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Lecture 6

Lecture 6. Applications of Nuclear Physics Fission Reactors and Bombs. 6.1 Overview. 6.1 Induced fission Fissile nuclei Time scales of the fission process Crossections for neutrons on U and Pu Neutron economy Energy balance A simple bomb 6.2 Fission reactors Reactor basics Moderation

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Lecture 6

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  1. Lecture 6 Applications of Nuclear Physics Fission Reactors and Bombs Nuclear Physics Lectures, Dr. Armin Reichold

  2. 6.1 Overview • 6.1 Induced fission • Fissile nuclei • Time scales of the fission process • Crossections for neutrons on U and Pu • Neutron economy • Energy balance • A simple bomb • 6.2 Fission reactors • Reactor basics • Moderation • Control • Thermal stability • Thermal vs. fast • Light water vs. heavy water • Pressurised vs. Boiling water • Enrichment • 6.3 Fission Bombs • Fission bomb fuels • Suspicious behaviour Nuclear Physics Lectures, Dr. Armin Reichold

  3. DEsep≈6MeV per nucleon for heavy nuclei Very slow n DEf=Energy needed to penetrate fission barrier immediately ≈6-8MeV Neutron 6.1 Induced Fission(required energy) Nucleus Potential Energy [MeV] A= 238 Neutrons Nuclear Physics Lectures, Dr. Armin Reichold

  4. 6.1 Induced Fission(required energy) • Spontaneous fission rates low due to high coulomb barrier (6-8 MeV @ A≈240) • Slow neutron releases DEsepas excitation into nucleus • Excited nucleus has enough energy for immediate fission if Ef - DEsep >0 • But due to pairing term … • even N nuclei have low DEsep • odd N nuclei have high DEsep • Fission yield in n-absorption varies dramatically between odd and even N Nuclear Physics Lectures, Dr. Armin Reichold

  5. 6.1 Induced Fission(fissile nuclei) • DEsep(n,238U) = 4.78MeV only  • Fission of 238U needs En>Ef-DEsep≈1.4 MeV • Must be provided by n-kinetic energy • Call this fast fission • Thermally fissile nuclei, Enthermal=0.1eV @ 1160K • 23392U, 23592U, 23994Pu, 24194Pu • Fast fissile nuclei En=O(MeV) • 23290Th, 23892U, 24094Pu, 24294Pu • Note: all Pu isotopes on earth are man made • Note: only 0.72% of natural U is 235U Nuclear Physics Lectures, Dr. Armin Reichold

  6. 6.1 Induced Fission (Reminder: stages of the process) t=0 <# prompt n> n=2.5 t≈10-14 s t>10-10 s <n-delay> td=few s <# delayed n> nd=0.006

  7. 6.1 Induced Fission (the fission process) • Energy balance in MeV: • Prompt: • Ekin(fragments) 167 • Ekin(prompt n’s) 5  3-12 from X+nY+g • E(prompt g) 6 • Subtotal: 178 (good for power production) • Delayed • Ekin(e from b-decays) 8 • E(g following b-decay) 7 • Subtotal: 15 (bad, spent fuel heats up) • Neutrinos 12 (invisible) • Grand total 205 Nuclear Physics Lectures, Dr. Armin Reichold

  8. 6.1 Induced Fission(fission crossections) • 23592U does O(85%) fission starting at very low En • 23892U does nearly no fission below En≈1.4MeV • Consistent with SEMF-pairing term of 12MeV/√A≈0.8MeV between • odd-even= 23592U and even-even= 23892U unresolved, narrow resonances unresolved, narrow resonances 235U 238U

  9. energy range of fission neutrons 23892U(n,g) 23892U(n,g) 23592U(n,f) 23892U(n,g) 23892U(n,f) 23592U(n,g) 23592U(n,f) 23592U(n,g) fast thermal 6.1 Induced Fission(fission probabilities in natural Uranium) absorbtion probabilit per 1 mm

