Performing diagnostics on the spallation neutron source vision beam line to eliminate high vibration levels and provide sustainable operation

The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) provides a range of neutron energies for a variety of experiments. The neutrons proceed down beam lines in the experiment hall, which houses a variety of experiments and test instruments. Each beam line has one or more neutron choppers which filter the neutron beam based on neutron energy by using a rotating neutron absorbing material passing through the neutron beam. Excessive vibration of the Vision beam line, believed to be caused by the T0 chopper, prevented the Vision beam line from operating at full capacity. This problem was expected to be specific to the installation at the Vision instrument and not the T0 chopper itself. Vibrations of the T0 chopper were 3 times higher installed at Vision than they were when the T0 chopper was installed in the chopper shop. To determine the cause of the high vibration, dynamic testing was performed. Twenty-seven channels of data, both accelerometers and motor current, were collected during drive up, drive down, coast down, and steady-state conditions; resonance testing and motor current signature analysis were also performed. The data was analyzed for traditional mechanical/machinery issues such as misalignment and imbalance using time series analysis, frequency domain analysis, and operating deflection shape analysis. The analysis showed that the chopper base plate was experiencing an amplified response to the excitation provided by the T0 neutron chopper. The amplified response was diagnosed to be caused by higher than expected base plate flexibility, possibly due to improper grouting or loose floor anchors. Based on this diagnosis, a decision was made to dismantle the beam line chopper and remount the base plate. Neutron activation of the beam line components make modifications to the beam line especially expensive and time consuming due to the radiation handling requirements, so this decision had significant financial and schedule implications. It was found that the base plate was indeed loose because of improper grouting during its initial installation. The base plate was modified by splitting it into multiple sections, isolating the T0 chopper from the rest of the beam line, and each section was then reinstalled and re-grouted. After these modifications, the vibration levels were reduced by a factor of 30. The reduction in vibration level was sufficient to allow the Vision beam line to operate at full capacity for the first time since its completed construction date.