Some Facts About Neutron and Reactor Physics - The Risks of Nuclear Energy Technology - page 9

Chapter 2
Some Facts About Neutron and Reactor
Physics

Abstract Chapter 2 describes some facts about neutron and reactor physics needed
for the understanding of Chaps. 3 - 10 . It starts with the radioactive decay and the
definitions of the decay constant and the half-life. It continues with the explanation
of the fission process for fissile nuclear isotopes, e.g. U-233, U-235, or Pu-239 and
the fission energy release by creation of fission fragments (products), prompt fission
neutrons and delayed neutrons and radiation ( -rays and antineutrinos).

This is followed by the definition of reaction rates of neutrons with other atomic
nuclei, the presentation of measured microscopic cross sections for absorption,
capture and fission as well as the definition of the macroscopic cross section and
the neutron flux.
In LWR cores the fuel is arranged heterogeneously in lattice cells together with a
moderator (water) in order to slow down the fission neutrons with high kinetic
energy to kinetic energies in the range of 0.025 eV (thermal energy). This is most
effective if the enriched uranium fuel is put in cylindrical rods which are arranged
in e.g. a square grid. The optimization of the geometrical distance between the fuel
rods leads to important safety characteristics of LWR cores: the negative fuel
Doppler coefficient and the negative coolant (moderator) coefficient.
The definition of the criticality factor or effective multiplication factor, k eff ,
allows a characterization whether the reactor core is operated in steady state
condition or whether it is subcritical or even supercritical. The criticality or
effective multiplication factor, k eff , can be changed by moving or by insertion or
withdrawing of absorber material (boron, cadmium, gadolinium, indium, silver,
hafnium, erbium) in the core. This allows control of the reactor. The reactor core is
controlled always in a k eff range where the delayed neutrons are dominating. The
delayed neutrons are therefore of highest importance for the control of the reactor.
During reactor operation over months and years the initially loaded U-235 in the
low enriched uranium fuel will be consumed, neutron absorbing fission products
will build up or other heavy nuclei with masses above U-235 and Pu-239 will be
created. This decreases the criticality of the effective multiplication factor k eff .This
burnup effect on the criticality factor k eff is accounted for by the design of the
reactor core. The enrichment of the initially loaded fuel is increased such that k eff

β -particles, γ