The Spallation Neutron Source (SNS) is currently in its third outage period, and the transitions between operations and maintenance continue to become smoother. The work during this six-week outage includes repairs to one or two cryomodules (depending on what is found during diagnostic tests) and a rather large effort pulling cables. As has been true in the past, we have little or no history at performing these jobs and pre-outage dose estimates are difficult (local dose rates, man-hours, and exact locations of the work area are all uncertain for different reasons). For purposes of maintaining doses as low as reasonably achievable (ALARA), most of the higher-dose jobs have been scheduled late in the outage to take advantage of decay - this also allows more time for planning based on measured (rather than projected) local dose rates. In most cases, the workers have figured out how to plan the work with predicted doses that are low enough so that formal ALARA reviews are not required (or sensible). We consider worker-initiated ALARA improvements in the job-planning process to be ideal since they know the work best - fortunately SNS is blessed with very experienced and dose-conscious workers.
SNS will eventually operate at almost ten times the power of the previously most powerful pulsed neutron source. What I did not realize earlier is that we would take over the record for high-power pulsed neutron operation while operating at only 15% of the initial planned full power! More important than the power record was the improved reliability that allowed initial user experiments to go pretty smoothly during the last run period. One additional instrument was commissioned right at the end of the run period, bringing the total number of active instruments to four. Five more instruments are scheduled for commissioning during the next year, so operational reviews will be a major activity during the next year. The reviews include final shielding design approvals (and confirmation of proper installation), fault study plan reviews, and numerous additional safety reviews. Radiation control technicians (RCTs) survey the beamline shielding of new instruments and surrounding areas during initial operation and then again while a worst-case sample is exposed to beam. Surveys are most important since theoretical models don't always include the effects of minor variations in shielding effectiveness, and almost never predict the effects of the inevitable cracks between shielding blocks. To date, the improvements indicated by these surveys have been simple and minor, and the Monte Carlo (MCNPX) model calculations have generally been modestly conservative.