U.S. Nuclear Regulatory Commission
Technical Assistance Request Regarding the Auxiliary Building Ventilation System at Zion Nuclear Power Station
HPPOS-323 PDR-9308260238
Title: Technical Assistance Request Regarding the
Auxiliary Building Ventilation System at Zion Nuclear Power
Station
See the memorandum from J. A. Zwolinski to E. G. Greenman
dated June 23, 1993. This NRR memo contains the NRR
responses to questions asked by Region III regarding the
auxiliary building ventilation system at Zion Nuclear Power
Station. The licensee had taken the position that the
UFSAR contains two types of information: descriptive and
design. They indicated that paragraphs labeled "system
description" are general design and operating features
intended to provide an understanding of the overall plant
operation. The licensee also stated that only paragraphs
labeled "design basis" can be considered as design basis.
This issue is concern at Zion and is generic to other
nuclear power plants.
Question 1: Is the whole UFSAR considered in the design
basis of the plant, or only sections specifically labeled
as such?
The definition of Design Bases in 10 CFR 50.2 means that
information that identifies the specific functions to be
done by a structure, system, or component of a facility and
the specific values or range of values chosen for
controlling parameters chosen for controlling parameters as
reference bounds for design. These values may be
restraints derived from generally accepted "state of the
art" practices for achieving functional goals, or
requirements derived from analysis of the effects of a
postulated accident for which a structure, system or
component must meet its functional goals. Regardless of
what a paragraph in an UFSAR or FSAR is called, if a
specification was assumed in an accident analysis, then it
is part of the design basis.
Question 2: Is the concept that NRC only cares about
maintaining negative pressure within contaminated cubicles
in the auxiliary building the design basis or is
maintaining a negative pressure within the whole auxiliary
building the design basis?
The design basis and the licensing basis for the auxiliary
building ventilation system serving all areas of the
auxiliary building and the spent fuel pool building are to
maintain the auxiliary building at a negative pressure of
about 0.25 inch of water relative to ambient under normal
and abnormal operation and to maintain the cubicles at a
negative pressure of about 0.25 inch of water relative to
the auxiliary building; hence, a negative pressure of about
0.5 inch of water relative to the outside. The objective
is to maintain the auxiliary building at a negative
pressure with respect to all adjacent areas so that
contamination is not transported to areas that are at a
lower pressure than the auxiliary building.
Question 3: Does the auxiliary building wall / door have any
function with regard to keeping contaminated airborne
material inside?
The design functions of the outer walls and doors serve in
situations not involving an accident are structural and
missile protection and control of the spread of
contamination by allowing the required vacuum to be
maintained. Auxiliary building access doors should not
routinely be left open during normal operations since this
may affect the normal ventilation flow path and/or function
of maintaining a negative pressure of about 0.25 inch of
water in the auxiliary building. This negative pressure is
designed to prevent the release of radioactive material
from the auxiliary building. The proper system flow
balance is required to prevent the spread of airborne
radioactive material from areas of high concentration to
areas of lower concentration. Question 4: Can licensees
justify operability with PRA and can licensees use PRA to
delay a test or an operability determination?
These practices are unacceptable.
Question 5: Is there some design function for the
auxiliary building outer walls relating to the confinement
of radioactive materials that may be present in the
auxiliary building during non-accident conditions?
The design function of the outer walls and doors not
involving an accident are structural and missile protection
and control of the spread of contamination by allowing the
required vacuum to be maintained. Maintaining 0.25 inch of
negative pressure in potentially contaminated areas serves
to confine radioactive materials to the auxiliary building
under non-accident conditions.
Question 6: Is the "interfacing system LOCA" considered a
postulated accident and is the occurrence of such an event
considered part of the design basis?
The answer is no to both questions.
Guidance was also sought on the role of PRA in the
preparation of 10 CFR 50.59 safety evaluations by
licensees. 10 CFR 50.59 identifies the use of probability
in reference to the determination of an unreviewed safety
question. Prior to PRA, the increase in probability of
occurrence for a 10 CFR 50.59 evaluation was judged on
design basis considerations and engineering judgement.
With the current PRA methods, reliability data, and plant
specific PRAs, it is reasonable to expect these to be used
to estimate changes in probability associated with proposed
plant modifications. However, the results of licensee 10
CFR 50.59 evaluations should not be based solely on bottom
line PRA numbers. Other considerations such as engineering
judgement and operating experience should be factored in
when appropriate.
Regulatory references: 10 CFR 50.2, 10 CFR 50.59
Subject codes: 5.0, 5.5
Applicability: Reactors