United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9902230163


LICENSEE EVENT REPORT (LER) PAGE: 1 OF 3
FACILITY NAME: VERMONT YANKEE NUCLEAR POWER CORPORATION
DOCKET NUMBER: 05000271
TITLE: Scram Discharge Volume Valve closing Time Excessive due to Undersized Actuators
EVENT DATE: 12/11/98 LER #: 98-025-01 REPORT DATE: 02/10/99
OTHER FACILITIES INVOLVED: N/A DOCKET NO: 05000
OPERATING MODE: N POWER LEVEL: 100
THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR SECTION: 50.73(a)(2)(vii) & OTHER "Part 21"
LICENSEE CONTACT FOR THIS LER:
NAME: Michael A. Balduzzi, Plant Manager TELEPHONE: (802) 257-7711
COMPONENT FAILURE DESCRIPTION:
CAUSE: B SYSTEM: AA COMPONENT: LCV MANUFACTURER: B237
REPORTABLE TO EPIX: Y
SUPPLEMENTAL REPORT EXPECTED: NO


ABSTRACT:

On 12/11/98 with the plant at 100% power, it was determined that one Scram Discharge Volume (SDV) drain valve (CRD-LCV-33B) did not meet the stroke time requirements of the In-Service Test (IST) Program. Both the North and the South SDV's have two drain valves in series, CRD-LCV-33A/C on the North and CRD-LCV-33B/D on the South. The drain valve, CRD-LCV-33B, was subsequently declared inoperable and the upstream manual isolation valve was isolated and tagged. This maintained primary containment integrity requirements and allowed the SDV to be manually drained. CRD-LCV-33B was subsequently refurbished, and tested to ensure that the IST timing requirements were met. This was completed on 12/13/98.

Prior quarterly surveillance testing on 12/7/98 determined that CRD-LCV-33D failed to close. It was refurbished, tested and returned to service on 12/10/98. Further investigation revealed a potential common-cause failure, reportable under 10CFR50.73(a)(2)(vii), whereby the valve actuators used on all the SDV drain valves were of potentially insufficient size to operate the valves within the required times, during all conditions. Subsequent testing determined that SDV drain valve CRD-LCV-33C also did not meet the IST stroke time requirements. These drain valves are required to go shut on a scram signal to isolate the SDV and act as a primary containment isolation valve when the scram valves are open. New larger actuators have since been installed on each valve which eliminated the problem.

The root causes of this event are an inadequate actuator sizing calculation by the vendor under the design conditions specified in the procurement specification, and inadequate manufacturing Quality Assurance controls to ensure specification requirements were maintained. Contributing causes included conflicting information in the design package that was not identified by Vermont Yankee, and closing forces of the actuator at either end of the valve stroke were apparently not as designed.

Immediate corrective action closed and tagged the upstream manual isolation valve. The system arrangement is such that the SDV can be drained locally through the manual isolation valve. The valves were refurbished and tested to ensure that the IST timing requirements were met. Larger actuators were subsequently installed to ensure proper valve operation under all design conditions. As only one drain valve failed to close, the remaining drain valve in that line would have closed and limited any reactor water loss which would fulfill the safety function of the valves. Therefore, this event is not considered to have presented an increased risk to the health and safety of the public.

This report also constitutes a Part 21 notification in accordance with 10CFR21.2(c).

END OF ABSTRACT


TEXT PAGE 2 OF 3

DESCRIPTION

On 12/11/98 with the plant at 100% power, during accelerated testing due to a previous failure of another Scram Discharge Volume (SDV) (EIIS=AA) drain valve, it was determined that one drain valve (CRD-LCV-33B) did not meet the stroke time requirements of the In-Service Test (IST) Program. Both the North and the South SDV's have two drain valves in series, CRD-LCV-33A/C on the North and CRD-LCV-33B/D on the South. The drain valve, CRD-LCV-33B, was subsequently declared inoperable and the upstream manual isolation valve was isolated and tagged. This maintained primary containment requirements and still allowed the SDV to be manually drained. CRD-LCV-33B was subsequently refurbished, and tested to ensure that the IST timing requirements were met. This was completed on 12/13/98.

Prior quarterly surveillance testing on 12/7/98 determined that CRD-LCV-33D, a drain valve on the South SDV, failed to close. It was subsequently refurbished, tested and returned to service on 12/10/98. Further investigation revealed a potential common-cause failure, reportable under 10CFR50.73(a)(2)(vii), whereby the valve actuators used on all the SDV drain valves were of insufficient size to operate the valves within the required times, during all conditions. Subsequent testing determined that SDV drain valve CRD-LCV-33C, a drain valve on the North SDV, also did not meet the IST stroke time requirements. The actuators and valves were installed under Engineering Design Change EDCR 97410 in April of 1998 and were sized in accordance with the vendor's recommendations. These drain valves are required to go shut on a scram signal to isolate the SDV and act as a primary containment isolation valve when the scram valves are open.

This issue was addressed by the installation of larger actuators, on 12/21/98, via the Minor Modification process, which were properly sized to operate the valves under any design conditions at Vermont Yankee. This report also constitutes a Part 21 notification in accordance with 10CFR21.2(c) and NUREG 1022, Revision 1.

CAUSE OF EVENT

The root causes of this event are an inadequate actuator sizing calculation by the vendor under the design conditions specified in the procurement specification, and inadequate manufacturing Quality Assurance controls to ensure specification requirements were maintained. Contributing causes included conflicting information in the design package that was not identified by Vermont Yankee, and closing forces of the actuator at either end of the valve stroke were apparently not as designed.

