Spent fuel reprocessing: A vital link in Indian nuclear power program

The success of the three stage Indian nuclear energy program is inter-linked with the establishment of an efficient closed fuel cycle approach with recycling of both fissile and fertile components of the spent fuel to appropriate reactor systems. The Indian reprocessing journey was started way back in 1964 with the commissioning of a plant based on PUREX technology to reprocess aluminum clad natural uranium spent fuel from the research reactor CIRUS. After achieving the basic skills, a power reactor reprocessing facility was built to reprocess spent fuel from power reactors. Adequate design and operating experience was gained from these two plants for mastering the reprocessing technology. The first plant, being the maiden venture, based on indigenous technology had to undergo many modifications during its operation and finally needed refurbishment for continued operation. Decommissioning and decontamination of this plant was carried out meticulously to allow unrestricted access to the cells for fresh installation. A third plant was built for power reactor spent fuel reprocessing to serve as a design standard for future plants with the involvement of industry. Over the years, spent fuel reprocessing based on PUREX technology has reached a matured status and can be safely deployed to meet the additional reprocessing requirements to cater to the expanding nuclear energy program. Side by side with the developments in the spent natural uranium fuel reprocessing, irradiated thoria reprocessing is also perused to develop THOREX into a robust process. The additional challenges in this domain are being addressed to evolve appropriate technological solutions. Advancements in the field of science and technology are being absorbed to meet the challenges of higher recovery combined with reduced exposure and environmental discharges.     

Scour Downstream of an Apron Due to Submerged Horizontal Jets

The results of an experimental investigation on scour of noncohesive sediment beds (uniform and nonuniform sediments) downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are presented. Attempts are made to explain the similarity existing in the scour process and profiles (including dune in the downstream of the scour hole). The scour profiles at different times follow a particular geometrical similarity and can be expressed by the combination of two polynomials. Using experimental scour depth at different times, the time variation of scour depth is scaled by an exponential law, where time scale increases linearly with densimetric Froude number. The equilibrium scour depth, related to the sediment size relative to the sluice opening, decreases with increase in sediment size and sluice opening. On the other hand, the equilibrium scour depth increases with increase in densimetric Froude number. The variation of equilibrium scour depth with tailwater depth indicates a critical tailwater depth corresponding to a minimum equilibrium scour depth. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to formation of an armor layer, and therefore prompted study of the reduction of scour depth by a launching apron placed downstream of the rigid apron. The results show that the average reduction of scour depth by placing a launching apron was 39%, having a maximum of 57.3% and a minimum of 16.2%. The characteristic parameters affecting maximum equilibrium scour depth are identified based on the physical reasoning and dimensional analysis. Equation of maximum equilibrium scour depth obtained empirically agrees well with the experimental data.     

Thermal Transport in BPSCCO System Between 20 and 170 K

Thermal conductivity of superconducting (Bi,Pb)2223 pellet in the temperature range 20–170 K is reported. Electronic contribution to thermal conductivity in the normal state is estimated to be 25%. Considering both phonon and phonon + electron approach, we attempted to examine the observed nature of the temperature dependence of thermal conductivity. Our analysis strongly supports the role of phonons as well as electrons in the origin of the thermal conductivity peak in the superconducting state. Some of the microscopic quantities evaluated from the best-fit parameters obtained from phonon + electron approach give reasonable values.     

Thermoelectric Power Studies on YBa2?xCaxCu3O7?δ System

The temperature dependence of the thermoelectric power of calcium substituted YBa_2–x Ca _x Cu₃O_7– pellets with 0x1.5 is presented between 60 K and 300 K. A metal–insulator transition was reported earlier by us in this system and was attributed to the ionic size-dependent localization effect. While the sign of thermoelectric power of all the calcium substituted samples was found to be positive, its magnitude increases significantly with calcium content in YBCO. The normal state thermoelectric power data of substituted YBa_2–x Ca _x Cu₃O_7– (0x1.5) are discussed in light of a two-band model originally proposed by Gottwick et al. for heavy fermion systems and later modified by Forro et al.     

