05000245/LER-1996-031, :on 960425,incorrect Fuses Caused Automatic Depressurization Sys to Be Inoperable.Cause Has Not Been Determined.All DC Circuits within Fuse Control Program Will Have Fuses W/Properly Certified DC Rating

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:on 960425,incorrect Fuses Caused Automatic Depressurization Sys to Be Inoperable.Cause Has Not Been Determined.All DC Circuits within Fuse Control Program Will Have Fuses W/Properly Certified DC Rating
ML20132C689
Person / Time
Site: Millstone Dominion icon.png
Issue date: 12/16/1996
From: Robert Walpole
NORTHEAST NUCLEAR ENERGY CO.
To:
Shared Package
ML20132C678 List:
References
LER-96-031, LER-96-31, NUDOCS 9612180439
Download: ML20132C689 (4)
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LER-1996-031, on 960425,incorrect Fuses Caused Automatic Depressurization Sys to Be Inoperable.Cause Has Not Been Determined.All DC Circuits within Fuse Control Program Will Have Fuses W/Properly Certified DC Rating
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(i)
10 CFR 50.73(a)(2)(viii)
10 CFR 50.73(a)(2)(ii)
10 CFR 50.73(a)(2)(x)
10 CFR 50.73(a)(2)(iii)
10 CFR 50.73(a)(2)(v), Loss of Safety Function
10 CFR 50.73(a)(2)(vii), Common Cause Inoperability
10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor
10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
2451996031R00 - NRC Website
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text

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I NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 315:o41o4 4 95)

EXPtRES o4/3o/98 No TSo CULIEC'T YOUrSF 50 AS E

TED t'M"'?o'L0a"f "^!'amo 7'oMi!a"V"!WJo ^24!

LICENSEE EVENT REPORT (LER)

IS"!Vf '~"u'c"2""t,'o%",^ef80"fJ,s"s?^ L"E="&

oEEElm" foe 0sEANoiEe"TI E4YnET"oN."So'sP" (See reverse for required number of j

digits / characters for each block) j FACILITY NAME (1)

DOCKET NUMBER (2)

PAGE (3)

Millstone Nuclear Power Station Unit 1 05000245 1 of 4 TITLE 14) incorrect Fuses Caused the Automatic Depressurization System to be Inoperat'e 1

f EVENT DATE (5)

LER NUMBER (6)

REPORT DATE (7)

OTHER FACILITIES INVOLVED (8)

J MONTH DAY YEAR YEAR SEQUENTIAL REVISION MONTH DAY YEAR FACluTY NAME DOCKET NUMBER NUMBER 3

04 25-96 96 031 01 12 16 96 OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 5: (Check one or more) (11) i MODE (9)

N 20.2201(b) 20.2203(a)(2)(v) 50.73(a)(2)(i) 50.73(a)(2)(viii)

POWER LEVEL (10) 000 20.2203(aH1) 20.2203(a)(3)(i) 50.73(a)(2)(ii) 50.73(a)(2)(x) 20.2203(aH2Hi) 20.220s,,(3Hii) 50.73(a)(2)(iii) 73.71 20.2203(a)(2Hii) 20.220Mt4) 50.73(a)(2Hiv)

OTHER

........u..

20.2203(a)(2)(iii) 50.36(c)(1) 50.73(a)(2)(v) specify in Abstract below 20.2203(aH2)(iv) 50.36(cH2) 1 50.73(a)(2)(vii)

LICENSEE CONTACT FoR THIS LER (12) i NAME TELEPHONE NUMBER (include Area Codel Robert W. Walpole, MP1 Nuclear Licensing Manager (860)440-2191 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13) i

CAUSE

SYSTEM COMPONENT MANUFACTURER REPOR T ABLE

CAUSE

SYSTEM COMPONENT MANUFACTURER REPORTABLE i

TO NPRDS To NPRDS t

4 l

1 SUPPLEMENTAL REPORT EXPECTED (14)

EXPECTED MONTH DAY YEAR i

SUBMISSloN g

YES NO (if yes, complete EXPECTED SUBMISSION DATE).

t l

ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) (16; i

i On April 25,1996, at 1515 hours0.0175 days <br />0.421 hours <br />0.0025 weeks <br />5.764575e-4 months <br />, with the plant shutdown and the reactor in the COLD SHUTDOWN condition, it was determined that fuses installed in the Millstone Unit No.1 DC control systems are not appropriately certified for thrir application.

