Sandia ‘RAP artists,’ White Sands techs free stuck radiation source
The cylinder was small — the size of a restaurant salt shaker — but it emitted enough gamma rays to kill a person in half a minute. And it was stuck.
Technicians at DoD’s White Sands Missile Range Gamma Irradiation Facility ordinarily use pneumatic air to propel the little cylinder from its insulated location to its exposed test position and back again, like drive-up banking facilities use pneumatic tubes to shuttle cylinders between customer and teller. The method had worked satisfactorily for decades.
But on Oct. 2, after the irradiator finished a test sequence, a switch along the cylinder’s path caught in one of its ribbings and would not release. The cylinder, emitting 20 rads/second at a distance of one foot, wouldn’t move, says White Sands health physicist Douglas McDonald.
Five hundred rads is considered a lethal dose; half the people receiving it die in 30 days.
Researchers significantly increased pneumatic pressure on the 3-by-1-inch cylinder, but it would not budge.
Warning horns blared. Warning lights flashed. They would do so for almost three weeks.
The facility’s primary mission is to irradiate electrical circuit boards to test their survivability under extreme circumstances. But because vehicles are sometimes irradiated as well, emissions intentionally extend beyond the building through a raised overhead door into an already cordoned-off area outside. These emissions were now unceasing.
“We’re required to alert staff [24 hours a day], using a visual and audible signal, to the existence of a radiation area,” Richard Williams told the Lab News. Williams is White Sands’ associate director for its Survivability, Vulnerability, and Assessment Directorate.
The facility also had to be staffed around the clock, he said, to warn security personnel and other potential visitors not to enter the area because radiation was present.
Because of these precautions and the lab’s layout, there was no danger to staff or the environment. But the approximately 3,000-square-foot test facility was now inoperative.
The White Sands team’s options, say McDonald and Williams, were expensive, labor-intensive, and time-consuming. No human was strong enough to carry a shield heavy enough to protect him or her. There were robots on the East Coast that might be located and flown in. The facility also had the capability to design, manufacture, transport, and maneuver in a very heavy lead shield on a front-end loader to block and then surround the errant source. Technicians drilling through the shield might send in a rod to free the switch.
But the quickest, simplest option was to work with NNSA’s local RAP (Radiological Assistance Program) team, DOE’s usual responder-of-choice in assessing and resolving emergency radiological situations.
On Oct. 3, White Sands personnel called Sandia RAP — exactly the right thing to do, says Richard Stump (12345), Sandia RAP leader. “Part of RAP’s mission is to help out in jobs like this,” he says.
Able to draw upon many Sandia resources, Richard called robotics manager Phil Bennett (6644) and explained the problem.
Phil said his group had a robot that might do the job. The 600-pound, five-foot-long robot, now unofficially known as M2, rolled on treads, could maneuver around obstacles, and had a long, multi-jointed gripper arm. It had the dexterity to reach into awkward places and apply force to drills and screwdrivers. It could remember positions, important in starting with tools at the right height and depth.
But radiation that can kill a human can kill a robot. Phil estimated M2 could withstand intense radiation for only 50 minutes. That might be long enough, perhaps, to free the cylinder.
The plan was to drill through the protective 3/16-inch steel plate covering the switch. By inserting a wire through the drilled hole, the switch — which rotated on a hinge pin, similar to a teeter-totter — could be nudged to change its alignment. A bomb-disposing robot might have been capable of this in simpler circumstances. But this plate was four feet off the floor, three feet away from any vertical position, and sat on a 45-degree angle. The height, depth of extension, and angle were beyond the capabilities of ordinary explosive ordnance disposal robots to drill.
Problems overcome, one after another
But when the call came, M2 was down with a faulty motor control board in its forearm. A call to DOE provided immediate funding to get it working; a replacement part was built and shipped from Agile Manufacturing in Waterloo, Ontario. Manufacturing, shipping, installation, testing, and querying White Sands took two weeks.
“Our people at first wondered what the holdup was,” says White Sands’ Williams, “and then we saw how well [the Sandia RAP team], with all their questions to us, had prepared.”
Because the robot lacked a trigger finger to depress and release a drill control, the Sandia team stalked the aisles of local hardware stores, buying cordless drills and other equipment they modified into remotely operated drills, hooks, and grippers. On tests performed at Sandia by Bob Anderson and Jim Buttz (both 6644) on a mock-up sent north from White Sands, M2 performed perfectly.
