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para hydrogen system under irradiation. [13] In the case of either source pulsing or hydrogen circulation in and out of the radiation field, we find that there are relatively small changes in the levels of ortho-hydrogen, and larger changes in the neutronically-insignificant levels of trihydrogen and monohydogen in the system, and only then at power levels significantly higher than we anticipate in any spallation source or reactor environment. 4. FUTURE WORK The preliminary nature of this model points the way to significant additional refinements we find necessary. First of all, there is no energy balance condition. Ortho-hydrogen converting to para-hydrogen at 20 K releases some 703 kJ/kg, significantly greater than the latent heat of vaporization (443 kJ/kg). The energy released in this process must be included with other energy source and sink terms in a temperaturedependent refinement to our model. This will introduce significant additional feedback loops to the system. Furthermore, many practical hydrogen systems at neutron sources do involve exchange with a gaseous volume of hydrogen, and the extension of our model to such a two-phase system is very similar to the work done by Nelms and Carter; we anticipate no great difficulty doing so here as well. We do not at the current time foresee a need to make a multi-point model, in which we would consider multiple volume elements of either liquid or gaseous material, but such an extension should be straightforward, if tedious. Further, the parameters and constants used in the model have been assembled from different sources, each of which concerned different efforts done at different times by different groups. An irradiation study conducted specifically to measure the parameters used in this model would likely result in significantly more reliable estimates of the key system parameters. Finally, the most important work we anticipate doing with this model is to test its application to real systems, both currently operating neutron sources (ISIS, the Lujan Center, the NIST reactor) and historical experiments as found in the literature. 5. CONCLUSIONS Although our model is both limited in its scope and not fully tested, we find that it provides a number of qualitative insights into the likely behavior or liquid hydrogen systems under irradiation. In typical neutron source configurations, our model indicates that 1. irradiation can provide meaningful levels of catalysis for ortho-to-para relaxation, 2. recombination of radiolytically-produced mono-hydrogen can create a dynamic equilibrium level of ortho-hydrogen significantly shifted from the thermodynamic equilibrium level (that is, without radiation), 3. neither trihydrogen nor monohydrogen levels are neutronically significant, and 4. all effects, including those desired from external catalysis, can be considered “averaged over time” in a pulsed source with a flowing hydrogen system where the hydrogen flows out of the neutron field. These conclusions are qualitative in nature, and will not alter given significant changes in the numerical values of the constants used to analyze the system, or given minor changes in the nature of the kinetics model. In the specific case of the liquid hydrogen moderators at the Spallation Neutron Source, controlling ortho- and para-hydrogen levels is a question of catalysis and power density. The dynamic equilibrium value of the ortho-hydrogen fraction may 56
e up to 0.30 with low levels of system catalysis, and be nearly identical to thermodynamic equilibrium with high levels of catalysis. Finally, any attempt to measure ortho-hydrogen concentrations in an operating neutron source system must account for the “residual catalyst,” from slowly-decaying trihydrogen, that can cause nearly instantaneous back conversion to higher-temperature-equilibrium ortho-hydrogen levels in the measurement apparatus. ACKNOWLEDGEMENTS The Spallation Neutron Source is managed by UT-Battelle, LLC, under contract DE-AC05- 00OR22725 for the U.S. Department of Energy. Argonne National Laboratory is operated by the University of Chicago for the Department of Energy under contract W-31-109-ENG-38. REFERENCES [1] L. L. Daemen and T. O. Brun, “Ortho- and para-hydrogen in neutron thermalization,” in Proceedings of the International Workshop on Cold Moderators for Pulsed Neutron Sources (J. M. Carpenter and E. B. Iverson, eds.), pp. 171–180, OECD, 1998. [2] R. E. Williams, J. M. Rowe, and P. Kopetka, “The liquid hydrogen moderator at the NIST reactor,” in Proceedings of the International Workshop on Cold Moderators for Pulsed Neutron Sources (J. M. Carpenter and E. B. Iverson, eds.), pp. 79–86, OECD, 1998. [3] D. J. Picton, T. D. Benyon, and T. A. Broome, “Hydrogen moderator performance calculations,” in Proceedings of the International Workshop on Cold Moderators for Pulsed Neutron Sources (J. M. Carpenter and E. B. Iverson, eds.), pp. 181–189, OECD, 1998. [4] G. J. Russell, P. D. Ferguson, and E. J. Pitcher, “Sensitivity studies of the ortho-para H2 concentration on the neutron performance of the Lujan Center liquid H2 moderator using state-of-the-art computational tools,” in Proceedings of the International Workshop on Cold Moderators for Pulsed Neutron Sources (J. M. Carpenter and E. B. Iverson, eds.), pp. 155–162, OECD, 1998. [5] L. W. Nelms and H. G. Carter, “Nuclear-radiation-induced conversion of parahydrogen to orthohydrogen,” Tech. Rep. FZK 351-2, NASA, 1968. [6] T. O. Brun, “Specta and pulse shapes of a decoupled liquid hydrogen moderator,” in Proceedings of the International Workshop on Cold