  10. 6.1 Induced Fission(a simple bomb) • Uranium mix • 235U:238U =c:(1-c) • rnucl(U)=4.8*1028 nuclei m-3 • average crossection: • mean free path: • mean time between collisions =1.5*10-9 s @ Ekin(n)=2MeV • Simplify to c=1 (the bomb mixture) • prob(235U(npromptf)) @ 2MeV ≈ 18% • rest of n scatter, loosing Ekin  prob(235U(n,f)) grows • most probable #collisions before 235U(nf) = 6 (work it out!) • 6 random steps of l=3cm  lp=√6*3cm≈7cm in tp=10-8 s Nuclear Physics Lectures, Dr. Armin Reichold

  11. 6.1 Induced Fission(a simple bomb) • After 10-8 s 1n is replaced with n=2.5 n • Let probability of new n inducing fission before it is lost = q • (others escape or give radiative capture) • Each n produces on average (nq-1) new n’s in tp=10-8 s (ignoring delayed n’s as bombs don’t last for seconds!) • if nq>1exponential growths • For 235U, n=2.5  if q>0.4 you get a bomb Nuclear Physics Lectures, Dr. Armin Reichold

  12. 6.1 Induced Fission(a simple bomb) • If object dimensions << 7cm  most n’s escape through surface  nq << 1 • If Rsphere(235U)≥8.7cm  M(235U)≥52 kg  nq = 1  explosion in < tp=10-8 s  little time for sphere to blow apart  significant fraction of 235U will do fission Nuclear Physics Lectures, Dr. Armin Reichold

  13. 23892U(n,g) 23892U(n,g) 23592U(n,f) 23892U(n,g) 23892U(n,f) 23592U(n,g) 23592U(n,f) 23592U(n,g) 6.2 Fission Reactors(not so simple) • Q: What happens to a 2 MeV fission neutron in a block of natural Uranium (c=0.72%)? • A: In order of probability • Inelastic 238U scatter • Fission of 238U (5%) • rest is negligible • Eneutron decreases • s(23892U(n,g)) increases and becomes resonant • s(23892U(n,f)) dercreases rapidly and vanishes below 1.4 MeV • only remaining change for fission is s(23592U(n,f)) wich is much smaller then s(23892U(n,g)) • Conclusion: piling up natural U won’t make a reactor. I said it is not SO simple Nuclear Physics Lectures, Dr. Armin Reichold

  14. 6.2 Fission Reactors(two ways out) • Way 1: Thermal Reactors • bring neutrons to thermal energies without absorbing them = moderate them • use low mass nuclei with low n-capture s as moderator material. (Why low mass?) • sandwich fuel rods with moderator (and coolant) layers • when n return from moderator energy is so low that it will predominantly cause fission in 235U Nuclear Physics Lectures, Dr. Armin Reichold

  15. 6.2 Fission Reactors(two ways out) • Way 2: Fast Reactors • Use fast neutrons for fission • Use higher fraction of fissile material, typically 20% of 239Pu + 80% 238U • This is self refuelling (breeding) via: • 23892U+n  23992U + g • 23993Np + e- + ne • 23994Pu + e¯ + ne • Details about fast reactors later Nuclear Physics Lectures, Dr. Armin Reichold

  16. 6.2 Fission Reactors (Pu fuel) • 239Pu fission crossection slightly “better” then 235U • Chemically separable from 238U (no centrifuges) • More prompt neutrons n(239Pu)=2.96 • Fewer delayed n & higher n-absorbtion, more later

  17. 6.2 Fission Reactors (Reactor control) • For bomb we found: • “boom” if: nq > 1 where n was number of prompt n • Reactors use control rods with large n-capture s (B, Cd) to regulate q • Lifetime of prompt n: • O(10-8 s) in pure 235U • O(10-3 s) in thermal reactor (long time in moderator) • Far too fast to control • … but there are also delayed n’s Nuclear Physics Lectures, Dr. Armin Reichold

