Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 85-03
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
January 15, 1985
Information Notice No. 85-03: SEPARATION OF PRIMARY REACTOR COOLANT
PUMP SHAFT AND IMPELLER
Addressees:
All pressurized water power reactor facilities holding an operating license
(OL) or construction permit (CP).
Purpose:
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
required.
Description:
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.
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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.
Discussion:
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.
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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
office.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Bill Jones, IE
(301) 492-7613
Attachment: List of Recently Issued IE Information Notices