Civil Engineering Reference
In-Depth Information
Table 2.2
Different components of energy release per fission of some heavy nuclei in MeV by
incident neutrons of different kinetic energy (in the eV or MeV range) [
10
]
Heavy nucleus
Incident neutron energy E
f
E
n
E
β
E
γ
E
ν
Q
tot
Q
th
U-235
0.025 eV
169.75 4.79 6.41 13.19
8.62 202.76 194.14
0.5 MeV
169.85 4.8
6.38 13.17
8.58 202.28 193.7
U-238
3.10 MeV
170.29 5.51 8.21 14.29 11.04 206.24 195.2
Pu-239
0.025 eV
176.07 5.9
5.27 12.91
7.09 207.24 200.15
0.5 MeV
176.09 5.9
5.24 12.88
7.05 206.66 199.61
Pu-240
2.39 MeV
175.98 6.18 5.74 12.09
7.72 206.68 198.96
10
10
Jor2
10
4
kWh
:
:
:
7
96
21
or
0
92 MWd
th
thermal energy
For other fissile materials like U-233 or Pu-239 the energy release per fission is
similar. Also fission by neutrons with thermal energies (0.025 eV) or by energies of
0.5 MeV leads to almost equal energy releases.
Therefore, it is usually assumed that the fission of the mass of 1 g of fissile
material, e.g. U-235 or Pu-239 produces roughly 1 MWd
th
and the measure of
burnup in MWd
th
per tonne of fuel also corresponds roughly to the number of grams
of, e.g. U-235 fissioned in 1 ton of spent fuel [
12
].
2.3 Neutron Reactions
Neutrons produced in nuclear fission have a certain velocity or kinetic energy and
direction of flight. In a fission reactor core, e.g. with U-235/U-238 fuel they may be
scattered elastically or inelastically or absorbed by different atomic nuclei. In some
cases the absorption of neutrons may induce nuclear fissions in heavy nuclei (U-235
etc.) so that successive generations of fission neutrons are produced and a fission
chain reaction is established.
2.3.1 Reaction Rates
!
) is the number of neutrons at point
!
, with velocity v and the direction
If n (
!
,v,
Ω
!
, then these neutrons can react within a volume element dV with N
of flight
dV
atomic nuclei (N being the number of atomic nuclei per cm
3
of reactor volume).
The number of reactions per second e.g. scattering or absorption, is then propor-
tional to
Ω
∙
