Information Notice No. 85-03: Separation of Primary Reactor Coolant Pump Shaft and Impeller

                                                            SSINS No.: 6835 
                                                            IN 85-03       

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              January 15, 1985 

                                   PUMP SHAFT AND IMPELLER 


All pressurized water power reactor facilities holding an operating license 
(OL) or construction permit (CP). 


This information notice is provided to alert recipients of an event 
involving primary reactor coolant pump impeller separation from the pump 
shaft. 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 


On September 16, 1984, the Palisades Nuclear Plant had been operating for 
approximately three days in the process of initial power ascension following
a refueling outage. The plant was at approximately 57 percent power. At 3:45
a.m. (EDT), reactor coolant pump (RCP) seal cavity pressures indicated that 
the first (lower) and second (middle) seals had failed simultaneously, and 
an orderly shutdown was commenced. At 5:20 a.m., the third (upper) seal 
failed. The reactor was taken off-line and, at approximately the same time, 
the pump vibration level reached the "danger" level (10 mils). The pump was 
then secured. In the 3-hour period prior to securing the pump, the electric 
current to the pump had increased by approximately 10 to 15 percent. During 
the event, the flow in the loop driven by the affected pump remained normal.
During the examination to determine the extent of the seal failure and to 
evaluate required repair, major damage to pump components was discovered. 
The bolts and guide pins that secure the impeller to the shaft had been 
severely damaged. Unexpected wear on the pump impeller/shaft assembly and 
pump internals was found. The impeller is normally fastened to the shaft by 
eight cap screws and four guide pins. However, when the pump was examined, 
all eight cap screws and two of the four guide pins were found broken. The 
impeller had been kept in rotation by the two guide pins that had remained 
intact. Stationary pump parts showed unexpected wear in 360 degrees of arc; 
rotating pump parts showed wear in 180 degrees of arc. 


                                                          IN 85-03        
                                                          January 15, 1985 
                                                          Page 2 of 3     

The RCP at Palisades is a Byron Jackson 850 rpm, single stage, centrifugal 
pump 1 with a 42-inch diameter impeller and a 4,000-hp Allis-Chalmers motor.
The pump has four seal stages with controlled bleed off (and no seal 
injection) for cooling. Because an impeller was damaged during 1983 by an 
apparently unrelated failure, another impeller and pump shaft assembly was 
installed as a unit in February 1984. Procedures prescribed by the pump 
manufacturer were, followed for the installation of the impeller/shaft 
assembly; however, no manufacturer's representative was present when the 
installation took place. This assembly had been stored horizontally in the 
plant store room for approximately 12 years after it had been used in the 
1972 time-frame during initial hot functional testing. It is this 
impeller/shaft assembly that failed on September 16, 1984, after the pump 
had accumulated approximately 1,300 hours running time before the event. 

The analysis of the failure of the eight cap screws and the two guide pins 
indicated that failures resulted from fatigue and impact loading. The two 
out of four guide pins that did not fail were bent, which caused the pump 
shaft to be forced upward and the impeller to be forced downward. No bolt 
corrosion was found. Although a sheared RCP shaft occurred at Surry 1 in 
November 1973 as a result of a manufacturing defect, the event described 
here is the first involving the potential separation of a primary coolant 
pump impeller from its shaft. 


Events leading to the pump impeller/shaft failure cannot be precisely 
determined; however, examinations of the cap screws indicate that abnormal 
stress caused them to fail. It is believed that the pump impeller/shaft 
assembly caused the abnormal cap screw stresses that ultimately led to the 
failure. It is now believed that the improper torquing of the pump screws 
was the root cause. 

The event and the above conclusion regarding the cause of the failure raise 
three issues which should be emphasized. (1) Disassembly and reassembly of 
primary reactor coolant pumps is an operation which should be done in a 
rigorous manner employing manufacturer's recommendations and proper 
procedures. (2) The nearly simultaneous failure of the two seal stages is 
indicative of the seal package under abnormal stress. This stress, 
concurrent with high pump vibration is indicative of possible severe pump 
damage. Operator response to these indications led to the pump being shut 
down before the final two guide pins failed, thus, preventing a more severe 
event. This action was a prudent response to the situation. (3) Although the 
event described here is apparently isolated, it demonstrates the credibility 
of a pump failure event which could lead to a rapid flow decrease transient 
of the type expected with a sheared shaft event. Most PWRs have a licensing 
basis analysis for that event or the similar seized rotor event. These 
analyses generally assume an automatic response of the plant's reactor 
protection system which generates a reactor trip as a result of low reactor 
coolant flow.

                                                          IN 85-03        
                                                          January 15, 1985 
                                                          Page 3 of 3     

Plants which sense primary flow by pump shaft rotation rather than a fluid 
flow measurement for this automatic trip function are cautioned to the 
vulnerability of the protective system to a failure of the pump impeller. No
specific action or written response to this information notice is required. 
If you need additional information about this matter, please contact the 
Regional Administrator of the appropriate NRC regional office or this 

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

Technical Contact:  Bill Jones, IE
                    (301) 492-7613

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