U.S. Reactor Containment Technology: a Compilation of Current Practice in Analysis, Design, Construction, Test, and Operation, Volume 1 Page: 1-02
220.127.116.11 Maximum Credible Accident
In considering the safety of any reactor system a large number of
potential accidents are examined in terms of their probable consequences.
The maximum credible accident (mca) (also sometimes referred to as the
maximum design accident) is that accident wnose consequences, as measured
by the radiation exposure of the surrounding public, would not be exceeded
by any other accident whose occurrence during the lifetime of the facility
would appear to be credible. It is immediately obvious that this defini-
tion is singularly dependent upon what is considered "credible," and upon
further reflection it will be seen that this credibility must be ascer-
tained at least twice for any reactor accident, once for the credibility
of the initiating event and again for the credibility of the performance
of the various consequence-limiting safeguards.2 In the former evaluation
a double-ended primary pipe rupture, but not a gross pressure vessel fail-
ure, is frequently considered to be the initiating event; and in the latter
the continuous integrity of the containment envelope is assumed credible,
but the operation of the core cooling system, although likely, may not be
deemed to be sufficiently certain for credit to be confidently assigned
in the event of an accident.
The maximum credible accident is generally the accident that releases
the greatest amount of fission products to the containment. However, this
is not necessarily so, inasmuch as the particular safeguard features in
use at a given installation may be more effective for some accidents than
1.1.2 Need for Containment
The need for a containment system in the large power reactor instal-
lation is well established by convention and precedent in the United States,
and the specific design requirements are determined by the reactor safety
analysis. Philosophically, containment is provided so that the risk
that cannot be dissociated from the operation of a particular reactor can
be reduced to acceptable proportions with respect to the corresponding
gain that is expected to result from its operation. However, such a
balance of gain versus risk is impossible to attain on a quantitative
basis, and only the risk enters into the evaluation that is made in con-
nection with every reactor safety analysis. The specific function of the
containment system is to reduce the consequences of the maximum credible
accident so that a particular facility may fulfill siting requirements
as defined in the Code of Federal Regulations.1 On this basis, contain-
ment systems may be called upon to effect a reduction in activity released
in an accident by a factor of 102 to 105.
1.1.3 Evolution of Containment3
As defined, containment is the provision for controlling the pub-
lic exposure following an mca to acceptable levels. This "control"
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Cottrell, William B. & Savolainen, A. W. U.S. Reactor Containment Technology: a Compilation of Current Practice in Analysis, Design, Construction, Test, and Operation, Volume 1, report, August 1965; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc101033/m1/30/ocr/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.