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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
7.2.1 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 documented.
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.
7.2.2 Double Contingency
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.
7-4


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