05000263/LER-2012-001, Regarding Degraded Voltage Transfer Scheme Not in Compliance with T.S. 3.3.8.1
| ML12187A774 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 07/05/2012 |
| From: | Schimmel M Xcel Energy, Northern States Power Co |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-MT-12-064 LER 12-001-00 | |
| Download: ML12187A774 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2632012001R00 - NRC Website | |
text
@ Xcel EnergyB July 5, 2012 Monticello Nuclear Generating Plant 2807 W County Road 75 Monticello, MN 55362 L-MT-12-064 10 CFR 50.73 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-001 Monticello Nuclear Generating Plant Docket 50-263 Renewed Facility Operating License No. DPR-22 LER 2012-001 "Degraded Voltage Transfer Scheme Not in Compliance with T.S. 3.3.8.1" A Licensee Event Report (LER) for this occurrence is attached.
Summarv of Commitments This letter contains no new commitments and no revisions to existing commitments.
Mark A. Schimmel 6'
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 State of Minnesota
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION (10-2010)
LICENSEE EVENT REPORT (LER) ee reverse for re uired number of (Sdigltslcharacters?or each block)
- 1. FACILITY NAME Monticello Nuclear Generating Plant APPROVED BY OMB NO. 3150-0104 EXPIRES 1013112013
, the NRC may not conduct or sponsor, and a person is norrequired to respond to, the information collection.
- 2. DOCKET NUMBER 05000 - 263
- 4. TlTLt Degraded Voltage Transfer Scheme Not in Compliance with T.S. 3.3.8.1
- 3. PAGE 1 OF 3
- 8. OTHER FACILITIES INVOLVED
- 9. OPERATING MODE 1
- 10. POWER LEVEL 100%
FACILITY NAMt FACILITY NAME
- 7. REPORT DATE
- 5. EVENT DATE
- 11. THlS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 5: (Check all that apply)
C]
20.2201(b)
C] 20.2203(a)(3)(i) 50.73(a)(2)(i)(C)
[XI 50.73(a)(2)(vii) 20.2201 (d)
C] 20.2203(a)(3)(ii)
C] 50,73(a)(2)(ii)(A)
C] 50.73(a)(2)(viii)(A)
C] 20.2203(a)(I)
C] 20.2203(a)(4)
[XI 50.73(a)(2)(ii)(B)
C] 50.73(a)(2)(viii)(B) 20.2203(a)(2)(i)
C] 50,36(c)(l)(i)(A)
C] 50.73(a)(2)(iii)
[XI 50.73(a)(2)(ix)(A)
C]
20.2203(a)(2)(ii)
C] 50,36(c)(l)(ii)(A)
C] 50.73(a)(2)(iv)(A)
C] 50.73(a)(Z)(x)
C] 20.2203(a)(2)(iii)
C] 50.36(~)(2)
[XI 50.73(a)(2)(v)(A)
C] 73.71 (a)(4)
C] 20.2203(a)(2)(iv) 50.46(a)(3)(ii)
[XI 50.73(a)(2)(v)(B)
C] 73.71 (a)(5)
C]
20.2203(a)(2)(v) 50,73(a)(2)(i)(A)
[XI 50.73(a)(2)(~)(~)
C] OTHER Specify in Abstract below or in C]
20.2203(a)(2)(vi)
[XI 50,73(a)(2)(i)(B)
[XI 50.73(a)(2)(v)(D)
EVENT DESCRIPTION
Monticello Nuclear Generating Plant (MNGP) was in Mode 1 at 100% power prior to the event. There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event.
U.S. NUCLEAR REGULATORY COMMISSION (10-2010)
At approximately 0900 CDT on May 8, 2012, it was determined that the Monticello Nuclear Generating Plant (MNGP) did not meet Technical Specification Limiting Condition for Operation (LCO) 3.3.8.1 because the requirement of Table 3.3.8.1-1 Function 2.b. for the 4.16 KV Essential Bus Degraded Voltage time delay transfer to the Emergency Diesel Generators (EDGs) [DG] of <=9.2 seconds could not be met under all postulated conditions. The degraded voltage transfer scheme has the ability to transfer to a standby offsite source (IAR Transformer [XFMR]) which under a degraded voltage condition would delay the transfer to the EDGs an additional 5 seconds. Both EDGs were subsequently declared inoperable.
