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Metallic fuels hold numerous advantages over conventional uranium dioxide fuels and are a key component of several liquid metal-cooled advanced reactor concepts including sodium fast reactors. These fuels undergo rapid swelling during early burnup; consequently, they spend most of their reactor lifetime in a porous state. The presence of this porosity alters many of the mechanical properties of the fuel including creep impacting fuel deformation during axial swelling. This work investigates the creep behavior of the porous fuel using a spark plasma sintering technique. Creep tests were performed for the first time on porous α-phase uranium and uranium with 10 wt. % zirconium (U-10Zr) samples. The samples of α-phase uranium and U-10Zr were fabricated from depleted uranium by spark plasma sintering and subjected to uniaxial compressive creep testing. Calculated stress exponents were found to be 2.6 ± 1.6 and 5.7 ± 1.4 for α-U and U-10Zr, respectively, and calculated activation energies were found to be 61.6 ± 1.1 kJ / mol for α-U. The creep data were also used to evaluate existing porosity inclusive in creep models. [ABSTRACT FROM AUTHOR]

TurboRVB is a computational package for ab initio Quantum Monte Carlo (QMC) simulations of both molecular and bulk electronic systems. The code implements two types of well established QMC algorithms: Variational Monte Carlo (VMC) and diffusion Monte Carlo in its robust and efficient lattice regularized variant. A key feature of the code is the possibility of using strongly correlated many-body wave functions (WFs), capable of describing several materials with very high accuracy, even when standard mean-field approaches [e.g., density functional theory (DFT)] fail. The electronic WF is obtained by applying a Jastrow factor, which takes into account dynamical correlations, to the most general mean-field ground state, written either as an antisymmetrized geminal power with spin-singlet pairing or as a Pfaffian, including both singlet and triplet correlations. This WF can be viewed as an efficient implementation of the so-called resonating valence bond (RVB) Ansatz, first proposed by Pauling and Anderson in quantum chemistry [L. Pauling, The Nature of the Chemical Bond (Cornell University Press, 1960)] and condensed matter physics [P.W. Anderson, Mat. Res. Bull 8, 153 (1973)], respectively. The RVB Ansatz implemented in TurboRVB has a large variational freedom, including the Jastrow correlated Slater determinant as its simplest, but nontrivial case. Moreover, it has the remarkable advantage of remaining with an affordable computational cost, proportional to the one spent for the evaluation of a single Slater determinant. Therefore, its application to large systems is computationally feasible. The WF is expanded in a localized basis set. Several basis set functions are implemented, such as Gaussian, Slater, and mixed types, with no restriction on the choice of their contraction. The code implements the adjoint algorithmic differentiation that enables a very efficient evaluation of energy derivatives, comprising the ionic forces. Thus, one can perform structural optimizations and molecular dynamics in the canonical NVT ensemble at the VMC level. For the electronic part, a full WF optimization (Jastrow and antisymmetric parts together) is made possible, thanks to state-of-the-art stochastic algorithms for energy minimization. In the optimization procedure, the first guess can be obtained at the mean-field level by a built-in DFT driver. The code has been efficiently parallelized by using a hybrid MPI-OpenMP protocol, which is also an ideal environment for exploiting the computational power of modern Graphics Processing Unit accelerators. [ABSTRACT FROM AUTHOR]

We describe an efficient algorithm to compute forces in quantum Monte Carlo using adjoint algorithmic differentiation. This allows us to apply the space warp coordinate transformation in differential form, and compute all the 3M force components of a system with M atoms with a computational effort comparable with the one to obtain the total energy. Few examples illustrating the method for an electronic system containing several water molecules are presented. With the present technique, the calculation of finite-temperature thermodynamic properties of materials with quantum Monte Carlo will be feasible in the near future. [ABSTRACT FROM AUTHOR]

The quantum effects on the thermodynamics of the easy-plane triangular antiferromagnet, displaying classically both an Ising and a Berezinskii–Kosterlitz–Thouless phase transition, are evaluated by means of the pure-quantum self-consistent harmonic approximation, which allows one to deal with any spin value through classical Monte Carlo simulations. Data for the specific heat, the static spin correlation functions, and the in-plane correlation length are presented in a broad range of temperatures, for different values of the spin and of the exchange anisotropy. Our results for the quantum transition temperatures are in good agreement with the few available theoretical and numerical estimates. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate the ground state properties of the S=1/2 Heisenberg antiferromagnet on the triangular lattice using mainly exact diagonalization (for lattices up to N=36 sites) and the recently developed Green Function Monte Carlo with stochastic reconfiguration (up to N=144). A systematic size scaling of the order parameter and of the spin gap gives strong evidences for the presence of long-range Ne´el order in the ground state of this highly frustrated model. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on a study on the thermodynamics of the classical Heisenberg antiferromagnet with easy-plane anisotropy on a triangular lattice. Use of Monte Carlo simulations; Topological frustration; Anisotropy of exchange interaction.

Studies the thermodynamics of two-dimensional XXZ easy-plane quantum Heisenberg magnets. Use of Hamiltonian method for simulations of the ferro and antiferromagnets; Evaluation of effects of quantum fluctuations.