05000528/LER-2017-001

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LER-2017-001, Essential Chiller B Inoperable Due to Refrigerant Leak Resulting in Safety System Functional Failure
Palo Verde Nuclear Generating Station Unit 1
Event date: 04-11-2017
Report date: 06-14-2017
Reporting criterion: 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications

10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat

10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
5282017001R00 - NRC Website
LER 17-001-00 for Palo Verde Nuclear Generating Station, Unit 1 Regarding Essential Chiller B Inoperable Due to Refrigerant Leak Resulting in Safety System Functional Failure
ML17165A445
Person / Time
Site: Palo Verde, Palo Verde Nuclear Generating Station Unit 1 Arizona Public Service icon.png
Issue date: 06/14/2017
From: Lacal M L
Arizona Public Service Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
102-07510-MLL/RAC
Download: ML17165A445 (6)


comments regarding burden estimate to the Information Services Branch (T-2 F43). U.S. Nuclear Regulatory Commission. Washington. DC 20555-0001 or by e-mail to Infocollects Resource@nrc gov. and to the Desk Officer Office of Information and Regulatory Affairs NEOB-10202 (31500104). Office of Management and Budget. Washington. DC 20503 If a means used to impose an information collection does not display a currently valid OMB control number. the NRC may not conduct or sponsor and a person is not required to respond to. the information collection Palo Verde Nuclear Generating Station 05000-528 REV - 001 - 00 2017 All times are Mountain Standard Time and approximate unless otherwise indicated.

1. REPORTING REQUIREMENT(S):

This Licensee Event Report (LER) is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) for operation which was prohibited by the Technical Specifications. The Unit 1 train B essential chilled water (EC) system (EIIS Code: KM) chiller was inoperable for approximately 7 days from April 11, 2017, to April 18, 2017, which exceeded the Technical Specification (TS) Limiting Condition for Operation (LCO) 3.7.10 required action completion time. TS LCO 3.7.10 Condition A states that one inoperable EC system train shall be restored to operable status within

72 hours
8.333333e-4 days
0.02 hours
1.190476e-4 weeks
2.7396e-5 months

. If the required action for Condition A is not met, TS LCO 3.7.10 Condition B states that the plant shall be in Mode 3 within

6 hours
6.944444e-5 days
0.00167 hours
9.920635e-6 weeks
2.283e-6 months

and Mode 5 within

36 hours
4.166667e-4 days
0.01 hours
5.952381e-5 weeks
1.3698e-5 months

.

This LER is also being submitted pursuant to 10 CFR 50.73(a)(2)(v)(B) and 10 CFR 50.73(a)(2)(v)(D) for a condition that could have prevented the fulfillment of the safety function of systems that are needed to remove residual heat and mitigate the consequences of an accident. The 7-day period of inoperability for EC chiller train B also rendered the associated train B supported systems inoperable including low pressure safety injection (LPSI) (EIIS Code: BP). On April 13. 2017, LPSI train A was declared inoperable for approximately 17 minutes during the performance of a routine surveillance test. Having both LPSI train A and train B inoperable during this 17-minute period represents a condition that could have prevented the fulfillment of a safety function.

This condition was discovered on April 17. 2017, when a maintenance technician identified a low refrigerant level in EC chiller train B. EC chiller train B was determined to be inoperable following the completion of preventive maintenance on April 11, 2017, when a refrigerant leak was introduced to the system. Therefore, the event date is April 11. 2017.

2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S), AND COMPONENT(S):

The EC system consists of two 100% capacity, redundant, safety-related chilled water trains. Each train is a closed-loop system which circulates chilled water to the engineered safety feature (ESF) air handling units that provide cooling to the following safety-related equipment rooms:

