Examples of Categorization Using System Interaction Rules

DOE-STD-1021-93

(and their rack, connections, and the surrounding room structure) have been assigned to NPH

Performance Category 2, in accordance with the provisions of Section 2.5 of this standard.

Initially, the water storage tank might be considered as NPH Performance Category I (i.e.,

preliminary performance category). However, a systems interaction check discloses that UPS

batteries will short out during water immersion if only 1000 gallons of water flood the room.

Thus, in accordance with the criterion given in Paragraph (a)(i) of Subsection 2.5 of this

standard, the 2000-gallon tank should have the same NPH performance category as the UPS

batteries, that is, PC-2.

But what if the water were stored instead in ten 200-gallon tanks? The individual failure of

each tank would not cause the UPS to fail. However, if "multiple common-cause failure" is

considered, one could reason that all ten tanks would be affected in the same way by the NPH

(an earthquake in this case), and simultaneous failure of several tanks might occur, leading to

flooding of the batteries. Thus, each 200-gallon tank should also be placed in NPH

Performance Category 2, in accordance with the provisions of Subsections 2.3(c) and 2.5 of this

standard.

(c)

Example 3

Consider a 100-foot tall laundry building smoke stack at a DOE site that is not part of any

safety system. However, its failure (from winds or earthquakes) would be costly and could

injure workers, so assume that initially it was classified as NPH Performance Category 1.

Consider that there is a single PC-2 safety system component (say an outside pump) that is 50

feet from the base of the stack. Assume that a systems interaction analysis showed that the

stack would fail during a seismic event and fall in essentially one piece and would cause the

pump to fail if it hits it. But the stack is equally probable to fall in any radial direction, and the

target size of the pump is small, fitting into a 2 degree angle. It is concluded that the potential

for interaction is low. Thus, in accordance with Paragraph (c)(ii) of Subsection 2.5 of this

standard, the stack can be retained in PC-1.

(d)

Example 4

Consider a Hazard Category I facility that relies heavily on operator actions, rather than

seismically-qualified instrumentation and automatic control systems, to maintain a safe state

following a design basis earthquake. Assume that, according io Section 2.5 of this standard,

safety system SSCs of this facility have been placed in PC-4. In addition, SSCs needed to

permit required operator actions following a design basis earthquake must also be classified as

PC-4 (see Paragraph b, Subsection 2.3).

As an example, assume that one earthquake procedure written for this facility requires an

operator to go inside the pump room to read a water level gauge (which is seismically qualified)

and then relay this information to the control room via a system of walkie-talkies (assume that

inside telephone lines are not seismically qualified). Items needed to permit this action, and

which must therefore meet NPH Performance Category 4 criteria, include all access doors

(deformations of the door frames may be critical), emergency lighting and communication

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