Civil Engineering Reference
In-Depth Information
Fig. 2.1
Fission of U-235 nucleus by a thermal neutron
Reactors
1
(Fig.
2.2
). They depend slightly on the kinetic energy of the incident
neutrons causing fission and on the type of heavy nuclei (U-233, U-235, Pu-239).
In addition to the fission products (fragments), 2-3 prompt neutrons are emitted
during the fission process. These prompt fission neutrons appear within some
10
14
s. They are created with different kinetic energies following a certain
distribution curve around an average neutron energy of about 2 MeV. In some
heavy nuclei with even mass numbers, e.g. Th-232 and U-238, nuclear fission can
only be initiated by incident neutrons with a certain, relatively high, threshold
kinetic energy (Table
2.1
), whereas the uneven heavy nuclei, e.g. U-233, U-235,
Pu-239 etc. can be fissioned by neutrons with all kinetic energies
0 eV. However,
the even-uneven rule is not a rigorous one, e.g. Am-242m can also be fissioned by
thermal neutrons.
The fission products can either be solid, volatile or gaseous. Many of the fission
products decay further emitting so-called delayed neutrons,
>
-rays and
antineutrinos. The delayed neutrons resulting from the decay of particular fission
products—called precursors—represent less than 1 % of all released neutrons.
β
-particles,
γ
1
1eV
10
19
J is the kinetic energy acquired by an electron passing through a potential
gradient of 1 V. 1 keV is equal to 10
3
eV and 1 MeV is equal to 10
6
eV. The energy of 0.0253 eV
corresponds to a neutron velocity of 2,200 m/s.
¼
1.602