  • Main control room
  • Direct current equipment rooms
  • Essential cooling water (EW) pump rooms
  • High pressure safety injection pump rooms
  • Low Pressure Safety Injection pump rooms
  • Containment spray pump rooms
  • Engineered Safety Feature switchgear rooms
  • Electrical penetration rooms Estimated ourden per response to comply with this mandatory collection request 80 hours
    9.259259e-4 days
    0.0222 hours
    1.322751e-4 weeks
    3.044e-5 months
    Reported comments regarding burden estimate to the Information Services Branch (T-2 F43), U S. Nuclear Regulatory Commission. Washington. DC 20555-0001. or by e-mail to Infocollects Resource@nrc gov and to the Desk Officer. Office of Information and Regulatory Affairs NEOB-10202. (3150-0104) Office of Management and Budget. Washington. DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number. the NRC may not conduct or sponsor. and a person is not required to respond to. the information collection Palo Verde Nuclear Generating Station 05000-528 1 Unit 1 l 2017 - 001 - 00 The key components in each of the EC system's two trains include a 100% capacity chiller (refrigeration unit), a 100% capacity chilled water circulation pump. related piping, tanks, valves, instrumentation, and controls. The refrigeration unit consists of a compressor, evaporator. refrigerant condenser/receiver unit, instrumentation, and controls. The EC system does not typically function during normal plant operation, and the system automatically starts upon receipt of one of the following ESF actuation signals:
  • Safety Injection Actuation Signal
  • Containment Spray Actuation Signal
  • Loss of Power
  • Control Room Essential Filtration Actuation Signal
  • Control Room Ventilation Isolation Actuation Signal
  • Auxiliary Feedwater Actuation Signal Cooling water for the EC chiller condenser is supplied by the respective train's EW system (EIIS Code: BI). a closed loop cooling water system that provides cooling water to ESF systems.

The refrigerant side of the chiller condenser is equipped with an air purge vent line to remove non-condensable gases from the closed refrigerant cycle. The air purge vent line consists of a manual isolation valve, a NUPRO air purge filter (model number SS-6TF-F6-90), and an automatic purge valve. The filter is also equipped with a Swagelok plug (model number SS-600-P) to seal the open tube port in the filter housing. Maintenance technicians perform a weekly inspection of the EC chillers to ensure the refrigerant levels are maintained within the manufacturer's specifications.

3. INITIAL PLANT CONDITIONS:

On April 11, 2017, PVNGS Unit 1 was in Mode 1 (Power Operation) at 100 percent power. normal operating temperature, and normal operating pressure. There were no other structures, systems. or components out of service that contributed to this event.

4. EVENT DESCRIPTION:

On April 17, 2017, a maintenance technician identified a low refrigerant level in EC chiller train B during the performance of a routine weekly inspection. Operations personnel immediately declared EC chiller train B inoperable and entered the action statement for TS LCO 3.7.10 Condition A. The maintenance technician identified leakage occurring at the air purge filter housing cap downstream of the air purge manual isolation valve. Inspection of the housing cap determined a required plug had not been installed on the air purge filter housing cap during maintenance. The leakage from the filter housing cap resulted in the low refrigerant level in EC chiller train B. On April 17. 2017. the leak was corrected and EC chiller train B was refilled with refrigerant to within the manufacturer's specifications. Operations personnel declared the system operable upon completion of a retest performance run on April 18, 2017.

Based on a review of maintenance history, the filter housing cap without the required plug was likely installed in 2007.

However, refrigerant leakage did not occur at that time following installation of the deficient housing cap because the air purge line, including the filter, was isolated from the system prior to April 11, 2017. On April 11, 2017, the air purge line manual isolation valve was repositioned to be normally open which then allowed refrigerant to begin leaking from the filter through the deficient housing cap. Repositioning of the manual isolation valve to be normally open was a planned.

new system configuration which was consistent with the manufacturer's specifications. Prior to April 11. 2017, the manual isolation valve remained closed unless maintenance personnel were performing a controlled manual vent of the chiller condenser. On April 11, 2017, when the manual isolation valve was repositioned open. refrigerant level dropped to a point whereby the EC system was incapable of performing its specified safety function. Thus. EC chiller train B was Estimated ouraen per response to comply with this mandatory collection request

80 hours
9.259259e-4 days
0.0222 hours
1.322751e-4 weeks
3.044e-5 months

Reported comments regarding burden estimate to the Information Services Branch (T-2 F43) U S Nuclear Regulatory Commission. Washington, DC 20555-0001. or by e-mail to Infocollects Resource@nrc gov. and to the Desk Officer. Office of Information and Regulatory Affairs.

NEOB-10202. (3150-0104). Office of Management and Budget Washington. DC 20503 If a means used to impose an information collection does not display a currently valid OMB control number the NRC may not conduct or sponsor. and a person is not required to respond to. the informaton

  • collection

NO

- 00 inoperable for approximately 7 days from April 11, 2017, to April 18, 2017, which exceeds the required action completion time for TS LCO 3.7.10.