ANALYSIS OF EVENT

The SDV's are used to limit the loss of and contain the reactor vessel water from all control rod drives during a scram. These volumes are provided in the scram discharge header. During normal plant operation the volumes are empty with all the drain and vent valves open. Upon receipt of a scram signal, the vent and drain valves close. While scrammed, the control rod drive seal leakage continues to flow to the discharge volumes until the discharge volume equals reactor pressure. Following a scram, when the scram signals are cleared and the Reactor Protection System (RPS) logic is manually reset the vent and drain valves are opened and the SDV drained.

Three of the four drain valves failed the IST timing tests with one failure to completely close. The design maximum closure time for these valves is 30 seconds. With the exception of CRD-LCV-33D, all valves met the 30 second design requirement and would have closed and provided their respective function. With respect to valve CRD-LCV-33D, primary containment isolation requirements would still have been met due to the redundant valve, CRD-LCV-33B in series being able to isolate the system and provide its function, Once the initial volume of water from the insertion of the control rods is drained to the SDV, additional leakage to the SDV would be from the control rod drive seals. If a scram had occurred such that the drain valves were required to close but all failed open, approved operating procedures are in place which would instruct operators to take the necessary steps to mitigate the event. Additionally, if this scenario were to occur, the operators would still have the ability to manually isolate the drains via local manual valves. Indication of failed drain valves could be detected by the operators based on remote valve position indication, vessel level indication, Condensate Storage Tank level or Radwaste tank levels.

As only one drain valve failed to close, the remaining drain valve in that line would have closed and limited any reactor water loss which would fulfill the safety function of the valves. Although the redundancy of the system was degraded, this event is not considered to have presented an increased risk to the health and safety of the public due to the capability to make-up to the reactor and availability to detect failure and manually isolate the line.

CORRECTIVE ACTIONS

IMMEDIATE:
1. Each valve was declared inoperable upon not having met its criteria, and corresponding Technical Specification requirements were met.
2. An Event Report was written to investigate the potential common-cause failure.
3. Daily stroke time tests were initiated to monitor for degradation, on an interim basis, for all the SDV drain valves.
4. A Basis for Maintaining Operation (BMO) was written and approved that justified continued plant operation.
5. New larger valve actuators were installed on the four drain valves.


TEXT PAGE 3 OF 3

CORRECTIVE ACTIONS (cont.)

SHORT TERM
1. The Vice President of Engineering held a briefing with all engineering managers regarding the development of procurement specifications during the design change process. This information was, in turn, relayed to the engineering staff. This has been completed. This immediately addressed contributing cause number 1.
2. The old valve actuators will be returned to the actuator manufacturer for testing to determine why the in-field testing showed an apparent degradation in torque values at each end of the stroke. The expected completion date is 2/26/99. This action will provide information for addressing the apparent contributing cause number 2.
LONG TERM
1. Although this issue was addressed by the installation of larger actuators, upon receipt of any additional recommendations from the valve manufacturer, Vermont Yankee will take additional actions as appropriate. The expected receipt of the recommendations is 3/16/99. This addresses both root causes.
2. Upon receipt of the actuator testing conclusions from the actuator manufacturer, Vermont Yankee will take additional actions as deemed necessary. The expected receipt of these conclusions is 6/18/99. This will address the apparent contributing cause number 2.
3. Evaluate the design change process for any changes needed during development of a procurement specification. The expected completion date is 3/12/99. This will address contributing cause number 1.
4. Vermont Yankee Quality Assurance will evaluate the adequacy of the vendors design control in reference to their approved Appendix B program. The expected completion date is 4/1/99. This addresses both root causes.

ADDITIONAL INFORMATION:

Similar events have been reported in the past as:

LER 96-021 - "Inadequate Procedure Controls of MOV Limit Switch Settings Resulting in a Potential Common Cause Failure Mode with the Capacity to Affect Multiple Safety Significant Components"
LER 97-010 - "Individual Control Rod Drive Scram Times Greater than Normal due to Manufacturing Defect in Scram Solenoid Pilot Head Assembly"
LER 98-006 - "An Inadequate Maintenance Procedure for Safety Class Breakers Establishes Conditions which could have Led to Failure of Multiple Safety Class Breakers"
LER 98-012 - "Failure to Adequately Define Standards for Reducing Contact Forces Caused by Sharp Edges on MOV Internals Results in Six Primary Containment Isolation Valves being Declared In operable"

These LER's all deal with common cause failure but address equipment other than valve actuators.


ATTACHMENT TO 9902230163 PAGE 1 OF 1

VERMONT YANKEE
NUCLEAR POWER CORPORATION
VY

P.O. Box 157, Governor Hunt Road
Vernon, Vermont 05354-0157
(802) 257-7711

February 10, 1999
BVY 99-015

U.S. Nuclear Regulatory Commission
ATTN: Document Control Desk
Washington D.C. 20555

Subject: Vermont Yankee Nuclear Power Station
License No. DPR-28 (Docket No. 50-271)
Reportable Occurrence No. LER 98-025, Rev. 1

As defined by 10CFR50.73, we are reporting the attached Reportable Occurrence as LER 98-025, Rev. 1.

Sincerely,

VERMONT YANKEE NUCLEAR POWER CORPORATION

Michael A. Balduzzi
Plant Manager

cc: USNRC Region I Administrator
USNRC Resident Inspector - VYNPS
USNRC Project Manager - VYNPS
VT Dept. of Public Service

*** END OF DOCUMENT ***


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