Embed Size (px)
Citation preview
OH #1
Month, 1999 (R000)file name.ppt
11
Module 7Fission
OH #2
Month, 1999 (R000)file name.ppt
22
Neutron
U-235
Fission
Fission
Fragments
MoreNeutrons
Gamma
About 200 MeV of energy per fission
Fission - Before
Before the neutron “falls” into the U-235 nucleus:235 bound particles, average mass 1.000 u each1 free neutron with a mass of about 1.009 u
total mass about 236.009 u
234 or 233 particles (two fragments) 0.999 u each2 or 3 free neutrons (typical): mass 1.009 u eachSome gamma rays (no mass)total mass about 235.8 u
The missing mass (0.2 u ≈ 200 MeV) shows up mostly as kinetic energy of the fission products.
Fission - After
After:
Where does the energy come from?
E = mc2 Mass “Disappears” and Energy Appearsin its place.
OH #6
Month, 1999 (R000)file name.ppt
66
Chain Reaction
OH #7
Month, 1999 (R000)file name.ppt
77
Controlled Chain Reaction
Parasitic Absorption
Fission
Leaka
ge
OH #8
Month, 1999 (R000)file name.ppt
88
Units of Reactivity
Neutron Multiplication Factor
generation previous in the neutrons ofnumber generation in this neutrons ofnumber k =
Reactivity
kk
k1k ∆
=−
=ρ
Units of Reactivity
ρ310mk 1 −=
ρ-510pcm 1 =
OH #9
Month, 1999 (R000)file name.ppt
99
Where do neutrons come from?
Beta
Unstable nucleus
GammaDaughterNucleus
Neutron
Delayed Neutrons
Neutron
U-235
Fission
Fragments
Induced Fission
MoreNeutrons
OH #10
Month, 1999 (R000)file name.ppt
1010
U-238
Fission
Fragments
MoreNeutrons
Spontaneous Fission
Where do more neutrons come from?
DeuteriumPhoto-neutrons
OH #11
Month, 1999 (R000)file name.ppt
1111
More about neutrons
Induced FissionMost of the neutrons at high power99.35% of all neutrons
Delayed NeutronsHalf-lives up to about 1 minuteDelay changes in power0.65% of neutrons
Photo-neutronsA decade less than delayed neutronshold power to about 5x10-5 FP after 1
day shutdownLongest lived decay chain with 2.2
MeV gammas is 15 daysUsually considered part of the source
neutrons
OH #12
Month, 1999 (R000)file name.ppt
1212
So What ????
Even more about neutrons
Spontaneous FissionVery small number of neutronsContribution is about 10-12%F.P.Only source in newly fuelled cores and after long shutdowns
Neutron Population in CANDUs is relativelyhigh for significant time periods following shutdown
OH #13
Month, 1999 (R000)file name.ppt
1313
Camel Curve
OH #14
Month, 1999 (R000)file name.ppt
1414
FastSlow or thermal Epi-thermal
Neutron Energies
OH #15
Month, 1999 (R000)file name.ppt
1515
Fast Spectrum of fission neutrons 1-10 MeV, 2 MeV is typical
Slow or thermal Slowed to thermal equilibrium0.0253 eV at 20°C
Epi-thermalanything in between
Neutron Energies
OH #16
Month, 1999 (R000)file name.ppt
1616
Probability of a specific nuclear reactionDepends on
reactiontarget nucleusneutron energy
Cross Sections
OH #17
Month, 1999 (R000)file name.ppt
1717
Cross sections vary with neutron energy
U-235 to a fast neutron
U235 to an epi-thermal neutron
U235 to a slow neutron
OH #18
Month, 1999 (R000)file name.ppt
1818
Cross sections vary with target nucleus
U-238 to a slow neutron
U235 to slow neutron
OH #19
Month, 1999 (R000)file name.ppt
1919
Cross sections vary with specific reaction
U-235 (n,γ) σγ
U235 to fission (n,f) σf
U235 to absorption σa
σa= σγ+ σf
OH #20
Month, 1999 (R000)file name.ppt
2020
Resonance Capture
OH #21
Month, 1999 (R000)file name.ppt
2121
Effect of Enriching Fuel
Natural Uranium Enriched Uranium
0.7 % U-235 2-4% U-235
Chance of a neutron capture in fuel causing a fission is much increased.
Reactors with enriched fuel do not need to return as many atoms to the fuel to achieve criticality
