HomeMy WebLinkAbout851194.tiff SSINS No. : 6835
IN 85-76
UNITED STATES a nrt
NUCLEAR REGULATORY COMMISSION 'ME1D CCil4fl Cn1,'; ; ,;.RS
OFFICE OF INSPECTION AND ENFORCEMENT 2 r r�1z
WASHINGTON, D. C. 20555 U 4�
September 19, 1985 SEP 2 71985
GREELEY. COLO.
IE INFORMATION NOTICE NO. 85-76: RECENT WATER HAMMER EVENTS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
This notice is to inform recipients of recent water hammer events in steam
supply lines to auxiliary feedwater (AFW) pump turbines, a steam exhaust line
from a high pressure coolant injection (HPCI) pump turbine, and discharge lines
from feedwater pumps (involving pump start logic). It is expected that recipi-
ents will review the information for applicability to their facilities and
consider actions, if appropriate, to preclude a similar problem occurring at
their facilities. However, suggestions contained in this information notice do
not constitute NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances:
In the past, the NRC has studied water hammer events in its Unresolved Safety
Issue (USI) A-1. Most of these events involved either the feedwater or steam
generator systems in pressurized water reactors (PWRs). In publishing its
technical findings relevant to this issue (Reference 1) , the NRC realized that
total elimination of water hammer is not feasible, because of the possible
coexistence of steam, water, and voids in various nuclear plant systems. The
frequency of events forming the subject of USI A-1 peaked in the mid-70' s but
then decreased as corrective equipment designs and procedures came into use.
Recently, additional events have been reported indicating an increased
frequency.
AFW Pump Turbine Steam Supply Lines
Water hammer events have been previously reported in the steam supply lines to
HPCI and reactor core isolation cooling (RCIC) turbines in boiling water
reactors (BWRs), as discussed in Reference 1, but have not been previously
reported with any frequency in steam supply lines to AFW (or equivalent system)
turbines in PWRs.
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851194
IN 85-76
September 19, 1985
Page 2 of 4
On May 8, 1985, at Waterford 3, an event occurred during an AFW surveillance
test that damaged 7 struts and 1 snubber supporting the steam supply line to
the turbine driven pump. The system passed the surveillance test satisfactorily.
These steam supply lines at Waterford employ heat tracing circuits to keep the
empty portions of pipe above 280°F to prevent water collection. The licensee
observed that two heat tracing circuits on a long run of empty pipe had not
been operating properly and identified two low points as possible sources of
water slugs.
While not all of the licensee' s investigations have been completed, the most
likely explanation of the event is water hammer from a steam driven water slug
produced by condensed weepage past the closed isolation valve into the normally
empty pipe. The licensee' s corrective actions include assuring the heat
tracing circuitry is keeping the pipe above 280°F and adding drains to the low
points.
Similar events have been reported at Diablo Canyon 1, on June 6, 1985, and at
San Onofre 3, on April 16, 1985. In both these events, damage was restricted
to pipe support snubbers and both systems passed the surveillance tests satis-
factorily. At Diablo Canyon, steam traps on the supply line had been inadver-
tently left isolated. At San Onofre, the cause could not be established, but
the licensee has decided to replace the existing steam traps with orifices to
reduce the possibility of collecting condensation in the pipe.
Before March 21, 1985, at Davis-Besse, damage was found to pipe hangar supports
on long, unheated, approximately horizontal sections of the crossover supply
lines to the turbine driven pumps. This damage is likely due to acceleration
of water slugs formed from condensation of steam in these lines. While not
definitely concluding so, the licensee suspects that the formation might also
have contributed to overspeed trips of both turbines on June 9, 1985, and other
irregularities in turbine speed characteristics before that date. (Information
Notice No. 85-50 provides further information about the Davis-Besse event of
June 9, 1985. )
HPCI Pump Turbine Steam Exhaust Line
Water hammer events in the steam exhaust lines of HPCI and RCIC turbines in
BWRs have been discussed in References 1 and 2. The following events, while
similar in some respects, introduce new information relating the events to the
operation of the turbine.
On April 2, 1985, at Pilgrim, the licensee found a failed inner rupture disk on
the HPCI turbine exhaust line and a damaged snubber near the torus penetration
of the line. Believing that a water hammer had occurred from trapped conden-
sate because the exhaust line had not been purged with nitrogen before a prompt
manual restart after a turbine trip, the licensee decided to purge the exhaust
line for a longer time after system operation and to inspect the line supports
following system operation.
