Spallation Neutron Source Runs Into More Problems
NS&D Monitor
10/31/2014
Problems keep mounting at the Spallation Neutron Source, with yet another target-vessel failure and lost research time at one of the Department of Energy’s top research facilities. On Oct. 27, just a week after the SNS was restarted following a couple of other significant problems, the systems were brought down again because of the apparent failure inside the stainless-steel vessel that holds the mercury target where neutrons are produced as it’s repeatedly struck by a power-packed proton team. Kevin Jones, the director of Oak Ridge National Laboratory’s Research Accelerator Division, said the SNS would be shut down for at least another two-and-a-half weeks, extending one of the most difficult periods in the facility’s eight-year history. “It’s a big source of frustration for everybody,” Jones said. “We’ve just got to work through it, solve the problem and move on. That’s what we’ve got to do.”
Jones compared this situation to a period in 2012 when the SNS endured back-to-back premature failures of the target vessel. On the morning of Oct. 27, sensors inside the stainless-steel target vessel—which holds and circulates 20 tons of mercury—indicated there was a potential mercury leak into the “interstitial region” between the mercury containment and the water shroud, Jones said. Those sensors are typically how the lab learned of an impending failure of the vessel after a lengthy run.
‘We’re Pretty Puzzled’
This time, however, the SNS had only been running for a short time, and there had been plenty of checks before restarting the operations systems. “That’s why we’re pretty puzzled,” Jones said. After the sensor alert, the SNS was shut down as required for inspection and the upcoming change of the target vessel. “All systems functioned as designed and there was no risk to personnel or the environment,” Jones said.
The string of problems at SNS began in early September, only a few weeks after operations had resumed following a summer outage for maintenance and upgrades. The first issue was the premature failure of the target vessel that forced the shutdown. Workers quickly worked to replace the vessel, but even before the SNS could be restarted another big issue came to management’s attention. This time it was a water leak in a section of the facility’s linear accelerator, which required extensive repairs and refurbishments. That kept the SNS out of operation until Oct. 19.
After that, the SNS operated for only 167 megawatt hours—the second shortest power time in the facility’s history. The previous shutdowns forced the ORNL team to postpone and reschedule about 150 experiment, and that number will now grow. Scientists with experiments scheduled for the next couple of weeks have already been alerted, and the research community in general is watching what happens next at the SNS. Scientists come to SNS from around the globe to use the unique source of neutrons for experiments that explore the essence of materials, including the structure and properties.
Investigation Team to Examine Failures
Jones said ORNL will appoint an investigation team that includes experts from other divisions at the Oak Ridge lab as well as specialists from other sites outside Oak Ridge. The restart is going to be exacerbated by logistical issues. As target vessels are removed from the operating position, the steel vessels are highly radioactive and must be kept in a shielded cell for a while to cool. That also allows time for evaluation. Currently, three formerly used vessels (Nos. 8, 9, and 10) are taking up all the room, so one of them must be shipped off-site for disposal before there will be room to take out the newly failed vessel (No. 11) and install No. 12.
Jones said it is hoped that vessel No. 8 can soon be shipped to the Nevada National Security Site. One concern is that once vessel No. 12 is installed at SNS, there will only be one spare available. The next one being manufactured isn’t due to arrive until April 15, and that could force the team to be more cautious and operate the SNS at reduced power levels to try to preserve the integrity of the target vessel for a longer time. The SNS had been operating with a beam power of about 1.1 megawatts. Jones said a decision hasn’t been made on future power levels at the Spallation Neutron Source.
