United States Nuclear Regulatory Commission - Protecting People and the Environment

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. 



8411060254  
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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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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 

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