Soliton mechanism of the uranium nitride microdynamics and heat conductivity at high temperatures
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ATTICE DYNAMICS AND PHASE TRANSITIONS
Soliton Mechanism of the Uranium Nitride Microdynamics and Heat Conductivity at High Temperatures V. A. Semenov, O. A. Dubovsky, and A. V. Orlov State Scientific Center of the Russian Federation Leipunsky Institute for Physics and Power Engineering, Obninsk, Kaluga oblast, 249033 Russia email: [email protected] Received December 28, 2010
Abstract—The microdynamics of soliton waves and localized modes of nonlinear acoustic and optical oscil lations in uranium nitride has been investigated. It is shown that, upon heating, the energies of solitons in the gap between the optical and acoustic phonon bands increase, while the energies of local modes decrease. The experimentally observed quasiresonance features, which are shifted in the gap with a change in temperature, can be manifestations of the revealed soliton waves and local modes. The microdynamics of uranium nitride heat conductivity with the stochastic generation of the observed solitons and local modes at remote energy absorption have been investigated. The temperature dependence of the heat conductivity coefficient has been determined from the temperature gradient and energy flux within the standard approach (which is to be gen eralized). DOI: 10.1134/S1063774511070297
Currently, the experimental study of hightemper ature nuclear fuel based on uranium nitride (UN) ceramics on the DIN2 neutron spectrometer is under way. Figure 1 shows the energy dependences of the spectral density of vibrational states of UN ceramics, which were obtained by processing the experimental spectra of inelastic neutron scattering from the sam ples of this ceramic [1]. The experiments were per formed on the DIN2PI IBR slowneutron spectrom eter (Dubna) using a hightemperature thermostat at four temperatures in the ascending order: 293, 623, 923, and 1273 K. One can see two strong resonances in Fig. 1 which correspond to the acoustic phonon band in the range of 0–20 meV and to the optical band in the range of 40–60 meV. These resonances are due to the vibra tions of heavy uranium and light nitrogen atoms, respectively. With allowance for the large difference (by a factor of almost 20) in the uranium and nitrogen atomic masses, the plateau with quasiresonance fea tures in the gap between the acoustic and optical states appears to be anomalously high. Calculations within the harmonic approximation showed that the optical and acoustic phonon bands are separated by a wide spectral gap, and no features can be formed in this gap on the monotonic tails of the branches due to only temperature broadening. As can be seen in Fig. 1, the UN frequency spectrum softens and the features in the range of 30–40 meV become more pronounced with an increase in temperature. The quasiresonance gap features are shifting with an increase in temperature, obviously not to lower energies, as in the case of the main resonances. It is shown that the quasiresonance
features observed can be spectrally represented by nonlinear soliton waves and local modes. It is pla
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