4.4.X [Applicable Safety-significant System, Structure, or Component]
Identify the safety-significant SSC.
This subsection states the reason for designating the SSC as a safety-significant
SSC, followed by specific identification of its preventive or mitigative safety
function(s) as determined in the hazard and accident analysis. Do not discuss
Safety functions are top- level statements that express the objective of the SSC
in a given accident scenario. For example, the safety function of a hydrogen
detector in a dissolver vessel offgas line could be stated as: "To monitor
hydrogen concentration in the dissolver offgas and provide a signal to
shutdown the dissolving operation before explosive concentrations of
hydrogen are reached."
The specific accident(s) or general rationale associated with the safety function
should be identified. Safet y-significant SSCs are designated for overall
purposes such as defense- in-depth, for which even normal operation
considerations are involved. There may, or may not be, a single accident that,
by itself, completely defines the safety function.
This subsection provides a description of the safety-significant SSC and the
basic principles by which it performs its safety function (e.g., sensor and
interlock for hydrogen detector discussed in section 4.3.X.1). Describe its
boundaries and interface points with other SSCs relevant to the safety function.
Identify SSCs whose failure would result in a safety-significant SSC losing the
ability to perform its required safety function. These SSCs would also be
considered safety-significant SSCs for the specific accident conditions or
general rationale for which the safety-significant designation was made
When describing the SSC, provide a basic summation of the physical
information known about the SSC, including simplified system drawings. If
known, summarize pertinent aspects of manufacturer's specifications.
Pertinent aspects are considered to be those that directly relate to the safety
function (e.g., diesel generator load capacity, time to load if critical) as
opposed to general industrial equipment specifications that fall out from these
capabilities (e.g., starting torque, motor insulation, number and type of
windings). Such lower tier details should be implicitly included only by
reference to the overall specifications.