@article{shutayfi_raj_eapen_shannon_2023, title={Design, Modeling, and Analysis of a Compact-External Electromagnetic Pumping System for Pool-Type Liquid Metal-Cooled Fast Reactors}, volume={193}, ISSN={["1873-2100"]}, DOI={10.1016/j.anucene.2023.109997r}, journal={ANNALS OF NUCLEAR ENERGY}, author={Shutayfi, Mohammed and Raj, Anant and Eapen, Jacob and Shannon, Steven}, year={2023}, month={Dec} }
 @article{shutayfi_raj_eapen_shannon_2023, title={Design, Modeling, and Analysis of a Compact-External Electromagnetic Pumping System for Pool-Type Liquid Metal-Cooled Fast Reactors}, volume={193}, ISSN={["1873-2100"]}, url={https://doi.org/10.1016/j.anucene.2023.109997}, DOI={10.1016/j.anucene.2023.109997}, abstractNote={Current pool-type Liquid Metal-Cooled Fast Reactors (LMCFRs), either under development or operational, immerse main reactor components in the primary coolant, (i.e. sodium), that includes heat exchangers, shielding structures, and pumping systems. Proposed main pumping systems, for some reactors that are under development, use electromagnetic pumps (EMPs) for primary coolant circulation. Annular linear induction pumps (ALIPs) are the preferred type since they are known for their advantages over mechanical centrifugal pumps (MCPs) when used to circulate liquid metals. This is due to ALIPs' absence of moving parts such as shafts and impellers, seals and bearings, auxiliary lubrication systems, and simplicity of flow and pressure control mechanism. The immersion of reactor components in the primary sodium in the reactor vessel minimizes the likelihood of radioactive coolant leakage and loss of coolant accidents. However, since there is only one access point to reactor components, the immersion of ALIPs prevents additional potential advantages. Online pump inspection and replacement, reduction of negative effects on pump components due to the high temperature and radiation environment, an additional heat removal mechanism for self-cooled ALIPs, and simple decommissioning procedures are some possible advantages. This paper discusses a study conducted to investigate the possibility of using large EMPs, ALIP type, that are located outside the reactor vessel and connected in parallel, instead of in vessel sodium immersed ones for pool-type LMCFRs. The large, outside-vessel EMP idea is tested on a liquid metal-cooled test reactor design using an experimentally validated multiphysics finite element analysis tool. Specifically, the steady state reactor's primary circuit cooling and pressure requirements are used to design two, in-vessel ALIPs and then two outside-vessel ALIPs. The two pumping systems are compared in terms of their geometries, performance characteristics, and impact on overall reactor design. It is found that the outside-vessel EMP design provides the same performance requirements and offers additional advantages compared to the in-vessel pump with only minimal reactor vessel modification and a slight drop in efficiency.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Shutayfi, Mohammed and Raj, Anant and Eapen, Jacob and Shannon, Steven}, year={2023}, month={Dec} }
 @article{raj_eapen_2019, title={Deducing Phonon Scattering from Normal Mode Excitations}, volume={9}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-019-43306-3}, abstractNote={AbstractWhile the quantum scattering theory has provided the theoretical underpinning for phonon interactions, the correspondence between the phonon modes and normal modes of vibrations has never been fully established; for example, the nature of energy exchange during elementary normal mode interactions remains largely unknown. In this work, by adopting a set of real asymmetric normal mode amplitudes, we first discriminate the normal and Umklapp processes directly from atomistic dynamics. We then demonstrate that the undulating harmonic and anharmonic potentials, which allow a number of interaction pathways, generate several total-energy-conserving forward and backward scattering events including those which are traditionally considered as quantum-forbidden. Although the normal mode energy is proportional to the square of the eigen-frequency, we deduce that the energy exchanged from one mode to another in each elementary interaction is proportional to the frequency – a quantum-like restriction. We anticipate that the current approach can be utilized profitably to discover unbiased scattering channels, many traditionally quantum forbidden, with complex anharmonicities. Our discovery will aid in the development of next-generation Peierls-Boltzmann transport simulations that access normal mode scattering pathways from finite temperature ab initio simulations.}, journal={SCIENTIFIC REPORTS}, author={Raj, Anant and Eapen, Jacob}, year={2019}, month={May} }
