UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

October 11, 1996

NRC INFORMATION NOTICE 95-04, Supplement 1: EXCESSIVE COOLDOWN AND

DEPRESSURIZATION OF THE

REACTOR COOLANT SYSTEM

FOLLOWING LOSS OF OFFSITE

POWER

Addressees

All holders of operating licenses or construction permits and vendors for nuclear power

reactors.

Purpose

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

to alert addressees to an excessive cooldown and depressurization of the reactor coolant

system and the main steam system following a loss of offsite power at Catawba Nuclear

Station, Unit 2. It is expected that recipients will review their programs to determine if similar

conditions exist at their facilities. However, no specific action or written response is required.

Background

The main generator at Catawba Unit 2 normally supplies power to the onsite electrical

distribution system through auxiliary transformers. Power to the offsite electrical grid is

supplied by way of two half-size stepup transformers (designated 2A and 2B) using a dualoutput circuit-breaker design (Figure 1). When the main generator is shut down, the two

output circuit breakers are opened and offsite power is supplied (backfed) to onsite loads by

means of two separate circuits via the two stepup transformers. An isolated-phase bus

system connects the generator and transformers.

In Information Notice 95-04, the staff described a December 27, 1993, loss of offsite power

event at McGuire Unit 2. The event was initiated by the failure of a 525-kV insulator.

Reactor coolant pumps tripped on the loss of offsite power, resulting in core cooling by

natural circulation. Emergency diesel generators (EDGs) started and provided power to vital

buses. After the reactor trip, the reactor coolant system rapidly cooled down and

depressurized because of a reduction in energy input and an increase in energy removal by

full, unthrottled auxiliary feedwater (AFW) flow and several steam flow paths. The steam flow

paths included the AFW pump turbine and open steamline drainlines.

The continuing steam flow caused secondary system pressure to drop rapidly, and

unthrottled AFW flow continued to lower steam pressure and temperature. Continued

9609180255
IN 95-04, Supp. 1

October 11, 1996

Page 2 of 4

secondary cooling caused the reactor coolant system temperature to drop. A safety injection

signal on low pressurizer pressure occurred 7 minutes and 32 seconds into the event. In

Licensee Event Report No. 93-008-00 (Accession No. 9402080199), the licensee stated that

the McGuire event was bounded by events described in Chapter 15 of the McGuire Final

Safety Analysis Report. Specifically it was bounded by the complete loss of reactor coolant

flow and the steam line break events. The licensee also stated that overall response of the

plant, from a safety point of view, was satisfactory.

In an evaluation of the McGuire event, Duke Power Company, the licensee for the McGuire

and Catawba plants, concluded that steam loads and AFW flow caused the rapid secondary

side depressurization and cooldown before the main steam isolation. The licensee performed

detailed modeling of the relative contributions to the cooldown of the unthrottled AFW flow,

the AFW pump turbine steam load, and the open steam line drains and determined that the

open steam line drains were the primary contributor. The licensee modified the McGuire

steamline drain valves upstream and downstream of the main steam safety valves to fail

closed on a loss of power. The modification was made to help slow the secondary side

depressurization rate and the reactor coolant system cooldown rate during a loss of offsite

power event, giving operators increased time to address the event. At the time of the event

described below, the licensee had not yet made the design modification to the Catawba units

that had been implemented at McGuire.

Description of Circumstances

A loss of offsite power and safety injection occurred at Catawba Unit 2 on February 6, 1996,

when two potential transformer resistor bushings in the 22-kV isolated-phase bus system

simultaneously shorted to ground. This caused a phase-to-phase fault that resulted in a

generator lockout followed by a turbine trip and reactor trip. The lockout interrupted power

supply to the 22-kV isolated-phase buses 2A and 2B and led to the loss of offsite power.

Reactor coolant pumps tripped when offsite power was lost. Core cooling was maintained by

natural circulation with heat removal via AFW and steam generator relief valves steaming to

the atmosphere. EDG 2A started and energized essential bus 2ETA. However, before the

event occurred, EDG 2B had been taken out of service for unplanned maintenance on its

associated battery charger. Consequently, power to essential bus 2ETB was initially

unavailable. Within 3 hours, EDG 2B was restored to service and essential bus 2ETB was

reenergized. EDG 2B was secured approximately 5.5 hours into the event when offsite

power was made available to essential bus 2ETB via a crosstie from the operating Catawba

Unit 1. An additional crosstie from Unit 1 was established to power the redundant essential

bus 2ETA, allowing EDG 2A to be secured about 7.5 hours into the event.

