1) Prevent neutrons from escaping 2) Increase probability of fissions 3) Prevent neutron absorption IMPLOSION TYPE BOMB Done with explosive Lenses causing implosionRECALL: Crowded room analogy Increase the density ofnuclei so the probability of fission increasesbefore neutrons escape!
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For Plutonium need to use implosion technique! Increase Density! 2 3 6 HEU or Pu is 4 1 7 5 compressed to Critical Mass (bring atoms Closer together)
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Think aboutCompressing aWater balloon Between your fingers! Not easy!
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We can get HEU by enriching natural U what about Pu?
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Pu Production in Reactors U-238 For Gun Type Bomb U-235 For Implosion n + U-238 U-239 Np-239 Pu-239 Type Bomb 24 min 2.4 days Fissile Material Fertile Material (can sustain chain rxn)(can’t sustain chain rxnbut can become fertile) U-238 U-235 Pu-239 is Produced
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• Fissionable = Nuclide undergoes fission after neutron capture• Fissile = Nuclide can sustain a chain reaction (U-235, Pu-239)• Fertile = Nuclide can become fissile after irradiation in a reactor (ex: U- 238)
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Trick: Pu-239 Absorption of neutron Pu-239 94 p 145 n + = Pu-240 94 p 146 n HIGHTo produce WG Pu SF RATEneed to remove Pu fuel (not good for(generally U fuel is 1% Pu) NW’s)WG = <7% Pu-240 so that SF rate is relatively low!
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Plutonium Production in Nuclear Reactors Plutonium is produced in Nuclear Reactors whereas uranium is found in nature Neutron activation on Pu-239 produces Pu-240 (bad for NW)% Pu-239/Pu-240
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A Policy Analysts Rule of Thumb1 megawatt-day of operation produces ~ 0.9 gram of plutonium in any reactor using 20-percent or lower enriched uranium Power = 2650 MW Mass Pu-239 = 0.9*2650MW *45 days /1000 g/kg = 106 kg not quite 80 kg
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EXAMPLE• A company is making a proposal to sell one turn-key LWR’s (2700 MW) to Burma. You are the one responsible for deciding whether a reactor project should proceed from the point of view of proliferation of the nuclear technology to that country. The official in a condescending way claimed that this particular reactor will in one month not produce any Plutonium so that there are absolutely “no risks or worries that should concern you”. What would you say (in a polite way) in response to the official about how much Pu is produced in this reactor/month in terms of SQ?• 1kg = 1000 g, And the rule of thumb is: 1 megawatt-day of operation produces ~ 1 gram of plutonium in any reactor using 20-percent or lower enriched uranium 60
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Answer:• Since the rule of thumb is 1 g produced per MW days then in 30 days the reactor produces 2700 MW * 30 days =80,000 MW days• Applying the rule of thumb = 80,000 MW days = 80,000* MW days * (1 g Pu/MW days) =80,000 g Pu and 1 kg = 1000 g =80,000 g Pu * (1 kg / 1000 g)• =80 kg Pu• In terms of SQ this is 10 SQ which is enough roughly for 10 bombs!“Excuse me but your assertion that a 2700 MW reactor will produce only 100 gPu/month is simply not correct. In fact, if you go through the calculation which Ijust did while you were talking, you will find that the reactor you proposeproduces 10 IAEA Significant Quantities per month!” (And the whole room will besilent because they will be impressed at your knowledge) 61
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QUESTION: In the satellite pictures below why is the lack of steam significant?
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“I remembered the line from the Hindu scripture, the Bhagavad-Gita: Vishnu is trying to persuade the Prince that he should do his duty and to impress him he takes on his multi-armed form and says, “Now I am become death, the destroyer of worlds” I suppose we all thought that, one way or another” R. Oppenheimer, pg 676, R. Rhoades, “The Making of the Atomic Bomb” On the Trinity Shot : first test of an atomic weapon.
