Page 27 - Dynamics and Control of Nuclear Reactors

18 CHAPTER 3 The point reactor kinetics equations

3.2 Delayed neutrons
3.2.1 Delayed neutrons from fission products
Not all of the neutrons resulting from a fission reaction emerge immediately. In a
reactor, a small fraction of the neutrons appears upon radioactive decay of a fission
product. Some fission products have too many neutrons for nuclear stability. They
can relieve this excess by either of two decay possibilities: emission of a beta particle
or emission of a neutron. Delayed neutrons come from neutron emissions from
elements that result from previous beta decay, but still have too many neutrons.
See Fig. 3.1 for the general scheme for producing delayed neutrons.
As long as the contribution of delayed neutrons is essential to maintain the chain
reaction, transients must “wait” for release of the delayed neutrons. If reactor oper-
ation relied entirely on fission neutrons, transients would be too fast to tolerate.
Nuclear bombs are designed to rely on fission neutrons and clearly things happen
very quickly.
Delayed neutrons appear with lower kinetic energy than fission neutrons (around
0.5MeV). This lower energy at birth influences the importance of delayed neutrons
in causing subsequent absorption by target nuclei and fission reactions. Also, delayed
neutrons have a smaller probability of leaking out of the reactor core compared to
prompt neutrons. We shall see that delayed neutrons are very important in making
reactors controllable.
Delayed neutrons may be treated with a model that includes six neutron precursor
groups. A delayed neutron precursor group is the collection of certain fission frag-
ment that decays to a stable isotope by giving rise to a delayed neutron. The yield of
each precursor group depends on which fuel material is involved. The total fraction
of delayed neutrons ranges from 0.0022 to 0.007, depending on the fissioning isotope
involved. We shall see that these small fractions have a major impact on reactor oper-
ation. Delayed neutron data for thermal fissions in three fissile materials and for fast
fissions in U-238 (fissionable only with fast neutrons) appear in Tables 3.1–3.4.

Fission product

Prompt neutrons

Neutron
Fissile Delayed neutron
material Delayed
Precursor β-decay
Emitter
Stable nucleus
FIG. 3.1
Production of delayed neutrons.