Technical Control Factors
Plutonium isotopes include 238Pu, 239Pu, 240Pu, 241Pu, and 242Pu. All these radionuclides
are fissionable materials; however, 239Pu and 241Pu are referred to as fissile materials,
a subset of fissionable materials. Fissile materials are capable of sustaining a neutron
chain reaction with thermal neutrons and fast neutrons and, as such, have lower
critical masses than other plutonium isotopes.
Single-parameter limits for plutonium solutions, oxides, and metals are presented in
ANSI/ANS-8.1 (ANSI, 1983b) and ANSI/ANS-8.15 (ANSI, 1981) and are
summarized in Table 7.1. A single-parameter limit means that if any one of the
parameters for a given material is maintained less than its limit, then a criticality
event is impossible. For example, for a 239Pu(NO3)4 solution, as long as the 239Pu mass
in the solution is less than 0.48 kg, the other parameters can exceed their limits (e.g.,
the solution concentration could be greater than 7.3 g/L) and a criticality incident is
not possible. The reader is referred to ANSI/ANS- 8.1 (ANSI, 1983b) for a
discussion of multiparameter control.
For plutonium solutions and metals in an isolated system, use of favorable geometry
is the preferred method of criticality control. An isolated system is far enough
removed from other systems such that neutron leakage from a nearby system will not
contribute to the likelihood of a criticality excursion. Where geometry control is not
feasible, the preferred order of controls is (1) other passive engineering controls (e.g.,
mass control), (2) active engineering controls, and (3) administrative controls. DOE
Order 420.1A requires that the basis for not selecting geometry control be
Other technical control factors used to control nuclear criticality risks include density
controls, spacing controls (sometimes referred to as interaction), neutron absorbers,
moderation controls, and neutron reflection. Spacing controls become particularly
important in the storage and transport of fissionable materials. ANSI/ANS-8.1
provides additional discussion of technical control factors.
DOE Order 420.1A addresses the concept of the application of double contingency in
nuclear criticality safety. This principle applies the technical control parameters
above to ensure nuclear criticality safety. This is referred to as double contingency
and must be adhered to at DOE facilities.