 @article{raj_eapen_2019, title={Exact diagonal representation of normal mode energy, occupation number, and heat current for phonon-dominated thermal transport}, volume={151}, ISSN={["1089-7690"]}, DOI={10.1063/1.5099936}, abstractNote={Collective excitations of crystal vibrations or normal modes are customarily described using complex normal mode coordinates. While appropriate for calculating phonon dispersion, the mixed representation involving the complex conjugates does not allow the construction of equivalent phonon occupation number or modal dynamical quantities such as the energy or heat current specific to a wave-vector direction (q). Starting from a canonical solution that includes waves going to the left and right directions, we cast the Hamiltonian, normal mode population, and heat current in an exactly diagonalizable representation using real normal mode amplitudes. We show that the use of real amplitudes obviates the need for a complex modal heat current while making the passage to second quantization more apparent. Using nonequilibrium molecular dynamics simulations, we then compute the net modal energy, heat current, and equivalent phonon population in a linear lattice subjected to a thermal gradient. Our analysis paves a tractable path for probing and computing the direction-dependent thermal-phononic modal properties of dielectric lattices using atomistic simulations.}, number={10}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Raj, Anant and Eapen, Jacob}, year={2019}, month={Sep} }
 @article{raj_eapen_2019, title={Phonon dispersion using the ratio of zero-time correlations among conjugate variables: Computing full phonon dispersion surface of graphene}, volume={238}, ISSN={["1879-2944"]}, DOI={10.1016/j.cpc.2018.12.008}, abstractNote={We present a robust family of methods (ZTR) to compute the phonon dispersion curves based on the ratio of zero-time correlations of conjugate variables in the reciprocal space. This general technique extracts the normal mode frequency corresponding to wave vector q and polarization p, from the ratio of the correlation of the nth derivative of displacement to the n-1th derivative in reciprocal space at zero time. A particular version of this method using the ratio of velocity to displacement (n=1) is previously known but seldom employed in atomistic simulations. For n=2, the method involves velocities and accelerations — dynamical variables that are more well-defined than equilibrium displacements in atomistic simulations. We test the ZTR methods and demonstrate that both ZTR methods (n=1,2) can accurately resolve the phonon mode frequencies while offering a significant improvement to the computational speed. We also illustrate the ability of the ZTR methods to handle anharmonicity and phonon softening at high temperatures. Finally, we demonstrate the power of the ZTR approach by computing the full phonon dispersion surface for graphene across the entire Brillouin zone with 3600 wave vectors and six polarizations at finite temperatures — a challenging task for the traditional methods.}, journal={COMPUTER PHYSICS COMMUNICATIONS}, author={Raj, Anant and Eapen, Jacob}, year={2019}, month={May}, pages={124–137} }
 @article{raj_eapen_2018, title={Computing Phonon Dispersion using Fast Zero-Point Correlations of Conjugate Variables}, volume={3}, ISSN={["2059-8521"]}, DOI={10.1557/adv.2018.288}, abstractNote={Time correlations of dynamic variables in the reciprocal space offer a rich theoretical setting for computing the phonon dispersion curves, particularly for systems with marked anharmonic interactions. Present techniques primarily rely either on the equipartition of energy between the phonon modes or on the oscillation of the time correlation of the normal mode projections. The former can lead to numerical errors due to deviation from equipartition while the latter usually requires long simulations for computing the time correlations. We investigate a different approach using the ratio of the normal mode expectation value of two conjugate variables – velocity and acceleration. Since only the correlations at the initial time (t=0) are needed, this approach is computationally attractive. In this work, we employ this method to extract the full Brillouin zone phonon dispersion for graphene.}, number={10}, journal={MRS ADVANCES}, author={Raj, Anant and Eapen, Jacob}, year={2018}, pages={531–536} }
 @article{sarkar_eapen_raj_murty_burchell_2016, title={Modeling irradiation creep of graphite using rate theory}, volume={473}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2016.01.036}, abstractNote={We have examined irradiation induced creep of graphite in the framework of transition state rate theory. Experimental data for two grades of nuclear graphite (H-337 and AGOT) have been analyzed to determine the stress exponent (n) and activation energy (Q) for plastic flow under irradiation. We show that the mean activation energy lies between 0.14 and 0.32 eV with a mean stress-exponent of 1.0 ± 0.2. A stress exponent of unity and the unusually low activation energies strongly indicate a diffusive defect transport mechanism for neutron doses in the range of 3–4 × 1022 n/cm2.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Sarkar, Apu and Eapen, Jacob and Raj, Anant and Murty, K. L. and Burchell, T. D.}, year={2016}, month={May}, pages={197–205} }