INL's nuclear research and development and maintenance work lend themselves to myriad challenges in the health and safety realm.
The nuclear site in eastern Idaho where the Idaho National Laboratory (INL) is located was once home to the world’s first electricity-producing nuclear facility and first liquid-metal-cooled reactor. More than 50 different reactors have been housed at the site over its lifetime.
The site’s long history also includes the commissioning and activation of the Hot Fuels Examination Facility (HFEF). Originally designed to handle pre- and post-irradiated reactor fuel for examinations and experiments within an argon-filled “hot cell”—a containment designed to protect workers from radioactivity and other hazards by allowing operators to work with remotely handled tools using windows for visibility—HFEF currently performs experiments on post-irradiated fuel. These efforts allow scientists to better understand characteristics that, among other things, can be exploited to develop accident-tolerant nuclear fuels.
Imagine that your daily work is looking into a hot cell that is 30 feet wide by 70 feet long and 25 feet tall (see Figure 1). To perform experiments on post-irradiated fuel, you must utilize remotely handled tools that resemble robot arms, extend through the four-foot-thick concrete walls, and mirror the commands and controls in your hand (which remains in a safe environment outside the cell). The work being performed can range from the manipulation of items larger than paint cans to items one-eighth of an inch in size.
Despite a long history of great accomplishments, HFEF had never before removed and replaced a hot cell window until October 2016. The replaced window was leaking mineral oil, which is required to maintain transparency in a window made of several slabs of thick leaded glass. Without the mineral oil, the window cannot provide the visibility necessary for work to continue.
THE WINDOW
INL’s nuclear research and development and maintenance work lend themselves to myriad challenges in the health and safety realm. The replacement of this hot cell window, initially installed circa 1975, demonstrated the complex health and safety challenges that the operations and maintenance staff can face.
The job involved hazards that include, but were not limited to, the following:
- high noise
- radiation/contamination
- manual material handling
- silica exposure
- lead handling
- heavy lifting, hoisting, and rigging
- elevated working surfaces
HFEF includes fifteen windows arrayed along the four sides of the cell wall. Each window is several layers of thick leaded glass, 4 feet deep, 3 feet tall, 4 feet wide, and weighs roughly 14,000 pounds. Mineral oil is used to fill voids between the layers of glass for optical clarity. The windows are contained inside steel frames that utilize lead wool and lead sheeting to seal small gaps between the frame and window and are held in place by high-density, lead-painted grout (219 lb/ft3). The windows themselves are divided into three separate pieces: the “A slab,” the “B slab,” and the tank unit. These comprise several layers of glass and mineral oil. The A slab is the radiologically contaminated side of the window—the side facing the interior of the hot cell, with its high-radiation fields.
The A slab of the window to be replaced was changed out in late 2015. The tank unit and B slab were designed to be removed as a single unit (see Figure 2); however, for this job, they were eventually separated.
Removal of the window included the following tasks:
- placement of a shield inside the hot cell (to reduce any potential radiation from sources in the cell)
- removal of electrical and mechanical interferences
- removal and draining of radiologically contaminated mineral oil contained within the window
- establishment of a containment tent
- removal of all grout
Windows are designed to be as thick as the hot cell wall to provide protection from radiation, contamination, and other hazards within the cell. For additional protection when the window was removed, a 9,000-pound steel shield was held in place by a crane on the inner side of the hot cell. The outer side of the window was covered with plywood to prevent accidental striking of the window.
WINDOW REMOVAL
To contain the large amounts of dust from the grout and any potential exposure to radiological contamination, and to protect the workers removing the grout and those occupying surrounding areas from dust and debris, a special containment tent and ventilation system were designed and erected.
The 2,000-cubic-foot containment tent was constructed out of Herculite over a rigid frame. The tent comprised two compartments, one for work and one for removing PPE. An air mover, attached to the main compartment of the tent, created negative pressure from the ambient environment. Rated for 1,000 cfm, the air mover had two inlets that measured 8 inches in diameter and were capable of adjusting flow rate. The two inlets were piped, using semi-rigid materials, to a high-efficiency particulate air (HEPA) filter unit, which was piped to the tent using the same materials. The inlets on the tent were covered with roughing filters to alleviate bulk-dust buildup and the clogging of the HEPA filtration system further downstream (see Figure 3).
