05000298/LER-1982-006, Forwards LER 82-006/03L-0.Detailed Event Analysis Submitted

Forwards LER 82-006/03L-0.Detailed Event Analysis Submitted

ML20052A347
Person / Time
Site:	Cooper
Issue date:	04/21/1982
From:	Lessor L NEBRASKA PUBLIC POWER DISTRICT
To:	Jay Collins NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML20052A348	List: 05000298/LER-1982-006, Forwards LER 82-006/03L-0.Detailed Event Analysis Submitted 05000298/LER-1982-006-03, /03L-0:on 820322,while Inerting Drywell,Ductwork Between Primary Containment & Reactor Bldg Ventilation Found Failed in Several Places,Preventing Oxygen Concentration & Differential Pressure from Being Established
References
CNSS820187, NUDOCS 8204280243
Download: ML20052A347 (3)

LER-1982-006, Forwards LER 82-006/03L-0.Detailed Event Analysis Submitted

Event date:
Report date:
2981982006R00 - NRC Website

text

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COOPER NUCLEAR 5TATtoN Nebraska Public Power District

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CNSS820187 April 21, 1982 h

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Mr. John T. Collins, Regional Administrator U.S. Nuclear Regulatory Commission q

Region IV

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\\b 611 Ryan Plaza Drive Suite 1000 Arlington, Texas 76011 l

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Dear Sir:

This report is submitted in accordance with Section 6.7.2.B.2 of the Technical Specifications for Cooper Nuclear Station and discusses a reportable occurrence that was discovered on March 22, 1982. A licensee l

event report form is also enclosed.

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j Report No.:

50-298-82-06

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IQ Report Date:

April 21, 1982

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l Occurrence Date: March 22, 1982

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m' '"y Facility:

Cooper Nuclear Station

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Identification of Occurrence:

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A condition which resulted in operation in a degraded e-per-u mitted by Sections 3.7.E.1 and 3.7.A.5 of the Technica Cshecif-p ications.

/4, g g Conditions Prior to Occurrence:

The reactor was critical at less than 5% of rated thermal power during plant startup.

Description of Occurrence:

The ductwork between the drywell isolation valves and the air purge inlet valve was found to have failed. As a result, the drywell was not inerted and a differential pressure was not established between the drywell and torus in the time required by the Technical Specif-ications.

Designation of Apparent Cause of Occurrence:

Complete failure of the ductwork between the drywell isolation valves and the air purge inlet valve caused by a rapid release of pressure in the ductwork. This ductwork is not required for pri-mary containment integrity.

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w Mr. John T. Collins April 21, 1982 Page 2.

Analysis of Occurrence:

The primary containment cystem is designed to provide a protective barrier between the reactor core and the environment. The section of ductwork that failed is not a part of this system but it does provide a path to the drywell for the following functions:

1.

Supplies purge air for de-inerting the drywell.

2.

Supplies nitrogen for inerting the drywell.

3.

Supplies nitrogen make up to the drywell during plant operation.

The failed section could have been isolated from the drywell by two isolation valves either automatically on an isolation signal or manually.

The following is a sequence of events and description of what is believed to be the cause of the failed ductwork. On March 20, 1982, a scram occurred. The scram caused a Group 6 isolation, closing all primary and secondary containment isolation valves.

The Group 6 isolation signal was cleared within a few minutes and the group isolation was reset.

The primary containment valves remained shut. Since this ductwork is used for nitrogen makeup, while the drywell isolation valves were shut, nitrogen continued to flow into the ductwork pressurizing it to a pressure of approx-imately 150 psig. This occurred because nitrogen makeup is not normally secured on a group isolation signal. The nitrogen makeup piping is protected from overpressure by a relief valve set at 225 psig. The design pressure of the ductwork is about 75 psig with no associated relief valve.

(It is recognized that the relief on the makeup line should have been set at a pressure commensurate with the design pressure of the ductwork.) The ductwork remained in this pressurized state for approximately two. hours. The operator then prepared to de-inert the drywell in preparation for a short maintenance outage. The appropriate procedure called for opening the air purge valve.

When this valve was opened, the pressure was rapidly released. Unlike the normal opening of the valve wich 1 to 2 psid across the disc, this rapid pressure release was somewhat like the firing of a cannon.

The massive release literally ripped the ductwork from its hangers between the drywell isolation valves and the air purge valve.

(The ductwork, the air purge valve, and the high pressure nitrogen had formed a gas shock tube.)

s' Mr. John T. Collins April 21, 1982 Page 3.

The failed ductwork was found on March 22, 1982 while trying to inert the drywell with nitrogen after a reactor startup. The drywell was immediately isolated by shutting the drywell isolation valves. At this time it was realized that the drywell would not be inerted in the 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> required ~by CNS Technical Specifications.

A temporary Technical Specification change was then requested and granted from the NRC to continue operation while inerting via an alternate path. The drywell was inerted in 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br /> versus 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and the differential pressure was established in 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br /> versus 26 hours3.009259e-4 days <br />0.00722 hours <br />4.298942e-5 weeks <br />9.893e-6 months <br />. The Technical Specification change ~ allowed 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.

Due to the fact that primary _ containment integrity was maintained throughout this event, this occurrence presented no adverse conse-quences from the standpoint of public health and safety.

Corrective Action

The ductwork that failed was replaced. The replacement material that_was used is of a heavier gauge than the original and the field joints were socket welded. A modification is_being implemented to eliminate the nitrogen makeup function of this ductwork by the installation hard piping to the drywell through the ACAD System which has the necessary isolation valves. This modification will incorporate a relief valve. The modification will be made during the bby 1982 outage.

In the interim, the control-room operators have been briefed to secure nitrogen upon isolation of containment..

Sincerely, ff

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L. C. Lessor i

Station Superintendent Cooper Nuclear Station LCL:cg Attach.

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