Spallation Neutron Source shut down to replace target vessel, but the news isn’t all bad

SNS Building MarkerThe Spallation Neutron Source is one of Oak Ridge National Laboratory’s signature research facilities.

The Spallation Neutron Source is shut down for the next few weeks while workers replace the research facility’s target vessel, which failed on Friday after a record-setting run of success. As many are familiar, the sturdiness of the stainless-steel vessel — which holds the 20 tons of mercury that’s a target hit by a proton beam many times a second to produce neutrons needed for experiments — has been a major issue and interfered with Oak Ridge National Laboratory’s ability to maximize research capabilities at the SNS.

The bad news, of course, is that the target vessel failed, requiring that operations be brought to a halt while the vessel is replaced. That means that a whole bunch of active experiments had to be put on hold, and they will be rescheduled for some point in the future. However, the SNS management team had planned to let the existing vessel operate to failure, so it wasn’t a total surprise. And, in the process, Vessel No. 12 — the twelfth in SNS history — established a few operational records for longevity, etc.

“We’ve just set several records with Target 12, which was installed in November 2014,” SNS operations manager Kevin Jones said via email. “We ran quite well since resuming operation in mid-August (following a summer maintenance period), with a beam power to target of just over 1.3 MW.  Last week we set a new one-week record for energy delivered, for the first time delivering more than 200 MW-Hrs to target (205.3 MW-Hr) for neutron production over 7 consecutive days.  Also on Sept. 16, 2015 Target 12 passed the previous exposure record of 4,195 MW-Hr set by Target 9, which was in use from October 2013 through June 2014.”

In other words, despite all the problems that the SNS has had with premature failures of target vessels, the recent experience was pretty good.

According to Jones, Target No. 12 reached its “end of life” on Friday, when a leak of mercury was detected in the sealed area between the target vessel and its protective water shroud.

“This is the normal scenario that we have experienced a number of times before,” Jones said. “We have begun the process of replacing the target and expect to resume operation in mid-October.

Jones said workers will install a new target with “instrumentation that could help us understand the mechanical response of the mercury vessel to the beam during the first few hours of operation.”

He added: ” We are looking forward to learning more about our targets with this new capability.”

Scientists and engineers have been perplexed by some of the premature vessel failures, and one line of thought has been that high-power operations during the early run with a new target vessel may have caused material damage leading to failures. That, however, has not been proven. Nonetheless, the SNS management team has opted to begin operations at lower power — with some success — to possibly season the vessel before ramping up the power to maximum levels.

Jones said the plan with the next restart — after installing Target Vessel No. 13 — will be to operate with beam power at about 850 kilowatts for about three weeks and then crank it up to about 1.3 megawatts.

ORNL’s user group is working to reschedule times for researchers to conduct their experiments at the Spallation Neutron Source.

“About 95 experiments involving up to 400 users will be affected (by the shutdown),” Jones said, “and we are working to accommodate these experiments as we move forward into the new fiscal year.”

Fiscal 2016 begins on Oct. 1.

Jones said restart of the SNS is expected sometime between Oct. 14 and Oct. 19.

He said the science research at the Oak Ridge facility has been ongoing and good, with the last week before shutdown having a reliability rating of 98.4 percent.

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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.