Information Notice No. 83-04: Failure of ELMA Power Supply Units 1

                                                            SSINS No.: 6835 
                                                            IN 83-04       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                             February 18, 1983 

Information Notice No. 83-04:   FAILURE OF ELMA POWER SUPPLY UNITS 1 

Addressees: 

All holders of a nuclear power reactor operating license (OL) or 
construction permit (CP). 

Purpose: 

This information notice is provided as an early notification of potentially 
significant problems pertaining to 24-V DC power supply units manufactured 
by ELMA Engineering of Palo Alto, California. Because of the potential 
safety significance and related generic implications of this problem, 
addressees are expected to review the information for applicability to their 
facilities. No specific action or response is required at this time. 

NOTE:     All power supply units, mentioned, discussed, and referred to 
          hereafter are power supply units manufactured by ELMA Engineering.

Description of Circumstances: 

On June 19, 1982 a loss-of-24-V DC-power test performed at Peach Bottom Unit
2 Nuclear Plant resulted in startup of high-pressure coolant injection 
(HPCI), reactor core isolation cooling (RCIC), and residual heat removal 
(RHR) systems on the Unit 3 plant while it was operating at 100% power. The 
initiation of the emergency core cooling system (ECCS) equipment caused a 
reactor power increase to 113% of normal full power and a reactor water 
level increase of seven inches before the feedwater flow was automatically 
reduced and the operator tripped the ECCS system. 

The initiation of the Unit 3 ECCS system was caused by the degraded 
operation of one power supply unit when the backup power supply was lost 
during the loss-of-power testing. 

Each ECCS logic train of Peach Bottom Units 2 and 3 has two power supply 
units in parallel. One of Unit 3 power supply units is powered from the 4-kV
emergency bus of Unit 2. This power supply was de-energized as part of the 
loss-of-power test. The parallel (battery powered) power supply unit had an 
undetected degradation which caused the voltage to drop and cause false 
"double low" and "triple low" reactor water level signals which subsequently
initiated the ECCS. 

This was the third failure of a power supply unit within a year. Subsequent 
testing of all remaining power supply units at Peach Bottom identified one 
more degraded power supply unit. In each case, the power supply degradation 

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                                                         February 18, 1983 
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was attributed to a faulty capacitor. Degraded operation of these power 
supply units is characterized by a high ripple voltage. As a result of this 
problem Peach Bottom decided to test the power supply units monthly, rather 
than semiannually as before. 

On May 13, 1982 the auxiliary operator at Vermont Yankee noted anomalous low
water level readings from the reactor vessel sensors. The instruments were 
declared inoperable and a channel trip (half scram) was instituted. 
Subsequent investigation revealed that a degraded power supply unit was the 
cause of anomalous readings. The power supply unit was replaced. Preventive 
maintenance checks of other power supply units at Vermont Yankee revealed 
that one more power supply unit was close to failure, evidenced by a 
decreased output. This power supply unit was also replaced. In both units 
the problem appears to have resulted from a faulty capacitor. 

Previous problem history with 10-ampere output power supply units caused 
Vermont Yankee to switch to 6-ampere output units at the vendor's 
recommendation. This apparently decreased the heating problem that is 
discussed in the following paragraph. 

Licensees should note that ELMA Engineering reported recently to the NRC 
that extensive testing at the factory indicates that improper, vertical 
mounting of power supply units can result in overheating of the capacitor 
adjacent to the ferro-resonant transformer and could cause capacitor 
failures. Further, ELMA Engineering pointed out that the ferro-resonant 
transformers are substantially less efficient when operating at partial 
rather than at full power. Therefore, if two power supply units in parallel 
share the load, their heat output into their enclosure(s) will be greater 
than the heat loss output of a single unit supplying the entire load. Power 
supply units which are not perfectly matched will result in unequal load 
sharing and further increased heat output. Therefore, ELMA Engineering 
indicates that if two or more power supply units are used in parallel, the 
system must be analyzed for load sharing, efficiency at low load, and proper 
cooling. 

If you have any questions regarding this matter please contact the 
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:  W. Laudan
                    301-492-9759

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