  18. Energy 6.2 Fission Reactors (Reactor control) • Fission products all n-rich  all b- active • Daughters of some b- decays can directly emit n’s (see table of nuclides, green at bottom of curve) • several sources of delayed n’s • typical t≈O(1 sec) • Fraction nd ≈ 0.6%

  19. 6.2 Fission Reactors (Reactor control) • Since fuel rods “hopefully” remain in reactor longer then 10-2 s  must include delayed n fraction nd • New control problem: • keep (n+nd)q = 1 • to accuracy of < 6% • at time scale of few seconds • Doable with mechanical system but not easy Nuclear Physics Lectures, Dr. Armin Reichold

  20. 6.2 Fission Reactors (Reactor cooling) • As q rises, power produced in reactor rises  • cool reactor and drive “heat engine” with coolant • coolant will also act as moderator • Coolant/Moderator choices: Nuclear Physics Lectures, Dr. Armin Reichold

  21. 6.2 Fission Reactors (Thermal Stability) • Want dq/dT < 0 • Many mechanical influences via thermal expansion • Change in n-energy spectrum • Doppler broadening of 238U(n,g) resonances  large negative contribution to dq/dT • Doppler broadening of 239Pu(n,f) in fast reactors gives positive contribution to dq/dt • Chernobyl No 4. had dq/dT >1 at low power Nuclear Physics Lectures, Dr. Armin Reichold

  22. 6.2 Fission Reactors (Thermal vs. Fast) • Fast reactors • need very high 239Pu concentration   Bombs • very compact core   hard to cool   need high Cp coolant likeliq.Na or liq. NaK-mix   don’t like water & air &  must keep coolant circuit molten &  high activation of Na • High coolant temperature (550C)  good thermal efficiency • Low pressure in vessel   better safety • can utilise all 238U via breeding   141 time more fuel • High fuel concentration + breading   Can operate for long time without rod changes • Designs for 4th generation Pb or gas cooled fast reactors exist. Could overcome the Na problems Nuclear Physics Lectures, Dr. Armin Reichold

  23. Nuclear Physics Lectures, Dr. Armin Reichold

  24. Nuclear Physics Lectures, Dr. Armin Reichold

  25. 6.2 Fission Reactors (Thermal vs. Fast) • Thermal Reactors • Many different types exist • BWR = Boiling Water Reactor • PWR = Pressure Water Reactor • BWP/PWR exist as • LWR = Light Water Reactors (H2O) • HWR = Heavy Water Reactors (D2O) • (HT)GCR = (High Temperature) Gas Cooled Reactor exist as • PBR = Pebble Bed Reactor • other more conventional geometries Nuclear Physics Lectures, Dr. Armin Reichold

  26. 6.2 Fission Reactors (Thermal vs. Fast) • Thermal Reactors (general features) • If moderated with D2O (low n-capture)   can burn natural U   now need for enrichment (saves lots of energy!) • Larger reactor cores needed   more activation • If natural U used  small burn-up time   often need continuous fuel exchange   hard to control Nuclear Physics Lectures, Dr. Armin Reichold

  27. 6.2 Fission Reactors (Light vs. Heavy water thermal reactors) • Light Water •  it is cheap •  very well understood chemistry •  compatible with steam part of plant • can not use natural uranium (too much n-capture)   must have enrichment plant   bombs • need larger moderator volume   larger core with more activation • enriched U has bigger n-margin   easier to control Nuclear Physics Lectures, Dr. Armin Reichold

  28. 6.2 Fission Reactors (Light vs. Heavy water thermal reactors) • Heavy Water •  it is expensive •  allows use of natural U • natural U has smaller n-margin   harder to control • smaller moderator volume   less activation • CANDU PWR designs (pressure tube reactors) allow D2O moderation with different coolants to save D2O Nuclear Physics Lectures, Dr. Armin Reichold

