Spallation Neutron Source shut down prematurely; problem with newly installed target

SNS Building Marker

 ORNL photo

The Spallation Neutron Source returned to operation Aug. 15 following its annual five-week summer maintenance period, but the SNS was shut down prematurely late last week after a problem was identified with a new target vessel that had been installed during the summer outage.

“We have begun the target replacement process, and should resume operation for users within two weeks,” Ron Crone, acting assistant lab director, said via email. “Users who were scheduled during this unplanned down time have been notified; we are working to accommodate them by rescheduling their beam time.”

The exact cause of the target failure has not been determined at this point, but it could be related to a new design for the vessel. “As part of our plans for continuous improvement, we installed a target with a modified design that included a new internal mercury flow pattern intended to minimize the internal cavitation damage at higher power operation,” Crone said.

He said he didn’t want to speculate on the cause, but he said the failure may be related either to the design or the manufacturing process.

“This new design includes a removable outer water shroud that will enable us to pinpoint the exact cause more easily,” Crone said.

After restart in mid-August, the SNS was operating with a beam power of about 1.15 megawatts and “user operations were in full swing, supporting a robust science program,” the ORNL executive said.

Sustained, reliable operations at the Spallation Neutron Source are largely dependent on the length of service of the vessel holding the mercury target — where neutrons are produced in vast quantities every time the mercury is pounded by the system’s proton beam.

The SNS in recent years has had problems with the stainless-steel vessels failing prematurely, forcing up-and-down operations and — at one time — limiting the power levels in order to maintain consistency for the researchers using the neturon streams to perform material experiments. However, welding issues were identified as the problem and those reportedly were fixed with newly manufactured target vessels, and that permitted the SNS to amp up the power levels — ultimately reaching 1.4 megawatts in June of this year.

Each of the target vessels costs about a $1 million. Crone said the lab currently has three spare vessels available for backup.

Although there have been problems with the target vessels failing prematurely in the past, Crone noted that each of the previous two targets ran for more than six months, “both progressively setting new records for accumulated beam exposure.”

He added: “We had just begun the post-irradiation examination of the target we removed during the summer, and have suspended that work to replace the current target.  Once we resume operation, we will complete that examination, followed by the analysis of the most recent target to determine the exact cause of failure.  We anticipate that this process will take at least 2 months to complete and that we will learn important new information.”

The newly designed target installed during the summer outage only completed about 12 percent of its intended beam exposure, Crone said. The target that’s currrently being installed at SNS is of the old design, he said.

Unless there are any more unplanned issues, restart of the SNS is expected during the last week of September, he said. Upon restart, the plan is to operate it at 1.15 megawatts.

During the summer outage, workers performed a number of maintenance activities at the ORNL research facility. Those included two remote-handling activities to replace both the proton beam window — which separates “the beam transport vacuum from the target core vessel” — and the target itself,” Crone said.

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About Frank Munger

Senior Writer Frank Munger covers the Dept. of Energy's Oak Ridge facilities and many related topics — nuclear weapons, nuclear waste and other things nuclear, environmental cleanup and science of all sorts. Atomic City Underground is, first and foremost, a news blog, but there's room for analysis, opinion and random thoughts that have no place else to go.