Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 3, 1988
Information Notice No. 88-55: POTENTIAL PROBLEMS CAUSED BY SINGLE FAILURE
OF AN ENGINEERED SAFETY FEATURE SWING BUS
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 caused by single failure of an engineered safety features swing bus.
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:
On January 28, 1988, while Robinson Unit 2 was operating at full power,
Carolina Power and Light reported to the NRC Operations Center, in compliance
with 10 CFR 50.72, that a single failure in the system supplying electrical
power to the three safety injection pumps could cause two of them to fail to
perform their intended function. On the following day, the licensee reported
to the NRC Operations Center that Robinson Unit 2 was being shut down in
compliance with its technical specifications.
As shown in Figure 1, each of two class 1E buses supplies power to a safety
injection pump. One of these buses also supplies power to a third safety
injection pump via a swing bus. The swing bus is normally supplied with power
by bus E1; however, if bus E1 fails to supply power to the swing bus, then the
swing bus is automatically transferred to bus E2. An interlock prevents the
swing bus from being supplied simultaneously from both of its supply buses.
As originally designed, a safety injection signal causes all three safety
injection pumps to start automatically.
During the licensee's review of the emergency electrical distribution system,
including dc control power, which was done in response to an NRC request, the
licensee identified several postulated single failures that could result in
loss of two of the three safety injection pumps. For example, loss of dc
control power for train B would cause loss of safety injection pumps B and C.
However, the analysis of record for the loss-of-coolant accident (LOCA)
8807280019
. IN 88-55
August 3, 1988
Page 2 of 2
assumed that a single failure would cause only one safety injection pump to
fail. Pending resolution of this problem, the auto-start capability for the
swing pump was defeated, the plant was derated to 60 percent of full power,
and the allowable power-peaking factor was reduced from 2.32 to 2.26.
Subsequently, the licensee performed a new LOCA analysis assuming that only
one safety injection pump operates to mitigate the consequences of a LOCA.
Calculation of emergency core coolant flow was based on system test
performance degraded by 5 percent. Using acceptable models, the licensee
found that peak cladding temperature, local metal-water reaction, and total
metal-water reaction would be within the acceptance criteria of 10 CFR 50.46
with the reactor operating at 100 percent power with the power peaking factor
at 2.32 before the accident. On the basis of these results, NRC permitted the
licensee to resume full power operation after removing the auto-start
capability of the swing pump and after the limiting conditions for operation
were changed to require that two safety injection pumps be operable, each
capable of automatic initiation from a separate emergency bus.
No specific action or written response is required by this information notice.
If you have 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: Roger Woodruff, NRR
(301) 492-1180
Peter Kang, NRR
(301) 492-0812
Attachments: 1. Figure 1 - Robinson 2, Power Supply for Safety Injection
Pumps
2. List of Recently Issued NRC Information Notices
. Attachment 2
IN 88-55
August 3, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-54 Failure of Circuit Breaker 7/28/88 All holders of OLs
Following Installation of or CPs for nuclear
Amptector Direct Trip power reactors.
Attachment
88-53 Licensee Violations of NRC 7/28/88 All manufacturers
Regulations, Which Led to and distributors
Medical Diagnostic of radio-
Misadministrations pharmaceuticals
for human use,
nuclear
pharmacies, and
medical licensees.
88-52 Failure of Intrauterine 7/27/88 Medical licensees.
Tandem of Fletcher
Applicator Brachytherapy
Devices During Patient
Treatment
88-46, Licensee Report of 7/26/88 All holders of OLs
Supplement 1 Defective Refurbished or CPs for nuclear
Circuit Breakers power reactors.
88-51 Failures of Main Steam 7/21/88 All holders of OLs
Isolation Valves or CPs for nuclear
power reactors.
88-50 Effect of Circuit 7/18/88 All holders of OLs
Breaker Capacitance or CPs for nuclear
on Availability of power reactors.
Emergency Power
88-49 Marking, Handling, 7/18/88 All holders of OLs
Control, Storage and or CPs for nuclear
Destruction of Safe- power reactors and
guards Information all other licensed
activities
involving a
formula quantity
of special nuclear
material.
88-48 Licensee Report of 7/12/88 All holders of OLs
Defective Refurbished or CPs for nuclear
Valves power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
