Technical Control Factors - doe-std-1128-98

DOE-STD-1128-98

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.

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