Reactor Theory (Neutron Characteristics)
DOE-HDBK-1019/1-93
PROMPT AND DELAYED NEUTRONS
Rev. 0
Page 29
NP-02
PROMPT AND DELAYED NEUTRONS
Not all neutrons are released at the same time following fission. Most neutrons
are released virtually instantaneously and are called prompt neutrons. A very
small fraction of neutrons are released after the decay of fission products and are
called delayed neutrons. Although delayed neutrons are a very small fraction of
the total number of neutrons, they play an extremely important role in the control
of the reactor.
EO 3.1
STATE the origin of prompt neutrons and delayed neutrons.
EO 3.2
STATE the approximate fraction of neutrons that are born as
delayed neutrons from the fission of the following nuclear fuels:
a.
Uranium-235
b.
Plutonium-239
EO 3.3
EXPLAIN the mechanism for production of delayed neutrons.
EO 3.4
EXPLAIN prompt and delayed neutron generation times.
EO 3.5
Given prompt and delayed neutron generation times and
delayed neutron fraction, CALCULATE the average generation
time.
EO 3.6
EXPLAIN the effect of delayed neutrons on reactor control.
Neutron Classification
The great majority (over 99%) of the neutrons produced in fission are released within about 10-13
seconds of the actual fission event. These are called prompt neutrons. A small portion of fission
neutrons are delayed neutrons, which are produced for some time after the fission process has
taken place. The delayed neutrons are emitted immediately following the first beta decay of a
fission fragment known as a delayed neutron precursor. An example of a delayed neutron
precursor is bromine-87, shown below.
