Human factors review for nuclear power plant severe accident sequence analysis Page: 4 of 17
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event diagnosis by having operators verify and
maintain the adequacy of important safety
functions. One advanta^s of an event-based
procedure, however, is that operators may
immediately relate causes and consequences of
off-normal conditions and subsequently act to
directly mitigate accident progression.
SASA analysts made the recommendation that
the emergency procedures for an ATWS be
separated from the EPGs. The human factors
analysis assisted in defining some of the
problems operators may experience with
instructions in the EPGs. One of these
problems is that certain operator actions
called for in response to an ATWS are
substantially different from actions appropri-
ate to other accidents. Some of these actions
are also contrary to operational practices on
which operators are trained. SASA analysts
noted their assumption that the signature of an
ATWS is so distinguishable that operators would
quickly diagnose the event and that a separate
ATWS procedure would expedite operator
One specific example of a problem related
to an ATWS is the instruction in the EPGs to
manually lower and maintain reactor vessel
water level at the top of the active fuel (TAF)
in order to reduce power. For all other
accidents, low vessel level would be an
off-normal condition and the EPGs would
instruct operators to restore vessel level to
within more acceptable bounds.
From a human factors standpoint, the
instructions in the EPGs present some difficul-
ties for operators in relation to an ATWS.
However, the solution proposed by SASA analysts
to separate those instructions relevant solely
to an ATWS may or may not be entirely
satisfactory. Operator performance during a
transient would be based on several factors
including training and operator aids. These
factors and others should be considered across
a range of accidents before targeting the
restructuring of procedures to address problems
related to one specific accident sequence.
The identification of critical operator
actions was coordinated with SASA analysts.
Inputs to the selection process included: (1)
review of the EPGs, (2) consideration of
operator actions included in computer code
models used for accident sequence analysis, (3)
review of operator actions observed during
exercises of ATWS perturbations on the
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Krois, P.A. & Haas, P.M. Human factors review for nuclear power plant severe accident sequence analysis, article, January 1, 1985; Tennessee. (digital.library.unt.edu/ark:/67531/metadc1070510/m1/4/: accessed February 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.