Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

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Even at h=0.01 mm which is too small value for a bead of 2 mm diameter, δT ≈ 5÷10 K. This estimate proves high probability of recombination wave propagation from one bead to another. Moreover, this statement is confirmed by interaction with the URAM-2 experiments with beads of methane hydrate and beads of ice in argon matrix where ignition of burps occurred always in a whole pack of beads. Actually, taking in mind that thermal conductivity of solid argon is much better than that of liquid hydrogen, one may conclude that cooling of interface between two beads in the case of stagnant liquid hydrogen is worse than in the case of solid argon (calculation of thermal expansion of ice beads in solid argon matrix showed that at 20 K beads are compressed in the matrix). Then, temperature rise in a contact area should be higher in liquid hydrogen, and propagation of recombination wave from one bead to another would come easier. If one accepts mechanical nature of propagation (“percolation” as it is defined in [11]) of ignition of clusters of radicals, then positive answer for the question “would a recombination wave propagate from one bead to another?” would be evident. And the last pessimistic conclusion: Even if one assumes that no spontaneous burps is expected in ESS solid methane moderator cooled with liquid hydrogen, any small perturbation of temperature in any single bead (+2÷3K) can cause fast recombination process in a whole assembly resulting in evaporation of cooling hydrogen. 3.2 Burps in water ice for ESS irradiation condition For the ESS moderator conditions for spontaneous burping in ice can be derived from URAM-2 data with correction for higher dose rate and big volume of ice. Time to burping will be 9.3 times less according to dose rate (for 1 MW ESS target station) and dispersion will be much less. Unlike the solid methane case, γ-factor for big samples of water ice is unknown. But one statement is evident: critical concentration of radicals (or amount of stored energy) can’t be less than minimal concentration to make an induced burp, which is 65-70 J/g. Accepting this value, we have γ ≈ 0.15. Estimated values of times between succeeding spontaneous burps in small samples of ice prepared by slow freezing of pure water (diameter < 30 mm) versus temperature are given in Fig. 4. Because of dispersion in burping condition, there are two curves t(Tirr), and expected times are confined between them. For the case of the ESS ice moderator (many beads) only lower curve should be taken into account. Released energy in spontaneous burps will be 65-70 J/g, and the sample will be heated up to 130 K. Samples of ice prepared by freezing water in liquid nitrogen (diameter ∼ 5 mm) would behave, probably, like hydrates but this is not fully confirmed. 3.3 Burps in hydrates for ESS irradiation condition For the ESS moderator made from THF or methane hydrate ice one can expect the next burping condition: • temperature is less than 25 K, • time between succeeding burps - 1.4÷1.6 hour, • stored energy ≥ 80-100 J/g, • heating - up to 170 K. 188
Figure 4. Recommended values of maximum allowable residence time of ice beads in the ESS ice moderator (lower border line of the gray area). 4. PROBABILISTIC MODEL OF SPONTANEOUS RELEASE OF ENERGY (“BURP”) IN IRRADIATED ICES 4.1 Conception of the model. A basis of this model was outlined in previous papers of the author [12,13]. There it was stated that a key to understand features of fast spontaneous reaction of recombination of radicals (SB) is in taking into account irregularities in space distribution of radicals and in energy deposited. Radicals are generated in the tracks of recoil protons, and therefore, they are arranged mainly in lines or in spurs. Because diffusion of radicals at low temperatures (10-30K) is too slow to get them away from their origins, we come to the conclusion about non-uniform distribution of radicals and their concentration in clusters. Besides their original distribution due to tracks formation, clusters of radicals may occupy structure defect sites as well. Structure defects can be caused by radiation or generated in the course of freezing of a sample. Two-dimensional computer simulation of proton track distribution revealed high dispersion of concentration of radicals on a small scale base: three-fourfold increase in local density of radicals as compared to averaged over a sample [12]. As rate of recombination reaction is square proportional to density of radicals, such increase in local density of radicals may lead to initiation of auto-catalytic reaction in this local area of high density of radicals. Therefore, clustered structure of distribution of radicals decreases critical density of radicals as compared to those determined by thermal instability of a sample. A simplified mathematical approach to this model was derived in [18] with no account to a stochastic character of initiation of a burp. This paper gives more detailed insight into the 189
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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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Pepe M. 43 Petriw S. 43 Picton D.J.
