Los Alamos Neutron Science Center researchers are collaborating with colleagues from Oak Ridge National Laboratory to study and mitigate target erosion problems that could affect the success of Oak Ridge’s Spallation Neutron Source.
“It’s a very nice example of labs working together on an experiment,” said Steve Wender of Neutron and Nuclear Science (LANSCE-NS).
Oak Ridge scientist Bernie Riemer agreed. “This is a wonderful collaboration,” he said.
The SNS is the world’s most powerful pulsed accelerator-based neutron source for materials and biological research. One of the greatest collaborative efforts in U.S scientific history involving six Department of Energy national laboratories -- Los Alamos, Argonne, Brookhaven, Jefferson, Lawrence Berkeley, and Oak Ridge -- the project accelerates a proton beam at more than 1 megawatt to produce intense pulsed neutron beams for scientific research, Wender said.
“The SNS accelerator-storage ring concept was originally developed at LANSCE and is still operational here,” Wender explained. “The LANSCE accelerator produces approximately 100 kilowatts of proton beam power directed onto a solid target made of tungsten and delivered to LANSCE’s Lujan Center. As Oak Ridge researchers developed plans to increase the SNS beam power to more than 1 megawatt, they recognized that the greater power would lead to cooling problems with a solid target." To get around this problem, they proposed and built a flowing liquid metal target to remove the deposited heat, he said.
ORNL researchers decided to substitute tungsten with pure mercury because of the metallic element’s high density, large atomic number, and other thermodynamic properties, Wender said. While a mercury target has several advantages over a solid target, the short pulse (less than 1 micro-second) of the proton beam bombarding the mercury may lead to a significant problem for the containment vessel.
“A mercury target can handle the increased power, but pulses are a concern,” Wender said. “During previous Weapons Neutron Research (WNR) experiments, Oak Ridge scientists found that when the pulse of the beam hits the mercury, cavitation bubbles are created. When these occur near the surface of the vessel, they collapse, gouge into the container material, and erode its surface.”
Damage caused by cavitation erosion decreases the lifetime of the target container and may require more frequent exchanges of the container, Wender said. This becomes more of a concern as operation of the SNS approaches its full power potential.
Oak Ridge researchers decided to come to LANSCE for several reasons: the Los Alamos pulsed proton beam has an energy density similar to the SNS beam; LANSCE/WNR possesses a unique facility where the required test apparatus can be set up and operated in the beam; and running the experiment at Oak Ridge would have significantly impacted the facility’s neutron science user program, Riemer said.
“We couldn’t have done this at Oak Ridge. LANSCE was a great place for us to come,” he said.
The researcher added that the experiment had been in the works for a long time. “It’s been a tremendous effort for both Oak Ridge and LANSCE,” said Riemer, the SNS mercury target development team leader. “It’s taken the better part of a year to prepare.”
Initial tests conducted at LANSCE confirmed cavitation damage to the mercury target. The present set of experiments seeks to study cavitation as well as the consequences of target geometry, fluid flow, and energy density on the cavitation rate to determine the magnitude of the effects and to develop techniques to mitigate the problem.
In addition to researchers from Oak Ridge, many other scientists participated in the measurements, including Günter Bauer, considered the “father of the mercury target,” and scientists from the University of Maryland, Boston University, Japan, and the proposed European spallation project. Laboratory researchers included Leo Bitteker, who was in charge of working with ORNL; technicians Gregg Chaparro and Art Bridge of LANSCE; radiological control technicians Paul Ortega, David Romero, Scott Fulton, and Gilbert Coriz of Health Physics Operations (RP-1); Joe Price and George Evans of Industrial Hygiene and Safety (IHS); and end of experiment survey crew, Markilee Martinez and Crystal Gallegos of RP-1.
