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 lower-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 cutting tools would spread debris and radiation everywhere. A more controlled disassembly was the only option.
Early in the disassembly process the radiation levels at the site reached over 1500 mR/hr (milli Rem per hour of radiation). PD Design Group developed a protective shield to reduce personnel exposure at the work level. Behind the shield, and standing 6 feet to the side, radiation levels to the technicians were reduced by 750 times.
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 was removed from the reactor, it is taken to a container to be shipped off site. Depending on the level of radioactivity and material, components 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 was loaded into small metal boxes or large 20-foot long truck containers called “Sealands”. Higher level items went in either Class A or Class B/C containers. The highest level waste is known as “Greater Than Class C” (GTCC) and required the highest integrity containers available. The higher the waste classification the higher the price tag for disposal at a burial site.
The WTS – PD 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 increased container efficiency. The original estimates planned for this waste to go into more than three liners. By carefully planning the packaging, efficiency was doubled and the amount of liners to be buried was noticeably decreased. This resulted in a very large dollar savings in the total burial costs.
Disassembly of the Plum Brook reactor was completed efficiently and safely. Technician exposure was far below what was originally anticipated and packaging efficiency in the waste liners resulted in a noticeable reduction to expected burial costs. Unknown experiments and unexpected extra components were successfully handled in real time. The dry disassembly of the Plum Brook reactor was a major success story in nuclear facility decommissioning.