MXenes for Plasmonic Photodetection

MXenes have recently shown impressive optical and plasmonic properties associated with their ultrathin‐atomic‐layer structure. However, their potential use in photonic and plasmonic devices has been only marginally explored. Photodetectors made of five different MXenes are fabricated, among which molybdenum carbide MXene (Mo₂CT_x) exhibits the best performance. Mo₂CT_x MXene thin films deposited on paper substrates exhibit broad photoresponse in the range of 400–800 nm with high responsivity (up to 9 A W^?1), detectivity (≈5 × 10¹¹ Jones), and reliable photoswitching characteristics at a wavelength of 660 nm. Spatially resolved electron energy‐loss spectroscopy and ultrafast femtosecond transient absorption spectroscopy of the MXene nanosheets reveal that the photoresponse of Mo₂CT_x is strongly dependent on its surface plasmon‐assisted hot carriers. Additionally, Mo₂CT_x thin‐film devices are shown to be relatively stable under ambient conditions, continuous illumination and mechanical stresses, illustrating their durable photodetection operation in the visible spectral range. Micro‐Raman spectroscopy conducted on bare Mo₂CT_x film and on gold electrodes allowing for surface‐enhanced Raman scattering demonstrates surface chemistry and a specific low‐frequency band that is related to the vibrational modes of the single nanosheets. The specific ability to detect and excite individual surface plasmon modes provides a viable platform for various MXene‐based optoelectronic applications.     

Thermal hydraulic investigations of intermediate heat exchanger in a pool-type fast breeder reactor

Intermediate heat exchanger (IHX) in a pool-type liquid metal cooled fast breeder reactor is an important heat exchanging component as it forms an intermediate boundary between the radioactive primary sodium in the pool and the non-radioactive secondary sodium in the steam generator (SG). The thermal loads during steady state and transient conditions impose thermal stresses on the heat exchanger tubes and on the shells which hold the tube bundle. Estimation of these thermal loads and achieving uniform temperature distribution in the tubes and shells by having uniform flow distributions are the major tasks of thermal hydraulic investigations of IHX. Through multi-dimensional thermal hydraulic investigations performed using commercially available computer codes such as PHOENICS, the flow and temperature distributions in the tubes and shells and in its secondary sodium inlet and outlet headers are obtained with and with out provisions of flow distribution devices. The effectiveness of these devices in achieving acceptably uniform flow and temperature distributions has been assessed and thermal loads on the tubes and shells for thermo mechanical analysis of the IHX have been defined. The predictions of the computational studies have been validated against simulated experiments.     

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

Bipolar compounds (referred to in general as btza ) containing a benzo[1,2,5]thiadiazole core and peripheral diarylamines and/or 4‐tert‐butylphenyl moieties have been synthesized via palladium‐catalyzed cross‐coupling reactions of 4,7‐dibromobenzo[1,2,5]thiadiazole with appropriate stannyl compounds. These compounds are fluorescent and the emission color ranges from green to red. The fluorescence of the compounds originates from a charge‐transfer process and exhibits solvatochromism. These red‐light‐emitting materials are amorphous and devices of different configurations were fabricated: I) ITO/ btza /TPBI/Mg:Ag; II) ITO/ btza /Alq₃/Mg:Ag; III) ITO/ btza /Mg:Ag (where ITO = indium tin oxide, TPBI = 1,3,5‐tris(N‐phenylbezimidazol‐2‐yl)benzene, and Alq₃ = tris(8‐hydroxyquinoline)aluminum). The performance of some of the red‐light‐emitting devices appears to be very promising.     

Thermal hydraulic investigations of primary coolant pipe rupture in an LMFBR

High quality for primary coolant pipes in fast reactors is ensured through utmost care taken in the design and manufacture. Demonstration of high structural reliability of them by extensive experimental and theoretical studies renders the double-ended guillotine rupture (DEGR) of a primary pipe a highly improbable event. However, as a defense in depth approach instantaneous DEGR of one of the pipes has been considered in design. Thermal hydraulic analyses of this event in a typical liquid metal cooled fast breeder have been carried out to study its consequences and to establish the availability of safety margins. Various uncertainties relevant to the event have been analysed to evaluate the sensitivity of each parameter. For this purpose, one-dimensional plant dynamics studies using thermal and hydraulic models of core subassemblies and primary sodium circuit have been performed. Validity of the assumptions made in the one-dimensional model like, uniform flow through all subassemblies in core under pipe ruptured condition and non possibility of sodium boiling by flashing have also been investigated through detailed three-dimensional and pressure transient studies. Analyses indicate the availability of good margins against the design safety limits in all the parametric cases analysed.     

A computational study of flow mal-distribution on the thermal hydraulic performance of an intermediate heat exchanger in LMFBR

The flow and thermal non-uniformities occurring in the intermediate heat exchanger (IHX) of a liquid metal-cooled fast breeder reactor have been characterized through numerical simulations. For modeling the primary and secondary sodium flow through the IHX, an equivalent anisotropic porous medium approach has been used. The pressure drop in the equivalent porous medium is accounted through the inclusion of additional pressure drop terms in the Navier–Stokes equations, with the help of standard correlations for cross flow or parallel flow over tubes. For secondary sodium flow, the effects of a flow distributor device with orifices and baffles at the inlet have also been included, in addition to axial flow through the tubes. The heat exchange between primary and secondary streams is incorporated in the form of a volumetric heat source or sink term, which is corrected iteratively. The resulting flow distributions are in reasonable agreement with available experimental results. The study shows that the temperature of the secondary sodium flow at the exit can be made more uniform by exchanging less heat near the inner wall of IHX, as compared to the region close to the outer wall, using suitable flow distribution devices.     