A low steam pressure safety injection signal initiated approximately 8 minutes after the start

of the Catawba event. Similar to the McGuire event, the low reactor coolant system pressure

condition resulted from high steam loads and unthrottled AFW injection. The steam loads

included the turbine-driven AFW pump and steaming through main steamline drains. As

stated above, at Catawba, the licensee had not implemented the design modification to have

the steamline drain valves fail closed on loss of power.
IN 95-04; Supp. I

October 11, 1996

Page 3 of 4

Because of the loss of offsite power, charging flow and seal injection flow discharge valves

failed open, increasing the mass addition to the reactor coolant system. With normal letdown

isolated on the safety injection signal, the reactor became water solid. Reactor coolant

system pressure increased to the lift setpoint of one of the three pressurizer power-operated

relief valves. Over a period of about 6 hours and 15 minutes, the relief valves opened

approximately 74 times to control pressure. The pressurizer relief tank rupture disc relieved

and reactor coolant system water discharged to the containment.

In Licensee Event Report No. 96-001-00 (Accession No. 9603120449), the licensee stated

that this cooldown event was bounded by cooldown events evaluated in Section 15.1 of the

Catawba Final Safety Analysis Report. In addition, characteristics that potentially exacerbate

overcooling events (i.e., small shutdown margin, highly negative moderator temperature

coefficient, and adverse core power distribution due to stuck control rod) were not present

during this event.

With respect to the electrical fault initiating the Catawba Unit 2 event, the licensee found

evidence of arcing within the failed resistor bushings. Internal degradation of the bushing

was postulated to have resulted from moisture ingress. The degraded bushing in the

presence of external moisture and other contaminants may have initiated the ground faults.

In the one phase of the bus, the heaters were not working because of a failed thermostat.

Consequently, moisture buildup and dirt may have occurred, resulting in the X-phase fault.

When the degraded bus resistor busing faulted, the current was initially limited to

approximately 12 amperes by the high impedance in the generator neutral to ground.

Simultaneously, and as a result of the ground fault, voltages in the other two phases

increased significantly. This increase in voltage helped to trigger the catastrophic failure of

the degraded resistor bushing in another phase. Both failures are considered to have, as a

common cause, a degraded bushing in the presence of moisture and other contaminants.

During previous preventive maintenance tests, degraded insulator bushings and moisture

contamination were noted as problems at Catawba. Similar failures of resistor bushings were

experienced at McGuire, Zion Station, and San Onofre Nuclear Station. The resistor

bushings, originally supplied by Westinghouse, were hollow with an internally installed

resistor. At Catawba Unit 2, all resistor bushings have been replaced by bushings that use

an externally mounted resistor which the vendor believes to be a better design.

Probabilistic risk assessments of this event performed by the NRC staff and the licensee

indicate that the potential station blackout sequences dominate the high core damage

probability (greater than 1E-4) because the event involved an extended loss of offsite power
IN 95-04, Supp. 1

October 11, 1996

Page 4 of 4

with one of the two EDGs out of service. Operator actions that reduced the risk of this event

included: establishing crossties with Unit 1, restoring EDG 2B within 3 hours, and staffing the

safe shutdown facility. The crosstie capability between the two Catawba units is particularly

important in assessing the core damage probability of this event because 1.5 days were

required to restore the 2B main transformer and 4.5 days to restore the 2A transformer.

Without the crossties, the EDGs would have been required to operate for an extended period

of time. The conditional core damage probability for this type of event could exceed 1 E-3 for

plants that do not have crossties available and that do not maintain a safe shutdown facility

similar to the one at Catawba.

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 regional office.

Thomas T. Martin, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: M. Miller, RII D. O'Neal, NRR

(404) 331-5550 (301) 415-3706

Email: mnm~nrc.gov Email: dmo~nrc.gov

F. Burrows, NRR

(301) 415-2901

Email: fhb~nrc.gov

Attachments:

1. Figure 1. Catawba 2 Electric Power Distribution System

2. List of Recently Issued NRC Information Notices
TO(FROM MAIN TO(FROU MAIN

SWITCHYARD MAIN GENERATOR #2 SWITC1HYARD

Figure 1. Catawba 2 Electric Power Distribution System
Attachment 2

IN 95-04, Supp. 1

October 11, 1996

Page 1 of 1

LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

96-40,

Supp. 1

96-52

92-68,

Supp. 1

96-51

96-50

96-49

Deficiencies in

Material Dedication

and Procurement

Practices and in

Audits of Vendors

Cracked Insertion

Rods on Troxler

Model 3400 Series

Portable Moisture

Density Gauges

Potentially Substandard Slip-On,

Welding Neck, and

Blind Flanges

Residual Contamination Remaining in

Krypton-85 Handling

System After Venting

Problems with

Levering-In Devices

in Westinghouse

Circuit Breakers

Thermally Induced

pressurization of

of Nuclear Power

facility Piping

Motor-Operated Valve

Performance Issues

10/07/96

09/26/96

09/16/96

09/11/96

09/04/96

08/20/96

08/21/96

All holders of OLs or CPs

for nuclear power reactors

All U.S. Nuclear Regulatory

Commission portable gauge

licensees and vendors

All holders of OLs or CPs

for nuclear power reactors

All material licensees

All holders of OLs and CPs

for nuclear power plants

All holders of OLs or CPs

for nuclear power reactors

All holders of OLs or CPs

for nuclear power reactors

96-48

OL = Operating License

CP = Construction Permit