System Interaction Effects ("Two Over One Protection")

DOE-STD-1021-93

additional requirements will depend on the type of source behavior that causes adverse

interaction with the target during or following an NPH event. System interaction may be

between one SSC and another SSC or one facility as a whole with another facility, in which

case similar consideration should be given for design of the facilities appropriately.

(b)

If the source behavior that causes adverse interaction is within the acceptable behavior limits

of the source, i.e., if the adverse interaction occurs before source failure, adequate measures

shall be taken to preclude such interaction and to ensure that the performance goal for the

target is preserved. For example, assume that the postulated seismic deflection of a PC-1

cabinet (source) is within its own acceptable behavior limits, but the cabinet can potentially

impact and cause the failure of a PC-2 fire-suppression component (target). To prevent this

adverse interaction, the cabinet support system or the cabinet itself can be

stiffened/strengthened in such a way that the calculated deflection of the cabinet towards the

target, when subjected to a seismic level corresponding to the performance category of the

target, is less than the available clearance by a factor equal to the applicable design margin

for the target. Alternatively, a barrier can be provided to preclude the adverse interaction and

to protect the target. Such a barrier shall be designed to withstand NPH effects combined

with the interaction effects from the source (in this case the impact from the PC-1 cabinet).

To ensure that the target performance goal is preserved, the barrier shall be placed in the

same performance category as the target (in this case PC-2).

(c)

If the adverse interaction is possible only after the source fails or exceeds its acceptable

behavior limits, either of the following two requirements shall be met to preclude adverse

interaction:

(i)

The source shall have additional NPH requirements corresponding to the performance

category of the target, if the failure potential of the target, given the failure of the

source, is assessed to be high. However, these additional requirements can be

restricted to the source failure mode related to the adverse interaction effects. If the

target failure potential is assessed to be low, no additional NPH mitigation

requirements need to be applied (see Table 2-1).

(ii)

Adequate measures shall be taken to preclude adverse interaction and to ensure that

the performance goal for the target is preserved. Examples of acceptable measures

are: stiffening/strengthening of the source structure or support system, relocating the

source and/or the target, installing barriers, installing new components, modifying

existing components, or any combination of these measures.

(d)

If the behavior or failure of a source can adversely affect the performance of more than one

target, the source shall have additional NPH requirements corresponding to the highest

performance category that is determined by applying the rules provided in Paragraphs (a), (b),

and (c) above separately for each target.

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