Information Notice 2000-14, Non-vital Bus Fault Leads to Fire and Loss of Offsite Power

Non-vital Bus Fault Leads to Fire and Loss of Offsite Power
	ML003748744
Person / Time
Issue date:	09/27/2000
From:	Marsh L; Operational Experience and Non-Power Reactors Branch
To:	;
	Goodwin E
References
	IN-00-014
	Download: ML003748744 (6)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D. C. 20555-0001 September 27, 2000

NRC INFORMATION NOTICE 2000-14: NON-VITAL BUS FAULT LEADS TO FIRE AND

LOSS OF OFFSITE POWER

Addressees

All holders of licenses for nuclear power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform

addressees of equipment and design issues identified following a recent transient at the Diablo

Canyon nuclear power plant. The aspect of the transient considered noteworthy was the failure

of bus duct, a passive component of known high reliability which often receives little preventive

maintenance or attention. It is expected that recipients will review the information for

applicability to their facilities and consider actions, as appropriate, to avoid similar problems.

However, suggestions contained in this information notice are not NRC requirements; therefore, no specific actions or written response is required.

Description of Circumstances

On May 15, 2000, at Diablo Canyon Unit 1, a phase-to-phase electrical fault occurred in a

12-kV non-Class 1E electrical bus duct from the unit auxiliary transformer to the switchboards

that supplied the reactor coolant pumps and the circulating water pumps. The fault caused a

turbine trip and consequent reactor trip. As this section of bus could not be isolated from the

main generator, the fault lasted for 4 to 8 seconds until the main generator electrical field

voltage decayed. The 12-kV bus fault occurred at a point at which the bus duct passes under

the 4-kV non-Class 1E bus from the startup transformer. The original fault and the resultant

arcing and smoke caused another fault, this time in the 4-kV bus duct directly above the original

failure.

The 12-kV circuit breaker that supplied the 4-kV startup transformers and the faulted 4-kV bus

duct downstream of the transformer tripped in response to this second fault. This trip resulted

in a loss of power to all 4-kV vital (safety-related) and non-vital buses. All three diesel

generators started and all vital loads were re-energized. However, the combination of the two

faults disabled both power supplies to the non-vital 4-kV buses.

Besides de-energizing all non-vital 4-kV power within the plant, the loss of both sources of

non-vital power also caused a loss of the 480-Vac power supply to the switchyard control

building. This failure led to a loss of power to the charger for the switchyard batteries; the

eventual depletion of the switchyard batteries would have led to a loss of control power in the

switchyard serving both Diablo Canyon units. The loss of control power would have disabled

remote control of the switchyard high voltage circuit breakers.

The licensee installed a portable generator to restore power to the charger before the

switchyard batteries were depleted. On May 16, 2000, after 33 hours3.819444e-4 days <br />0.00917 hours <br />5.456349e-5 weeks <br />1.25565e-5 months <br />, plant personnel

energized the 4-kV and 480-Vac non-vital buses by backfeeding through Auxiliary

Transformer 1-2.

Discussion

Switchgear Room Arrangement and Bus Duct Construction

The auxiliary and startup transformers are connected to the onsite distribution switchgear by

bus bars with a 1/2 - by 6-inch cross-sectional area. All three phases are enclosed in a single

aluminum duct (nonsegregated). The startup and auxiliary 12-kV and 4-kV non-vital switchgear

are located within a common room. To connect the two sources of offsite power to multiple

switch boards within the room, there are many crossing bus ducts above the switchgear. The

non-vital 4-kV bus ducts from both auxiliary and startup power are in close proximity for

extended runs. Since none of the bus ducts was designed as safety grade, no regulatory

separation criteria apply.

The bus work in the room was a combination of aluminum and copper bus bars connected with

aluminum splice plates secured by four 1/2-inch bolts. The bus bars and splice plates were

silver-plated at the connection points to ensure conductivity. The bus bars had a nominal

3/16-inch gap between them at the splice plate to allow for thermal expansion. The 12-kV bus

bars had a 6-inch air gap between phases, which is slightly below the required air gap for

uninsulated conductors, so bus bars and connections were insulated with a combination of

sleeves and wraps. The 4-kV bus bars were similarly insulated.

Root Cause

The licensees evaluation concluded that a center bus bar overheated at a splice joint, which

caused a polyvinyl chloride boot insulator over the splice joint to smoke. Eventually, heat-induced failure of fiberglass insulation on adjacent phases resulted in phase-to-phase

arcing. The fault and resultant fire destroyed any direct physical evidence of the root cause;

however, the factors discussed below could individually or jointly have led to the failure. They

include inconsistent silverplating, currents approaching bus capacity, undersized splice plates, torque relaxation of connecting bolts, and undetected damage from a 1995 explosion of

Auxiliary Transformer 1-1.