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MODULE 2 , CHAPTER 3Material Advantage Disadvantag e U (HEU) Testing not Difficult to Necessary get HEU Pu Easy to Testing Produce Necessary
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Power Reactors: NW: HEU,Pu LEU Large Amount of Energy Large Amount of Energy Slow, Controlled Release of Energy FAST , Uncontrolled Release of Energy
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Energy is expendedSlowly over the period of many months.
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Now the Details!How Do You Get A Self-Sustaining Reaction : An Analogy
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• Neutron moving slow will easily fission U-235 but not U-238• Neutron moving fast will easily fission U-238 but also U-235• Neutron are produced through fission at high energy• It is almost as if the neutron at low energy is completely different from a neutron travelling at high energies
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• Like people meeting..• When neutron encounters an isotope, it must make a decision how it interacts when it meets 1) Fission 2) Bounce off 3) Be absorbed (give off a gamma) 4) Not interact at all Probabilities described by Size of the target (cross-section)
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U-238 Neutron (Very small target neutron will miss) U-235 Neutron(Large targetneutron will hit)
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• Neutrons can also do other things besides fission (bounce off, be absorbed, not interact)• It all has to do with the size of the target which changes for different neutron energies• You have to think like a neutron
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• Graph of ‘Energy’ or speed of the Neutron vs ‘Cross-section’ or probability of interaction or size of the targetCROSS-SECTION PROBABILITY NEUTRON SPEED OR ENERGY
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Target size of U-235 1000X bigger than at 1 MeV Energy Region of neutrons produced by FISSION Size of the target For U-238Energy Region of neutronsslowed down Thermal Neutrons
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Gamma Ray MeV Incoming Neutron 2-3 Neutrons Fission free to fission U-235Slow Neutron (Thermal) MeV Incoming Neutron Gamma U-238 Ray Tend to absorb No Fission Not fission! Radiative Capture
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INCREASE U-235 (Red!) Will Fission! 50% U-238, 50% U-235 : PROBABLY SELF SUSTAINING Will Fission! Fission! Enough U-235 aroundto continue process Fission! Will Absorb!
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Natural Uranium = 0.711 % U-235! Need to get to 50% Need to enrich $
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Neutrons from fission (MeV) Appear to require 50% U-235and 50% U-238 – this is expensive!THE TRICK: Slow the neutrons down! Moderate them!
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Essentially turn Into From a Neutron point of view(Neutron is travelling slowly and feels theattraction of the neutron for a longer time)
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• Possible to have self-sustaining chain reaction by using a moderator.
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Need less U-235 because they appear bigger for Slow/thermal neutrons Instead of 50% U-235with trick can use 3-5% U-235
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U-235 atoms have a higher fissioninteraction probability than U-238 capture Need only 3-5% to Start a sustained Chain Reaction
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Go from here to here In a few bouncesModeration TRICK! FISSION ENERGIES
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FUEL (assembly/rods) 3% 3% 3% LEU LEU LEU UOxide UOxide UOxide Nuclear Reactor has Fissile Material (LEU) just below a Critical Mass (Controlled Chain Reaction) Fission Energy =Heat Produced!Moderator: Slows down neutrons This is the TRICK! so fission with U-235 can occur
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Moderator CharacteristicWater (H2O) Good moderator but can also capture neutrons. Can use 3-5% U-235. Most reactors around the world use H2O as moderatorHeavy Water (D2O) Excellent moderator. Low probability of neutron capture. Can start with natural (0.7%) uranium.Carbon (Graphite) Not a great moderator (mass A is high) but very small capture probability. Can use natural (0.7%) uranium as long as very pure and no neutrons present.
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• Isotopes that absorb neutrons. Why would it be useful? Gamma Incoming Neutron Gd-155 Ray No Fission Looks HUGE for incoming Radiative Capture neutron!
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Contain Gd rather than UOx Dropped in by gravity
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Pressurized Water Power Reactor (PWR)So far we have concentrated on the core 98
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Partial Batch Refueling A = Removed after 1 cycle B A C B= Removed after 2 cycles C= Removed after 3 cycles Neutron FluxLess at Outside LIKE A BARBECUE!
