Speaker
Description
The hydrogen moderator design for MW-class pulsed neutron source is undertaken for the next generation of high-intensity neutron sources with high brightness, such as European Spallation Source (ESS) and Oak Ridge National Laboratory Second Target Station Project. Such a design is chosen based on the fact that the performance of a liquid hydrogen moderator operating at around 20 K depends on the spin states of the diatomic molecule, the ortho and para-states. This is because the total-neutron-scattering cross-section of the parahydrogen state is lower than the ortho hydrogen state by two orders of magnitude for the neutron energies below 14.5 meV. It is, therefore, important to keep the parahydrogen fraction close to 99.5% in liquid hydrogen moderators, which achieves thermal equilibrium at 22.7 K. At the ESS neutron source, the moderator system consists of a water pre-moderator and two liquid hydrogen cold moderators. Two flat butterfly-shaped cold moderator vessels have been designed and optimized to achieve maximum brightness of the neutron beams for scientific experiments, under the requirement of parahydrogen fraction to be higher than 99.5%.
A cryogenic moderator system (CMS) has been designed to continuously supply subcooled liquid hydrogen with a temperature of 17 K with a parahydrogen concentration of more than 99.5% and to remove huge nuclear heating of 6.7 kW across the two moderators. The CMS is equipped with an ortho-parahydrogen catalyst (IONEX Type OP), primarily composed of Fe2O3, to ensure the desired parahydrogen fraction of above 99.5% in the cold moderators. The catalyst bed is designed for the conversion efficiency of 0.8 at a space velocity (SV) of 1800 min-1. The catalyst needs to be regenerated by a hot dry nitrogen purge to eliminate absorbed water. Additionally, the ortho-to-parahydrogen fractions at the inlet and outlet of the moderators will be measured using an in-situ measurement system using a Raman spectroscopy.
In this study, the IONEX Type OP catalyst with a volume of 13.7 ml (19.03 g) was regenerated at temperatures, T_R, of 24 and 100 ℃ by a heater. A dry nitrogen flowed through the catalyst tube with an inner diameter of 10.2 mm at a flow rate of 3.5 liter/min. Normal hydrogen (297 K and 0.35 MPa) passed through the regenerated and non-regenerated catalyst immersed in liquid nitrogen bath, changing the flow rates. The conversion efficiencies were measured using the developed Raman optical system. The efficiency declined in direct proportion to the SV. At T_R= 24 ℃, the efficiency was estimated to be 0.55, significantly below the required value, considering the nominal operational condition (SV=1800 min-1). Even at T_R= 100 ℃, the efficiency improved to 0.8 under the same condition. There was still an insufficient margin. Therefore, catalyst regeneration should be conducted at temperatures exceeding 100 ℃.
| Submitters Country | Sweden |
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