Speaker
Description
The Second Target Station (STS) at Oak Ridge National Laboratory will be a 700 kW, 15 Hz pulsed spallation neutron source designed to provide the world’s highest brightness cold neutron beams to 18 beamlines. In order to produce the required neutron beam performance, two compact liquid hydrogen moderators are located above and below the tungsten spallation target within the Moderator Reflector Assembly (MRA). The lower tube moderator is a novel triangular 3 tube arrangement feeding 6 beamlines while the upper cylinder moderator feeds the remaining 12 beamlines. The characteristic moderator dimensions, the tube diameter and the cylinder height, were determined to be 30 mm in order to maximize neutron brightness while providing appropriate sample illumination. The hydrogen vessels are surrounded by an insulating vacuum layer to reduce the heat load to the cryogenic moderator system (CMS), which supplies the moderators with less than 20 K hydrogen. Except for the neutron extraction ports and hydrogen supply and return lines, the moderators are surrounded by a light water premoderator and then a beryllium reflector. The dimensions of the moderators, premoderators, and reflectors were determined by a parametric neutronics optimization which included engineering feedback to provide geometries with appropriate vessel wall thicknesses. The moderator and reflector vessel designs provide robust implementation of the neutronics optimization geometry which minimize aluminum welding, sometimes at the expense of complicated machining. The reflector vessels are held in place around the target by a water cooled stainless steel backbone. An upper shield block, water piping, and hydrogen transfer lines are added to the backbone to complete the MRA. The neutronic performance is dependent on the alignment of the moderators to the neutron guide entrances, thus the MRA is designed to be remotely replaced in a precise location using kinematic mounts. Prototypes of the most critical MRA hardware, the upper and lower hydrogen vessels and vacuum vessels, have been manufactured successfully, demonstrating feasibility and providing opportunities for design improvement.
