05000263/LER-2014-003, Regarding Torus to Drywell Vacuum Breaker Dual Indication During Testing
| ML14098A460 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 04/08/2014 |
| From: | Fili K Northern States Power Co, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-MT-14-030 LER 14-003-00 | |
| Download: ML14098A460 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| LER closed by | |
| IR 05000263/2017001 (19 April 2017) | |
| 2632014003R00 - NRC Website | |
text
Xcel Energy April 8, 2014 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket 50-263 Renewed Facility Operating License No. DPR-22 Monticello Nuclear Generating Plant 2807 W County Road 75 Monticello, MN 55362 L-MT-14-030 10 CFR 50.73 LER 2014-003 "Torus to Drywell Vacuum Breaker Dual Indication during Testing" A Licensee Event Report (LER) for this occurrence is enclosed.
Summary of Commitments This letter contains no new commitments and no revisions to existing commitments.
Karen D. Fili Site Vice President, Monticello Nuclear Generating Plant Northern States Power Company-Minnesota Enclosure cc:
Regional Administrator, Region Ill, USNRC Project Manager, Monticello Nuclear Generating Plant, USNRC Resident Inspector, Monticello Nuclear Generating Plant, USNRC
NRC FORM 366 U.S. NUCLEAR REGULA TORY COMMISSION APPROVED BY OMB: NO. 3160-0104 EXPIRES: 0113112017 (02-2014)
, the NRC may not conduct or sponsor, and a person is not required to respond to, the lnfomnation collection.
YEAR 2014
- 6. LER NUMBER I
SEQUENTIAL I NUMBER 003 REV NO.
00 2
- 3. PAGE OF 3
On February 11, 2014, the plant was in Mode 1 at approximately 88% power. At 1430 during performance of the monthly torus to drywell vacuum breaker check procedure, vacuum breaker valve A0-2382A [VACB]
indicated closed in the control room but provided dual indication locally in the reactor building. Each vacuum breaker is provided with dual position switches which operate status lights in the control room and reactor building. The vacuum breaker was declared inoperable, and Technical Specification (TS) Action 3.6.1.7.8 was entered. An alarm response procedure for an open torus to drywell vacuum breaker was entered and the valve was cycled four times until it was verified closed at both locations at 1436.
After the torus to drywell vacuum breaker indicated closed, the vacuum breaker was declared operable and the TS Action 3.6.1.7.8 was exited.
EVENT ANALYSIS
The event is reportable in accordance with 10 CFR 50.73 (a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident and in accordance with 10 CFR 50. 73(a)(2)(ii)(B) as an unanalyzed condition.
The event was reported in Event Notification 49819.
A review of the event determined that a safety system functional failure did not occur as defined in Nuclear Energy Institute (NEI) 99-02 Revision 7. An engineering analysis was performed which determined that the torus to drywell vacuum breaker was capable of performing its design basis function.
SAFETY SIGNIFICANCE
Vacuum breakers are provided to equalize the pressure between the suppression chamber (i.e., torus) and the drywell to prevent a backflow of water from the suppression chamber pool into the vent header system.
An open vacuum breaker would allow communication between the drywell and suppression chamber airspace, and as a result, there would be a potential for suppression chamber over pressurization due to this bypass leakage if a loss of coolant accident were to occur. During performance of the monthly torus to drywell vacuum breaker check procedure, vacuum breaker valve A0-2382A stroked closed therefore, safety function was preserved.
CAUSE
Analysis has shown the cause of the observed intermittent dual indication for vacuum breaker valve A0-2382A after cycling was inconsistent operation of the valve limit switches. Based on failure modes and effects analysis, the closed limit switches did not consistently indicate closed when the valve was, in fact, closed.
CORRECTIVE ACTION
U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
- 2. DOCKET
- 6. LER NUMBER
- 3. PAGE OF YEAR I SEQUENTIAL I REV NUMBER NO.
05000-263 3
3 2014 003 00 Vacuum breaker valve A0-2382A, its limit switches, and close indication circuit will be inspected to confirm the results of the failure mode and effects analysis during the next refueling outage or earlier opportunity, if available.
PREVIOUS SIMILAR EVENTS
LER 2014-002 reported a similar occurrence of valve A0-2382A not indicating closed during performance of the torus to drywell monthly vacuum breaker check procedure on February 7, 2014. Initial investigation indicated the occurrence may have been a result of the short time between inerting containment and performance of the test, as such it was decided it was acceptable to re-perform the monthly torus to drywell vacuum breaker check on February 11, 2014. Subsequent to the event on February 11, 2014, it was determined that the indication was inconsistent operation of the valve limit switches.