United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-76: Recent Water Hammer Events

                                                         SSINS No.:  6835
                                                            IN 85-76 

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

                             September 19, 1985

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 high pressure coolant injection (HPCI) pump turbine, and discharge 
lines from feedwater pumps (involving pump start logic). It is expected that 
recipients 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 sys-
tem) turbines in PWRs. 



8509170373 
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                                                       IN 85-76 
                                                       September 19, 1985 
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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 satisfac-
torily. These steam supply lines at Waterford employ heat tracing circuits 
to keep the empty portions of pipe above 280F to prevent water collec-
tion. 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 280F and adding drains
to the low points. 

Similar events have been reported at Diablo Canyon 1, on June 6, 1985, and, 
at Onofre 3, on April 16, 1985. In both these events, damage was restricted 
to pipe support snubbers and both systems passed the surveillance tests sat-
isfactorily. At Diablo Canyon, steam traps on the supply line had been inad-
vertently left isolated. At San Onofre, the cause could not be established, 
but the licensee has decided to replace the existing steam traps with ori-
fices 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 cross-
over 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 pene-
tration of the line. Believing that a water hammer had occurred from trapped
condensate 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. 

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                                                       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 start-
ing 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 over-speed 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 surveillance 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 re-
quired 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 
condensate booster pump discharge pressure gauges. The licensee is modifying 
procedures 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. 

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                                                       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 L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and 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 
     Analytical Evaluation of Operational Data, U.S. Nuclear Regulatory 
     Commission, January 10, 1984. 

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