United States Nuclear Regulatory Commission - Protecting People and the Environment

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

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