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DOE-STD-1027-92
has already been performed (see Section 4.1.1) is considered sufficient for
identifying release mechanisms.
Single-Failure Electro-Mechanical Systems
2)
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
3)
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
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