nuclear reactors, bau, 1 st semester, 2007-2008 (saed dababneh). 1 neutron attenuation (revisited)...
Post on 26-Mar-2015
216 Views
Preview:
TRANSCRIPT
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
1
Neutron Attenuation (revisited)
Xeraction
Xeractionno
t
t
eXP
eXP
1)(
)(
int
int
Recall t = N t
Probability per unit path length.
X
I0 I
Probability
mfp for scattering s = 1/s
mfp for absorption a = 1/a total mfp t = 1/t
XteIXI 0)(
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
2
Recall Ft = t I N = I t Simultaneous beams, different intensities, same energysame energy.
Ft = t (IA + IB + IC + …) = t (nA + nB + nC + …)vIn a reactorreactor, if neutrons are moving in all directionsall directions
n = nA + nB + nC + …
Ft = t nv
neutron flux = nv
Reaction Rate Rt Ft = t = /t (=nvNt)
Neutron Flux and Reaction Rate
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
3
Different energiesDifferent energiesDensity of neutrons with energy between E and E+dEn(E)dEReaction rate for those “monoenergetic” neutronsdRt = t(E) n(E)dE v(E)
0
)( dEEnn
0
)()( dEEEn
00
)()()()()( dEEEdEEEnER ttt
0
)()( dEEER ii
Neutron Flux and Reaction Rate
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
4
Neutron Flux and Reaction Rate
In general, neutron flux depends on:• Neutron energy, E.• Neutron angular direction, • Neutron spatial position, r.• Time, t.
Various kinds of neutron fluxes (depending on the degree of detail needed).
Time-dependent and time-independent angular neutron flux. ),,,( tE Ωr
),,( ΩEr
In Thermal ReactorsThermal Reactors, the absorptionabsorption rate in a “medium” of thermal (Maxwellian) neutrons
Usually 1/v cross section, thus
then
The reference energy is chosen at 0.0253 eV. • Look for Thermal Cross Sections.• Actually, look for evaluated nuclear data.
000000 )()()()( EnvEdEEnvER aa
Thermal
aa
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
5
Neutron Flux and Reaction Rate
Thermal
aa dEEvEnER )()()(
)()(
)( 0
0 Ev
v
E
E
a
a
Reference
2200 m/s flux2200 m/s flux
Show that, after elasticelastic scattering the ratio between the final neutron energy E\ and its initial energy E is given by:
For a head-on collision:
After n ss-wave-wave collisions:where the average change in lethargy lethargy is
HW 6HW 6
2
222
2
2\
)1(
sincos
)1(
cos21
A
A
A
AA
E
E CM
2
min
\
1
1
A
A
E
E
nEEn lnln \
1
1ln
2
)1(1ln
2
\
A
A
A
A
E
Eu
av
6Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Neutron Moderation
)ln( EEu M
Reference
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
7
Neutron Moderation HW 6 HW 6 (continued)(continued)
• Reproduce the plot.• Discuss the effect of the thermal motion of the moderator atoms.
Neutron Moderation HW 6 HW 6 (continued)(continued)
Neutron scattering by light nuclei then the average energy loss and the average fractional energy loss
• How many collisions are needed to thermalize a 2 MeV neutron if the moderator was:
1H 2H 4He graphite 238U ?• What is special about 1H?• Why we considered elastic scattering?• When does inelastic scattering become important?
8Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
EE )1(21\
EEEE )1(21\
)1(21
E
E
Nuclear Fission
~200 MeV
Fission
Fusi
on
Coulomb effectSurface effect
9Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission• B.E. per nucleon for 238U (BEU) and 119Pd (BEPd) ?• 2x119xBEPd – 238xBEU = ?? K.E. of the fragments 1011 J/g• Burning coal 105 J/g• Why not spontaneous?• Two 119Pd fragments just touching The Coulomb “barrier” is:
• Crude …! What if 79Zn and 159Sm? Large neutron excess, released neutrons, sharp potential edge, spherical U…!
MeVMeVfm
fmMeVV 2142502.12
)46(.44.1
2
10Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
• 238U (t½ = 4.5x109 y) for -decay.• 238U (t½ 1016 y) for fission.• Heavier nuclei??• Energy absorption from a neutron (for example) could form an intermediate state probably above barrier induced fission.• Height of barrier above g.s. is called activation energy.
11Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
Liquid Drop
Shell
Act
iva
tion
Ene
rgy
(MeV
)
12Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
Surface Term Bs = - as A⅔
Coulomb Term BC = - aC Z(Z-1) / A⅓
3
3
4R
2
3
4ab=
1
)1(
Rb
Ra23 abR
...)1( 252
...)1( 251
Volume Term (the same)
32
31
52
51 )1( AaAZZa SC fission
47~2
A
Z
Crude: QM and original shape could be different from spherical.
13Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
48300
)120( 2
Extrapolation to 47 10-20 s.
Consistent with activation energy curve for A = 300.
14Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
235U + n93Rb + 141Cs + 2nNot unique.
Low-energy fission processes.
15Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
Z1 + Z2 = 92Z1 37, Z2 55A1 95, A2 140Large neutron excess
Most stable:Z=45 Z=58Prompt neutronsPrompt neutrons within 10-16 s.Number depends on nature of fragments and on incident particle energy.The average number is characteristic of the process.
16Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
The average number of neutrons is different, but the distribution is Gaussian.
17Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Delayed neutronsDelayed neutrons
Higher than Sn?
~ 1 delayed neutron per 100 fissions, but essential for control of the reactor.
Follow -decay and find the most
long-lived isotope (waste) in this
case.
18Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
19Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Fission
1/v
235U thermal cross sectionsfission 584 b.scattering 9 b.radiative capture 97 b.
Fast neutrons should be moderated.
Fission Barriers 20Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
21
Nuclear Fission
• Q for 235U + n 236U is 6.54478 MeV.• Table 13.1 in Krane: Activation energy EA for 236U 6.2 MeV (Liquid drop + shell) 235U can be fissioned with zero-energy neutrons.
• Q for 238U + n 239U is 4.??? MeV.• EA for 239U 6.6 MeV MeV neutrons are needed.• Pairing term: = ??? (Fig. 13.11 in Krane).• What about 232Pa and 231Pa? (odd Z).• Odd-N nuclei have in general much larger thermal neutron cross sections than even-N nuclei (Table 13.1 in Krane).
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
22
Nuclear Fission
f,Th 584 2.7x10-6 700 0.019 b
Why not use it?Why not use it?
23
Nuclear Fission
• 235U + n 93Rb + 141Cs + 2n• Q = ????• What if other fragments?• Different number of neutrons.• Take 200 MeV as a representative value.
66 MeV 98 MeV
miscalibrated
Heavyfragments
Lightfragments
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
24
Nuclear Fission• Mean neutron energy 2 MeV.• 2.4 neutrons per fission (average) 5 MeV average kinetic energy carried by prompt neutrons per fission.
• Show that the average momentum carried by a neutron is only 1.5 % that carried by a fragment. • Thus neglecting neutron momenta, show that the ratio between kinetic energies of the two fragments is the inverse of the ratio of their masses.
1
2
2
1
m
m
E
E
140
95
98
66
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
25
Nuclear Fission
Distribution of fission energy
Krane sums
them up as
decays.Lost … !
Enge
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
26
Nuclear Fission
Segrè
Lost … !
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
top related