A note on the linear instability of the Blasius flow

Summary It is found that in a ribbon-excited Blasius boundary layer, a wave Reynolds number defined here on the basis of phase speed and wave number of the disturbance remains nearly independent of the local mean flow Reynolds number, and so also of the streamwise distance, under the parallel flow approximation. Consequently, a limited similarity feature of the Orr-Sommerfeld equation has been found to exist for the streamfunction in the outer region of the boundary layer.     

Design of testable sequential circuits by repositioning flip-flops

This paper presents a technique to enhance the testability of sequential circuits by repositioning flip-flops. A novel retiming for testability technique is proposed that reduces cycle lengths in the dependency graph, converts sequential redundancies into combinational redundancies, and yields retimed circuits that usually require fewer scan flip-flops to break all cycles (except self-loops) as compared to the original circuit. Our technique is based on a new minimum cost flow formulation that simultaneously considers the interactions among all strongly connected components (SCCs) of the circuit graph to minimize the number of flip-flops in the SCCs. A circuit graph has a vertex for every gate, primary input and primary output. If gatea has a fanout to gateb, then the circuit graph has an arc from vertexa to vertexb. Experimental results on several large sequential circuits demonstrate the effectiveness of the proposed retiming for testability technique in reducing the number of partial scan flip-flops.     

Dispersion reduction in open-channel liquid electrochromatographic columns via pressure-driven back flow

Application of electrokinetic forces to drive the mobile phase diminishes analyte dispersion in open-channel liquid chromatographic columns due to minimization of shear in the flow field. However, the retentive layer coating the inner walls of such devices slows down the average convective velocity of solute molecules in its vicinity, inherently causing dispersion of analyte bands. In this article, we explore the possibility of reducing such dispersion in electrochromatographic columns by imposing a pressure-driven back flow in the system. Analysis shows that although such a strategy introduces shear in the flow field, the overall dispersion in the mobile phase is reduced. This occurs as the streamline velocity in such a system is greater near the channel walls than that in the center of the conduit, thereby allowing fluid dispersion to counteract wall retention effects. For an optimally chosen magnitude of the back flow, hydrodynamic dispersion of any target species in the mobile phase may be shown to diminish by a factor of 3 and 10/3 in a circular tube and a parallel-plate geometry, respectively. A similar reduction in slug dispersion is also realized in rectangular conduits for all aspect ratios. In trapezoidal geometries with large wedge angles or isotropically etched profiles, this reduction factor may attain values of 10 or greater.     

Scour below a High Vertical Drop

Experimental results on scour below a high vertical drop (drop height/critical depth >1) in uniform sands and gravels are presented. The experimental results are used to describe the effects of important parameters, identified from the dimensional analysis, on equilibrium scour depth. The important observations are that the equilibrium scour depth increases with increase in densimetric Froude number, whereas the scour depth decreases with increase in sediment size and tailwater depth. The time scale of scour depth that follows an exponential law is determined. The nondimensional time scale decreases with increase in densimetric Froude number.     

Diamond like carbon coatings deposited by microwave plasma CVD: XPS and ellipsometric studies

Diamond-like carbon (DLC) films were deposited by microwave assisted chemical vapour deposition system using d.c. bias voltage ranging from −100 V to −300 V. These films were characterized by X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry techniques for estimating sp ³/sp ² ratio. The sp ³/sp ² ratio obtained by XPS is found to have an opposite trend to that obtained by spectroscopic ellipsometry. These results are explained using sub-plantation picture of DLC growth. Our results clearly indicate that the film is composed of two different layers, having entirely different properties in terms of void percentage and sp ³/sp ² ratio. The upper layer is relatively thinner as compared to the bottom layer.     

Transactions Letters - Outage Capacity and Optimal Power Allocation for Multiple Time-Scale Parallel Fading Channels

In this paper, we address the optimal power allocation problem for minimizing capacity outage probability in multiple time-scale parallel fading channels. Extending ideas from the work of Dey and Evans (2005), we derive the optimal power allocation scheme for parallel fading channels with fast Rayleigh fading, as a function of the slow fading gains. Numerical results are presented to demonstrate the outage performance of this scheme for lognormal slow fading on two parallel channels.