Last Updated：2022/12/24
Although nearly all neutrons produced by fission are emitted as soon as the atom splits (within 10-14 sec or so), a very small proportion of neutrons (0.65% for U-235, 0.25% for Pu-239) are emitted by fission fragments with delays of up to a few minutes. In delayed criticality these neutrons are required to maintain the chain reaction. These long delays mean that power level changes can only occur slowly. All nuclear reactors operate in a state of delayed criticality.
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Although
nearly
all
neutrons
produced
by
fission
are
emitted
as
soon
as
the
atom
splits
(within
10^-14
sec
or
so),
a
very
small
proportion
of
neutrons
(0.65%
for
U-235,
0.25%
for
Pu-239)
are
emitted
by
fission
fragments
with
delays
of
up
to
a
few
minutes.
In
delayed
criticality
these
neutrons
are
required
to
maintain
the
chain
reaction.
These
long
delays
mean
that
power
level
changes
can
only
occur
slowly.
All
nuclear
reactors
operate
in
a
state
of
delayed
criticality.