Thsse fuses were found in vario'us control circuits, including those associated with the automatic depressurization i

1 system (ADS). This event was reported, pursuant to 10CFR50.73(a)(2)(v)(A), as a condition which alone could have 3

prevented the fulfillment of a safety function required to shutdown the reactor and maintain it in a safe shutdown condition. This event was also reported, pursuant to 10CFR50.73(a)(2)(ii)(B) as a condition outside of the design basis of the plant, and was promptly reported on April 25,1996, pursuant to 10CFR50.72(b)(2)(i) as a condition which resulted in the nuclear power plant being seriously degraded. Due to this condition, the ADS was believed to be inoperable for several operating cycles; therefore, this event was also reported pursuant to j

10CFR50.73(a)(2)(i) as a condition prohibited by the plant's Technical Specifications. Additionally, this event was believed to represent a common mode failure, and therefore was reported pursuant to 10CFR50.73(a)(2)(vii) as a condition which caused independent trains or channels to become inoperable in a system designed to shutdown the reactor and maintain it in a safe shutdown condition.

I On July 11, 1996, Overload Testing was conducted at higher DC voltages that assured that the protective fuses ware operable.

9612180439 961216 PDR ADOCK 05000245 S

PDR

NRC FORWI 366A U.S. NUCLEAR REGULATORY COMMISSloN I4-95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 1 05000245 NUMBER NUMBER 2 of 4 96 031 01 TEXT (If more space is required, use additional copies of NRC Form 366A) (17) 1.

Descrintion of Event On April 25,1996, at 1515 hours0.0175 days <br />0.421 hours <br />0.0025 weeks <br />5.764575e-4 months <br />, with the plant shutdown and the reactor in the COLD SHUTDOWN condition, it was determined that fuses installed in the Millstone Unit No.1 DC control systems are not appropriately sized for their application.

These fuses were found in various control circuits, including those associated with the ADS. This event was reported, pursuant to 10CFR50.73(a)(2)(v)(A), as a condition which alone could have prevented the fulfillment of a safety function required to shutdown the reactor and maintain it in a safe shutdown condition. This event was also reported, pursuant to 10CFR50.73(a)(2)(ii)(B) as a condition outside of the design basis of the plant, and was promptly reported on April 25,1996, pursuant to 10CFR50.72(b)(2)(i) as a condition which resulted in the nuclear power plant being seriously degraded. Due to this condition, the ADS was believed to be inoperable for several operating cycles; therefore, this event was also reported pursuant to 10CFR50 73(a)(2)(i) as a condition prohibited by the plant's Technical Specifications.

Additiona!!y, this event was believed to represent a common mode failure, and therefore was reported pursuant to 10CFR50.73(a)(2)(vii) as a condition which caused independent trains or channels to become inoperable in a system designed to shutdown the reactor and maintain it in a safe shutdown condition.

On July 11,1996, Overload Testing was conducted at higher DC voltages that assured that the protective fuses were operable.

11.

Cause of Event

Upon further investigations, it has been determined that this event is not reportable.

Ill. Analysis of Event During the field inspection for the Millstone Unit 1 Fuse Control Program, Cooper Bussmann type FNA, FNM and MIN fuses were found in DC control circuits that operate the safety / relief valves (SRVs). The 10 ampere FNA, FNM and MIN fuses have a tested DC rating of 125 volts. The fuse vendor had indicated that at higher voltages, the fuse might be late to clear a fault, or else will not clear the fault at all.

The SRVs are the principle component of the ADS. Following a small break loss of coolant accident (SBLOCA), the ADS rapidly depressurizes the reactor vessel so that low pressure emergency core cooling systems (e.g., low pressure coolant injection and core spray) can initiate their safety functions. The failure of the ADS during a SBLOCA, without any high pressure injection systems (e.g., feedwater coolant injection), would result in the loss of core cooling. The SRVs are also used by the operator to depressurize the reactor vessel during non-LOCA scenarios to help mitigate multiple failures. For example, the emergency operating procedures will direct the operator to depressurize the reactor vessel by opening 4 of the SRVs in i

a main steam isolation valvo closure event if uncovering of the core is imminent. If the control power fuses are assumed to fail when the SRVs are required to open, the operator action to open the SRVs will be delayed until power to the SRVs is restored; however, additional methods exist for depressurizing the reactor vessel.

RRC FORM 366A (4 95)

.,U.S. NUCLEAR REGULATORY COMMISSloN (4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION

)

FACILITY NAME (1)

DOCKET NUMBER (2)

LER NuM"lR (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 1 05000245 NUMBER NUMBER 3 of 4 96 031 01 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)

The DC system is an ungrounded 125 V system, having two pole circuit breakers and double fuses in all load circuits. The maximum voltage will not be across one fuse (exceeding its voltage rating) urless there is a ground on the DC system and a second fault causes the fault current to flow in only one fuse.

