Logic Tree Analysis

DOE-STD-1052-93

(1)

Is the occurrence of a failure evident or visible to the operating crew while

performing their normal duties?

This question divides failures into two groups, evident and hidden:

Yes - Those failures that are obvious to the operators during their normal day-today

activities. It is not necessary that the operators know precisely what is wrong,

only that something is wrong and that they may take the steps required to

repair the component.

No -

Those failures that are discovered when operation of infrequently used

equipment is attempted or when protective, standby, or backup systems fail

to operate on demand. These failures are called hidden failures and may be

especially critical to safe and efficient operation.

(2)

Does the failure cause a loss of function or secondary damage that has a direct

and adverse effect on the safety of the plant?

NOTE:

Considering the consequences of a failure of safety system components

as if redundant equipment is not available is a conservative assumption

that may result in many components being classified as having a safety

consequence. There are analytical techniques used to determine the

significance of failures of redundant components. These techniques are

typically used in probabilistic risk assessments that quantify risk and

reliability. Although usually manpower-intensive, using these

techniques may reduce the number of components that require FMEA

or change the priority of the components to be evaluated.

This question divides the functional failure into two groups, safety and nonsafety:

Yes - Those failures that directly affect operating safety. Safety relates to essential

functions needed to protect the health and safety of the public. These should

be direct threats, not improbable combinations of events that have minor impact

on operating safety or are highly unlikely.

No -

Those failures that do not impact the operating safety as described above.

These failures have economic or operational impacts that restrict the operator

from using installed equipment.

D - 34