1. ! Nuclear Fission 23592U + 10n ► 9037Rb + 14455Cs + 210n
Thermal neutron fission of 235U forms compound nucleus that splits up in more than 40 different ways, yielding over 80 primary fission fragments (products).
23592U + 10n ► 9037Rb Cs n
23592U + 10n ► 8735Br La n
23592U + 10n ► 7230Zn Sm + 410n !
The fission yield is defined as the proportion (percentage) of the total nuclear fissions that form products of a given mass number. Revisit thermal and look for fast.
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 1

2. Nuclear Fission Remember neutron excess. (A,Z)  (A,Z+1) or (A-1,Z).
Asymmetry
Remember neutron excess.
(A,Z)  (A,Z+1) or (A-1,Z).
Only left side of the mass parabola.
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

3. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
 165 MeV average kinetic energy carried by fission fragments per 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
Light
fragments
Heavy
fragments
miscalibrated
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

4. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
 neutrons emitted per fission.
 depends on fissioning nuclide and on neutron energy inducing fission.
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

5. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
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.
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

6. Nuclear Fission Enge Distribution of fission energy Lost … !
Krane sums them up as  decays.
Lost … !
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

7. Nuclear Fission Segrè Distribution of fission energy
Lost … !
How much is recoverable?
What about capture gammas? (produced by -1 neutrons)
Why c < (a+b) ?
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

8. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
 and  emissions from radioactive fission products carry part of the fission energy, even after shut down.
On approaching end of the chain, the decay energy decreases and half-life increases. Long-lived isotopes constitute the main hazard.
Can interfere with fission process in the fuel. Example? (poisoning).
Important for research.
-decay favors high energy  ~20 MeV compared to ~6 MeV for .
Only ~ 8 MeV from -decay appears as heat. Why?
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

9. ! Nuclear Fission f - (n,) - (n,)
A-1, Z j
A, Z i
A, Z-1 k -
(n,)
A+1, Z
A, Z+1
dNi/dt = Formation Rate - Destruction rate - Decay Rate
Ni saturates and is higher with higher neutron flux, larger “fission yield” and longer half-live. !
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 9

10. Nuclear Fission
HW 7
Investigate the activity, decay and gamma energies of fission products as a function of time. Comment on consequences (e.g. rod cooling).
Shutdown
HW 8
Investigate both and
giving full description for the buildup and decay of fission fragment i.
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 10

11. Nuclear Fission Fission product activity after reactor shutdown?
per watt of original operating power.
T = time of operation.
Fission product activity after reactor shutdown?
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 11

12. Nuclear Fission The fission gamma radiation
Prompt within 0.1 s and with average energy of 0.9 MeV.
 delayed gammas.
Investigate how prompt
gammas interact with
water, uranium and lead.
HW 9
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 12

13. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
HW 10
The experimental spectrum of prompt neutrons is fitted by the above equation. Calculate the mean and the most probable neutron energies.
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

14. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
Recoverable energy release  200 MeV per 235U fission.
Fission rate = 2.7x1021 P fissions per day. P in MW.
3.12x1016 fissions per second per MW, or 1.2x10-5 gram of 235U per second per MW (thermal).
Burnup rate: 1.05 P g/day. P in MW.
The fissioning of 1.05 g of 235U yields 1 MWd of energy.
Specific Burnup = 1 MWd / 1.05 g  MWd/t (pure 235U !!).
Fractional Burnup = ???
Thermal reactor loaded with 98 metric tons of UO2, 3% enriched, operates at 3300 MWt for 750 days.
 86.4 t U. Specific burnup  MWd/t.
Not all fissions from 235U.
Fast fission of 238U.
238U converted to plutonium  more fission.
Actually much less.
Work it out, NOW!
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

15. Nuclear Reactors, BAU, 1st Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
Capture-to-fission ratio:
Consumption rate: 1.05(1+) P g/day.
Read all relevant material in Lamarsh Ch. 4. We will come back to this later.
Two neutrinos are expected immediately from the decay of the two fission products, what is the minimum flux of neutrinos expected at 1 km from the reactor.
4.8x1012 m-2s-1
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

16. Nuclear Fission 3.1x1010 fissions per second per W.
In thermal reactor, majority of fissions occur in thermal energy region,  and  are maximum.
Total fission rate in a thermal reactor of volume V
Thermal reactor power (quick calculation)
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 16

17. Some technicalities, so far!
Nuclear Fission
Some technicalities, so far!
It is necessary to evaluate the potential hazards associated with an accidental release of fission products into the environment.
It is required to determine a proper cooling time of the spent fuel (before it becomes ready for reprocessing) that depends on the decay times of fission products.
It is necessary to estimate the rate at which the heat is released as a result of radioactive decay of the fission products after the shut down of a reactor.
The poisoning is needed to be calculated (the parasitic capture of neutrons by fission products that accumulate during the reactor operation).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).
Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 17