Information Notice No. 84-80: Plant Transients Induced by Failure of Non-Nuclear Instrumentation Power
SSINS NO.: 6835
IN 84-80
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
November 8, 1984
Information Notice No. 84-80: PLANT TRANSIENTS INDUCED BY FAILURE OF
NON-NUCLEAR INSTRUMENTATION POWER
Addressees:
All Babcock and Wilcox Company (B&W) designed power reactor facilities
holding an operating license (OL) or construction permit (CP).
Purpose:
This information notice is to alert recipients of potentially significant
problems caused by failure of non-nuclear instrumentation (NNI) power
supplies at B&W designed reactor facilities. It is expected that recipients
will review the information for applicability to their facilities and
consider actions, if appropriate, to preclude similar problems occurring at
their facilities. However, suggestions contained in this information notice
do not constitute NRC requirements and, therefore, no specific action or
written response is required.
Description of Circumstances:
Rancho Seco: Repeated Partial Loss of NNI Power on March 19, 1984
The plant was operating at 92% power when a ground fault on a land steam
exhauster caused the supply breaker to a motor control center (MCC) to trip.
This supply breaker could not be reclosed. The loss of the MCC resulted in a
loss of the hydrogen side seal oil pump for the main generator that is
powered from that MCC. The plant continued operation as the licensee
procedures and technical manual of the turbine-generator manufacturer
(Westinghouse) allow full-load operation with the hydrogen side seal oil
pump shut down. When operators initiated implementation of a procedure to
restore the seal oil system, a small hydrogen explosion occurred and a
reactor power reduction was initiated. Shortly thereafter, with the plant at
85% power, a large explosion and fire occurred and the turbine was
immediately tripped. The reactor was automatically tripped by the recently
installed anticipatory trip circuit.
Following the large explosion, the fire burned for several minutes and was
extinguished by the CO2 system. The explosion resulted in the generator
housing lower skirts being thrown into some neighboring equipment. The
missiles also broke two large handwheels for valves and damaged some small
test valves and non-safety-related level instruments.
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November 8, 1984
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Shortly after the reactor trip, the operators proceeded to start auxiliary
boilers to provide for the auxiliary steam loads. The small boiler was
successfully fired. When the control power for the big boiler was turned on,
in preparation for its start, the small boiler tripped and control power to
both boilers was lost. At the same time, several alarms were annunciated in
the control room indicating NNI power failure and nonvital power bus
trouble. Investigation by the licensee revealed that the voltmeters on the
non-Class 1E J inverter were pegged low. The J inverter provides alternate
power to NNI-X and Y, and control power for auxiliary boilers. The licensee
believes that some of the equipment damaged during the fire and explosion
caused several grounds in the 125-V dc supply bus to the J inverter,
resulting in voltage degradation.
During restoration of the J inverter to operability, + - 24-V dc power of
the NNI-X power distribution system was lost for approximately 4 minutes as
a result of deenergization of the inverter along with the spurious loss of
the -24-V dc bus. The licensee believes that the loss of the -24-V dc bus
was caused by drift of the low output voltage set point of the power supply.
The loss of NNI-X + - 24-V dc was perceived to be a total loss of NNI power
in the control room because numerous instrumentation failed to midscale and
"Power On" lights on NNI cabinets appeared to be off. Based on these
indications, operators implemented the "Complete Loss of NNI Power"
procedure which required manual initiation of the safety features actuation
system (SFAS). SFAS initiation started the high pressure injection (HPI)
pumps and opened the HPI valves. As the reactor coolant system pressure
began to increase, a pressurizer code safety valve opened prematurely (about
140 psi below the 2,500 psig set point). The safety valve opened twice and
reseated both times during the event. At Rancho Seco, the power-operated
relief valve (PORV) is kept in the locked closed position. During the event,
an atmospheric dump valve also opened as a result of NNI-X loss, but it was
recognized and closed from the control room.