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET The plant non-essential4.16 KV buses may be supplied from either of two sources. The normal source is 2R Transformer supplied from the 345 KV Substation. The alternate source is 1 R Transformer supplied from the 11 5 KV Substation. The 2R Transformer and 1 R Transformer have two separate secondary windings designated as x and y. The y winding of each transformer supplies Bus 13 and Bus 14. Buses 15 and 16 are normally supplied by Buses 13 and 14, respectively. In the event of degraded or loss of voltage to Buses 15 and 16, an essential bus transfer separates the Essential Buses from Buses 13 and 14 and switches them to an alternate source. These sources include the voltage regulated IAR Transformer or the EDGs if IAR Transformer is not available, Technical Specification 3.3.8.1 is the specification for Loss of Power Instrumentation (LOP). LC0 3.3.8.1 states, "The LOP instrumentation for each Function in Table 3.3.8.1-1 shall be Operable." Table 3.3.8.1-1 gives the required function for the time delays to be Operable as being able to fulfill their functions of separating from the offsite sources in as little as 8.8 seconds or as long as 9.2 seconds. Although the entire transfer scheme is tested under a TS surveillance procedure, there is no mention in Table 3.3.8.1-1 of the 5 seconds that it takes to check bus voltages while the logic checks IAR Transformer as an acceptable offsite source.
- 3. PAGE 2 OF 3
- 1. FACILITY NAME Monticello Nuclear Generating Plant Following installation in 1981, the degraded voltage scheme would directly transfer to the EDGs after 10 seconds of persistent degraded voltage. In 1984, the MNGP installed a modification that transferred essential buses to IAR Transformer for 5 seconds prior to transferring to the EDGs under degraded voltage conditions, resulting in a total potential 15 second time delay to separate from Offsite power. During the 201 2 Component Design Bases Inspection, it was determined that the MNGP was not compliant with Technical Specification 3.3.8.1 time delay for Function 2.b. This led to both EDGs subsequently being declared inoperable and entry into LC0 3.8.1.E on May 8, 2012 at 1000. IAR Transformer was removed from service at 1014, removing the additional degraded voltage time delay of 5 seconds and restoring Technical Specification compliance.
EVENT ANALYSIS
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(A-D) Event or Condition that Could Have Prevented Fulfillment of a Safety Function; 10 CFR 50.73(a)(2)(i)(B) Operation or Condition Prohibited by Technical Specifications; 10 CFR 50.73(a)(2)(ii)(B) Unanalyzed Condition; 10 CFR 50,73(a)(2)(vii)
Common Cause Inoperability of Independent Trains or Channels; and 10 CFR 50.73(a)(2)(ix)(A) Single Cause that Could Have Prevented Fulfillment of the Safety Function of Trains or Channels in Different Systems, This event was considered a safety system functional failure. 2. DOCKET 05000-263
- 6. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
SAFETY SIGNIFICANCE
The total transfer time from offsite power to EDGs, including the additional time to transfer to IAR Transformer, is within SAFERIGESTAR Loss Of Coolant (LOCA) Analysis for MNGP of 15 seconds.
Additionally, to satisfy the Technical Specification compliance issue related to the degraded voltage logic timing, IAR Transformer was removed from service. As determined by Probabilistic Risk Analysis, this reduction in offsite AC power redundancy leads to a continuous estimated Core Damage Frequency increase of approximately 1 E-O7lyear.
U.S. NUCLEAR REGULATORY COMMISSION (10-2010)
CAUSE
During the development of the 1980's modification that added the IAR time delay degraded voltage requirements, the TS submittal did not include the total time delay required to disconnect the Essential Buses from all Offsite Sources.
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
- 1. FACILITY NAME Monticello Nuclear Generating Plant
CORRECTIVE ACTION
The interim corrective action was to remove IAR Transformer from service which disabled the additional 5 second degraded time delay and restored Technical Specification compliance.
- 2. DOCKET 05000-263 A license amendment request has also been submitted which, pending NRC approval, would remove the 5 second time delay for IAR Transformer and directly transfer the Essential Buses to the EDGs under a degraded voltage condition. This would allow IAR Transformer to be returned to service.
- 3. PAGE 3 0 F 3
- 6. LER NUMBER I
PREVIOUS SIMILAR EVENTS
A review of Licensee Event Reports determined there have been no previous similar events in the last three I
years.
REV YEAR I SEQUENTIAL NUMBER NO.