  29. 6.2 Fission Reactors (PWR = most common power reactor) • Avoid boiling   better control of moderation • Higher coolant temperature   higher thermal efficiency • If pressure fails (140 bar)   risk of cooling failure via boiling • Steam raised in secondary circuit   no activity in turbine and generator • Usually used with H2O   need enriched U •  Difficult fuel access  long fuel cycle (1yr)   need highly enriched U • Large fuel reactivity variation over life cycle   need variale “n-poison” dose in coolant

  30. 6.2 Fission Reactors (BWR = second most common power reactor) • lower pressure then PWR (70 bar)   safer pressure vessel •  simpler design of vessel and heat steam circuit • primary water enters turbine   activation of tubine   no access during operation (t½(16N)=7s, main contaminant) • lower temperature   lower efficiency • if steam fraction too large (norm. 18%)   Boiling crisis = loss of cooling

  31. 6.2 Fission Reactors (“cool” reactors)

  32. 6.2 Fission Reactors (“cool” reactors) • no boiling crisis • no steam handling • high efficiency 44% • compact core • low coolant mass

  33. 6.2 Fission Reactors (enrichment) • Two main techniques to separate 235U from 238U in gas form UF6 @ T>56C, P=1bar • centrifugal separation • high separation power per centrifugal step • low volume capacity per centrifuge • total 10-20 stages to get to O(4%) enrichment • energy requirement: 5GWh to supply a 1GW reactor with 1 year of fuel • diffusive separation • low separation power per diffusion step • high volume capacity per diffusion element • total 1400 stages to get O(4%) enrichment • energy requirement: 240GWh = 10 GWdays to supply a 1GW reactor with 1 year of fuel Nuclear Physics Lectures, Dr. Armin Reichold

  34. 1-2 m 15-20 cm O(70,000) rpm Vmax≈1,800km/h = supersonic! & gmax=106g  difficult to build! Nuclear Physics Lectures, Dr. Armin Reichold

  35. 6.2 Fission Reactors (enrichment) Nuclear Physics Lectures, Dr. Armin Reichold

  36. 6.3 Fission Bombs (fission fuel properties) • ideal bomb fuel = pure 239Pu

  37. 6.3 Fission Bombs (where to get Pu from?) a. Pu-241 plus Am-241. c. Plutonium recovered from low-enriched uranium pressurized-water reactor fuel that has released 33 megawatt-days/kg fission energy and has been stored for ten years prior to reprocessing (Plutonium Fuel: An Assessment (Paris:OECD/NEA, 1989) Table 12A). d. Plutonium recovered from 3.64% fissile plutonium MOX fuel produced from reactor-grade plutonium and which has released 33 MWd/kg fission energy and has been stored for ten years prior to reprocessing (Plutonium Fuel: An Assessment(Paris:OECD/NEA, 1989) Table 12A). Nuclear Physics Lectures, Dr. Armin Reichold

  38. 6.3 Fission Bombs (drawbacks of various Pu isotopes) • 241Pu: decays to 241Am which gives very high energy g-rays  shielding problem • 240Pu: lots of spontaneous fission n • 238Pu: decays quickly  lots of heat conventional ignition explosives don’t like that! • in pure 239Pu bomb, ignition timed optimally during compression using burst of n  maximum explosion yield • … but using reactor grade Pu, n from 240Pu can ignite bomb prematurely  lower explosion yield but still a very bad bomb • Reactor grade Pu mix has drawbacks but can “readily” be made into a bomb. Nuclear Physics Lectures, Dr. Armin Reichold

  39. 6.3 Fission Bombs (suspicious behaviour) • Early removal of fission fuel rods  need control of reactor fuel changing cycle! • Building fast breaders if you have no fuel recycling plants • Large high-E g sources from 241Am outside a reactor • large n fluxes from 240Pu outside reactors very penetrating  easy to spot over long range Plutonium isotope composition as a function of fuel exposure in a pressurized-water reactor, upon discharge. Nuclear Physics Lectures, Dr. Armin Reichold

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