Aerial view of the Spallation Neutron Source at Oak Ridge National Laboratory. (ORNL photo)
The Spallation Neutron Source restarted last week following a month-and-a-half winter maintenance period, and the operations report at the experimental facility seemed to be all positive.
“I’m pretty happy,” SNS Operations Manager Kevin Jones said Tuesday.
Last fall, after a mercury target vessel failed and required some experiments to be postponed, the SNS resumed operations in early October and reportedly produced neutrons in an efficient way through through Dec. 21, 2015, when the systems were shut down for the winter maintenance.
In fact, according to Jones, the SNS set a record for beam power in mid-November, running for 16 days with good reliability at 1.4 megawatts (the design max for the current set-up). The SNS had previously achieved 1.4 MW during a 30-hour demonstration.
In December, the power was scaled back to 1.2 MW and operations continued without issue until the maintenance outage.
The big, ongoing issue at the Spallation Neutron Source is the unpredictability of the target vessels, which have failed expectedly on several occasions over the years. Jones indicated that the SNS team is keenly sensitive to that and doing everything possible to balance risks with the high-power operations needed to produce the desired amount of neutrons for researchers.
Jones said most of the scientists who lost beam time last fall due to the target failure were later given makeup time to do their experiments.
In the meantime, when a new target vessel was installed — No. 13 since operations began in 2006 — it was equipped with new diagnostic devices to help the team understand what kind of mechanical responses may be taking place with the stainless-steel vessel when first hit with the powerful proton beam.
Jones said the diagnostic devices will only survive a short time in the highly radioactive environment, “but we were able to do a very careful and productive set of measurements with the first 70 or so well-controlled beam pulses to obtain data.” That data is proving very valuable in comparing the engineering model for the system and the actual mechanical response to the beam pulses, he said.
Before ramping up to high power in November, the system ran for weeks at a reduced power level to “break-in” the new target vessel.
The planned maintenance outage, which started in late December, “was exceptionally productive,” the SNS official said.
The work included a number of upgrades to key systems, Jones said, as well as improvements to some of the neutron-scattering research instruments.
“One key accomplishment was deployment of a technique developed here at ORNL/SNS that allows us to improve the accelerating gradient in superconducting cavities called plasma processing,” Jones said in an email statement. “We implemented this process on one cryomodule in the accelerator tunnel during the outage, and were able to significantly improve the accelerating field in that unit without having to remove it from the beam tunnel.”
As a result of those changes, the accelerator is now operating at the highest energy — not power — that it ever has during neutron production, he said.
“This is a very important accomplishment for us, and we expect that further work of this kind on other cryomodules planned for the next several outages will further increase the beam energy,” Jones said.
The SNS is currently operating with a beam power of 1.2 megawatts.
“Our operating plan for the spring is to operate at 1.2-1.3 MW through the end of April, and then to operate at 1.4 MW for the month of May,” Jones said. “We will turn off for the summer outage at the end of May and resume operation for users in mid-July.”
The power strategy is part of the overall plan to protect the target vessel while maintaining operational reliability for the researchers. If at all possible, the management team wants to avoid an unplanned outage due to the failure of the target vessel.
“It’s a balancing act,” Jones said.
There are currently two back-up pressure vessels in storage, and four more are in the manufacturing stages. At least three of those will probably be delivered to ORNL before the end of the year. Each of those vessels costs more than $1 million, so premature failure is not just an operational and research concern, but an added expense as well.
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