Page 40 - Nuclear Plant Journal Digital Edition: January - February 2013
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40
Nuclear Plant Journal, January-February 2013
Reactor
Repair
Using
Robotics
By Edward Petit de Mange, Diakont.
Edward Petit de Mange
Edward Petit de Mange is the Managing
Director of Diakont’s Advanced
Technologies
Division, and
a member of
Diakont’s Board
of Directors. In
this role, he is
responsible for
leading Diakont’s
continuing
expansion of
supplying of
equipment and
services to the US
nuclear power
industry. Mr.
Petit de Mange
holds a degree
in Mechanical Engineering from the
University of Maryland.
Aging nuclear reactor fleets
worldwide are undergoing upgrades and
repairs to extend their operational life. The
RBMK reactors in Russia are examples of
nuclear power plants that need extensive
restoration to extend their life, some of
which required repairs to critical reactor
vessel and internals components. In
order to minimize outage duration, these
repairs were often conducted in-situ, in
the confined spaces nearby or inside the
reactor core, where high radiation levels
necessitated that they be completed using
a remote manipulator or robotic system.
Diakont was recently commissioned
to repair the fuel guide tubes on a reac-
tor at the Leningrad Nuclear Power Plant.
The fuel guide tubes of the RBMK-1000
reactor were built with several hundred
fuel guide tubes which expand and retract
through
differing
temperatures
and
operating
condi-
tions. However, due
to structural chang-
es in the graphite
moderator blocks
supporting the fuel
guide tubes, many of
the lower portions of
the guide tubes have
begun to sag, result-
ing in a scenario
where the telescopic
joints completely
disengage; introduc-
ing the possibility
that reactor or fuel damage could occur if
the joints contract in a misaligned orien-
tation. These guide tubes are an integral
component of the reactor, and are com-
pletely inaccessible by personnel due to
high radiation levels, even when all fuel
is offloaded. Figure 1 shows a diagram of
fuel guide tubes in RBMK-1000 reactor
along with the graphite moderator blocks
that had begun to sag.
Historically, the repair of RBMK fuel
guide tubes required the deconstruction
of the upper reactor and the complete
replacement of the damaged equipment.
This repair method produced large
amounts of radioactive waste, exposed
plant operators to high dose uptakes, and
required that the reactor be offline for a
lengthy duration while the repair took
place.
Repair Solution
In order to remedy the situation,
Diakont worked with the chief designer of
the RBMK reactor to develop specialized
mechanical collars for installation
around the guide tube telescopic joints.
These collars provided a crucial vertical
extension, so that the telescopic joints
could not extend beyond the appropriate
travel range.
To test the mechanical collar design,
Diakont worked with the RBMK reactor
designer to create a stand simulating
the
mechanical
and
temperature
impacts existing in the reactor. These
simulations confirmed the feasibility of
the new mechanical collar technology;
the mechanical reliability testing of the
lengthening collars showed operational
performance of more than 25 years
and strength tests were confirmed with
a coefficient of more than 40 times.
The final lengthening collars were
designed with the full scope of the
design, manufacture & test overseen by
Rostechnadzor, the Federal Service for
Ecological, Technological and Nuclear
Supervision.
Repair Obstacles
The guide tubes were located in an
inaccessible location within the reactor,
adjacent to the core. The only entry point
into the reactor was a sixteen foot long
vertical shaft with an inner diameter of
twenty two inches. This shaft also had
protruding bolts that had to be negotiated.
Secondly, the reactor floor was covered
with bolts, gaps and weld beads. Lastly,
the expansion joints of the fuel guide
tubes were located near the top of the
guide tubes, with minimal clearance.
Robotic Installation
To install these mechanical extension
collars, Diakont developed a complex
system comprised of several remotely-
controlled robots; a loadingunit, a delivery
robot and an installation robot. Working
together, these robots were designed
to perform the delivery, alignment and
installation of the specialized mechanical
collars.
After examining all possible
options of entering into the reactor
area to install the mechanical collars,
Diakont engineering decided to deliver
the installation robot through a 16 foot
