Fertile nuclei are characterized by very large capture resonances. In
heterogeneous reactors fissile and fertile nuclei are concentrated in fuel
rods. Neutrons having an energy within a resonance width are strongly absorbed
on a short distance. As a result all fertile nuclei do not participate to the
capture process. The apparent width of the resonances, as it seen by the
neutron moving in the medium, is increased over its natural value by the
thermal motion of the nuclei. Higher temperatures lead to larger widths, and
thus, to a broader energy region where all neutrons are captured. A larger
number of captured neutrons means a smaller number of neutrons available for
inducing fission. Thus the Doppler effect leads to a negative value of the
temperature coefficient, at least in heteregeneous reactors4.11.