(1) Silverplating

Many of the bus bars and splice plates had only a thin layer of silverplating. Laboratory

analysis determined that the silverplating on one splice plate had partially separated from the

base aluminum, and corrosion products were found on the aluminum surface. If this separation

had existed at the point of the fault, it would have created higher resistance and, therefore, more heat at the connection. The laboratory stated that the most likely source of corrosive

compounds was the polyvinyl chloride insulating boot. Silverplating was also observed flaking

off the aluminum bus bars at two other splice joints not directly affected by the fault. (2) Heavy Bus Loading and Splice Joint Configuration

The 12-kV 6-inch aluminum bus bars were rated at 2250 amps. The bus was routinely loaded

to 2100 amps with an actual worst case operating load of about 2250 amps. The vendor stated

that all bus bars supplied to Diablo Canyon met the design requirements of Institute of Electrical

and Electronics Engineers (IEEE) 37.20-1969, IEEE Standard for Switchgear Assemblies

Including Metal-Enclosed Bus, which stipulates an operating temperature limit of 65 C.

Vendor-supplied test data reported a maximum temperature rise of 46 C at 2000 amps and

63 C at 2200 amps for the bus bar type that failed. Since the test temperature increased 17 C

for a current increase of 200 amps, a temperature increase of only 2 C for an additional load of

50 amps seems improbably low and it is reasonable that the bus and its insulation had

exceeded design conditions for some time. In addition, the vendor heat rise tests of aluminum

bus bars for 2200 amps were conducted with two splice plates at the splice joint. The vendor

test used 3 - by 4-inch copper splice plates instead of the 21/2 - by 4-inch aluminum splice plates

used at Diablo Canyon.

The inspectors noted that the splice plates connecting the bus bars were considerably smaller

than the bus bars themselves. Some splice joints had two aluminum plates on each phase

sized 21/2 inches by 4 inches by ÿ inch. Also, the splice plates were not always centered

between the bus bars. The lack of centering of splice plates and splice plates smaller than the

tested configuration reduced the contact area, causing increased heat generation.

(3) Torque Relaxation

The as-found torque values for many of the splice plate connecting bolts was 10 to

20 foot-pounds indicating that thermal relaxation had occurred since initial installation. The

bolts had an initial torque value of 40 foot-pounds. Torque relaxation on the bolts could have

allowed the splice plate to lose contact with the bus bar, leading to arcing and overheating.

(4) Undetected Damage from 1995 Auxiliary Transformer 1-1 Explosion

A 1995 explosion of Auxiliary Transformer 1-1 had displaced the 12-kV busing several feet into

the turbine building. Most of the bus bar connections upstream of the failed connection had

been disassembled and repaired where necessary. Records for the failed connection were

incomplete. The inspectors determined that the failed joint had been visually inspected and

micro-ohm tested; however, no evidence was found to verify that the joint had been torqued.

Inadequate torque could have resulted in increased resistance and heat generation if the joint

became loose.

Corrective Actions

All damaged components were repaired and refurbished, accessible splice joints on the 12-kV

auxiliary bus and 4-kV startup bus were inspected and torqued, and post-maintenance tests

were conducted to ensure that the bus bars were properly restored. The licensee concluded

that a similar defect was unlikely to occur on Unit 2 because the splice plates for the Unit 2 bus, although similar in construction to the one that failed in Unit 1, had previously been inspected and torqued. The licensee also tested the affected startup and auxiliary transformers to ensure

no breakdown had occurred in the winding insulation.

The licensee examined the various design issues associated with the 12-kV and 4-kV buses.

The buses with operating currents near design limits were judged to be operable because the

expected temperature at the Diablo Canyon site was considerably lower than the ambient

temperature assumed in the vendor heat rise test acceptance criteria. The licensee grouped

renovation and maintenance of the undamaged runs of bus into three groups by decreasing

order of failure susceptibility to prioritize remedial actions: (1) bus sections where normal load

has little margin relative to the continuous duty design rating, (2) bus sections in which the

auxiliary buses pass near the startup buses, and (3) bus sections that cannot be isolated from

the main generator (i.e., have no breaker to quickly sense and interrupt the fault).

During upcoming refueling outages, planned actions to prevent recurrence include the

following: (1) inspect and torque the booted connections on the 3750-amp bus for Startup

Transformer 1-1 and replace splice plates on the booted connections with full-face copper

splice plates; (2) inspect and torque splice plate connections on the 4-kV auxiliary buses; and

(3) upgrade the 2250-amp 12-kV buses from aluminum to copper bars. The same inspection

and replacement activities will occur on taped connections during the subsequent Unit 1 refueling outage.

This information notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact one of the technical contacts

listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

/RA/Marvin M. Mendonca FOR

Ledyard B. Marsh, Chief

Events Assessment, Generic Communications

and Non-Power Reactors Branch

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical contacts: Edward Goodwin, NRR Dyle Acker, Region IV

301-415-1154 805-595-2354 E-mail: efg@nrc.gov E-mail: dga@nrc.gov

Gregory Pick, Region IV

817-860-8270

E-mail: gap@nrc.gov

Attachment: List of Recently Issued NRC Information Notices and torqued. The licensee also tested the affected startup and auxiliary transformers to ensure