Technical Control Factors - STD-1128-98master0208

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.

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