New Models for UO2 Fuel Structure Evolution under Irradiation in Fast Reactors

On the base of analysis of experimental observations and critical assessment of existing models for oxide fuel structure evolution under operation conditions of fast reactors, new models for fuel restructuring and coring are proposed. The restructuring model describes coherent motion in the temperature gradient of various voids (gas bubbles, sintering pores and large lenticular pores) and grain boundaries, to which the voids are attached. As a result, the model explains elongation of thermally growing equiaxed grains and formation of columnar grains, and predicts a rapid formation of extended columnar grain zone during a relatively short initial period of fast reactor irradiation. The coring model describes formation and growth of the central void in the fuel pellet, activated by mass transport from the inner to the outer zone of the pellet under stresses induced by inhomogeneous fuel densification in the initial period of irradiation.Comment: 17 pages, 7 Figure

Development of the Reiss theory for binary homogeneous nucleation of aerosols

The Reiss theory for binary homogeneous nucleation in binary gas mixtures is critically analysed and further developed. Based on the analysis of phase space trajectories in the supercritical zone of the phase transition, carried out within the framework of the theory of two-dimensional dynamical systems and supplemented by the flux matching condition at the boundary of the critical zone, it is shown how the theory should be modified. The proposed modification is equivalent to the earlier modifications by Langer and Stauffer, based on additional trial assumptions (ansatz) for solving the steady state equation for the non-equilibrium size distribution function, but reveals and substantiates the approximation underlying their approach. The extension of the Reiss theory to binary vapours in inert carrier (atmospheric) gases is justified.Comment: 14 pages, 1 figur

Behaviour of oxide layer of zirconium-based fuel rod cladding under steam starvation conditions

Das Verhalten der Oxidschicht von Zirkonium-basierten Brennstabhüllrohren unter Dampfmangelbedingungen Es werden die Ergebnisse von Dauerglühversuchen mit voroxidierten Zircaloy-Hüllrohren in Inertgasatmosphäre bei Glühtemperaturen zwischen 1250 und 1500 °C und ein entsprechendes Modell dargestellt. Ziel der Tests war die Untersuchung der Kinetik der Oxidschichtreduzierung während der Dampfmangelphase bei der Trockenlegung des Reaktorkerns während eines schweren Störfalles. Neben der Abnahme der Oxidschichtstärke wurden die homogene Entwicklung von α-Zr(O) Ausscheidungen innerhalb der Oxidschicht und die Entwicklung einer α-Zr(O) Schicht an der äußeren Oberfläche des Hüllrohres nachgewiesen. Das Phänomen sollte einen starken Einfluss auf die intensive Wasserstofffreisetzung während der folgenden Abschreckphase haben

Dislocation lines as the precursor of the melting of crystalline solids observed in Monte Carlo simulations

The microscopic mechanism of the melting of a crystal is analyzed by the constant pressure Monte Carlo simulation of a Lennard-Jones fcc system. Beyond a temperature of the order of 0.8 of the melting temperature, we found that the relevant excitations are lines of defects. Each of these lines has the structure of a random walk of various lengths on an fcc defect lattice. We identify these lines with the dislocation ones proposed in recent phenomenological theories of melting. Near melting we find the appearance of long lines that cross the whole system. We suggest that these long lines are the precursor of the melting process.Comment: 5 pages, 5 figures, accepted in Physical Review Letter

Microwave analysis of the interplay between magnetism and superconductivity in EuFe 2 (As 1 -x P x ) 2 single crystals

This paper presents a microwave analysis of the interplay between magnetism and superconductivity in an iron-based ferromagnetic superconductor. By comparing the complex rf susceptibility with magnetic force images, the authors discuss the nature of the observed phase transitions and the possible presence of a quantum critical point

Effect of controlled artificial disorder on the magnetic properties of EuFe2(As1−xPx)2 ferromagnetic superconductor

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe2 (As1−xPx )2 (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, Tc (H), shows an extraordinarily large decrease. It starts at Tc (H = 0) ≈ 24 K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only Tc moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in Tc to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe2 (As1−xPx )2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu2+ lattice act as efficient pairbreakers leading to a significant Tc reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of Tm (less than 1 K) after the irradiation drives Tc to below Tm . Our results suggest that FM and SC phases coexist microscopically in the same volume

Effect of controlled artificial disorder on the magnetic properties of EuFe2_2(As1−x_{1-x}Px_{x })2_2 ferromagnetic superconductor

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, $\textrm{EuFe}_{2}(\textrm{As}_{1-x}\textrm{P}_{x})_{2}$ (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, $T_{c}(H)$, shows an extraordinarily large decrease. It starts at $T_{c}(H=0)\approx24\:\textrm{K}$ in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Our results suggest that in $\textrm{EuFe}_{2}(\textrm{As}_{1-x}\textrm{P}_{x})_{2}$ superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered $\text{Eu}^{2+}$ lattice act as efficient pairbreakers leading to a significant $T_{c}$ reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of $T_{m}$ (less than 1 K) after the irradiation drives $T_{c}$ to below $T_{m}$. The results suggest that FM and SC phases coexist microscopically in the same volume