Airflow at the inlet was measured with an anemometer. Volumetric flow rate was calculated from the duct diameter and flow rates. Once this number was obtained, air changes per hour were calculated for the tent. The tent was adjusted several times to allow for appropriate intake of make-up air.
Grout removal was performed utilizing electric drills with chisel bits (see Figure 4). Approximately 1.6 cubic yards of high density grout were removed over the span of six days. The main health concern with this process was silica exposure; therefore, the workers inside of the tent wore powered air-purifying respirators with HEPA filters. HEPA vacuums were also used to assist in dust collection. As required, these vacuums were electrically grounded to protect against static charge built up by the large number of fine particles traveling through the vacuum hose.
For the upper portion of the removal, frame scaffolding was used for access to remove grout. Radiological Control personnel also used the frame scaffold to take surveys on exposed window surfaces during and after grout removal.
During grout removal, roughing filters were changed approximately every 20 minutes due to excessive loading. Radiation dose rates and radiological contamination were also a concern during this task. The Radiological Control department conducted surveys on all tools and equipment used. Surveys were performed using direct-read instrumentation for alpha and beta removable contamination and dose rates were taken for beta-gamma radiation. Smears of 100 cm2 were used to determine contamination level.
During the performance of this work, hot spots—high radiation areas—were identified. Techniques such as time, distance, shielding, and ALARA (As Low As Reasonably Achievable) practices were used to minimize worker exposure to the radiation fields. Radiation dose rates were highest during window removal and were measured to be 120 millirem/hr at 30 cm.
Once grout removal was complete (see Figure 5), the window was removed using a specialized cart that mated up to the cell wall (see Figure 6). The cart featured sliding rails and come-alongs. High levels of radiological contamination were found on the interior surfaces of the window flange. The highest level of contamination found on the flange was 1,993 disintegrations per minute/100 cm2 alpha, and 1,551,993 disintegrations per minute/100 cm2 beta-gamma.
The surfaces were decontaminated to an acceptable level over two decontamination periods using traditional methods of wet-wiping surfaces to remove the unwanted radioactive particulate material. The window was bagged and placed in a storage location to await further disassembly, and the new window was installed using a similar, though reversed, set of tasks.
GROUT REPLACEMENT AND WINDOW DISASSEMBLY
After the new window was bolted in place, concrete forms were erected for grout installation. Grout replacement was performed in two phases: a poured phase and a dry, “hand-packed” phase. The grout composition was a mix of medium aggregate, magnetite, and Portland cement that ensured high density upon setting and for handling purposes during the hand-packing phase. The lower portion of the window well was poured in place and continuously inspected by camera for air pockets because voids would weaken radiological protection in the area surrounding the window.
The second phase of grout replacement consisted of dry packing or hand-packing the top portion of the window well. Again, during grout replacement, it was imperative that no voids or bubbles were inadvertently created. Also, during both of the grout-installation phases, PPE was used to avoid concrete burns on the skin.
This ended the window replacement project. However, further disassembly of the window was required. This comprised removing the steel frame from the glass window portion. The steel frame weighed approximately 750 lbs. The gaps between the window and the frame were filled with lead wool and lead sheets to a depth of 9 inches and approximately three-quarters of an inch wide. Specialized tools were required to dislodge the lead wool from small gaps. This process proved to be quite tedious and difficult. Due to the age of the window and the amount of compression generated by the window’s weight, the lead wool was tightly packed within the small gaps.
Lead sheeting was removed after the steel frame was unbolted and removed. This process was facilitated using minimal controls due to the lack of oxidation on the lead sheeting and previously conducted negative exposure assessments and sampling during similar lead-handling tasks.
This window was sent off site to be refurbished for later use in the HFEF in case of future window change-outs.
SUCCESSFUL COMPLETION
Planning and coordination for removal of the hot cell window began in 2015. This entire job took 38 days to complete; however, the window removal and replacement themselves took only five days. Performance of this activity presented several health and safety challenges, and lessons learned will be applied to future window change-outs. For example, we identified a need to change the grout mixture specifications to create a mix that flows more readily for the bottom portion and a different mix that is more easily handled for the upper portion.
Several organizations and companies were needed to successfully accomplish this highly complex task. Moving forward, other windows will be replaced in the same facility, hopefully with the same successful outcome and without injuries.
KIMBERLY MORGAN, CIH, CSP,
is an industrial hygienist at the Idaho National Laboratory in Idaho Falls, Idaho. She can be reached at (208) 533-7431 or via email
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