Strömungsmechanische Simulation und experimentelle Validierung des kryogenen Wasserstoff-Moderators für die Europäische Spallationsneutronenquelle ESS

The European spallation neutron source ESS is currently under construction and should start part-load operation in 2023. With an average proton beam power of 5 MW, it will become the most powerful spallation neutron source worldwide. A key component of a spallation neutron source is the cold moderator. At the ESS, the cold moderator will be operated with liquid parahydrogen at a temperature and pressure around 20 K and 10 bar respectively and is intended to slow down (moderate) the fast neutrons, released by the spallation process, to the required low velocity level. Latest particle-transport-simulations show that the neutron yield can be increased by up to 30 % by optimizing the existing cold moderator. The present dissertation therefore examines the technical feasibility of this new moderator for full-load operation of the European spallation neutron source ESS. The primary goal is to verify whether the cold moderator can be operated at full proton beam power or up to which beam power a safe operation is possible. In addition, the feasibility from the structural mechanical and manufacturing point of view will be assessed. In order to investigate the flow behavior in the cold moderator, a numerical flow simulation was first carried out. The flow guiding has been optimized for the best possible heat transfer because the pulsed proton beam causes an enormous fluctuation in thermal load. Furthermore, sources of errors of the simulation were identified and minimized. For this purpose, the model error of the flow simulation was determined by particle image velocimetry (PIV) comparison measurements. As part of the parameter studies, it turned out that the cold moderator can only be safely operated up to a proton beam power of approx. 3.4 MW under the given requirements and with a conservative consideration of all errors. Therefore, a several additional options were shown, by which the proton beam power might be significantly increased, and the goal of 5 MW would still be possible. The structural mechanical part of this work, in which the cold moderator was designed according to the nuclear code RCC-MRx, showed that the pressure vessel withstands all static and dynamic loads. Thereby the radiation as well as all loads in normal and abnormal operation were considered. Finally, an initial prototype of the optimized cold moderator has been manufactured and tested. The joining technology for the selected aluminum alloy AW 6061-T6 was of special importance, since this alloy is generally difficult to weld. Electron beam welding was used because it leads to lowest possible distortions and minimized local heat input. Finally, non-destructive tests were carried out to confirm the high quality of the manufacturing, and thus the suitability of the cold moderator for a safe operation under the extreme operating conditions

Cryogenic hydrogen Moderator infrastructure at ESS

The European Spallation Source (ESS) in Lund, Sweden, is designed to become the most powerful spallation neutron source in theworld. As one subsystem of the Target Station, which was developed and built at Central Institute of Engineering, Electronics and Analytics –Engineering and Technology (ZEA-1) of Forschungszentrum Juelich, the cold Moderator slows down high-energy neutrons from the spallationprocess. To gain maximum neutron flux intensities along with high system availability for condensed and soft matter research, an optimizedliquid hydrogen Moderator circuit has been developed. Hydrogen with a pressure around 1 MPa, a temperature around 20 K, and a para-hydrogen fraction of at least 0.995 will be utilized to interact with neutrons in a unique cold Moderator vessel arrangement. Hydrogen conversionfrom ortho- to para-hydrogen will be controlled using a catalyst. Two turbo pumps are arranged in series and circulate the cryogen. A heliumrefrigerator, the Target Moderator Cryoplant (TMCP), continuously recools the hydrogen mass flow. Pressure stabilization is achieved by apressure control buffer. The individual ESS Cryogenic Moderator System (CMS) components, the first and second generation of hydrogenModerators (BF1 and BF2) and a first draft of a deuterium Moderator upgrade are presente

Cold moderators for the High Brilliance Neutron Source

Long-wavelength neutrons for the investigation of nano-scale materials are an indispensable tool in neutronresearch. With the decommissioning of several European nuclear research reactors in recent times compactaccelerator-driven neutron sources (CANS) are of interest in providing scientists with the necessary capacityof neutrons to conduct experiments.At the High Brilliance Neutron Source (HBS) project, multiple cold moderators will be positioned inside thesame Target-Moderator-Reflector unit (TMR), each providing its own instrument with cold or even verycold neutrons. All of these moderators can therefore be optimized in terms of material, operating temperatureand geometry, depending on the requirements of the instrument.In a first approach, two cryogenic moderator systems for a prototype TMR have been designed and arecurrently being manufactured at Forschungszentrum Jülich. While one is a closed-cycle liquid parahydrogensystem, the other one allows the batch-wise production of solid moderators, e. g. frozen methane. Bothmoderators are positioned as close to the target as possible by using so-called moderator plugs (MPs). Theseconsist of a vacuum-insulated cryostat with a detachable fluid transfer and moderator section, a neutronguide and surrounding radiation shielding.The planned operation of these cryogenic moderator prototypes from summer 2022 will enable theexperimental investigation of different cold moderator geometries, as well as various options for thesurrounding thermal moderator and reflector. The obtained results can then be used to validate andcomplement nuclear simulations, proof efficient operation and will allow more reliable future designs ofsuch cold neutron sources

Cryostat for the provision of liquid hydrogen with a variable ortho-para ratio for a low-dimensional cold neutron moderator