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MODULE 2 , CHAPTER 4• Basics of how a nuclear reactor works at the core level• The trick was Think like a neutron• Moderators and the moderation trick• Neutron poisons and control rods• Nuclear fuel• Fission products
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•Open pit mining•Underground Mining•In Situ Leaching•Sea Water Recovery
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Excavated uraniumore milled (crushed)processed into yellowcake
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Excavated uraniumore milled (crushed)processed into yellowcake
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Removal of500 t Yellowcakefrom Iraq (2008) Natural U contains 99% U-238 which is not very radioactive You could not make a dirty bomb out of natural uranium
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HF+fluorine gas mixed with yellowcake produces UF6 crystals(at room temperature it is a solid) At this point: Proliferation Risk
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3-5% for LWR 85-95% for Weapon Utilizing 1% mass difference between isotopes to separate themRemove U-238 until desired ratioU-235/U-238 is reached (93% is WG)
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Gas is “spun” in a supersonic centrifuge, forcing lighter U-235 to the top, where it can be “scooped off” This demands high strength materials and precision engineering. To reach speeds of 100,000 rpm, centrifuges need: Light, strong rotors Well-balanced rotors High-speed bearings (usually magnetic) to reduce friction
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Separation < 1 kg SWU/yrSeparation much better 300kg SWU/yr
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Pellets are placed in fuel rodsrods are combined into fuelassemblies
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Must be secured to prevent non-state actors from getting the material
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1.E+07 1.E+06 PYRO Product 1.E+05 NOT IN PYRO 1.E+04 Pu-Odd Pu-Even 1.E+03 Ci/MTHM Np-237 Cs-137 + Sr-90 1.E+02 Fission Products Excluding 1.E+01 Tc-99 + I-129 TOTAL 1.E+00 TOTAL LOW 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Am 1.E-01 1.E-02 1.E-03 Years After Discharge: The radioactivity profile of SNF throughout time calculated by the author with ORNL’s Scale6 code system [SCALE6] for a Westinghouse 50 MWd/kgHM and 4.5% enriched PWR fuel assembly. The dotted line indicates the time when the SNF cooled in reactor cooling ponds are moved to interimstorages such as dry casks. The actinides are generally represented by the thicker lines and thinner lines correspond to the actinides. Notice thatafter the fission products (especially Cs-137 and Sr-90) decay the actinides, and Tc-99 and I-129 will start to dominate the profile. Notice also thatthe pyroprocessing products will after several decades be at the level of hundreds of Ci and will be below the level that the IAEA considers self-protective [Kang and von Hippel] since the fission products that would produce a self-protective dose are removed in pyroprocessing (see NOT INPYRO label).
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Spent Fuel Problem• Spent fuel remain radioactive for many years• Very hot needs to be cooled down• Cooled in a spent fuel pond for 3-5 yrs• Further cooling in dry casks• Future: emplaced in geological repository or other measures• So far no solution while we have 250k MT waste
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Interim StoragePassive Air Cooling: Carries the heat not the radioactivity!
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• Closed fuel cycle – recycle plutonium produced in other fuel• “Russian policy is to close the fuel cycle as far as possible and utilize recycled uranium, and eventually also to use plutonium in MOX fuel. However, its achievements in doing this have been limited - in 2011 only about 16% of used fuel was reprocessed.” (WNA see: http://goo.gl/Zrtvn)• The United States does not reprocess fuel although does advocate research in this area• Controversial because of use of plutonium
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MODULE 2 , CHAPTER 5• Fuel cycle is complex• Open cycle – no reprocessing• Closed cycle – reprocess• Vulnerabilities – After enrichment step if cycle is closed and plutonium is used• Nations differ on policy for reprocessing (US does not reprocess, Russian Federation does)• More detail on all of this in the next Module!
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