Bent Lauritzen Risø DTU Fission og kernekraft Bent Lauritzen Risø DTU

Radioaktivt henfald, fission Kædereaktion og kritikalitet Overview Atomer og kerner Radioaktivt henfald, fission Kædereaktion og kritikalitet Neutronregnskab Reaktorer 2 2

Elektroner (negative) Ladning Masse Energi Proton +e 2 × 10-27 kg Atomer og kerner Elektroner (negative) 10-10 m 10-14 m Ladning Masse Energi Proton +e 2 × 10-27 kg 938 MeV Neutron 939 MeV Elektron -e 1 × 10-30 kg 0.511 MeV Atomkerne Neutron (elektrisk neutral) Proton (positivt elektrisk ladet) 3 3

Atomer og kerner Antal protoner Navn Symbol 1 brint H 2 helium He 26 jern Fe 47 sølv Ag 92 uran U Isotoper Brint, stabil, 1H Deuterium (D), stabil, 2H Tritium (T), ustabil, 3H proton neutron

Nuklidkortet Atomkernerne Antal protoner (Z) Antal neutroner (N)

Z=N 235U

Radioaktivt henfald, fission Reaktion Stråling a (alfa) (Z,N) (Z-2,N-2) helium kerne b- (beta –) (Z,N) (Z+1,N-1) elektron b+ (beta +) (Z,N) (Z-1,N+1) positron g (gamma) (Z,N) (Z,N) gamma foton a b+ b-

Uranium series a b

Stabilitet af kerner Nuclear and Coulomb forces

Bindingsenergi

Fission og fusion Nuclear binding energy n U Fission D T He n Fusion

Fission and fusion processes Nuclear binding energy n n U Fission Binding energy:

Fission and fusion processes Nuclear binding energy D T He n Fusion

Fission and fusion processes Mass per nucleon D T He n n U Fusion Fission

Energy released in fission of U-235 235U + n  144Ba + 90Kr + 2n + 200 MeV Fission energy: Fossil fuel: C + O2  CO2 + 4.1 eV

Energy released in fission MeV Kinetic energy of fission fragments 168 Prompt gamma radiation 5 Kinetic energy of fission neutrons Beta decay of fission products 7 Gamma decay of fission products 6 Neutrinos 11 Total 202

Energy released in fission Energi MeV Kinetisk energi af fragmenter 165 Prompte neutroner 5 Prompt gammastråling 7 Radioaktivt henfald 25 Total 202

Z=N 235U

Opgave: Fission af 1g U-235 Ved fission af et uran-235 atom frigøres ca. 200 MeV. Hvor mange MWd (mega-watt døgn) termisk energi fås ved spaltning af 1g 235U?

Kædereaktion og kritikalitet

Fissionstværsnit

Kædeproces DD

Nuclear chain reaction Effective multiplication factor (keff): No. fission neutrons in one generation / neutrons in previous generation keff < 1 subcritical keff = 1 critical keff > 1 supercritical Neutron economy: neutron leakage fission and non-fission cross sections lp = time between generations Safe reactor operation: keff < 1.001

Nuclear chain reaction Effective multiplication factor (keff): No. fission neutrons in one generation / neutrons in previous generation keff < 1 subcritical keff = 1 critical keff > 1 supercritical Start up : keff > 1 Safe reactor operation: keff < 1.001

no. of neutrons per fission fission vs. non-fission cross sections Neutronregnskab Neutron economy: no. of neutrons per fission fission vs. non-fission cross sections neutron leakage

Fission cross section – 235U

Fission cross section – 238U

Nuclear chain reaction Kædereaktion (kritikalitet) er ikke mulig i naturligt uran (0.7% 235U) Men er mulig med Hurtige neutroner, 10-20% beriget uran Moderator (H2O, D2O, grafit), termiske neutroner og naturligt/svagt beriget uran

Nedbremsning, indfangning Neutron-regnskab Hurtig fission e Nedbremsning, indfangning p Absorption i brændsel f Fission h

Neutron-regnskab

Reaktor kontrol Antal neutroner Prompte neutroner Forsinkede neutroner

lp = time between generations Reaktor kontrol Antal neutroner Prompte neutroner Reaktoren kan ikke kontrolleres af mennesker! lp = time between generations

Reaktor kontrol Antal neutroner Forsinkede neutroner Reaktoren kan kontrolleres!

n + 235U  fission fragments + n + n + … Formeringsreaktorer Fuel breeding: Neutron yield (h): No. produced neutrons per captured thermal neutron n + 235U  fission fragments + n + n + … n + 238U  239U  239Pa  239Pu Nuclide 233U 235U 239Pu Nat. uran. h 2.28 2.05 2.09 1.3

 Fast neutrons needed for breeder reactors η for 233U, 235U og 239Pu 233U 235U 239Pu  Fast neutrons needed for breeder reactors

Back Reaktorer

Vigtige reaktorkomponenter Reaktortank: brændsel (235U, 239Pu..) moderator (vand, grafit, tungt vand) kølemiddel (vand, gas, Na) kontrolstænger (bor, Cd, Hf) Afskærmning (beton, jern, bly, vand) Reaktorindeslutning (stål, beton)

Nuclear reactors by type Thermal (LWR, HWR, GCR) Fast (FBR, LMFBR) Fuel 3-5% U-235 Pu-239 10-20% U-235 Th-232 + U-233 Moderator H2O, D2O Graphite __ Coolant CO2, He Liquid metals, metal alloys

Boiling Water Reactor - BWR

Pressurized Water Reactor - PWR