POWER May 2016 - 20

4. Shroud head bolt stack-up. The
core shroud head bolt assembly includes several
pieces that work together to safely secure
the core shroud head on a boiling water
reactor (BWR). Courtesy: AREVA Inc.
5. Tool on the mock-up. The shroud
head bolt retainer tool reacts off of the top of
the shroud head bolt to help reposition the
retainer into the locked position. Courtesy:
AREVA Inc.
6. Simple but effective. The shroud
head bolt retainer tool includes a base or
frame, an engagement member, and a collar.
Courtesy: AREVA Inc.
ternals. During removal and installation
of the core shroud head, it is necessary
to detension and tension the core shroud
head bolts.
A typical shroud head bolt is 14 feet
manager for AREVA Inc. Although the tool
is fairly simple, it can save nuclear plant
owners time and money when a springloaded
retainer fails to lock back into
place during reactor reassembly.
What Is a Core Shroud Head Bolt?
The core shroud is a cylindrical structure
that surrounds the core in a BWR. It provides
a barrier to separate the downward
coolant flow through the annulus from the
upward flow through the core. The core
shroud also provides structural support,
and it aligns the fuel assemblies, steam
separator assemblies, and control rod
guide tubes.
A head sits on top of the shroud and
creates an upper plenum into which a twophase
steam/water mixture enters from
the core and is directed to steam separation
equipment. The core shroud head
must be removed to access the core for
refueling and inspection of the core in20
long
with a diameter of 1.75 inches. A
nut is screwed onto one end of the bolt
and a tee head is welded to the other
(Figure 4). A sleeve covers the rest of the
bolt, and the base of the sleeve is joined
to a collar that is welded to the shaft
near the tee head. A part of the collar is
cut out to provide space for an alignment
pin window.
A retainer locks the nut on the shroud
head bolt assembly, preventing the nut
from rotating (Figure 5). There is a spring
that pushes up against the retainer to
force engagement with the nut. After
many years in service and with the accumulation
of grit and grime, the retainer
may not return to the locked position with
only spring pressure.
A Tool for the Job
When the retainer fails to lock the nut
properly, the entire shroud head bolt assembly
must be replaced. In the past, an
L-pick on a long-handled pole has commonly
been used to release the retainer.
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The L-pick has multiple purposes, but for
this specific application, it was difficult
for operators to get enough leverage to
pull the retainer up, and the process was
very time-consuming.
The shroud head bolt retainer tool modification
was modest, yet the advanced
tool design resulted in a more effective
and efficient solution to complete the repair.
By gripping under the retainer and
reacting off of the top of the shroud head
bolt, the tool (Figure 6) can help free the
retainer and assist the spring in repositioning
it into the locked position, saving
the cost of a new bolt and the outage time
to install it.
On June 23, 2015, the U.S. Patent
and Trademark Office issued a patent for
the invention. With the announcement,
Cocke joined a list of more than 400
AREVA inventors credited with patented
technologies.
" Incremental improvements are usually
the best patents, " said Piotr Kolodziejczyk,
intellectual property manager for AREVA
Inc. " Jason's new tool is simple, but very
efficient, and a good example of AREVA's
commitment to offer the most innovative
solutions to our customers. "
The tool was designed for use on all
BWR plants and has already been used at
one facility in the eastern U.S. ■
-Aaron Larson is a POWER associate
editor.
POWER | May 2016
http://www.powermag.com
POWER May 2016

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