05000400/LER-2015-005
| Shearon Harris Nuclear Power Plant, Unit 1 | |
| Event date: | 06-16-2015 |
|---|---|
| Report date: | 08-17-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii) |
| 4002015005R00 - NRC Website | |
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Energy Industry Identification System (EllS) and component codes are identified in the text as [XX].
BACKGROUND
On June 16, 2015, Harris Nuclear Plant (HNP) was operating at 100% in mode 1. Maintenance work was scheduled in the Main Steam (MS) [SB] Tunnel and doors 1FP-D0010 [DR] and 1FP-D0011 [DR] (henceforth referred to as D10 and D11) were closed, separating the MS Tunnel (MST) from the Reactor Auxiliary Building (RAB) [NF] 261' elevation. The high energy line break (HELB) equipment qualification (EQ) analysis for HNP assumes these two doors are closed to prevent a potentially harsh environment in the MST from adversely impacting equipment in the RAB. D10 and D11 are also credited as fire protection doors.
There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event.
This event is reportable under 10 CFR 50.73(a)(2)(i)(B), "any operation or condition which was prohibited by the plant's Technical Specifications." This event is also reportable under 10 CFR 50.73(a)(2)(ii), "any event or condition that resulted in: (B) the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety." It is also reportable under 10 CFR 50.73(a)(2)(v), "any event or condition that could have prevented the fulfillment of the safety function or structures or systems that are needed to: (B) remove residual heat; (D) mitigate the consequences of an accident," and 10 CFR 50.73(a)(2)(vii), "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: (B) remove residual heat; (D) mitigate the consequences of an accident.
EVENT DESCRIPTION
On June 16, 2015, HNP doors D10 and D11 were blocked open for maintenance in the MST as an egress route out of the MST in case of an emergency. Doors D10 and D11 were open for approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />.
The NRC Senior Resident Inspector questioned the main control room (MCR) regarding the condition.
It was determined that doors D10 and D11 are assumed to be closed in the HELB EQ analysis and are therefore credited as high energy line break and internal flooding barriers. The opening of these doors resulted in HNP being in an unanalyzed condition.
An analysis was performed to determine the worst case environmental conditions in the RAB 261' elevation if a HELB in the MST had occurred concurrent with D10 and/or D11 blocked open. The analysis projected that both trains of air handlers AH-20 [AHU], and both trains of WC-2 chillers [CHU] and their respective pumps would be inoperable. This results in the inoperability of the Essential Services Chilled Water (ESCW) [KM] system, a common cause inoperability of independent trains and a condition that could have prevented fulfillment of a safety function.
HNP Technical Specification 3.7.13 requires two independent ESCW loops to be operable in modes 1-4. If two independent ESCW loops are inoperable, HNP Technical Specification 3.0.3 requires that action must be taken within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. Technical Specification 3.0.3 criteria was not met during this event since two independent chilled water loops were deemed inoperable when D10 and D11 were blocked open, resulting in HNP being in a condition prohibited by the plant's Technical Specifications.
D10 and D11 were opened on several occasions over the past three years. A review of this past data determined that the timeframe between June 2014 and June 2015 contained the longest aggregate duration over a 12-month period for D10 and D11 being open during modes 1 through 4. During this time interval, D10 and D11 were open for a combined total of approximately 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />.
CAUSAL FACTORS
The root cause of this event was that HNP Engineering failed to develop and implement control measures for hazard barriers credited for mitigating HELB events. Consequently, there is a lack of established control measures for HELB barriers during plant maintenance and modification activities.
The primary contributing cause was that HNP Engineering failed to adequately evaluate NRC Regulatory Issue Summary (RIS) 2001-09, "Control of Hazard Barriers." When the HNP evaluation of RIS 2001-09 was performed, inappropriate credit was taken for existing barrier controls under the Security, Radiological Protection, and Fire Protection programs.
SAFETY ANALYSIS
No high energy line break occurred during the time period that D10 and D11 were open, thus there were no safety consequences associated with the actual event. Similarly, there were no adverse impacts to public health and safety or to plant employees.
As outlined in NUREG-1038, "HNP Safety Evaluation Report related to the operation of Shearon Harris Nuclear Power Plant," Section 9.2.7, "Essential Services Chilled Water System," the ESCW system is an auxiliary system necessary for safe reactor operation or shutdown. It serves both safety-related and nonsafety-related systems, which include the control room air conditioning system, the spent fuel pool pump ventilation system, and RAB subsystems. The RAB subsystems include the RAB nonnuclear-safety ventilation system, the RAB engineered safety feature equipment cooling system, the RAB switchgear rooms ventilation system, and the RAB electrical equipment protection rooms ventilation system.
In the event of a HELB, either a MS line break (MSLB) or Feedwater [SJ] line break (FWLB), in the MST concurrent with doors D10 and/or D11 blocked open, ingression of the MSLB or FWLB fluid into the RAB 261' could cause the area to become harsh in terms of environmental quality for temperature, pressure, humidity, and flooding. It is possible that both safety trains of air handler AH-20 could be rendered inoperable due to flooding. It is also possible that both trains of the WC-2 chillers could be rendered inoperable due to moisture intrusion, which would result in the ESCW system being declared inoperable.
The actual impact of this event to safety-related equipment during a postulated plant event or accident is unknown. However, based upon the potential impact to the ESCW system as described above, this event and other instances where the doors were blocked open are considered to be a safety system functional failure.
CORRECTIVE ACTIONS
Completed Actions
1) Doors D10 and D11 were shut immediately.
2) Operations issued Standing Instruction 2015-024, which prohibits doors D10 or D11 being blocked open during modes 1-4 without permission from a shift supervisor. Prior to authorization by a shift supervisor of blocking open any plant doors, hatches, or breached penetrations, an evaluation of the impact of the associated boundary being breached with regard to the Current License Basis (CLB) is to be completed. This includes considerations for the fire protection program, radiation controlled area boundaries, the flooding analysis and the HELB analysis. Access through the doors in the steam tunnel in an emergency is still allowed, but the doors must be closed upon exiting. This instruction addresses the extent of both the cause and condition for this event.
Planned Actions
1) Develop and implement an engineering change that evaluates the required passive design features to support HELB analysis and overall licensing bases. This product will identify required passive design features and establish the necessary process to ensure barriers are appropriately identified and controlled.
2) Conduct a formal briefing with HNP Engineering to communicate the importance of internalizing and maintaining a strong technical conscience when reviewing operating experience, conclusions of the root cause evaluation, and the effects of a MST HELB on RAB equipment when either D10 or D11 is blocked open.
3) Establish a new preventative maintenance frequency for doors D10 and D11 so that functional verification (opening the doors) is not required during modes 1-4. Alternatively (if necessary), provide justification for functional verification in modes 1-4 based on RIS 01-009 requirements.
PREVIOUS EVENTS
There have been no related reportable events at HNP. HNP became aware of the issue on June 16, 2015.
COMMITMENTS
This report contains no regulatory commitments.