05000237/LER-2016-004

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LER-2016-004, Reactor Building Differential Pressure Less than Technical Specification Requirement
Dresden Nuclear Power Station, Unit 2
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
LER closed by
IR 05000237/2017001 (20 April 2017)
2372016004R00 - NRC Website
LER 16-004-00 for Dresden Nuclear Power Station, Unit 2, Regarding Reactor Building Differential Pressure Less than Technical Specification Requirement
ML17010A281
Person / Time
Site: Dresden Exelon icon.png
Issue date: 01/09/2017
From: Karaba P J
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
17-0001
Download: ML17010A281 (4)


Reported lessons leamed are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

05000-237

3. LER NUMBER

2016 - 00 004

PLANT AND SYSTEM IDENTIFICATION

Dresden Nuclear Power Station (DNPS), Unit 2, is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].

A. Plant Conditions Prior to Event:

Unit: 02 Reactor Mode: 1 Event Date: 11/08/16 Event Time: 0510 CST Mode Name: Power Operation Power Level: 100 percent

B. Description of Event:

On 11/8/16 at 0510 CST, during Under Voltage surveillance testing Bus 33-1 was secured causing Standby Gas Treatment (SBGT) [BH] to start, and the Reactor Building (RB) Ventilation system [VA] automatically isolated. This plant response was expected, but during the evolution RB pressure relative to outside was less than 0.25 inches water column (in WC) vacuum which is required by Technical Specifications (TSs). TS Limiting Condition for Operation (LCO) 3.6.4.1, "Secondary Containment", Condition A was entered. At 0532 CST, the Unit 2 RB Equipment Access Inner door was closed and RB differential pressure was observed to increase above 0.25 in WC vacuum, and Condition A was exited. At the time of the event, the Unit 2 RB Equipment Access Outer door was the boundary for secondary containment.

It should be noted that on 11/7/16 at 2200 CST, during another part of the Under Voltage surveillance testing SBGT was started with RB Ventilation isolated and differential pressure was successfully maintained.

TS LCO 3.6.4.1 was not applicable to Unit 3 which was in Mode 5 with no recently irradiated fuel moves or any operations with a potential for draining the reactor vessel (OPDRVs) in progress.

This event is reportable under 10 CFR 50.73(a)(2)(v)(C), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material.

C. Cause of Event:

The Apparent Cause was degraded sealing surface on the Unit 2 RB Equipment Access Outer door coincident with RB Ventilation isolation which resulted in excessive in-leakage. The SBGT system did not have the capacity required to maintain the differential pressure in this condition. A Contributing Cause was a lapse in procedure use and adherence which requires communication of degraded seal conditions. An additional Contributing Cause was the lack of prioritization of repairs due to improper risk perception associated with addressing longstanding issues with the seals.

Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Intocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

05000-237

3. LER NUMBER

2016 - 00 004

D. Safety Analysis:

The safety significance of this event was low as it did not adversely impact the health and safety of the public. During the evolution, the differential pressure of the RB remained negative relative to the outside at all times. Additionally, there was no release, or potential for release, of radioactive material during the 22 minutes in TS LCO 3.6.4.1 Condition A.

E. Corrective Actions:

The Unit 2 RB Equipment Access Outer door seals were replaced, and work has been scheduled to improve door operation. This issue will be reviewed to determine if changes to the surveillance testing procedures are appropriate given the potential impact of the open interlock door. Additionally, procedures will be revised to require communication of seal inspection results for documentation in Operator Logs.

F. Previous Occurrences:

On 12/15/15, Operations secured RB Ventilation for Unit 2 and RB differential pressure degraded rapidly due to the 3C exhaust fan vortex damper failing to open as expected which impacted secondary containment.

On 9/5/15, a failure of the latches on an access hatch resulted in reduction of RB Ventilation exhaust flow and subsequent loss of secondary containment.

G. Component Failure Data:

Manufacturer Model S/N Type N/A N/A N/A Door