Both 125 VDC power supplies (primary and backup) could be tripped if a fault did not successfully clear and was transferred to the backup supply. This is considered unlikely because of the circuit design, and the fact I

that each DC circuit has two fuses (one each in the positive and negative leg). Only one of the fuses must open to isolate the fault. When the circuit breaker opens to transfer power to the backup supply, there is a brief moment when no voltage is present to continue the arcing inside the fuse. This is expected to allow the circuit to be interrupted. Since it was not known at the time whether the fuses will fail to isolate a fault at a voltage higher than that previously tested, a test of the breakdown vc!tage was planned to determine the actual voltage the fuses will tolerate before failing to clear a fault.

On July 11, 1996, Overload Testing of MIN, FNA and FNM fuses was conducted by National Technical Systems (NTS) to determine the consequences of elevated DC voltage on the clearing times (open circuit) for these fuses which the manufacturer rates for 125 Volts DC. Fuses of five, ten and rifteen amperes of all three types were blown.it two to three times their current rating. The circuit voltage was raised until the l

power supply reached it' upper limit of 157.2 V at 30 Amperes. At the maximum voltage obtained, all of the l

fuses were clearing successfully in approximately 40 mihiseconds or less after the fuse link began to melt.

The overload test is more severe than a fault te'.* nhen evaluating a fuse for voltage withstand according to the fuse manufacturers. Therefore these tests were performed at current values to simulate the worst case overload clearing conditions. The maximum voltage limit of the power supply prevented the determination of the fuse breakdown voltage, however the results are conclusive for demonstrating the operability of the circuits that contained these fuses. The purpose of this testing was to show that these fusc.*.at a higher than rated DC voltage) could isolate an overload on one of the safety relief valves without causing a trip of the circuit breaker that powers all six valves.

The Millstone Unit No.1's 125 V system operates with the normal battery float voltage between 133 and 135 V DC and the maximum DC bus voltage of 143 Volts (charger trip voltage) when the batteries are being equalize charged. The actual fuse breakdown voltage could not be reached due to limitations of the test equipment. The NTS fuse testing demonstrated that the MIN, FNA and FNM fuses which were in service in the Millstone Unit No.1 DC circuks would have operated correctly with a margin of at least 14 volts above the maximum bus voltage during all plant operations.

i Although the actual fuse breakdown voltage was not achieved during these tests, :i.e ability of these fu es to operate at DC voltages above the maximum possible bus voltage provides assurance that overcurrent l

protection was always present, arm 'hc protective fuses were operable before the reporting was made to the l

NRC. No additional testing is plann-In addition, the fact that all DC circuits within the Fuse Control Program will have fuses with properly certified DC ratings, provides assurance that there will be overcurrent protection before the plant resymes operation.

j Breakdown voltage was reached during fuse testing r;t NTS for Bussmann NON-10 fases which were reported in LER 96-17. These fuses were also tested for DC use since they are not rated for DC service, j

These fuses began to have prolonged clearing times at 155 V de and at the maximum of 157.2 volts the 10RC FORM 366A (4 95) l I

4

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NRC FORM 362A U.S. NUCLEAR REGULATORY COMMISslON (4 95) l UCENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 1 05000245 NUMBER NUMBER 4 of 4 96 031 01 TEXT lif more space is required. use additional copies of NRC Form 366A) (17) clearing times were 100 and 158 milliseconds for two fuses compared to 30 to 40 milliseconds at lower voltages. This was indication of the threshold voltage where prolonged arcing occurred and is well above the maximum operating DC voltage.

IV. Corrective Action

All DC circuits within the Fuse Control Program will have fuses with properly certified DC ratings prior to startup for operating Cycle 16.

The broader and long term issue of fuse control is being addressed by the Fuse Control Program. This program will include a listing of all distribution fuses in use at Millstone Unit No.1 in a master fuse list. This list will include the unique identification number for each fuse, and it will describe the specific fuse to be j

used. Fuse specification will include manufacturer, type designation and ampere rating, in addition to the creation of the master fuse list, the Fuse Control Program will ensure that the proper information is captured on plant drawings. Unique identification numbers will be assigned to each fuse to ensure that workers in the future are able to quickly and accurately replace fuses. These numbers will be reflected in the master fuse list, on the drawings, and on plant equipment implementation of the Fuse Control Program is on-going, and is committed to in Commitments B15607-1 and B15607-2.

V.

Additional Information

I

Similar Events

A similar event was reportN on March 20,1996, as LER 96-017-00, documenting an event which occurred on February 16, 1996. LER 96-017-00 reported the existence of improper fuses installed in DC control I

circuits for the emergency gas turbine generator and the 480 voit load center breakers.

i Manufacturer Data None.

1

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WRC FORM 366A (4-95)

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