A Review of Criticality Accidents A Review of Criticality Accidents

235 U Spherical Critical Mass (kg)
100
10
1
9
0.8
20 100 500 1000 2000
Figure 38. Critical masses of water-reflected spheres of hydrogen-moderated U(93), U(30.3), U(5.00), U(3.00), and
U(2.00). The accidents are shown by numbered circles.
Fission Yields
Table 10 lists the estimated fission energy releases
for the 22 accidents. An attempt has been made to
categorize the spike yield in this edition in a manner
consistent with the prior two editions of this report and
definitions in the appendix. These definitions are
repeated here for convenience.
spike (in a prompt power excursion): The initial
power pulse of a prompt power excursion, limited by
the shutdown mechanism. See excursion, prompt
power.
excursion, prompt power: A nuclear excursion as
the result of a prompt critical configuration of fissile
material. In general, a sharp power spike followed by a
plateau that may be interrupted by smaller spikes.
From solution excursion experimental data, such as
many of the CRAC4 and SILENE3 experiments, it is
apparent that there is a smooth transition from excursions
in which the maximum reactivity did not reach
prompt critical to those in which it slightly exceeded
prompt critical. There is no significant distinction
H/ 235 U Atomic Ratio
U(2)F –Paraffin
2
U(3)F –Paraffin
2
U(5)O2F2 Solution
U(30.3)O F Solution
2 2
U(93)O F Solution
2 2
Calculated UO F –Water
2 2
Calculated UF –Paraffin
4
22
15
between the power histories of two excursions, one
having a maximum reactivity of $0.90 and the other
having a maximum reactivity of $1.10. They both
exhibit an initial spike followed by less energetic,
recurring spikes at approximately 10 to 20 second
intervals, eventually leading to a quasi–plateau. Only
when the maximum reactivity attained is about $0.50
or less is the traditional spike not present.
Another result of the CRAC 4 and SILENE 3 experiments
that can be compared to the accident yields
listed in Table 10 is the specific yield of the first
excursion, the spike. For experiments with a maximum
reactivity of about $0.50 or more, the specific yield of
the spike was always about 1.0 × 10 15 fissions per liter
except for very fast excursions, those that achieved
inverse periods much greater than 100 s –1 . For these
very fast excursions, specific yields up to several times
10 15 fissions per liter were measured. The accidents for
which a spike yield is given in Table 10 are consistent
with the specific yields of the CRAC 4 and SILENE 3
data in that none exceeded a few times 10 15 fissions
per liter. However, there are three reported spike yields
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