Civil Engineering Reference
In-Depth Information
Fig. 2.2
Fission product yield (%) for fission reaction of different isotopes by thermal and fast
(E
>
0.2 MeV) neutrons [
9
]
Table 2.1
Threshold kinetic energy for incident neutrons causing substantial fission in different
heavy nuclei [
10
]
Heavy nucleus
Th-232 U-233 U-234 U-235 U-238
Pu-239
Incident neutron kinetic energy [MeV]
>
1.3
>
0
>
0.4
>
0
>
1.1
>
0
β
The fraction
of delayed neutrons originating from fissioning by thermal neutrons
(0.0253 eV) of U-235 is
β ¼
β¼
0.22 % from fissioning of Pu-239.
They appear following decay constants of 0.01-3 s
1
for U-235 and 0.01-2.6 s
1
for Pu-239. These delayed neutrons are of absolute necessity for the safe control
and operation of nuclear fission reactors [
6
,
7
,
11
].
The total energy release per fission, Q
tot
, appears as kinetic energy of the fission
products, E
f
, of the prompt fission neutrons, E
n
,as
0.67 %, and
β
-radiation, E
β
,as
-radiation,
E
γ
, or as neutrino radiation, E
v
, (Table
2.2
). The neutrino radiation does not produce
heat in the reactor core due to the small interaction probability of neutrinos with
matter. Table
2.2
also shows the total energy, Q
tot
, and the thermal energy, Q
th
,
released during fission of a nucleus. Some of
γ
β
-radiation and
-radiation of the
fission products is not released instantaneously, but delayed according to the decay
of the different fission products.
On the average, about 194 MeV or 3.11
γ
10
11
J are released per fission of one
U-235 atom. Most of the fission energy is released instantaneously.
Since 1 g of U-235 meal contains 2.56
10
21
atoms, the complete fission of 1 g
of U-235 results in:
