05000263/LER-2022-001, Loss of Control Room Envelope Operability
| ML22189A111 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 07/07/2022 |
| From: | Domingos C Northern States Power Company, Minnesota, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-MT-22-025 LER 2022-001-00 | |
| Download: ML22189A111 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2632022001R00 - NRC Website | |
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(l Xcel Energy July 7, 2022 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket No. 50-263 Renewed Facility Operating License No. DPR-22 LER 2022-001-00, Loss of Control Room Envelope Operability 2807 County Rd 75 NE Monticello, MN 55362 L-MT-22-025 10 CFR 50.73 Pursuant to 10 CFR 50.73(a)(2)(ii)(B), 10 CFR 50.73(a)(2)(v)(D), and 10 CFR 50.73(a)(2)(vii),
Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), hereby submits Monticello Nuclear Generating Plant (MNGP) Licensee Event Report (LER) 2022-001-00.
Summary of Commitments This letter m k n
ew commitments and no revisions to existing commitments.
,.--n Christopher Do Site Vice President, Monticello and Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:
Administrator, Region Ill, USNRC Project Manager, Monticello, USNRC Resident Inspector, Monticello, USNRC Minnesota Department of Commerce
Abstract
On May 13, 2022, the Monticello Nuclear Generating Plant was operating in Mode 1 at 100% power. Following an inspection in the Steam Jet Air Ejector room, it was identified that the steam leakage from two valves exceeded the assumptions in the Alternate Source Term dose analysis calculation. As a result, the Control Room Envelope (CRE) was declared inoperable and LCO 3.7.4, "Control Room Emergency Filtration (CREF) System," Condition B was entered. The leaks were mitigated such that the CRE was declared operable on May 20, 2022.
This report is being submitted as an "event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety" per 10 CFR 50.73(a)(2)(ii)(B); as an "event or 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" per 10 CFR 50.73(a)(2)(v)(D); and as "any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to mitigate the consequences of an accident" per 10 CFR 50.73(a)(2)(vii).
EVENT DESCRIPTION
On May 13, 2022, the Monticello Nuclear Generating Plant was operating in Mode 1 at 100% power. Following an inspection in the Steam Jet Air Ejector (SJAE) room, it was identified that the steam leakage from two valves exceeded the assumptions in the Alternate Source Term (AST) dose analysis calculation. As a result, the Control Room Envelope (CRE) was declared inoperable and LCO 3.7.4, "Control Room Emergency Filtration (CREF) System," Condition B was entered. The leaks were mitigated such that the CRE was declared operable on May 20, 2022.
EVENT ANALYSIS
Steam leakage from the 12 SJAE Pressure Control outlet valve [PCV], MS-23-2, was originally identified on May 5, 2022.
As part of an investigation into the leakage, Operators opened the 12 SJAE Pressure Control bypass valve [PCV],
MS-24-2, to allow maintenance to remove insulation from MS-23-2. Upon entry into the SJAE room to remove the insulation from MS-23-2 it was discovered that the valve packing from MS-24-2 had failed and was also leaking steam.
After removal of the insulation from MS-23-2, it was determined that the leakage from MS-23-2 and MS-24-2 both exceeded the leakage rate assumptions of the AST dose analysis calculation and evaluation. As a result, the Control Room CRE was declared inoperable.
The CRE was inoperable from May 5, 2022 until May 20, 2022 when the leakage sources were mitigated.
This condition was determined to be reportable in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an "event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety, 10 CFR 50.73(a)(2)(v)(D), as an "event or condition that could have prevented fulfillment of a safety function to mitigate the consequences of an accident," and 10 CFR 50.73(a)(2)(vii) as "any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to mitigate the consequences of an accident."
This event will be reported as a Safety System Functional Failure in accordance with Nuclear Energy Institute 99-02, "Regulatory Assessment Performance Indicator Guideline".
SAFETY SIGNIFICANCE
There was no actual safety consequence from this condition. During the time the CRE was inoperable radiation levels in the Turbine Building and on the Wide Range Gas Monitors remained stable. Compensatory measures were put in place, including assigning a dedicated operator to promptly isolate the portion of the SJAE system that was leaking in the event of a design basis event. In addition, the control room operators were directed to don a self-contained breathing apparatus (SCBA) on breathing air if a design basis event were to occur. An engineering analysis determined that donning SCBAs would ensure control room operator doses remained below 5 Rem Total Effective Dose Equivalent (TEDE). Prior to the implementation of the compensatory measures described above on May 13, 2022, it is estimated the post-accident dose limit for the control room operators could have been exceeded. During the time the steam leakage in the SJAE Room exceeded the assumptions of the AST analysis both CREF subsystems were otherwise available.
CAUSE
The direct cause of the CRE being declared inoperable was that the accumulated leakage had exceeded the leakage rate assumptions of the AST dose analysis calculation. The margin to the assumed AST leakage was eroded by the leaks which were identified previous to MS-23-2 and MS-24-2. The additional steam leakage from MS-23-2 and MS-24-2 exceeded the leakage rate assumptions of the AST dose analysis calculation. The cause of the steam leak from MS-24-2 was improperly consolidated valve packing when the valve was installed. MS-23-2 developed a through body leak, the apparent cause of which cannot be determined without an inspection of the valve body.
CORRECTIVE ACTIONS
The leakage from MS-23-2 and MS-24-2 has been contained. Both valves will be replaced during the next refueling outage. Procedural clarifications are being developed to verify valve packing is properly installed and consolidated.
In addition, the current maintenance strategies will be reviewed against the consequences associated with AST.
PREVIOUS SIMILAR EVENTS
There were no previous similar licensee event reports in the past three years.
ADDITIONAL INFORMATION
The Institute of Electrical and Electronics Engineer codes for equipment are denoted by [XX]. Page 3
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