The licensee personnel continued troubleshooting the NNI power supplies when
a second partial loss of NNI power occurred approximately 3 hours after the
explosion. The 115-V ac power of the NNI-X power distribution system was
lost for approximately 5 minutes when the supply breaker from Class 1E D
inverter to NNI-X was manually tripped and the automatic bus transfer (ABT)
switch did not transfer the loads to the J inverter as it was still
operating at degraded voltage. The D inverter provides the normal power to
NNI-X and Y.
Crystal River Unit 3: Loss of NNI-Y Power on April 26, 1984
The plant was operating at 97% power when the dc power supply of the NNI-Y
power distribution system failed as a result of the failure of a capacitor
with wrong ratings installed by the manufacturer in the + - 24-V dc supply.
The loss of NNI-Y dc power caused erroneous signals to be sent to the
integrated control system (ICS). The ICS rapidly reduced the feedwater flow
to one of the steam generators causing an undercooling transient. The
reactor tripped on high reactor coolant system pressure. The atmospheric
dump valves (ADVs) and main steam safety valves (MSSVs) subsequently opened.
One ADV and several MSSVs failed to reseat resulting in a small radioactive
release since there was a
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IN 84-80
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previously identified small steam generator tube leak. The release did not
cause any regulatory limits to be exceeded. The ADV was manually isolated
and the MSSVs reseated after steam generator pressure was reduced.
The NNI-Y power failure caused erroneous indications and prevented the
automatic control of some non-safety-related systems. However, plant
operators were able to identify the erroneous indications immediately and
control the plant by using the redundant set of instrumentation powered from
the NNI-X power distribution system. Other systems affected by the loss of
NNI-Y included the condensate system, the plant computer, and a number of
annunciator event alarms.
Discussion:
These recent operating events at Rancho Seco and Crystal River Unit 3 have
again demonstrated that the failure of NNI power has the potential of
causing transients as well as challenges to an operator's capability to
mitigate those transients at B&W reactor plants. The NRC staff has met
recently with licensees and the B&W Owners Group to discuss the implications
of such events and the effectiveness of improvements made in responding to
transients caused by loss of NNI power supplies since publication of IE
Bulletin 79-27, "Loss of Non-Class IE Instrumentation and Control Power
During Power Operation." From the responses to the Rancho Seco and Crystal
River events, it was evident to the staff that significant improvements have
been made since the bulletin. In addition, the corrective actions taken by
both licensees after the events are expected to further reduce the frequency
of loss of NNI power events.
The corrective actions taken before the restart of Rancho Seco included
readjusting of NNI and ICS dc power supplies overvoltage and undervoltage
trip set points. The licensee's long-term actions include: consideration for
installation of an automatic transfer switch between the normal supply and
backup supply for the J 120-V ac bus to reduce the chance of loss of NNI dc
power on loss of a single inverter; investigation into the possibility of
providing a single computer manual action that would place the appropriate
computer points, unaffected by loss of NNI, on analog trend (the current
procedure takes several minutes); scheduling the calibration of + - 24-V NNI
and ICS power supply alarm and trip settings once every refueling outage;
revising operating procedure NNI drawings and plaques on the NNI panels to
more correctly indicate system functions; revising the plant training manual
to include more detailed NNI and ICS power supply descriptions; and
modification of NNI/ICS panels, as necessary, to eliminate physical contact
of the doors with power supply wiring.
For Crystal River Unit 3, the corrective actions included: inspection of
power supplies similar to the failed NNI-Y +24-V dc power supply for correct
capacitors; scheduling inspection of NNI-X power supplies; and future
installation of redundant power supplies for NNI-Y. The redundant power
supplies would prevent a transient, similar to the one caused by a failed
capacitor, from occurring as a result of a single 24-V dc power supply
failure. After this modification, both NNI-X and NNI-Y will have redundant
power supplies.
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No specific action or written response is required by this information
notice. If you have any questions regarding this matter, please contact the
Regional Administrator of the appropriate NRC regional office or this
office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: R. N. Singh, IE
(301) 492-8985
Attachment: List of Recently Issued IE Information Notices
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