Information Notice No. 85-76: Recent Water Hammer Events
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D,C. 20555
September 19, 1985
Information Notice No. 85-76: RECENT WATER HAMMER EVENTS
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
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.
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 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.
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
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.
September 19, 1985
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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
2. List of Recently Issued IE Information Notices
September 19, 1985
Page 1 of 1
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.
Page Last Reviewed/Updated Friday, May 22, 2015