The following is an artical for a company newsletter about
the segmentation of the Plum Brook Reactor. It is dated 2/24/04, before
segmentation of the reactor had been completed.
Opening
Although the dismantling of the nuclear reactor is
a small part of the entire D&D of the Plum Brook Reactor Facility,
it is on the main stage. While most of the Plum Brook Reactor Facility
is being dismantled by hand or up close with power tools, the reactor
disassembly must be done remotely. High levels of radiation and contamination
do not allow a technician to get close to the work. Using unique tools
operated 30 feet away by watching them on camera is the daily routine
for the WTS technicians performing reactor segmentation.
Challenges
NASA looked at decommissioning the reactor in the 1980's, but elected
to wait for radiation levels to drop before attempting disassembly. High
radiation levels require flooding the reactor with water to shield the
technicians. Underwater disassembly and cutting of the reactor was anticipated
to be difficult. The expected low-level radiation would allow for a dry
disassembly using standard tools. The facility committed to removing all
water and seal off the piping.
After sitting idle for 30 years protected
by a nitrogen blanket to prevent corrosion it was decided to start decommissioning
the reactor. A similar reactor disassembled just before the Plum Brook
reactor allowed for dry hands-on disassembly. However, radiation levels
in the Plum Brook reactor would be much greater than this other reactor.
These higher levels would not allow technicians to do hands-on disassembly.
Remote tools would need to be used.
Many commercial nuclear plants use under
water torches, saws, and abrasive water jet cutters to remove components
from those reactors. Because Plum Brook was a dry reactor now, the fumes
and debris from these types of "normal" cutting tools would
spread debris and radiation everywhere. A more controlled disassembly
was the only option.
Since September 2003 technicians have
been unbolting and cutting components inside the reactor vessel. After
five months a majority of these components were out of the vessel. All
of these components have been removed using long handle tools by technicians
working 25 feet up and 6 feet to the side. A simple operation such as
tuning a screw involves mounting a pneumatic wrench on a 30-foot pole,
lowering it down through a protective shield, and then using a 6-foot
horizontal pole to maneuver onto the bolt. Using cameras to watch the
tool as it swings over the bolt takes a great deal of patience and practice.
One component of the reactor has 138 bolts alone.
Early in the disassembly process the
radiation levels at the protective shield reached over 1500 mR/hr (milli
Rem per hour of radiation). Behind the shield, and standing 6 feet to
the side, radiation levels to the technicians were reduced by 750 times.
The usual daily limit for a technician is 100 mR per day. That means without
this shield the technicians would only be allowed to work 4 minutes each
day.
Before the dome covering the top of the
reactor was removed some of the components were removed from side penetrations.
Technicians practiced on "clean" mock-ups to quickly and safely
place remote tooling in position. Using cameras and remotely controlled
tools, items were pulled from the center of the reactor. The radiation
exposure to the technicians was much lower than expected in part by the
training they had done.
Packaging the Radiological Waste
After a component is removed from the reactor it is
taken to a container to be shipped off site. Depending on the level of
radioactivity and material, components may go in one of several different
containers. The lowest level components (such as the bolts on the reactor
dome) had no detectable radiation. Some facilities would "free release"
these items and discard them in the trash. At Plum Brook all items from
the reactor are being shipped off site in special containers.
Low level waste can go in small metal
boxes or the large 20-foot long truck containers called "Sealands".
Higher level items go in either Class A or Class B/C containers. The highest
level waste is know as "Greater Than Class C" (GTCC) and require
the highest integrity containers available. The higher the waste classification
the higher the price tag for disposal at a burial site.
The WTS and PlantDecommissioning Design
Team did detailed planning in the Segmentation and Packaging Plan to reduce
the amount of high level waste containers required. In addition real time
changes to the packaging plan have increased liner efficiency. At this
point in segmentation two Class B/C liners have been filled with high
level waste. The original estimates planned for this waste going into
over three liners. By careful planing the packaging efficiency has doubled
and the amount of liners to be buried is noticeably decreasing. This results
in a very large dollar saving in the total burial costs.
Early in the project it was anticipated
that the reactor would contain items that were "Greater Than Class
C" (GTCC) and require the highest integrity containers available.
To reduce the chances of having GTCC waste, segmentation of the reactor
was planned out well in advance. If a reactor component is to be cut into
3 pieces and the middle piece had the bulk of the radiation, it can cause
that middle piece to become GTCC. Looking at the drawings and running
computer models will give an idea what components this may apply to. These
and any other suspicious components are then checked real time for these
"hot spots". If found, care must be taken not to cut into this
area and avoid creating GTCC waste.
Closing
Disassembly of the Plum Brook reactor is proceeding
efficiently and safely. Technician exposure is far below what was expected.
Packaging of the waste in liners is more efficient than planned resulting
in a noticeable reduction to expected burial costs. Unknown experiments
and unexpected extra components have been handled real time and successfully.
The dry disassembly of the Plum Brook reactor will be a success story
in nuclear facility decommissioning.
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