United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-79: Degradation or Loss of Charging Systems at PWR Nuclear Power Plants Using Swing-Pump Designs

                                                            SSINS No.: 6835 
                                                            IN 86-79       

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              September 2, 1986

                                   AT PWR NUCLEAR POWER PLANTS USING 
                                   SWING-PUMP DESIGNS 


All nuclear power reactor facilities holding an operating license or a 
construction permit. 


This notice is provided to alert recipients of a possible degradation or 
actual loss of primary coolant charging systems when using swing-pump 
designs (i.e., one of three pump motors can be aligned to receive electrical 
power from either of two separate electrical buses). It is expected that 
recipients review this information for applicability and consider actions, 
as appropriate, to preclude this and similar problems from occurring at 
their facilities. However, suggestions contained in this notice do not 
constitute NRC requirements; therefore, no specific action or written 
response is required. 

Description of Circumstances: 

Surry Unit 1 

Surry Unit 1 has three charging pumps which also serve as high head safety 
injection pumps. The "A" pump is powered from the "A" bus, the "B" pump is 
powered from the "B" bus. The "C" pump is a swing pump and may be powered 
from either bus; however, its normal power supply is from the A bus. On June
26, 1985 Surry Unit 1 was operating at 100% of full power with the A 
charging pump out of service for maintenance. The C swing charging pump was 
being powered from the A bus. While in this configuration, the operators 
racked out the B charging pump motor breaker to perform maintenance on the 
pump. Subsequently, the normal feeder breaker for the operating C charging 
pump motor tripped as a result of an electrical interlock. With the A and B 
pumps out of service and the C charging pump motor, tripped, all makeup 
water (including high head safety injection) and reactor coolant pump seal 
injection flow were unavailable. The operator immediately racked in the B 
pump motor breaker, thereby clearing the interlock, and the C charging pump 
restarted. An electrical jumper was installed around the interlock in the B 
pump motor breaker cubicle to prevent the breaker for the C charging pump 
motor from tripping. The breaker for the B pump motor was then racked out. 


                                                         IN 86-79         
                                                         September 2, 1986 
                                                         Page 2 of 3      

The charging pump interlocking scheme at Surry Unit 1 is designed such that 
each of the two essential power source buses provides power to only one 
charging pump motor at a time. When the B pump feeder breaker was racked out
of service, the interlock design assumed that the A pump was being operated 
from the A bus (although, in fact, it was out of service) and tripped the C 
pump to prevent it from being powered by the A bus. There is no automatic, 
transfer of the C pump to the B bus; this prevents a postulated fault on the
C pump from tripping both buses. 

The cause of the event was attributed to inadequate precautions in the 
procedure to remove the B charging pump from service. The licensee 
reinstructed operating personnel on the operation of the swing pump design 
regarding the associated interlocking scheme in use at Surry Unit 1. Labels 
were attached to the breakers associated with the charging pump motors to 
provide warning information related to the existing interlocks. 

Millstone Unit 2 

On June 11, 1985 during routine testing on the Millstone Unit 2 simulator, 
an apparent design deficiency was identified in interlocking circuitry 
associated with the B charging pump motor. The B charging pump is the swing 
pump and, as such, its motor can be aligned to either of two electrical 
power buses. At Millstone Unit 2 the charging pumps provide makeup water to 
the primary system during normal plant operating conditions and ensure 
adequate shutdown margin during accident conditions. The problem identified 
on the simulator resulted in the B pump being rendered inoperable following 
a loss of power for the electrical bus opposite to the one to which the B 
charging pump motor was aligned. Subsequent investigation of the actual 
plant circuit design by the licensee confirmed that indeed electrical power 
must be available on both buses before the B charging pump can be started by 
either automatic or manual means. A modification to the circuit design was 
immediately implemented to prevent inoperability of the B charging pump 
under such conditions. A review of other potentially affected circuits at 
the Millstone plant was undertaken and no similar problems were identified. 


The events described above were identified during a systematic NRC study of 
licensee event reports. These events illustrate how the safety function of 
the charging systems using swing-pump designs can be degraded or lost as the
result of design deficiencies in interlocking circuitry or inadequacies in 
maintenance procedures. The NRC study of the generic implications 
surrounding these events did not identify any other similar event or 
situation. However, the study does raise the concern that degradation or 
actual loss of charging systems could occur at a time when makeup water to 
the primary system would be needed, either during normal or accident 
conditions. The study also concluded that the types of deficiencies 
identified at Surry Unit 1 and Millstone Unit 2 for swing-pump designs are 
not likely to be detected by normal design reviews and/or routine testing. 
It takes a specific set of circumstances or conditions to readily detect the 
deficiencies in interlock circuitry or maintenance procedures. The 
likelihood of the occurrence of such circumstances and/or conditions is 
small. This is evidence by the fact that Surry Unit 1 and Millstone Unit 2 

                                                         IN 86-79         
                                                         September 2, 1986 
                                                         Page 3 of 3      

for more than 10 years before the deficiencies were uncovered. Therefore, 
these or similar deficiencies may very well exist at other plants which use 
safety systems with swing-pump designs. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 

                                   Edward L. Jordan Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  Vincent D. Thomas, IE
                    (301) 492-4755

                    Frank Ashe, AEOD
                    (301) 492-4442

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