The 60MWt Pressurized Water Reactor (PWR) at NASA’s Plum Brook Facility (PBRF) operated from 1962 to 1973 with full power operations totaling 98,000 MW-days. Between August 2003 and February 2005, a 9-foot (2.7m) x 31-foot (9.4m) tall PWR and the Reactor Vessel Internals (RVI) were disassembled and packaged for removal from the site in Sandusky, Ohio, US.
The primary process for disassembly was mechanical “nut and bolt” removal. High dose rates and no water shielding made minimizing radiation exposure an ongoing challenge.
Shielding for Grave Danger
Typically when working over a reactor, personnel are on a refueling bridge or work platform. Tools are manipulated by long poles and cut components by J-Hooks or Grippers on poles. These Long Handle Tools (LHT’s) are common in reactor pool and fuel pool work. At Plum Brook, with the absence of water shielding, dose rates coming up from the reactor and the internals were so high that the general area was a Locked High Radiation Area. Inside the reactor vessel was a Very High Radiation Area and marked as “Grave Danger”. Entry into the vessel was never considered, and physically leaning over to look into the vessel was not allowed. Therefore any tool used in the reactor could not be operated with conventional long poles from above.
With the reactor head removed, a Reactor Pressure Vessel (RPV) is simply a hole in the floor. Safety railings are placed around this hole to prevent falling, but do nothing to prevent radiation from coming out. For Plum Brook, a “railing,” measuring approximately 9 inches (230mm) thick, provided shielding. During operation there had been a three-piece Shrapnel Shield covering the reactor head. A hole cut in the center of this “hat”-shaped shield allowed the long handle tools to enter the RPV. This created a worktable approximately 3 feet (1m) tall that not only shielded the workers but also prevented them from physically getting close to the center opening.
With the Shrapnel Shield in place, a tool at the bottom of a long handle pole could be positioned vertically into the reactor. However, since the operator would not be able to reach this, horizontal gripper poles were used to manipulate the tools in the horizontal plane. To raise and lower a tool vertically, electric hoists were installed above the opening on a set of jib cranes. This was a huge ALARA benefit as it kept the tool operators and their hands out of the vertical ray of radiation.
To help the operator learn how to manipulate a Long Handle Tool without actually touching it, a wooden mock-up of the Shrapnel Shield was constructed. Using the horizontal pole to vertical pole technique had not been anticipated or planned. Operators trained in a low dose area before working on the actual Shield. A high level of proficiency was achieved much sooner than anticipated, making this approach welcomed by all groups onsite.
A camera and ventilation system were mounted inside the Shrapnel Shield before they were installed over the vessel. The ventilation assured that air was always drawn into the opening to reduce the risk of airborne contamination exiting the vessel. The camera worked as an overview of the work going on inside the vessel. Video monitors on top of the Shrapnel Shield allowed operators the same view as if they looked directly into the opening. Additional cameras installed in the vessel also assisted with locating a tool on a component.
The Shrapnel Shields also provided a benefit for contamination control. Removable plastic sheeting covered the top of the Shield and could be replaced as needed. Any tool or pole that came out of the hole was surveyed and handled as if it had been in a flooded reactor pool. Although fully dressed out in radiological PPE for loose contamination, additional PPE (such as gloves and shoe covers) were worn around the Shrapnel Shield. Before leaving the Very High Radiation Area where the Shield was located, personnel removed their second set of PPE at a step-off pad. This practice of utilizing additional step-off pads and PPE was also used in other work areas.
Being a test reactor, Plum Brook’s RV was designed with many penetrations and tubes in which experiments could be placed. Several “Beam Tubes” channeled the neutrons toward the experiment. The high concentration of neutrons resulted in the Beam Tubes being highly activated, especially those that utilized high Nickel (Ni63) content.
Three of the Horizontal Beam Tubes were on the horizontal centerline of the reactor next to the Core Box that contained the fuel. Constructed of stainless steel, with one of them being filled with nickel components, the “HB Tubes” were expected to contain a large amount of the RVI’s total activation. Mounting a mechanical saw on them to size reduce and separate the waste levels was the original plan, however the lack of free space and the tight area around the Core Box presented a challenge. Many man-hours would be required to remove the components above them to gain access, and then lower the saw down for remote cutting.