On Oct. 21, the team — including DOE team leader Gregory Sahd from WIPP, and RAP team members Richard, Phil, Bob, Jim, and Al Horvath (12345) — made the trip to White Sands, where reality — as it often does — proved more complex than the dry run had led the team to expect.
Aided by M2’s video camera, Bob steered the robot around two free-standing radiation shields and stopped it at the work site. The robot drilled through the steel plate on target, leaving room for a probe that pressed down the back side of the teeter-totter.
The switch did not budge.
The robot drilled a second hole and applied the probe, with the same negative result.
A third hole, drilled through the switch’s hinge pin, failed to dislodge it.
Using a hook, the robot attempted to pull the switch by yanking at two linked wires; the wires came apart. Grasping one of the wires in its pincer, the robot only succeeded in detaching it.
By this time an hour and a half had gone by, and the team was temporarily out of ideas. Phil had estimated that the robot could remain ambulatory in the radiation field for only 50 minutes, and in fact the robot’s lower portion was no longer responding to commands.
Wouldn’t touch it with a 10-foot pole
The RAP team, working with White Sands personnel, as a precaution against this specific circumstance, had tied a rope to M2 before sending it into the work area. The rope, attached to a RAP team winch 100 feet outside the structure, insured the robot could be hauled out if radiation damaged its drive unit. But radiation shields now blocked a direct haul. M2, powerless, was hemmed in.
The radiation field fanned out like a flashlight beam, strongest at its center and weakest at its edges. Using a ten-foot-long pole and standing at the edge of the field, the team hooked and then tugged at the rope hauling M2. The deflection of the rope’s pull slid the robot around a moveable radiation shield without knocking it over. The RAP team’s winch then pulled the robot directly out.
Rebooting the robot and performing other maintenance, Bob and Jim found they could reactivate it, and the team finished the day ready to return the next morning.
The new plan was to unscrew six bolts securing the 3/16-inch steel plate that blocked the team’s direct access to the switch.
When they returned the next morning, however, the robot again would not start. The problem was traced to a damaged fiber optic line, apparently chewed by one of the numerous rabbits in the area.
“The orange fiber optic cable probably looked like a carrot,” said Williams of the line transmitting control data to the robot. “Fortunately, White Sands has another facility that uses fiber optic lines, and it was able to repair the cable.”
But it was Sunday morning. It took half a day to replace the damaged line.
The time was not wasted. The group, making frequent trips to the local Home Depot and Lowe’s, modified its tools. Three screws required Allen wrenches to turn them; the other three screws were single-slotted. The drills needed to turn on in reverse when pushed, yet move slowly enough to engage the screw heads at the start of each effort. The robot could not control the speed via the trigger, since it had only a pincer grip, and accuracy was difficult when trying to insert a turning screwdriver into a single-slot screw head. The team purchased a small, clear acrylic bubble that acted as a see-through guide for Bob to aim the screwdriver head; unfortunately, heat from the radiation source melted the plastic.
So ended the second day.
The third day
A metal guide bought from the hardware store the next morning was opaque, but small enough in diameter to satisfactorily seat the tool on the screw.
By counting revolutions, the team could estimate whether the screw had been rotated enough times to free it from the plate. The team then tried air pressure and hook tools to remove the plate. When neither worked, they steered the robot out of the area and outfitted special tips to the end of its gripper. This time M2 succeeded. A blast of air then blew the entire switch out of the cylinder’s pathway, and the radiation source at long last was blown back to its storage position.
Inspection revealed the problem: Forceful early attempts to blow the cylinder back apparently had bent the straight switch into a right angle. Nothing other than plate removal could have freed it.
It was at White Sands that the robot came to be affectionately (and unofficially) dubbed “M2,” for the cartoon character Mighty Mouse (“Here I come to save the day”).
“It would have been impossible to return the source to storage without removal of that switch,” says Richard.
Cleanup extended for another day.
The four-day on-site effort ended the problem, to the exuberance of the RAP team and relief of White Sands personnel.
“The warning lights and horns that could be heard for miles away finally stopped after 21 straight days of annoying personnel at White Sands,” says Richard.
Says McDonald, “The facility is being evaluated. We’re looking at what happened and considering what we can do to prevent similar incidents in the future.”