Single-Failure Electro-Mechanical Systems

DOE-STD-1027-92

has already been performed (see Section 4.1.1) is considered sufficient for

identifying release mechanisms.

Use Failure Modes and Effects Analysis (FMEA)

These systems include relatively simple electrical and mechanical devices

in which a single-failure mechanism causes a release of materials. Simple

one-step processes, single glove box operations, and small furnaces are

example of such devices. FMEA is a bottom-up approach that looks at the

failure of each element of a system or process and identifies the

consequence of each failure. FMEA is most appropriate for analysis of

small segments of a system or process when it is determined that failure of

single components in this segment could lead to system or process failure

or release of material.

FMEA has some limitations which must be recognized to ensure its

appropriate use. First, FMEA is not very efficient for large-scale systems

analysis because, by virtue of its bottom-up approach, it examines and

documents the effects of component failures having little, if any, relevance

to system failure or potential release. Second, FMEA considers only one

failure at a time and has no logical process for considering multiple or

combined failures. Third, FMEA is strictly equipment-oriented. It looks at

failure of equipment in different nodes and assesses their consequences

but does not look at failures of a process, which, by its very nature, may

have complexities and instabilities far beyond those which can be assessed

only by examining the failure of individual components.

Systems with Redundant Barriers or Requiring Multiple Failures

Use Event Tree Analysis (ET)

ET analysis is a simple approach to delineating sequences of events which

could lead to an undesired event. An undesired event could be uncontrolled

release of hazardous material from a facility or core damage in a reactor. In

the ET analysis, for each initiating event, various systems or barriers designed

to prevent the occurrence of the undesired event or to mitigate the progress of

the accident are identified. At each node, the success or failure of these

systems or barriers, known as event tree headings, is graphically shown.

The result is a pictorial representation of various combinations of systems or

barriers which succeed or fail to prevent the occurrence of the undesired

event or to achieve a final safe condition. ET analysis is most helpful for

delineation of sequences of events leading to release of material when there

are multiple or redundant barriers for mitigation of the progression of the

accident. Examples of such sequences include fire scenarios or seismic