Nuclear Plant Journal, September-October 2015 NuclearPlantJournal.com
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Figure 1: Block Diagram of ESWWater Hammer Mitigation Modification.
Figure 2: Pipe Chase Module.
experienced during a loss of offsite
power, and verifies that the emergency
systems respond appropriately.
To mitigate this effect, Wolf Creek
installed a check valve on each train’s
service water supply line, which prevents
water from draining back through the
service water pumps. We also installed
the new loop seal (see Figure 1), which
is meant to contain the water between
the loop seal and the check valves on the
supply line. To help mitigate the effect of
the vacuum on the system, we installed
vacuum breakers at the top of the loop
seal. The vacuum breakers let air in to
minimize the size of the void by allowing
water beyond the loop seal to drain to the
lake. A void in the system still exists, but
it is now between the loop seal and the
lake as opposed to being in containment.
The plant design determined to be
the optimum solution for ESW water
hammer was used. Several options were
considered for locating the two 100-feet
tall, 24-inch diameter pipe loops. No
existing location inside the plant was
found that had suitable space.
It was decided that building an
external pipe chase tower (the housing
around the vertical loop seal piping –
multiple stacked modules that were
fabricated and fitted up off site and
then transported to the site and erected)
alongside the plant control building was
the best answer. This involved excavating
down from plant grade approximately 35
feet, and building a reinforced concrete
vault up to grade level, designed to
seismic Class 1 requirements. For the
above grade portion of the design, pre-
fabricated steel modules were chosen.
Three dimensional modeling of the
steel modules was performed to provide
the strength to tolerate the worst case
tornado missile, meet seismic design
requirements and to provide a module that
was light enough to be pre-fabricated and
shipped to the site. The module design
that resulted was a tube steel frame with a
one-half inch thick plate skin.
The steel modules offered several
advantages over poured concrete. First,
the plant is located in Kansas and the
mitigation work was to be performed in
the November to February time frame.
Snow and cold winter temperatures
were a given, making pouring concrete
difficult. In addition, large quantities of
safety-related concrete was not readily
accessible since there is only one small
batch plant in the vicinity of Wolf Creek
and the cost of erecting a batch plant
on site was prohibitive. Further, the
time frame for design and construction
was quite compressed. Therefore, it
was advantageous to have the ability
to stack the modules as they were built
and delivered – which enabled work
to proceed inside the tower between
deliveries of the modules.
Work on the initial design began in
June 2014, and was ready by October
2014. Excavation and shoring began in
November 2014 and completed just prior
to the end of 2014.
Construction of the underground
vault began January 2, 2015 and the four-
foot thick reinforced base mat was poured
two weeks later. The concrete pours for
