The photo above shows the control room in the service area of the Spallation Neutron Source’s Target Facility, where cameras track what’s happening in the remotely operated hot cells. In the photo below, SNS staffer David Dunning wipes away excess lubricant on Target Vessel No. 12, which is being installed this weekend as preparations are made for the restart of operations shortly before Thanksgiving.
These are tough times at the Spallation Neutron Source, which hasn’t produced neutrons — its reason for being — on a regular basis since summer.
The shutdown has brought important experiments, exploring the very essence of materials and how they behave and interact with other materials, to a standstill.
But scientists and engineers are problems-solvers at heart, and the mood was surprisingly upbeat on Thursday as they talked about what’s gone wrong and how they hope to fix it.
“This will get worked out. This isn’t an engineering problem that can’t be solved,” Ron Crone, Oak Ridge National Laboratory’s acting associate lab director for neutron sciences, said during a tour of the research facilities.
Crone was talking about ongoing issues with the system’s target vessel, a stainless-steel container that holds and circulates about 20 tons of mercury.
The vessel is a critical component. When the SNS is operating, the nose of the target is struck about 60 times a second — more than 5 million times a day — by a high-energy proton beam that knocks the neutrons out of the mercury (the process is called “spalling,” thus the naming of the Spallation Neutron Source).
“There’s a lot of stress on this module,” said Kevin Jones, who heads the Oak Ridge lab’s Research Accelerator Division and oversees operations at SNS. “It wobbles around and expands and contracts.”
The vessel obviously has to be sturdy to withstand such a pounding, but recently there have been a couple of premature failures. One of the vessels lasted only a week before developing a leak. Ideally, the SNS team would like to change vessels only once, or maybe twice, a year. When a vessel fails, not only does it take time to replace it — shutting down operations and delaying research experiments — but it’s also costly. Each one costs more than $1 million. And, after an old vessel is removed from its target position, it is hotly radioactive and must be stored in the service bay for a while to cool down before it is shipped (inside a heavily shielded cask, which looks like a giant dumbbell) to the Nevada National Security Site for disposal.
A team of experts, including material scientists and computer simulation specialists, are on the team that’s studying the problem — or problems — with the target vessels.
The vessel failures of recent times are strikingly similar to a period in 2012, when back-to-back failures also prompted a comprehensive investigation. At that time, the team identified a welding issue during the manufacturing process as the culprit, creating a vulnerable area inside the vessel that was prone to leaks. That problem was fixed. Changes were made in the manufacturing of the vessel, and rigorous quality-assurance checks were instituted before a new vessel was installed.
That seemed to work fine for 2013 and much of this year, and there were a couple of long and successful runs. But soon after the SNS returned to operations following its summer maintenance outage in early September, sensors inside the target vessel — this one incorporating a new design that was supposed to alter the flow of mercury and, ironically, extend the life of the vessel — gave signals that a breach was occurring and failure was near.
That was followed by another problem, a water leak that damaged a section of the linear accelerator, and then the most recent failure of the target vessel, which was of the old design.
Jones admitted that the pattern of events was puzzling. Nobody knows for sure what’s causing some of the target vessels to have such a short life. But they’re working hard, six days a week, to find out.
On Thursday, a team from the lab used a video boroscope to take its first look inside Target No. 11 — the most recent one to fail — and were able to quickly pinpoint the area of failure. It was the same place where the vessels failed back in 2012. But that still doesn’t answer the question of why it failed.
Crone said he doesn’t think it’s a manufacturing problem.
“I feel confident that we’re building these exactly like they’re designed,” he said. “After this last failure we’re looking harder at how we’re operating, how we’re ramping up the power, the support systems, the cooling systems. We’re stepping back and doing a comprehensive view on what could be impacting the lifetime of these things.”
One thing that’s been noticed is that the vessels that have lasted the longest have been those in place when the SNS was operated for a while at a lower beam power before ramping up to 1 megawatt or all the way up to the maximum design power of 1.4 megawatts.
Jones said the current plan is to restart the Spallation Neutron Source a few days before Thanksgiving, and he said it will be operated at 850 kilowatts. That decision was made, in part, to help extend the life of the vessel because the SNS only has one spare target in reserve and another new one won’t arrive from the manufacturer — Metalex in Cincinnati — until April 2015.
Jones said the SNS team will reevaluate the power level after another backup vessel is available.
The team would prefer to operate SNS at its full power because that produces more neutrons and maximizes the research capabilities.
Despite all the problems this fall, Crone and Jones emphasized that the SNS remains a very successful research facility that’s much in demand from scientists across the United States as well as foreign countries.
“The demand for neutrons is high,” Crone said, noting that SNS had a record number of users last year and recently received a record number of proposals from researchers wanting to get future beam time at SNS to perform their material experiments.
Even during the shutdown periods, the SNS staff at Oak Ridge has been very busy, according to lab officials. “They’re working on scientific publications, they’re helping users analyze data,” Crone said. “Nobody is sitting around.”
Because of the unexpected shutdowns this fall, access time for about 250 experiments had to be rescheduled. That puts even more pressure on the SNS management team to get the facility up and running reliably.
Researchers use the penetrating streams of neutrons like a camera, gaining incredible understandings of how materials are put together and how they function, and Crone said the facility is producing science results with an impact.
Among the recent projects of note: using neutron imaging to study hydraulic fracturing of shale gas, a growing energy source in the U.S. and elsewhere; new insights on how drugs can bind to their targeted biological tissues, studies that could ultimately be applied to the fight against cancer; and using the neutron imaging capabilities to better understand how some unusual materials can spontaneously change from metals to insulators, which could lead to the future production of ultra-ultra-fast computer chips.
“With some discoveries it may take two or three years before people realize their impact,” Crone said. “It’s exciting times, no doubt.”
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