ML20137B531

Exemption from Requirements of 10CFR50.44(c)(3)(iii) Re Installation of Reactor Vessel Head Vent & Other Sys Required to Maintain Adequate Core Cooling If Accumulation of Noncondensible Gases Causes Loss of Function of Sys

ML20137B531
Person / Time
Site:	Rancho Seco
Issue date:	12/24/1985
From:	Schroeder F Office of Nuclear Reactor Regulation
To:	SACRAMENTO MUNICIPAL UTILITY DISTRICT
Shared Package
ML20137B520	List: ML20137B515 ML20137B531 ML20137B542
References
TAC-57558, TAC-65453, NUDOCS 8601150550
Download: ML20137B531 (5)

Text

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7590-01 ,

UNITED STATES OF AMERICA >

NUCLEAR REGULATORY COMMISSION In the Matter of )

SACRAMENTO MUNICIPAL UTILITY )

DISTRICT ) Docket No. 50-312 (Rancho Seco Nuclear Generating )

Station) )

EXEMPTION

• I.

Sacramento Municipal Utility District (the licensee) is the holder of Facility Operating License No. DPR-54 which authorizes the operation of the Rancho Seco Nuclear Generating Station (the facility) at steady-state power levels not in excess of 2772 megawatts thermal. The license provides, among other things, that it is subject to all rules, regulations and Orders of the Nuclear Regulatory Connission (the Commission) now or hereafter in effect.

The facility is a pressurized water reactor (PWR) located at the i licensee's site in Sacramento County, California.

II.

] On December 2,1981, the Commission published a revised Section 10 CFR 50.44, " Standards for Combustible Gas Control System in Light-Water-Cooled j

Power Reactors" (46 FR 58484). Section10CFR50.44(c)(3)(iii)ofthe regulation requires:

"To provide improved operational capability to maintain adequate core cooling following an accident, by the end of the first scheduled outage beginning after July 1,1982, and of sufficient duration to permit required modifications, each light-water nuclear power reactor shall be provided with high point vents for the reactor coolant system, for the 8601150550 851224 DR ADOCM O 2

7590-01 1

reactor vessel head, and for other systems required to maintain adequate core cooling if the accumulation of noncondensible gases would cause the loss of function of these systems."

By letter dated July 2, 1982, the licensee requested an exemption from the requirement of 10 CFR 50.44(c)(3)(iii) to install a reactor vessel head vent. The basis for the request was a proposed alternate technique for venting noncondensible gases from the reactor vessel head. The alternate technique involved the use of high point vents at the top of the hot leg U-bends as a means to vent noncondensible gases from the reactor vessel.

On July 25, 1983, the Commission issued an interim exemption to defer the implementation date for installation of a reactor vessel head vent until December 31, 1985. At that time, the Commission was unable to conclude that noncondensible gases could be safely vented by the hot leg high point vents '

4 alone. The primary reason for the Commission's conclusion was the lack of integral system test data which would demonstrate the feasibility of the proposed venting procedure. The interim exemption was granted in order to allow the licensee to perform the necessary integral system testing.

On April 12, 1985, the licensee provided the results of testing performed in the Once-Through Integral System (OTIS) test facility which was performed to demonstrate that a reactor vessel head vent is not necessary to (nsure adequate core cooling in the presence of significant quantities of noncondensible gases. Based upon these test results, the licensee requested a permanent exemption from the requirement to install a reactor vessel head vent.

7590-01 III.

The OTIS facility is an experimental test facility at Babcock & Wilcox's (B&W's) Alliance Research Center in Alliance, Ohio. The OTIS facility was designed to evaluate the thermal / hydraulic conditions in the reactor coolant system and steam generator of a raised-loop B&W reactor, during the natural circulation phases of a small-break Loss of Coolant Accident (LOCA). The OTIS facility is a portion of the Integral Systems Test Program sponsored by the Commission, EPRI, B&W Owners Group and B&W.

The Commission has reviewed the OTIS test results and the licensee's proposed procedures and found that: -

The hot leg vent can be used to remove noncondensible gases from the reactor vessel head during a plant cooldown and depressurization without interrupting natural circulation.

Even if noncondensible gases in the reactor vessel head expanded into the hot leg U-bend and interrupted natural circulation, feed-and-bleed cooling can be used to assure core cooling. In addition, opening of the het leg vent will also allow the restoration of natural circulation.

Venting procedures have been developed to limit gas expansion rates from the reactor vessel head in order to ensure that natural circulation is continuously maintained during a plant cooldown.

Procedures would specify initiation of feed-and-bleed cooling of the core, should natural circulation become interrupted, while' attempting to recover natural circulation by leaving the hot leg vents open. -

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7590-01 The reactor vessel head gas bubble which would remain following plant cooldown to actuation of the decay heat removal system will not interfere with core cooling. The licensee has developed various means to remove this gas bubble in the long term.

The OTIS tests were performed in a manner which assures that the results bound the expected Rancho Seco plant performance.

Based on the above discussion, we conclude that an exemption to 10 CFR ~

50.44(c)(3)(iii) can be granted. Details of the review may be found in the Commission's related Safety Evaluation dated December 24, 1985, which is available for public inspection at the Commission's Public Document Room, 1717 H Streat, N.W., Washington, D.C., and at the Sacramento City-County Library, 828 I Street, Sacramento, California.

IV. '

Accordingly, the Commission has determined that, pursuant to 10 CFR Part 50.12, an exemption is authorized by law and will not endanger life or property or the common defense and security and is otherwise in the public interest and hereby grants an exemption with' respect to the requirements of 10 CFR 50.44(c)(3)(iii), as follows:

The licensee is not required to install a reactor vessel head vent at the Rancho Seco Nuclear Generating Station.

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Pursuant to 10 CFR 51.32, for the reasons stated in the Environmental Assessment and Finding of No Significant Impact, published in the FEDERAL REGISTER on December 17, 1985, 50 FR 51492, the Commission has determined that the issuance of the exemption will have no significant impact on the environment.

This exemption is effective upon issuance.

FOR THE EAR REGULATORY COMMISSION Fran Schroeder, De uty Director i Division of PWR Licensing-B

Office of Nuclear Reactor Regulation Dated at Bethesda, Maryland this 24th day of December 1985.

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