Moderators for Pulsed Neutron Sources (J. M. Carpenter and E. B. Iverson, eds.), pp. 163–170, OECD, 1998. [7] R. E.Williams and J. M. Rowe, “The presence of ortho-hydrogen in the NIST liquid hydrogen cold neutron source.” Presented at the Annual Meeting of the American Nuclear Society, Milwaukee, Wisconsin, June 2001. [8] L. L. Daemen and P. D. Ferguson, “Characteristics of the partially-coupled liquid hydrogen moderator at the Manuel Lujan, Jr. Neutron Scattering Center.” Presented at the Annual Meeting of the American Nuclear Society, Milwaukee, Wisconsin, June 2001. [9] Y. Milenko, R. Sibileva, and M. Strzhemechny, “Natural ortho-para conversion rate in liquid and gaseous hydrogen,” Journal of Low Temperature Physics, vol. 107, no. 1-2, pp. 77–92, 1997. [10] E. B. Iverson and J. M. Carpenter, “Kinetics of irradiated liquid hydrogen,” in ICANS XVI, Proceedings of the International Collaboration on Advanced Neutron Sources, Neuss, Germany (H. Conrad, ed.), May 2003. To be published. [11] J. W. Conant, F. J. Edeskuty, J. E. Huston, and F. V. Thorne, “The conversion of para to ortho hydrogen in a gamma ray and neutron radiation field,” Cryogenics, vol. 15, no. 1, pp. 12–16, 1975. 57
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ACoM - 6 6th 6 International Worksh
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Forschungszentrum Jülich GmbH Inst
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Preface These are the Proceedings o
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List of participants, cont’d. Kü
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Page 12 and 13: Experimental background, results an
Page 14 and 15: • Light water ice as H(H2O) at T
Page 16 and 17: In Table 2 a survey is given about
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After melting phase I in the cryost
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4. SOLID PHASES OF MESITYLENE-D0 AN
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G(ν) [a.u.] 8 6 4 2 Phase II Phase
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A not quite as perfect cold spectru
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2.2 Inelastic neutron scattering Th
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Figure 4. Inelastic spectra at the
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3.2 Moderator performance The resul
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Figure 2. The methane pelletizer, m
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Figure 5. The cryogenic hopper fill
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ture
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The charging device scheme could be
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Table 1. Data of irradiation of met
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5. METHODS OF PROCESSING OF RAW EXP
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of radicals, that is, before a burp
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6.2 Condition for thermally stimula
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Table 4. Estimated values of an ene
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• Nine spontaneous burps were obs
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APPENDIX Some useful relations and
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APPENDIX. Graphs of burps. 148
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2. EXPERIMENTAL SETUP Fig. 1 will g
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All these parameters are kept const
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4. CONCLUSIONS We demonstrated that
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JESSICA (Juelich Experimental Spall
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The moderator system consists of th
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to change the temperatures and to p
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supplied from the high pressure bom
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10 9 10 8 10 7 10 6 Para35%[Experim
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2. TIME OF FLIGHT SPECTRA AND ENERG
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When applying this transformation t
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data were analyzed in more detail.
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First of all, we should conclude th
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The equation (a) may also describe
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Even at h=0.01 mm which is too smal
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model and some consequences of its
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methane and 0.7-1.5% for water ice
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case: the sample of infinite size.
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Phase Diagram of the CH4-H20 system
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Reaction Chamber for Hydrate Produc
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Analysis: X-ray diffraction pattern
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Comparison with non-hydrate forming
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Mechanical Tests: Apparatus 220
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Morphology of Indium Jacket 222
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Schriften des Forschungszentrums J
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Schriften des Forschungszentrums J
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