Some Facts About Neutron and Reactor Physics - The Risks of Nuclear Energy Technology - page 20

2.6 Neutron Poisons for the Control of the Reactor Power

Neutron poisons are used to control the number of neutrons and the power in the
reactor core. Such neutron poisons have high microscopic absorption cross sections
for neutrons. Neutron poisons are, e.g. Boron, Cadmium, Gadolinium etc. Figure 2.7
displays microscopic absorption cross sections for Boron, Cadmium and Gadolin-
ium isotopes.
The neutron poisons are inserted into the reactor core as, e.g. axially moveable
cylindrical control rods (Pressurized Water Reactors) or axially moveable cruci-
form control elements (Boiling Water Reactors). Another possibility is to add boric
acid (H 2 BO 3 ) to the cooling water, or to extract it from its solution in the cooling
water.
Withdrawing the, e.g. Cadmium or the Boron carbide control elements from the
reactor core changes the effective multiplication factor from k eff <

1tok eff > 1.

Inserting the control elements changes k eff from 1.0 to k eff < 1. This action changes

the number of fission reactions and the power of the reactor, correspondingly.
Similarly, the reactor can be controlled by the variation of the concentration of
the boric acid in the cooling water. However, all variations of the effective
multiplication factor k eff are limited by design such that they remain below about
half of the fraction of the delayed neutrons (see Sect. 2.10 ).

2.7 Fuel Burnup and Transmutation During Reactor
Operation

During reactor operation over months and years the initially loaded U-235 in the
low enriched uranium fuel will be consumed due to neutron fission and capture
processes. As a consequence also the initial criticality or effective multiplication
factor k eff decreases. Neutron capture in U-235 leads to U-236. Subsequent neutron
capture in U-236 leads to Np-237. Neutron capture in the fertile isotope U-238 leads
to U-239 and after decay to Np-239 and further decay to the buildup of the new
fissile isotope Pu-239. Subsequent neutron captures in Pu-239 lead to the higher
Pu-Isotopes Pu-240, Pu-241, Pu-242. After β -decay of Pu-241 americium is

created. Neutron capture in americium leads to curium. This increases somewhat
the criticality or effective multiplication factor k eff . Fission products originating
from the fission of fissile isotopes decrease the criticality or effective multiplication
factor k eff due to their absorption cross sections. The combination of these three
effects results in a time dependent change—usually a decrease—of the criticality
factor, k eff , during reactor operation.
This burnup effect on k eff is accounted for by design of the reactor core. The
enrichment of the initially loaded fuel is increased such that k eff becomes slightly
> 1. As the k eff shall be equal 1 during the whole reactor operation cycle, this is

balanced by absorber materials in the core (moveable absorber rods or special rods