Mitigating
Water
Hammer
By Jaime McCoy, Wolf Creek Nuclear
Operating Corporation.
Jamie McCoy
Dr. McCoy joined the Wolf Creek team
on Sept. 29, 2014 as vice president of
Engineering. He had served as director
of Engineering
at Entergy
Corporation’s
Arkansas Nuclear
One in Russellville,
Arkansas since
2010. Dr. McCoy
began his nuclear
career at Arkansas
Nuclear One and
held positions
of increasing
responsibility,
including manager of
programs and components and reactor
engineering superintendent. He earned
an undergraduate degree and a Ph.D.
in nuclear engineering from Missouri
University of Science and Technology in
Rolla, Missouri.
Wolf Creek Solves Legacy Water Hammer Issue
with Simple, Innovative Solution
48
NuclearPlantJournal.com Nuclear Plant Journal, September-October 2015
Wolf Creek Nuclear Generating
Station made several attempts over the
years to mitigate the effects of column
closure water hammer in the essential
service water system. In May of 2014,
the management team from the single-
unit pressurized water reactor in Kansas
committed to the Nuclear Regulatory
Commission that the effects of water
hammer would be mitigated before
starting up from its spring 2015 refueling
outage.
Having this solution in place prior
to startup from the spring 2015 outage
would require a fast track modification
and exceptional team-
work across Wolf
Creek
departments
and with the more
than 12 outside ven-
dors, including en-
gineering companies
and constructors.
In June 2014, a
team of engineers,
operators and main-
tenance and projects
personnel analyzed
alternatives with a
high level of technical rigor to ensure
the essential service water system water
hammer issue would be mitigated.
This review of alternatives included
discussion by the Engineering Peer Team
of the STARS Alliance. The alliance,
which includes Callaway, Diablo
Canyon, Palo Verde and Wolf Creek, is
a cooperative effort among these four
nuclear stations to leverage their natural
synergies to improve performance.
After evaluating multiple options, the
Wolf Creek team selected the preferred
solution in early July 2014.
The solution selected was a 100-
foot pipe loop on the discharge side and
additional check valves on the supply
side of the essential service water system
that would maintain the piping system
filled during a loss of power event.
By maintaining the essential service
water system piping system filled during
a loss of power, there is no column
separation when the pumps start and
reestablish cooling water flow. Therefore,
the “column closure water hammer”
event is eliminated in the vicinity of the
safety related equipment in the plant.
To understand how this works, first
it’s important to understand how Wolf
Creek’s emergency systems worked
under the old design.
The Emergency Service Water
System at Wolf Creek provides a source
of backup cooling water from the ultimate
heat sink (the Coffey County Lake) to
remove heat from safety equipment
in the plant. When normal offsite
power from the transmission system is
available, cooling water for plant safety
equipment is supplied by the normal
service water system. If off-site power
from the transmission system is ever lost,
one of the two on-site emergency diesel
generators starts up and provides electric
power through the on-site emergency bus
to plant emergency equipment, including
the emergency service water system.
With the old design, in the brief time
between losing power to the service water
pumps and the ESW pumps coming on,
the water that was already in the system
drained in two directions — back toward
the service water pumps and toward the
discharge into the lake.
That resulted in a vacuum effect that
created a vapor void in the system.
When the ESW pumps started, the
water was pumped from the ultimate heat
sink at the ESW pump house, causing the
column of water and vapor to collapse,
resulting in a water hammer effect in the
system. The forces created during the
water hammer caused leaks in mechanical
joints and pin-hole leaks in plant heat
exchanger tubes, especially where tubes
are thinned due to erosion/corrosion. This
most severely impacted the containment
coolers because they are at the high point
in the system.
The negative effects of the water
hammer had been observed during outages
when engineered safety feature actuation
system (ESFAS) testing was performed.
The test simulates the conditions
