A Review of Criticality Accidents A Review of Criticality Accidents

Material
Fissile Mass: Fissile mass is the mass of either 235U or 239Pu. Fissile type is designated below the mass
entry. Three uranium accidents, 9, 15, and 22 had
enrichments of 22.6%, 6.5%, and 18.8% by weight,
respectively. For these accidents the fissile mass
column also provides the hydrogen to 235U atom ratio.
For the plutonium eight accidents with the plutonium
was assumed to be 95% 239Pu by weight.
Fissile Density: Fissile density is the quotient of
fissile mass and fissile volume assuming a homogeneous
mixture.
Estimated Spherical Critical Mass: Entries in this
column represent the spherical critical mass as the
quotient of fissile mass and shape factor. These
estimated masses are used as the measure of consistency
or agreement of the accident re-constructions
with established conditions for criticality. For accidents
9, 15, and 22, the spherical mass was adjusted to
a fully water reflected value.
Critical Cylinder Mass / Critical Sphere Mass
6
5
4
3
2
1
Water
Reflector
Discussion
Traditionally, techniques of the type used to
generate the estimates presented in Table 9 have been
referred to as “back–of–the–envelope” calculations.
These calculations are characterized by their mathematical
simplicity and their results are better described
as estimates when contrasted to results from
computer calculations. In some cases these estimates
are sufficient and more elaborate computer code
calculations are not necessary. These results are
characterized as estimates in conformity with
Wheeler’s First Moral Principle: “Never make a
calculation until you know the answer. Make an
estimate before every calculation....” 36
Figures 37, 38, and 39 are selected and adapted
from Los Alamos report LA–10860. 34 These three
figures include points corresponding to fissile density
(or atom ratio) and the estimated spherical critical
mass for the twenty-one accident re-constructions in
Table 9. Figure 38 includes curves for systems with
uranium enrichment corresponding
H/ 235U Atomic Ratio
44.3 329 52.9 43.9 320
Unreflected
0
0.05 0.1 1 10
Cylinder Height / Cylinder Diameter
Figure 36. The ratio of cylindrical to spherical critical masses of U(93)O 2 F 2 solutions, unreflected and with water
reflector, as a function of cylinder height to cylinder diameter ratio.
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