Application of System Interaction Rules

DOE-STD-1021-93

(b) Approximate Method

In this method, the effects of source failure on a target are modeled approximately but

rationally, considering possible scenarios identified by a facility walkdown. Even though such

models are approximate, their evaluation provides qualitative information that is often

adequate for the purpose. Examples of this method are given in Section 3.9.

3.8

Application of System Interaction Rules

The consideration of adverse effects of system interaction of one component or system (source) on

another (target) is very important in determining the performance categories of SSCs. Adverse

interaction effects can be different for different systems. Examples of typical adverse interaction

effects are:

Structural Failure and Falling: Inadequately designed, inadequately anchored, and unanchored

(i)

components may fail, slide, and/or topple and fall on or bump into other components that are

not designed to withstand such interaction effects.

(ii) Proximity and Impact: Adjacent components may impact each other, causing damage if the

clearance between them is inadequate for NPH-induced deflections. Such adverse interaction

may occur even if the deflection of the source is within its design limits.

(iii) Differential Displacement: A target distribution system (e.g., vital cable trays, pipes,

ventilation ducts) may span between different structural systems (source). Differential

displacements between the structural systems may be within acceptable behavior limits for the

structures, but may still affect the distribution system adversely.

(iv) Mechanical or Electrical Failure: The failure of a source mechanical or electrical component

may impair the safety function of another component or system (e.g., the failure of a valve in a

non-safety water distribution system causing flooding that short circuits a safety-class

electrical motor).

The preliminary performance category of each component of the facility, as determined in Steps 4 and

5 (Subsections 3.5 and 3.6), must be modified to account for the above system interaction effects,

using the guidelines provided in Subsection 2.5.

Paragraph (b) of Subsection 2.5 provides the general requirements for precluding interaction that can

occur before the source fails or reaches its acceptable behavior limits. Paragraph 2.5(c) provides two

options to meet the requirements for precluding adverse interaction that can occur only when the

source fails. The following paragraphs provide additional discussions of these two options.

(a)

The first criterion in Paragraph (c)(i) of Subsection 2.5 is the more conservative of the two

options, because it imposes additional NPH requirements if the failure potential of the target,

given the failure of the source, is high. But it can also be more costly, since it may require

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