Some Facts About Neutron and Reactor Physics - The Risks of Nuclear Energy Technology - page 28

prompt and superprompt critical range

delayed critical range

range for
reactor control

reactor
shut down

subcritical range

Fig. 2.11 Different ranges of the effective multiplication (criticality) factor k eff for a typical LWR

(see Sect. 2.1 ). In this range the time difference of successive neutron genera-
tions is given by the lifetime of the prompt neutrons which is about 2.5 10 5 s

for LWRs with U-235 enriched uranium fuel. This means that the chain reaction
multiplies very fast. The very rapidly increasing number of fissions and the
reactor power as well as the fuel temperatures are, however, reduced by the
counteracting negative fuel-Doppler-temperature coefficient. This reduces
the power and after having attained a certain peak level the power drops again.
LWRs are designed such that the k eff is limited such that the energy released by
the power peak is small and limited.
- The range of k eff < 1 if control/absorber elements are inserted into the reactor

core or the concentration of boric acid is increased in the cooling water. In this
case the power drops rapidly within seconds to the afterheat level.

Figure 2.11 explains the three ranges of k eff which are important for the
description of the non-steady or instationary behavior of nuclear reactors. All
control procedures, e.g. withdrawal of control/absorber elements or increase of
the boric acid concentration in the coolant water are performed in the delayed
prompt critical range of k eff . The design of an LWR core must guarantee that the
super prompt critical range is never attained during normal operation. In case that
the super prompt critical range should be attained during accidental condition, then
the fuel-Doppler-temperature coefficient will limit the energy released in a power
peak. In addition rapid automatic reactor shut down by control/absorber rods will
limit the damage to the reactor core.
Similarly, the negative moderator/coolant-temperature coefficient or void coef-
ficient will drastically reduce the reactor power in case of depressurization of the
primary coolant system by large scale pipe breaks or faulty opening of valves in the
primary system.