A significant contribution to the enhancement of the neutron brilliance achievable with Compact Accelerator-driven Neutron Sources (CANS) can be made by an optimized cold moderator design. When using liquid para-H2 as the moderating medium, the concept of low-dimensional cold moderators can be employed to increase the neutron brightness (as currently foreseen at the European Spallation Source ESS). Para-H2 shows a drop in the scattering cross section by about one order of magnitude around 15 meV, resulting in a large deviation between the mean free paths of thermal and cold neutrons. Taking advantage of this effect, the cold moderator geometry can be optimized to allow the intake of thermal neutrons through a relatively large envelope surface and then extracting them in an efficient way towards the neutron guides. One drawback of this solution is the lack of thermalization of the cold neutrons. In the context of the HBS (High Brilliance Neutron Source) project, efforts are made to overcome this problem by increasing the scattering cross section of the H2 in a defined way. The idea is to admix small amounts of ortho-H2, which maintains its large scattering cross section in the region below 15 meV. Like this, the neutron spectrum can be shifted towards lower energies and adjusted for the needs of the respective instruments. In a cooperation between TU Dresden and FZ Jülich, an experimental setup has been created to prove the feasibility of this concept. The main component of the experimental setup is a LHe-cooled flow cryostat that enables the separate condensation of a para-H2 and a normal-H2 flow and a subsequent mixing of the two in precise proportions. The resulting LH2 mixture at 17 - 20 K is fed into a small cold moderator vessel (approx. 200 ml). In this work, the current status of the setup is presented. The construction and commissioning of the mixing cryostat have been completed and first test runs show that different ortho-para-H2 mixtures can be produced. In the near future, the system will be ready for measurements at a neutron source

Cryostat for the provision of liquid hydrogen with a variable ortho-para ratio for a low-dimensional cold neutron moderator

A significant contribution to the enhancement of the neutron brilliance achievable with Compact Accelerator-driven Neutron Sources (CANS) can be made by an optimized cold moderator design. When using liquid para-H2 as the moderating medium, the concept of low-dimensional cold moderators can be employed to increase the neutron brightness (as currently foreseen at the European Spallation Source ESS). Para-H2 shows a drop in the scattering cross section by about one order of magnitude around 15 meV, resulting in a large deviation between the mean free paths of thermal and cold neutrons. Taking advantage of this effect, the cold moderator geometry can be optimized to allow the intake of thermal neutrons through a relatively large envelope surface and then extracting them in an efficient way towards the neutron guides. One drawback of this solution is the lack of thermalization of the cold neutrons. In the context of the HBS (High Brilliance Neutron Source) project, efforts are made to overcome this problem by increasing the scattering cross section of the H2 in a defined way. The idea is to admix small amounts of ortho-H2, which maintains its large scattering cross section in the region below 15 meV. Like this, the neutron spectrum can be shifted towards lower energies and adjusted for the needs of the respective instruments. In a cooperation between TU Dresden and FZ Jülich, an experimental setup has been created to prove the feasibility of this concept. The main component of the experimental setup is a LHe-cooled flow cryostat that enables the separate condensation of a para-H2 and a normal-H2 flow and a subsequent mixing of the two in precise proportions. The resulting LH2 mixture at 17 - 20 K is fed into a small cold moderator vessel (approx. 200 ml). In this work, the current status of the setup is presented. The construction and commissioning of the mixing cryostat have been completed and first test runs show that different ortho-para-H2 mixtures can be produced. In the near future, the system will be ready for measurements at a neutron source

Parametric study and design improvements for the target of NOVA ERA

The results of a parametric study are presented which was conducted in the framework of the High Brilliance Neutron Source Project (HBS), in order to optimise the target dimensions for a Compact Accelerator driven Neutron Source (CANS). A thin disc shaped target cooled by a water jet was taken as design reference, which was recently published in the Conceptual Design Report for NOVA ERA (Neutrons Obtained Via Accelerator for Education and Research Activities).For a given target thickness, limited by the ion range in the target material, the cooling fluid pressure and the heat deposition of the ion beam, an optimal diameter of the target disc can be found, for which the occurring stresses are minimised. With the accelerator parameters of NOVA ERA (10 MeV protons with an average power of 400 W on the Target) and with the results of the parametric study, it was possible to design a target, where the occurring stresses are by a factor 3 smaller than the yield strength of the employed beryllium alloy, S-65C VHP

JULIC Neutron Platform, a testbed for HBS

The High-Brilliance neutron Source (HBS) project [1] develops a High-Current Accelerator-driven Neutron Source (HiCANS) with a pulsed proton beam, a peak current of 100 mA and an average power at the target of 100 kW. The concept of such a HiCANS was published some years ago [2] indicating the feasibility of such a facility with all of its components: high-current accelerator, target station with integrated moderator-reflector assemblies and neutron instruments. All components require engineering development and testing. The JULIC Neutron Platform was thus developed as a testbed for all components and the investigation of their interplay.The JULIC Neutron Platform uses a cyclotron providing a tunable pulsed proton beam with a low current but a variable frequency and pulse length to a spacious experimental area. A target station shielding is placed in its center with an empty inner core of 1 m3, able to accommodate different moderator-reflector assemblies as well as cryogenic moderators. The target station uses a tantalum target for the conversion of protons to neutrons and has eight spacious flexible ducts where moderator plugs for neutron extraction or blind plugs are placed.First beam on target was achieved in December 2022 with three instruments in operation: reflectometer, diffractometer and detector test stand. Further beamtime in 2023 is planned in order to investigate different cryogenic moderators, to estimate the performance of such a HiCANS and to perform further experiments.At UCANS, we will present the JULIC Neutron Platform, the experiments performed and the possibilities it offers.[1] P. Zakalek, et al, J. Phys.: Conf. Ser., 1401, 012010 (2020)[2] T. Brückel, et al. Conceptual Design Report Jülich High Brilliance Neutron Source (HBS), Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich (2020