Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

TABLE 12-3. Delayed Neutrons from the Fission of 235 U
CRITICALITY 653
YIELD, MEAN GENERATION
GROUP percent TIME, seconds YIELD × MEAN TIME
i ni Ti ni × Ti
1 0.0267 0.33 0.009
2 0.0737 0.88 0.065
3 0.2526 3.31 0.836
4 0.1255 8.97 1.125
5 0.1401 32.78 4.592
6 0.0211
ni = 0.6397
80.39 1.688
ni Ti = 8.315
and the power level increase in 1 second would be
n
n0
= e t
T = e 1
0.2 = 150 fold.
Such rapid increases in the power level as calculated in the example above would
make it extremely difficult, if not impossible, to control a reactor. Fortunately, the
calculation above is not applicable to a real reactor because, although the mean
lifetime of a single neutron from birth until absorption is about 0.001 second, the
mean lifetime of a whole generation of a large number of neutrons is much greater
than 0.001 second. This increased mean generation time is due to the fact that
0.6407% of fission neutrons are delayed, that is, they are emitted for as long as 80.39
seconds after fission. Six distinct groups of delayed neutrons are observed, each group
having its own mean delay time. Table 12-3 lists these groups, together with their
mean delay (or generation) time.
The mean generation time for all the fission neutrons of a given generation is
6
ni Ti
¯T
i=0
=
6
= 8.315 + (100.0000 − 0.6397) × 10−3
= 0.084 second.
100
ni
i=0
The group i = 0 is the group of prompt neutrons, whose yield is 99.359% and whose
mean generation time is 0.001 second.
If k, is equal to or greater than 0.006407, the reactor is said to be in the prompt
critical condition, since the chain reaction can be sustained by the prompt neutrons
alone. If k is less than 0.006407, the reactor is in the delayed critical condition,
because the delayed neutrons are essential to sustaining the chain reaction. In the
delayed critical condition, the reactor period is sufficiently long to allow the power
level to be easily controlled.
W Example 12.5
Compute the reactor period and the increase in power level in 1 second for the
case where the mean neutron generation time is 0.084 second for excess reactivity