Information Notice No. 89-16: Excessive Voltage Drop in DC Systems
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 16, 1989
Information Notice No. 89-16: EXCESSIVE VOLTAGE DROP IN DC SYSTEMS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems resulting from unexpected large voltage drops in cables between the
batteries and circuit breakers. 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.
Description of Circumstances:
Nine Mile Point Unit 1 was shut down in December 1987 for refueling. During
this shutdown, the licensee made modifications to its 125-Vdc power system. A
sub-sequent review of these modifications led the licensee to conclude that
there was a voltage supply problem associated with the operation of battery
charger (MG set) circuit breakers. The manufacturer of these circuit breakers
stated that the minimum voltage for operation of the circuit breakers was 90
volts. The technical specifications require a minimum of 106 Vdc at the
battery terminals for the batteries to be considered operable. However,
studies showed that with 114 Vdc at the battery terminals, the calculated
voltage at the breaker would be less than 90 Vdc because of line losses due to
current demand during breaker closure. A battery voltage of less than 114 Vdc
could exist at the time ac emergency power is required to be reconnected to
the battery chargers. Thus, if a loss of site ac power were to occur, the dc
voltage at the battery charger circuit breakers may not be adequate for the
battery chargers to be reconnected to emergency ac power. This situation
would cause the batteries to discharge. The decreasing dc voltage would lead
to degraded dc control power and a subsequent trip of the emergency diesel
generators. Therefore, a loss of normal ac power could lead to a loss of dc
power and the subsequent loss of emergency ac power.
Discussion:
Problems with electrical systems at commercial power reactors have been
identified with increasing frequency. These problems led to the issuance
8902100283
. IN 89-16
February 16, 1989
Page 2 of 2
of Generic Letter 88-15, "Electric Power Systems - Inadequate Control Over
Design Processes," on September 12, 1988. The above-described event is a
unique illustration of one of the types of problems presented in the generic
letter.
The Nine Mile Point Unit 1 licensee is not aware of any testing that could be
used to establish the adequacy of the original design, nor has the licensee
found documentation of the original design basis. The voltage drop could be
attributed to an inadequate original estimate of load, hence cable sizing;
load creep over the years without assessment of the consequences; or use today
of a potentially more conservative power profile as determined by current
practice for battery sizing. After identifying the problem, the licensee
assessed the actual loads and plans to replace the undersized cable to improve
the system's operability.
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 technical
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: J. Carter, NRR
(301) 492-1194
J. Lazevnick, NRR
(301) 492-0814
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 89-16
February 16, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
89-15 Second Reactor Coolant Pump 2/16/89 All holders of OLs
Shaft Failure at Crystal or CPs for nuclear
River power reactors.
89-14 Inadequate Dedication 2/16/89 All holders of OLs
Process for Commercial or CPs for nuclear
Grade Components Which power reactors.
Could Lead to Common Mode
Failure of a Safety System
89-13 Alternative Waste Management 2/8/89 All holders of NRC
Procedures in Case of Denial specific licenses.
of Access to Low-Level Waste
Disposal Sites
89-12 Dose Calibrator Quality 2/9/89 All NRC medical
Control licensees.
89-11 Failure of DC Motor-Operated 2/2/89 All holders of OLs
Valves to Develop Rated or CPs for nuclear
Torque Because of Improper power reactors.
Cable Sizing
89-10 Undetected Installation 1/27/89 All holders of OLs
Errors In Main Steam Line or CPs for BWRs.
Pipe Tunnel Differential
Temperature-Sensing Elements
at Boiling Water Reactors.
89-09 Credit for Control Rods 1/26/89 All holders of OLs
Without Scram Capability or CPs for test and
in the Calculation of the research reactors.
Shutdown Margin
89-08 Pump Damage Caused by 1/26/89 All holders of OLs
Low-Flow Operation or CPs for nuclear
power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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