Inside ORNL’s Second Target Station’s Cryogenic Moderator Design

The STS Moderator Reflector Assembly locates two liquid hydrogen moderators adjacent to the rotating tungsten target. Credit: ORNLThe Second Target Station (STS) is the future of forefront neutron scattering science at Oak Ridge National Laboratory, which started with the first neutron scattering measurements in the 1940s at the X-10 Graphite Reactor and continues today at the High Flux Isotope Reactor (HFIR) and Spallation Neutron Source (SNS). STS will be a 700 kW, 15 Hz pulsed-spallation neutron source designed to provide the world’s highest peak of brightness cold neutron beams, meeting the demand for neutron scattering resources for physical, chemical, biological, geological, materials and human health sciences. The STS will utilize one out of every four proton pulses from the SNS accelerator, delivering the 1µs pulses to a rotating water-cooled tungsten target and producing neutrons by the spallation process. To convert high energy spallation neutrons to high brightness cold neutron beams, two compact liquid hydrogen moderators are located adjacent to the target’s peak neutron production zone and surrounded by light water premoderators and beryllium reflectors to increase neutron flux. 

The STS moderator geometries were chosen to be a cylinder moderator, serving 16 neutron beamlines, and a tube moderator, featuring three tubes arranged in a triangle that will serve six beamlines. The dimensions of the moderators and reflectors have been optimized through parametric Monte Carlo neutron production simulations to maximize cold neutron brightness, resulting in a brightness gain of an order of magnitude compared to SNS. The cylinder moderator is 30 mm in height and 100 mm in diameter, while the tube moderator is 30 mm in diameter. The longest tube length is 170 mm. The spin state of the hydrogen in the moderators is critical to the STS moderator performance due to the large difference in neutron scattering cross-section for cold neutrons between ortho and parahydrogen and the large hydrogen depths of the moderators. Those cold neutrons which are moderated deep in the moderator are unable to efficiently exit the moderator if significant orthohydrogen is present due to the higher scattering cross-section. The tube moderator performance will drop by nearly 20% if the orthohydrogen content in the moderator is increased to 1% from the equilibrium orthohydrogen ratio of 0.2% at 20 K. Additionally, the neutron interactions with the hydrogen atoms drive orthohydrogen production in the moderator, resulting in higher orthohydrogen ratio than equilibrium at the operating temperature. This back conversion process is slow, but so is the conversion to equilibrium hydrogen without the presence of an ortho para converter. At SNS, where no ortho para converter is installed, the steady state orthohydrogen content is estimated to be 30% due to back conversion in the moderators, which would result in catastrophic performance losses to STS moderators.

The Cryogenic Moderator System (CMS) will supply the two STS moderators with 20 K or less hydrogen and a parahydrogen fraction of 99.8% or greater.  The two moderators will be connected in series in a single hydrogen loop cooled by a helium refrigerator with a cooling capacity of approximately 2.25 kW at 17 K. Because of the neutron production and associated radiological hazards in the target monolith, most of the CMS will be located in an adjacent building while hydrogen is supplied to the moderators via 40-meter-long transfer lines. The hydrogen loop, based on the CMS design of the SNS, will consist of a hydrogen circulator, hydrogen helium heat exchanger, ortho paraconverter, accumulator and heater. The hydrogen loop will have a constant flow rate of 0.5 L/s and remove a nuclear heat load of 860 W from the two moderators, which is deposited both directly in the hydrogen and the adjacent moderator structures. Because the nuclear heat load is accelerator-driven, the hydrogen system must remain stable when the heat load is removed instantaneously during beam trips. The hydrogen loop will operate at a constant hydrogen mass after cooldown with the heater maintaining temperature and the accumulator passively mitigating pressure excursions from temperature variations. General hydrogen temperature control is provided by controlling the flow rate of helium to the heat exchanger. The ortho para converter is being designed to guarantee maximum ortho hydrogen content of 0.2% and is located immediately downstream of the heat exchanger to take advantage of the lower equilibrium ortho hydrogen content at lower temperatures. STS CMS is in the early stage of preliminary design, and the current focus is evaluating component sizing and system stability during beam transients. 

SNS and HFIR are Department of Energy Office of Science user facilities.

Image: The STS Moderator Reflector Assembly locates two liquid hydrogen moderators adjacent to the rotating tungsten target. Credit: ORNL

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