An alternate approach was conceived that allowed removal of these components at the beginning of the project, prior to the vessel head removal. Original site drawings and procedures indicated that the tubes could be withdrawn from the reactor from the outside; walk downs and inspections of that area confirmed this. However, pulling a Beam Tube was normally done underwater with equipment that no longer existed. This plan was approved because of the exposure reduction to be gained.
A mock-up was constructed to test the tooling and to train the crew. Once the Beam Tubes were cut, all operations needed to be fully remote. Activities were performed out of normal sequence to minimize time working in high radiation fields. Items used last, such as final lifting devices, had to be installed long before they were needed. The crew trained for several days until the customer was satisfied. The training paid off because the actual dose exposure and task time was less than expected.
The highest dose rate components found in the reactor were the Control Rods. Even after the years of no fuel, the materials of construction of these items resulted in several items with radiation levels of over 1000Rem/hour (10,000MSv) at contact. Although constructed of stainless steel, there were other items attached to the main unit that carried high curries. The overall length of these items required size reduction in order to fit inside a liner. The challenge was to size reduce them with minimal personnel exposure and secondary waste.
Even characterizing the individual components was a challenge. Each item had to be lifted away from the high radiation field and surveyed. Once the radiation levels were “mapped,” a cut line was plotted on each item that would minimize the high level waste. As expected, all items were consistent in the cut line location. The cutting tool would require the capacity to cut the equivalent of 4-inch (100mm) heavy wall stainless steel pipe. To minimize the cutting debris, a hydraulic shear was chosen as the cutting device. This shear was mounted to a Long Handle Pole and sent down into the reactor.
Each control rod was cut and the lower dose level sections brought to the surface. Once characterized, they were placed in the appropriate waste liner. Due to the high levels of the remaining sections, they could not be brought out of the reactor in the same manner.
In an excellent ALARA practice, the high radiation lower sections were transported from the reactor vessel to the shielded liner with minimal personnel in the area. This was done during lunch break with all personnel onsite removed from the area. (Remember that there was no water shielding so the dose rates throughout the entire facility increased.) Site personnel could not exit through the portal monitors due to the increase in background levels. The crane operator and radiation protection personnel were the only ones in the immediate area.
In keeping with the ALARA goal, it was suggested that the amount of crane moves be minimized. This suggestion originated during a practice of the critical lift. Instead of moving one control rod section at a time, four of them were moved at one time. A specially-made Debris Containment Device (DCD or simply “debris box”) was placed into the vessel and individual items were loaded into the boxes. These boxes were sealed to prevent contamination or loose parts from falling off during transport. This minimized the risk of contamination spread while transporting to the liner and helped packaging efficiency.
A portable “Shadow Shield” was erected out of thick carbon steel plate (see Image 5). The personnel stood behind this during the “flying” (transporting to the liner by crane). A camera in the containment vessel was used to follow the load hanging from the polar crane. Another camera above the liner gave the operator a view of the destination. These cameras gave the operator the ability to watch most of the movements on a video monitor behind the Shadow Shield. When necessary, such as passing by obstacles, the crane operator came out of the shielded area for a few seconds for a perspective view.
The debris box idea was an excellent “Engineering Control” for ALARA, because it:
- Reduced the amount of personnel exposure by having less crane moves;
- Reduced the risk of contamination spread by being sealed;
- Reduced the chance of an item not being securely fastened to the hook, by having approved and known rigging to the box; and
- Helped liner packing efficiency by keeping components closer together.
While the debris box was never part of the original plan, a suggestion during the project and the crew’s questioning attitude allowed for this ALARA improvement. Additional boxes were made for many other items.
Mock-Up Practicing Worth the Time
Mock-up practicing in low dose areas definitely saved time and exposure. The easiest part of a work package is sometimes the hardest to complete because it is assumed to be easy. During a mock-up test, personnel on the crew were encouraged to stop and ask questions; and they did. Sometimes this delayed the start of work, but the work was accomplished successfully because everyone was ready and understood their scope.
About the Author:
Steve R. Larson started in nuclear power in 1983. He has served as a design engineer for several RV and RVI segmentations. For the Plum Brook RVI & RV segmentation he was one of the tool designers for the equipment. He also was onsite as a project manager for the reactor work.
The author would like to thank:
The NASA PBRF web page, www.grc.nasa.gov/WWW/pbrf/PlantDecommissioning.com