IN 85-76
September 19, 1985
Page 3 of 4
On May 18, 1985, the Pilgrim licensee found two damaged snubbers on the HPCI
steam exhaust line after another surveillance test. The licensee believes that
the snubber damage may have occurred when the HPCI turbine tripped, slowed, and
automatically restarted after only a few seconds during the test. The damage
occurred on the exhaust line near the torus penetration downstream of the
vacuum breaker and stop check valve.
To lessen the severity of transients caused by quick starts of the turbine, the
licensee revised HPCI procedures to manually control the speed on starting and
installed a bypass line around the governor' s actuator assembly to increase
hydraulic pressure downstream of the actuator' s internal pump. On testing, the
licensee found the initial spike in turbine speed to be lessened considerably.
These changes were designed to decrease the likelihood of overspeed tripping on
starting.
The water hammers likely were caused by water being siphoned into the exhaust
line from the suppression pool as steam in the exhaust line condensed. The
operation of the drain and vacuum breaker subsystems on the line may not have
been adequate for the short operating cycles experienced. For example, the
vacuum breaker is only of 1-inch size for a 20-inch exhaust line. The licensee
now is considering installing a bigger vacuum breaker on the line near the
torus penetration. This type of problem, which could be expected to occur in
an actual demand under accident conditions, might not be noticed on surveil-
lance testing if fast-start testing were not employed.
Logic Problems With Main Feedwater Pump Restart
On August 21, 1984, the licensee at McGuire 1 experienced a loss of offsite
power, which with the manual closing of the main steam isolation valves
required by the event resulted in a loss of all condensate and feedwater pumps.
The loss of power also caused the feedwater pump recirculation valves to the
condenser to fail open as designed. This partially drained the feed system.
When power was restored and the feed pumps manually restarted, water hammer
occurred.
Inspection revealed that the water hammer caused only minor damage to conden-
sate booster pump discharge pressure gauges. The licensee is modifying proce-
dures to minimize the possibility of water hammer on pump restart following a
loss of offsite power. This is an example of a system being designed to drain
on loss of power, creating the conditions for subsequent water hammer. Similar
conditions also have been reported for the auxiliary saltwater systems at
Diablo Canyon Units 1 and 2.
IN 85-76
September 19, 1985
Page 4 of 4
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 . Jordan, Director
Divis' n of Emergency Preparedness
an Engineering Response
Office of Inspection and Enforcement
Technical Contact: V. Hodge, IE
(301)492-7275
Attachments:
1. References
2. List of Recently Issued IE Information Notices
Attachment 1
IN 85-76
September 19, 1985
Page 1 of 1
References
1. "Evaluation of Water Hammer Occurrence in Nuclear Power Plants,"
NUREG-0927, Revision 1, Office of Nuclear Reactor Regulation,
U. S. Nuclear Regulatory Commission, March 1984.
2. "Water Hammer in Boiling Water Reactor High Pressure Coolant Injection
Systems," Engineering Evaluation Report No. AEOD/E402, Office of Analyti-
cal Evaluation of Operational Data, U.S. Nuclear Regulatory Commission,
January 10, 1984.
Attachment 2
IN 85-76
September 19, 1985
LIST OF RECENTLY ISSUED
IE INFORMATION NOTICES
Information Date of
Notice No. Subject Issue Issued to
85-75 Improperly Installed Instru- 8/30/85 All power reactor
mentation, Inadequate Quality facilities holding
Control And Inadequate Post- an OL or CP
modification Testing
85-74 Station Battery Problems 8/29/85 All power reactor
facilities holding
an OL or CP
84-70 Reliance On Water Level 8/26/85 All power reactor
Sup. 1 Instrumentation With A facilities holding
Common Reference Leg an OL or CP
85-73 Emergency Diesel Generator 8/23/85 All power reactor
Control Circuit Logic Design facilities holding
Error an OL or CP
85-72 Uncontrolled Leakage Of 8/22/85 All power reactor
Reactor Coolant Outside facilities holding
Containment an OL or CP
85-71 Containment Integrated Leak 8/22/85 All power reactor
Rate Tests facilities holding
an OL or CP
85-70 Teletherapy Unit Full 8/15/85 All material
Calibration And Qualified licensees
Expert Requirements (10 CFR
35. 23 And 10 CFR 35. 24)
85-69 Recent Felony Conviction For 8/15/85 All power reactor
Cheating On Reactor Operator facilities holding
Requalification Tests an OL or CP
85-68 Diesel Generator Failure At 8/14/85 All power reactor
Calvert Cliffs Nuclear facilities holding
Station Unit 1 an OL or CP
OL = Operating License
CP = Construction Permit
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