A Review of Criticality Accidents A Review of Criticality Accidents

to 2.0, 3.0, 5.0, 30.3, and 93.0. Accidents 9, 15, and 22
are superimposed on this figure. Since the curves in
Figure 38 are for water reflected systems, these points
have been adjusted downward since the actual accidents
were relatively unreflected.
Additional adjustments to fissile density and
estimated critical spherical mass could be performed.
For example, the effects of nitrate absorption and
organic versus aqueous base composition could be
included in the estimates. Of course, judgment is
required as to whether such adjustments are meaningful
and whether carrying out such adjustments would
lead to estimates in closer agreement with the curves
presented in Figures 37, 38, and 39. Such adjustments
cannot be justified. The absence of technical detail
provided in the accident descriptions prevents meaningful
refinement of the estimates. This lack of
technical information also precludes any attempt for
meaningful, more detailed, neutronic computer
modeling.
235 U Spherical Critical Mass (kg)
60
200
100
10
1
19
13
Data From Spheres
Data Derived From Cylinders
Calculated Metal–Water Mixtures
Calculated UO2 –Water Mixtures
4
6
14
0.16 cm Stainless Steel
Reflector
12
16
2
8
Density of 235 U (kg/ )
Conclusions
Considering the effects of partial reflection and
inherent uncertainties in the estimates, it is judged that
the position of 18 of the 21 points plotted in Figures 37,
38, and 39 are sufficient for establishing credible
agreement between the reported accident conditions and
known conditions for criticality. The estimates for
accidents 1, 7, and 9 appear to be somewhat questionable
in that more mass than reported in the accident would be
required for criticality under the hypothesized unreflected
accident conditions. However, accidents 1 and 7, would
be in reasonable agreement if the partial reflection
present during the accident were taken into account. It
should be noted that for these two cases, the “missing”
mass is no greater in magnitude than other accident reconstructions
(notably, accidents 12, 14, and 17) in which
the reported mass exceeds the known conditions for
criticality. The discrepancy surrounding accident 9 is also
consistent with the large reported uncertainty in the
amount of mass present. No systematic features are
distinguishable that differentiate the R.F., U.S., U.K., and
Japanese accidents.
Water
Reflector
Limiting Critical Density
Metal
0.5
0.01 0.1 1 10 20
Figure 37. Critical masses of homogeneous water moderated U(93.2) spheres. Solution data appear unless indicated
otherwise. The accidents are shown by numbered circles.
3