Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 31, 1989
Information Notice No. 88-86, SUPPLEMENT 1: OPERATING WITH MULTIPLE
GROUNDS IN DIRECT CURRENT
DISTRIBUTION SYSTEMS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This supplement to Information Notice No. 88-86, "Operating With Multiple
Grounds in Direct Current Distribution Systems," (IN 88-86) is provided to
alert addressees to a failure mode in which direct current (dc) grounds can
cause equipment to fail in the energized state. 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 do not constitute NRC
requirements; therefore, no specific action or written response is required.
Background:
IN 88-86 was issued October 21, 1988, to alert addressees to potential
problems caused by plants operating with ground(s) in vital dc distribution
systems (which are normally ungrounded systems).
Description of Circumstances:
A recent NRC Region II Operational Safety Team Inspection at the Virgil C.
Summer Nuclear Station (Inspection Report 50-395/88026) identified a pre-
viously unidentified potential failure mode. In this failure mode, dc grounds
can cause equipment to fail in the energized state instead of the deenergized
state necessary to satisfy protection system performance.
Attachment 1 shows a solenoid for an ASCO valve with a ground fault on the
positive side. The circuit for this solenoid contains only one actuation
contact which is located on the positive side of the solenoid. Also shown is
the licensee's installed ground detection system which consists of indicating
lights in the control room and at the battery chargers. A complete circuit
exists through the solenoid and the ground detector, assuming a ground on the
positive side of the solenoid.
8903270348
. IN 88-86, Supplement 1
March 31, 1989
Page 2 of 3
These ASCO solenoids experience dropout at approximately 20 volts, pickup at
approximately 40 volts, and require only milliamps of current for these ac-
tions. A harsh environment in the area of ASCO solenoids that were not sealed
for environmental qualification (EQ) could result in steam intrusion into the
solenoid. A resulting decrease in solenoid insulation resistance could then
generate an electrical leakage path between the positive side of the solenoid
and the plant ground. A ground fault on the positive side of the solenoid, or
anywhere between the solenoid and the actuation contact, coupled with a ground
on the positive leg of the dc buss of 4,000 ohms or less, could prevent the
solenoid from dropping out or cause it to pick up. Spurious, uncontrolled
valve actuation could result.
Even with the ground detection system disabled and the dc buss resistance to
ground maintained at greater than 5,000 ohms, the licensee determined that the
plant was still susceptible to this failure mode. This conclusion was drawn
because, in a high-energy line break/loss-of-coolant accident (HELB/LOCA)
environment, the dc system was very likely to become grounded because of the
many non-EQ portions of the system such as terminal blocks, limit switches,
and solenoid valves. Buss ground faults in the range of about 100 ohms to
4,000 ohms on the positive leg could occur for some periods of time. These
ground faults would not be large enough to blow fuses or trip breakers, but
would be large enough to accommodate ASCO solenoids failing energized. Thus,
the licensee determined that the EQ of additional solenoids was required.
Other equipment besides ASCO solenoids may have circuits that could fail
energized. The licensee identified the main turbine electro-hydraulic control
system as a potential example. The licensee's policy of maintaining greater
than 5,000 ohms of resistance to ground on the positive leg and ensuring
adequate equipment EQ should prevent such failures.
Discussion:
The importance of the dc power system to plant safety is discussed in IN
88-86. The safe recovery from various accident situations is dependent on the
availability and reliability of the battery and the dc power distribution
system. Licensees may have performed safety evaluations that assumed an
ungrounded, vital dc system and specific equipment failure modes during harsh
environments that may no longer be valid because of possible multiple grounds.
In addition, the use of installed ground detectors and portable ground
locating equipment and the policies on operating the facility with grounds on
vital dc systems may not maintain a minimum threshold resistance to ground for
dc power systems above which predicted system/component operation is ensured.
As noted, the licensee's evaluation determined that a HELB/LOCA event could
cause common mode failures of the type described. As a result of this deter-
mination, the licensee
1. Environmentally qualified approximately 90 solenoids that actuate
approximately 30 valves. The affected valves included all pressurizer
power-operated relief valves, main steam isolation valves, feedwater
isolation valves, reactor building cooling unit bypass dampers, reactor
building purge valves, and other containment isolation valves. These
items
. IN 88-86, Supplement 1
March 31, 1989
Page 3 of 3
previously had been exempted from EQ requirements because they had been
assumed to fail safe (i.e., deenergized).
2. Established a policy on operating the facility with grounds that requires
ground faults on the vital 125-volt dc busses to be promptly cleared if
they have a resistance of less than 5,000 ohms.
3. Disabled the installed ground detection system on the vital 125-volt dc
busses. These systems, as designed, caused a permanent ground of 1,500
ohms on both the positive and negative legs of the dc busses.
4. Began pursuing the installation of a better ground detection system and
the purchase of state-of-the-art portable ground locating equipment.
5. Planned to conduct further evaluations and to consider additional modifi-
cations, including converting to a grounded dc system.
6. Issued Licensee Event Report (LER) 50-395/88-12 on this subject.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the techni-
cal contacts listed below or the Regional Administrator of the appropriate
regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Robert P. Schin, Region II
(404) 331-3911
Frederick H. Burrows, NRR
(301) 492-0833
Attachments:
1. Diagram Description of Failure Mode
2. List of Recently Issued NRC Information Notices
. Attachment 2
IN 88-86, Supplement 1
March 31, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
89-35 Loss and Theft of Un- 3/30/89 All U.S. NRC
secured Licensed Material byproduct, source
and special
nuclear material
licensees.
89-34 Disposal of Americium 3/30/89 All holders of an
Well-Logging Sources NRC specific
license
authorizing well-
logging
activities.
89-33 Potential Failure of 3/23/89 All holders of OLs
Westinghouse Steam or CPs for PWRs.
Generator Tube
Mechanical Plugs
89-32 Surveillance Testing 3/23/89 All holders of OLs
of Low-Temperature or CPs for PWRs.
Overpressure-Protection
Systems
89-31 Swelling and Cracking 3/22/89 All holders of OLs
of Hafnium Control Rods or CPs for PWRs
with Hafnium
control rods.
89-30 High Temperature 3/15/89 All holders of OLs
Environments at or CPs for nuclear
Nuclear Power Plants power reactors.
89-29 Potential Failure of 3/15/89 All holders of OLs
ASEA Brown Boveri or CPs for nuclear
Circuit Breakers power reactors.
During Seismic Event
89-28 Weight and Center of 3/14/89 All holders of OLs
Gravity Discrepancies or CPs for nuclear
for Copes-Vulcan power reactors.
Air-Operated Valves
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
