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In the fission of
U235, about 99.35% of the neutrons
produced are prompt neutrons and the remaining 0.65% are delayed
neutrons.
Delayed
neutron data for thermal fissions in U235.
The
delayed neutron fraction can be discussed as both an actual
and effective fraction. See
discussion.
Neutron life-time
is the average time from the formation of the precursor until
a delayed neutron is emitted, or, that is to say,
In order
to obtain
The average
neutron life-time of delayed neutrons is about 13 sec, or
in other words, this is about the time lapse we must wait
for the average delayed neutrons to be released by their precursors.
In order to
obtain the average time for all prompt and delayed neutrons:
0.9935
· 10-14 s + 0.0065 · 13 s = 0.085 s
In order to
obtain the average neutron life-time for a given generation,
we must add the diffusion time (tD)
of the neutrons in the moderator:
L
= 0.085 + 0.001 = 0.086 s
Thus, even though
the delayed neutrons represent only a small portion of total
neutrons generated via fission, the average neutron life-time
of all neutrons is increased:
L
= 0.001 s to 0.086 s equals a factor of 86
In order to simplify
matters, let's assume that L = 0.1 s.
Example
Example
for the case of negative reactivity
Graphic
representation of these two examples
The rise in
power is much slower than in the case of the prompt neutrons
and thus it is possible to implement protection and regulation
systems.
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