Examples of Categorization Using System Interaction Rules

DOE-STD-1021-93

upgrading the source SSC to the higher performance category of the target. Hence, this

criterion should be used when:

(i) upgrading of the source does not involve a significant design change, or

(ii) the existing source design has an adequate margin to withstand the same NPH level as the

target.

(iii) The second option given in Paragraph (c)(ii) of Subsection 2.5 requires taking measures to

preclude interaction, e.g., by the use of a barrier to prevent the source from interacting

with the potential target. Very often this can be the most practical and cost-effective

option. The barrier must be placed in the same performance category as the target and be

designed to withstand the interaction effects from the source, in addition to the NPH loads.

3.9

Examples of Categorization Using System Interaction Rules

This subsection provides a few examples of the application of categorization rules considering system

interaction effects, as provided in Subsection 2.5.

(a)

Example 1

Consider an emergency diesel generator in a Hazard Category I facility that has been classified

as a "safety-significant" system using DOE-STD-3009-94 guidelines. Assume that the diesel

generator and all the support systems (e.g., fuel, lubrication, cooling water, and DC power

systems) that perform a safety function have been assigned to NPH Performance Category 2

(PC-2), in accordance with the provisions of Section 2 of this standard.

Now, consider a fluorescent light fixture (source) hung directly above the diesel generator

(target). For this case, assume that the light is not needed for required operator actions

following an NPH event. Hence the preliminary performance category for the light fixture is

PC-1. Diesels themselves are fairly rugged, and a lightweight falling object, like the light

fixture, is unlikely to damage them. However, there are some possible weak spots, particularly

in the peripheral support systems (e.g., lubrication lines), that might be damaged and result in

system failure. Assume that, in accordance with Section 3.7 of this standard, the failure

potential of the diesel resulting from the failing light fixture is assessed to be high. (This

assessment assumes the lighting fixture will fall during an NPH event, and no credit is taken at

this stage for its design strength, if any, against NPH events.) Following Paragraph (c)(i) of

Subsection 2.5 of this standard, the lighting fixture should then be placed in NPH Performance

Category 2 (PC-2).

(b)

Example 2

Consider a case in which batteries for an uninterruptible power supply (UPS) in a Hazard

Category 3 facility are in the same room with a 2000-gallon water storage tank. The UPS is

classified as a safety system, but the water storage tank is not. Assume that the UPS batteries

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