Speaker
Description
The Second Target Station (STS) at Oak Ridge National Laboratory will be a 700 kW pulsed spallation neutron source optimized to deliver 18 high brightness cold neutron beams. To supply the optimum neutron performance, two compact liquid hydrogen moderators are located in the peak neutron production zones, immediately above and below the rotating tungsten spallation target, resulting in a nuclear heat load of over 400 W in each moderator. To simplify the Cryogenic Moderator System (CMS), the hydrogen loop will supply the two moderators in series with less than 20 K and greater than 99.8% para hydrogen fraction hydrogen at a constant flowrate of 0.5 L/s. A total nuclear heat load of up to 850 W will be removed while providing required hydrogen temperature, density, and spin state to maximize neutron performance. The hydrogen loop consists of a hydrogen circulator, a helium hydrogen heat exchanger, ortho-para converter, accumulator, transfer lines, and the two moderators. The hydrogen loop will operate with a minimum pressure of 14 bara, just above the critical pressure, in order to reduce instabilities during system cool down and to provide margin against loss of density in recirculation zones in the moderators. The accumulator, which consists of a gaseous helium volume surrounded by the flowing hydrogen but separated by a bellows, allows thermal and pressure equilibrium between the hydrogen and helium volume. The coupling of the hydrogen and the helium volume allows the hydrogen loop to run in a constant pressure mode, where volumetric changes of the liquid hydrogen driven by beam transients are compensated by small changes to the hydrogen and corresponding helium volume temperature in the accumulator, providing system stability through beam transients which can rapidly vary the nuclear heat load. The ortho-para converter counteracts the radiation driven back-conversion of para to ortho hydrogen in the moderators by driving the moderator supply parahydrogen concentration to near equilibrium as required for optimal neutron performance. Both the required performance of the ortho-para converter and the ortho-para diagnostics, monitoring the parahydrogen fraction, have been demonstrated as part of the recent upgrades provided by the Proton Power Upgrade at the Spallation Neutron Source. The STS CMS design is maturing rapidly, and system requirements are being verified in order to demonstrate system performance.
