Interaction or Arrays

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

7.2.1.6 Interaction or Arrays

Interaction is the exchange of neutrons between separate containers containing uranium material.

An increase in the exchanged neutrons increases the fission reaction rate. Units that are subcritical

individually can be made into a critical array if brought near each other.

7.2.1.7 Neutron Poisons (Absorbers)

Neutron absorbers (poisons) are nonfissionable materials that capture neutrons, thus reducing the

number of neutrons available for a fission reaction. Cadmium, boron, and chlorine are examples of neutron

absorbers. Boron in borosilicate glass Raschig rings and chlorine in polyvinyl chloride (CPVC) rings are

poisons used in some applications.

7.2.1.8 Monitoring for Deposits for Nuclear Safety Control

One concern in many older facilities is the potential for accumulation of uranium compounds in

ventilation ductwork and process piping. A program must be in effect to routinely monitor such equipment

to identify uranium compound deposits in quantities that may present nuclear criticality safety concerns.

The need for such a program should be determined by nuclear criticality safety specialists, based on the

enrichment of material processed (both past and present) and the geometry of the ductwork or piping. Such

a review and survey should also be conducted prior to shutdown and decommissioning of uranium

facilities. In general, the use of NaI detectors, in conjunction with single or multichannel analyzers, can

often provide adequate sensitivity to determine holdup deposits. If intervening shielding reduces sensitivity

and/or background gamma radiation levels are too great, neutron detectors may be effective in identifying

uranium deposits, particularly for highly enriched uranium. Since the hold-up measurements are generally

taken in "cpm" for maximum sensitivity, it is useful to have a correlation from "cpm" to exposure or dose

units to facilitate an understanding of the relative radiological hazard.

7.2.2 Double Contingency Principle

DOE O 420.1A mandates the application of the double contingency principle in nuclear criticality

safety.

The double contingency principle, as defined in DOE O 420.1A, requires that process designs

incorporate sufficient factors of safety to require at least two unlikely, independent, and concurrent changes

in process conditions before an inadvertent, unplanned criticality could occur. Protection, or defense in

depth, shall be provided by either a) the control of two independent process parameters (which is the

preferred approach, if practical) or b) a system of multiple controls on a single parameter. In all cases, no

single failure shall result in the potential for a criticality accident. The basis for selecting either approach

shall be fully documented.

The two parameters that are controlled in the double contingency analysis process shall not be

subject to common mode failures. Judgment is required in determining whether the two events are

related and, consequently, whether they represent two contingencies or a single contingency. For

example, exceeding a storage limit and then flooding an area with water would constitute two

independent events. However, a fire followed by the flooding of a storage area with fire suppression

water would constitute a single event.

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