Page 254 - Dynamics and Control of Nuclear Reactors

APPENDIX

C
Basic reactor physics

C.1 Introduction
It is assumed that most readers are familiar with basic reactor theory. But for those
lacking that familiarity or those needing a refresher, this appendix provides a brief
overview of basic concepts relevant to a study of reactor dynamics. Greater detail
may be found in pertinent references [1–4].

C.2 Neutron interactions
Interaction of neutrons with matter results in various outcomes depending on the neu-
tron energy and the nature of target material (target nuclei). The interactions that are
important in reactor operation are as follows:
Elastic collision: This occurs when the neutron shares its energy with the target
nucleus without exciting the nucleus. In a collision between a target nucleus and a
neutron, the target nucleus recoils and the neutron continues with lower energy. This
is analogous to a “billiard ball collision”. The total kinetic energy (KE) is conserved.
This is the primary mode of slowing down of neutrons to thermal energies by inter-
actions with light nuclei of the moderator.
Inelastic collision: In an inelastic collision the target nucleus becomes excited,
emits a gamma ray, and emits a neutron with lower energy than the incident neutron.
Radiative capture: In radiative capture the neutron is absorbed by the target
nucleus, produces an excited nucleus that becomes stable by emitting gamma rays.
Transmutation: In a transmutation reaction, neutron absorption yields new iso-
topes. For example
10 1 7 4
B+ n ! Li + He αðÞ (C.1)
Fission: The essential reaction that takes place in a nuclear reactor is the fission reac-
tion, which occurs in certain heavy nuclei. 92 U-235 is the only naturally occurring
isotope of uranium that has this property for reactions with slow neutrons. The other
main isotopes that undergo fission by slow (or thermal) neutrons are 92 U-233,
94 Pu-239, and 94 Pu-241.
The natural abundance of the isotopes of uranium (as it is found in nature) is
as follows:
U-234 ¼ 0:006%
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