Regional Basic Professional Training Course in Korea

Three possible states could then be obtained depending to the value of k: k < 1
sub-critical system, with an exponential decrease of neutron population.
k= 1 critical system with a sustainable reaction chain.
k > 1 super-critical system with an exponential increase of neutron population.
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1. Nuclear Reactor Principles
Critical state is the only feasible operational conditions when the objective is to produce
energy in a usable way. A sub-critical state is compulsory when only manipulation or
transformation (without any aim to produce energy) of fissile material is desired.
Super-critical state is obviously a dangerous one (remember the substantial amount of
energy and of radiation released) encountered only in exceptional situations (weapons,
accidents…).
Reactivity ρ is another quantity often used to characterize the state of nuclear chain
reaction.
It measures the relative distance of a system from critical state: ρ= (k-1)/k is expressed in
pcm
(1pcm = 10 -5
).
One could imagine different possibilities to produce a sustainable fission. As discussed in
the introductory course to nuclear physics, neutron cross-sections vary with incident
energy. One may distinguish three energy domains:
MeV energies: This is where fission neutrons are produced; neutrons with kinetic energy
in this domain will be referred to as fast neutrons.
keV energies: This domain is characterized by heavy nuclei resonance cross-sections;