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Experimental background, results an
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• Light water ice as H(H2O) at T
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In Table 2 a survey is given about
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sigma-T in barns/molecule Figure 4.
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2.3 Liquid water Compared to the so
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presented. The corresponding experi
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3.2 Gaseous hydrogen and deuterium
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Rho(Omega) normalised 160 140 120 1
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elatively free and can also suffer
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Figure 16. Heat Capacity for Solid
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o-D2 and p-H2 at 5K for an ucn ener
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Figure 22. UCN Upscattering Rates i
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ho(omega) normalised 40 35 30 25 20
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derived from Walker [52]. The neutr
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Since there is only a small gain of
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7. CALCULATION OF SCATTERING LAW DA
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For the validation of these data se
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[36] Nielsen, M.: Phonons in Solid
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2. THE CASE OF MOLECULAR SOLIDS An
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γ r ( 0) ≅ 2 2 2 T erf e ( ) / 2
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4. PRELIMINARY APPLICATION Solid Me
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[5] Meeting on Moderator Concepts a
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state. Conversely, the monatomic hy
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Note that the equilibrium trihydrog
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para hydrogen system under irradiat
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[12] T. E. Fessler and J. W. Blue,
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tion of neutron intensities, partic
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varied between calculations but the
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5. THE OPTIMISATION OF MULTIPLY GRO
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Figure 7. A comparison of time dist
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4.8 cm hydrogen slab in front of a
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ACoM - 6 6 th Meeting of the Collab
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the moderator is annealed every 12
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The method requires two additional
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ABSTRACT ACoM - 6 6 th Meeting of t
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Heat transfer processes across phas
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3. TWO-PHASE FLOW PATTERN IN THE MO
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continuous phase dominating, and a
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Also the velocity pattern has chang
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Figure 8. Methane pellets concentra
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ACoM - 6 6 th Meeting of the Collab
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sample volume and the nominal densi
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The results presented in this paper
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Table 10. Summary of energy transfe
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2. EXPERIMENTAL Methane hydrate was
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Figure 4. Energy levels of methane
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102
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phase transitions from phase II to
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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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112
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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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122
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Figure 2. The methane pelletizer, m
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Figure 5. The cryogenic hopper fill
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128
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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
Page 142 and 143: of radicals, that is, before a burp
Page 144 and 145: 6.2 Condition for thermally stimula
Page 146 and 147: Table 4. Estimated values of an ene
Page 148 and 149: • Nine spontaneous burps were obs
Page 150 and 151: APPENDIX Some useful relations and
Page 152 and 153: APPENDIX. Graphs of burps. 148
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Page 160 and 161: 2. EXPERIMENTAL SETUP Fig. 1 will g
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Page 164 and 165: 4. CONCLUSIONS We demonstrated that
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Page 168 and 169: The moderator system consists of th
Page 170 and 171: to change the temperatures and to p
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Page 174 and 175: supplied from the high pressure bom
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Page 180 and 181: 2. TIME OF FLIGHT SPECTRA AND ENERG
Page 182 and 183: When applying this transformation t
Page 184 and 185: data were analyzed in more detail.
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Page 188 and 189: First of all, we should conclude th
Page 190 and 191: The equation (a) may also describe
Page 194 and 195: model and some consequences of its
Page 196 and 197: methane and 0.7-1.5% for water ice
Page 198 and 199: case: the sample of infinite size.
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Page 216 and 217: Phase Diagram of the CH4-H20 system
Page 218 and 219: Reaction Chamber for Hydrate Produc
Page 220 and 221: Analysis: X-ray diffraction pattern
Page 222 and 223: Comparison with non-hydrate forming
Page 224 and 225: Mechanical Tests: Apparatus 220
Page 226 and 227: Morphology of Indium Jacket 222
Page 228 and 229: Schriften des Forschungszentrums J
Page 230 and 231: Schriften des Forschungszentrums J
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