The 7-day period of inoperability for EC chiller train B also rendered the associated train B supported systems inoperable including LPSI. On April 13, 2017, while LPSI train B was inoperable due to the undiscovered refrigerant leak, LPSI train A was declared inoperable and Condition A of TS LCO 3.5.3 was entered for approximately 17 minutes during the performance of a routine surveillance test. LPSI train A was inoperable when a 1-inch vent valve on the discharge side of the LPSI pump was opened to align a temporary pressure gauge during the test. The surveillance test procedure required a dedicated operator to be continuously stationed at the vent valve to manually close the valve and immediately restore operability if the LPSI system is needed. As such, the LPSI system remained available during the test. However.

this 17-minute period represents a condition that could have prevented the fulfillment of a safety function while LPSI train B was affected by the inoperability of EC chiller train B.

5. ASSESSMENT OF SAFETY CONSEQUENCES:

This event did not result in a challenge to the fission product barriers or result in the release of radioactive materials. The event did not result in a potential transient more severe than those analyzed in the Updated Final Safety Analysis Report.

There were no actual safety consequences as a result of this event, and the event did not adversely affect the health and safety of the public.

The nuclear safety risk significance of the subject condition was minimal. The core damage and large early radioactive release risk achievement worth for EC chiller train B is 1.08 per the current Equipment Out of Service software. The risk significance of an EC chiller is small due to the redundancy with the alternate train EC chiller and normal chillers, the low likelihood of a loss of offsite power that would fail the normal chillers, and the slow heatup rate upon loss of room cooling for risk significant electrical and pump rooms cooled by these chillers. The incremental core damage and large early radioactive release probabilities for the 7-day period of unavailability of EC chiller train B were 3.4E-9 and 1.2E-10, respectively. These risks are two orders of magnitude below the risk values which would be characterized as small per NRC Regulatory Guide 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis.

EC system train A was available to perform its safety function from April 11, 2017. to April 18, 2017. Although LPSI train A was affected during a 17-minute period on April 13, 2017, LPSI train A was available based on a dedicated operator being continuously stationed to manually isolate the vent valve and restore operability. However, this event does constitute a condition that could have prevented the fulfillment of a safety function due to the inoperability of both trains of LPSI. All other train A systems supported by EC system train A remained operable during the 7-day period.

6. CAUSE OF THE EVENT:

The direct cause of this event was a refrigerant leak in EC chiller train B due to the prior installation of an air purge filter housing cap fitting without the required plug. The cause of the fitting with the missing plug being installed during maintenance was ineffective work instructions. The work instructions failed to identify the appropriate part number to be used during filter replacements. This resulted in maintenance personnel installing a replacement filter assembly without the required plug when the work order only intended to replace the filter element. The individuals involved in the development of the work instructions and the individuals involved in installation of the filter with the missing plug were utility personnel.

comments regarding burden estimate to the Information Services Branch (T-2 F43). U S. Nuclear Regulatory Commission. Washington DC 20555-0001. or by e-mail to Infocollects.Resource@nrc gov. and to the Desk Officer. Office of Information and Regulatory Affairs.

NEOB-10202. (3150-0104). Office of Management and Budget. Washington. DC 20503 If a means used to impose an information collection does not display a currently valid OMB control number. the NRC may not conduct or sponsor and a person is not required to respond to the information collection Palo Verde Nuclear Generating Station 05000-528 NUMBER NO Unit 1 2017 - 001 - 00

7. CORRECTIVE ACTIONS:

As immediate corrective actions, the EC chiller train B air purge filter was isolated from the system, the refrigerant levels were restored, and the inadequate air purge filter housing cap fitting was replaced to correct the leak and restore full compliance with the design of the system. Maintenance personnel also verified that no other EC system chillers at PVNGS have a filter installed with a missing plug.

To address the cause of the ineffective maintenance, the work instructions were revised to identify the part number for the air purge filter element specifically instead of identifying the part number for the entire assembly. Original work instructions specified the part number for the entire assembly when performing changes that only require the filter element to be replaced, which caused the maintenance personnel to neglect the required plug. This change will ensure that the existing plug remains in place during filter element replacement. A leak test was also added to the work instructions to verify that no refrigerant leaks are present following maintenance.

8. PREVIOUS OCCURRENCES:

No previous similar events have been reported by PVNGS. In 2010, PVNGS reported a condition prohibited by TS and a condition that could have prevented the fulfillment of a safety function when the Unit 2 EC chiller train B was inoperable for a period of 33 days (Agencywide Documents Access and Management System Accession Number ML100990161).

However, the condition in 2010 was caused by a bypass valve in the EW system in the incorrect position and is unrelated to the event in this LER. The corrective actions from the event in 2010 could not have prevented this condition from occurring.