Adjoint operator approach to functional reliability analysis of passive fluid dynamical systems

Reliability analysis of passive systems mainly involves quantification of the margin to safety limits in probabilistic terms. For systems represented by complex models, propagating input uncertainty to get the response uncertainty and hence probability information requires intensive computational effort. Here a computationally efficient method for the functional reliability analysis of passive fluid dynamical systems is presented. The approach is based on continuous adjoint operator technique to generate a response surface approximating the given system model from the sensitivity coefficients. A numerical application of this method to the reliability analysis of heat transport in an asymmetrical natural convection loop is demonstrated. Computational efficiency and accuracy compared with the direct Monte-Carlo and forward response surface methods.     

Turbulent natural convection of sodium in a cylindrical enclosure with multiple internal heat sources: A conjugate heat transfer study

Turbulent natural convection of liquid metal in a cylindrical enclosure with locally distributed heat source has been investigated for Boussinesq number in the range of 4.5 × 0¹¹– 6.25 × 10¹². The enclosure considered is an ideal model of the lower plenum of a fast reactor with tray(s) holding damaged core debris, which continuously generate heat. The focus of the study has been to assess the heat dissipation capacity of single and multiple trays in respecting the specific temperature limits on the tray(s). Heat conduction in the metallic trays and in the impervious core debris mixture and turbulent natural convection of the liquid sodium are solved as a conjugate heat transfer problem. The equations that govern the various heat transfer processes in 2-D axi-symmetric cylindrical polar coordinate system have been solved by the finite volume method. Turbulence has been modeled by the k–ε turbulence model, without the use of wall functions. The predictions of the numerical model have been validated against benchmark data reported in open literature. Also, experiments have been conducted in an ideal water model towards validation of the computational model. For typical enclosure dimensions representing a 500 MWe fast reactor, it is seen that the critical parameters are heat dissipation area of the source (area of trays) and the thickness of the heat source (debris thickness). With increase in the number of trays, the heat transfer area increases while the debris thickness reduces. Both these effects lead to reduction in the tray as well as source temperatures. The heat dissipation capacity exhibits a non-linear relationship with the number of plates.     

Dynamic scheduling of manufacturing job shops using genetic algorithms

Most job shop scheduling methods reported in the literature usually address the static scheduling problem. These methods do not consider multiple criteria, nor do they accommodate alternate resources to process a job operation. In this paper, a scheduling method based on genetic algorithms is developed and it addresses all the shortcomings mentioned above. The genetic algorithms approach is a schedule permutation approach that systematically permutes an initial pool of randomly generated schedules to return the best schedule found to date.A dynamic scheduling problem was designed to closely reflect a real job shop scheduling environment. Two performance measures, namely mean job tardiness and mean job cost, were used to demonstrate multiple criteria scheduling. To span a varied job shop environment, three factors were identified and varied between two levels each. The results of this extensive simulation study indicate that the genetic algorithms scheduling approach produces better scheduling performance in comparison to several common dispatching rules.     

Computational fluid dynamic investigations and experimental validation of frozen seal sodium valve assembly of a fast reactor

Frozen seal sodium valves are used in fast reactors. To achieve sodium freezing, horizontal fins are attached to the outer surface of valve sheath. Adjacent fins form open-ended cavities and natural convection of air in these cavities is investigated using the PHOENICS code for various values of fin length, spacing and root temperature. It is seen that convective air does not penetrate deep into shallow cavities leading to poor heat transfer coefficient, offsetting the enhancement in surface area. Penetration depth of air is a function of aspect ratio and Rayleigh number based on fin spacing and is independent of fin length. Generalized correlations are derived for Nusselt number in terms of Rayleigh number and aspect ratio. Using these correlations, temperature distribution in entire valve assembly is predicted using the HEATING5 code, to select an optimum design. Experiments have been conducted on a model valve of selected design in the SILVERINA facility available at this centre. Measured stem temperature distribution is found to compare satisfactorily with HEATING5 predictions, validating the correlations derived from computational fluid dynamic studies and integrated thermal analysis methodology.     

Overview of pool hydraulic design of Indian prototype fast breeder reactor

Thermal hydraulics plays an important role in the design of liquid metal cooled fast breeder reactor components, where thermal loads are dominant. Detailed thermal hydraulic investigations of reactor components considering multi-physics heat transfer are essential for choosing optimum designs among the various possibilities. Pool hydraulics is multi-dimensional in nature and simple one-dimensional treatment for the same is often inadequate. Computational Fluid Dynamics (CFD) plays a critical role in the design of pool type reactors and becomes an increasingly popular tool, thanks to the advancements in computing technology. In this paper, thermal hydraulic characteristics of a fast breeder reactor, design limits and challenging thermal hydraulic investigations carried out towards successful design of Indian Prototype Fast Breeder Reactor (PFBR) that is under construction, are highlighted. Special attention is paid to phenomena like thermal stratification, thermal stripping, gas entrainment, inter-wrapper flow in decay heat removal and multi-physics cellular convection. The issues in these phenomena and the design solutions to address them satisfactorily are elaborated. Experiments performed for special phenomena, which are not amenable for CFD treatment and experiments carried out for validation of the computer codes have also been described.