Numerical simulation of the thermohydraulics of reactor fuel assemblies with blockages

Computer codes for analyzing the thermohydraulics of fuel assemblies with blocked coolant flow cross sections are examined. The computational results obtained with these codes are analyzed, and a conclusion is drawn on the basis of this analysis about the most important computational codes in this field. The results of the analysis are illustrated. An extensive literature from the worldwide practice of the thermohydraulics of blocked fuel assemblies is presented. Further work required to improve the computer codes is indicated. 7 figures. 2 tables, 65 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power Engineering Institute. Translated from Atomanya énergiya, Vol. 87, No. 5, pp. 342–356, November, 1999.     

Carrier heating in quantum wells under optical and current injection of electron-hole pairs

Carrier heating in GaAs/AlGaAs quantum wells (QWs) under optical interband pumping in the spontaneous-emission mode has been studied. The electron temperature was determined as a function of the pumping intensity. The effect of the electric field on the photoluminescence spectrum was examined. The change in the carrier concentration with the drive current in the spontaneous- and stimulated-emission modes in InGaAsSb/InAlGaAsSb QWs was determined from electroluminescence spectra. The rise in the temperature of hot carriers, which results in the increase in the carrier concentration with the drive current, was roughly estimated.     

Optical anisotropy of InAs quantum dots

The optical anisotropy of InAs quantum dots (QDs) synthesized in the regime of either continuous or submonolayer deposition on a singular GaAs(100) surface have been studied using polarized photoluminescence measurements. It is established that an isolated array of QDs formed in a continuous deposition regime possesses a weak (<1–2%) optical anisotropy, whereas the vertical matching (coupling) of such QDs via less than 15-nm-thick spacer layers leads to an 8% linear polarization of PL along the [0$ \bar 1 $ \bar 1 1] crystallographic direction. QDs formed in the regime of submonolayer deposition exhibit a strong (17–20%) anisotropy of emission from the ground and excited states of QDs in the same crystallographic direction.     

Lasing at 1.5 µm in quantum dot structures on GaAs substrates

Lasing at 1488–1515 nm in the temperature range 20–83°C was obtained in structures with an active region based on multiply stacked arrays of self-organized quantum dots grown on GaAs substrates. The threshold current density of a laser with four cleaved facets was 800 A/cm2 at room temperature. The method of wavelength extension is based on the use of a metamorphic buffer layer with an In content of about 20% intended for relieving the lattice mismatch stress.     

Pilling-Resistant Polyester Fibre

The possibility of chemically modifying polyester using a polyfunctional compound as a modifier is examined. The technology developed was used to set up manufacture of pilling-resistant fibre with characteristics that satisfy consumer requirements. It was found that incorporation of components that do not chemically react with polyester during synthesis increases the pilling resistance and dyeability of the fibre with disperse dyes. The additional use of organic acids in synthesis makes it possible to flexibly regulate the dyeability of the fibre.     

The effect of a “Coulomb well” on the absorption and magnetoabsorption spectra of strained InGaAs/GaAs heterostructures

The optical and magnetooptical properties of strained InGaAs/GaAs quantum-well heterostructures grown by molecular-beam epitaxy were studied at T=1.7 K in magnetic fields B⩽7.5 T. The well-resolved oscillatory structure of the magnetoabsorption spectra makes it possible to reproduce the “fan diagrams” for transitions between Landau levels of the HH1E1 quantum-confined states, taking into account exciton binding energies calculated variationally. Based on these results, reduced cyclotron masses of carriers were calculated for quantum wells with various indium contents. A self-consistent variational solution to the exciton problem in the structure under study shows that for weak type-II potentials the effect of Coulomb localization of the hole leads to a relative increase in the oscillator strength of the LH1E1 exciton transition. In this case the LH1E1 and LH3E1 exciton transitions remain spatially direct and retain a considerable intensity. The calculated splitting of ∼9 meV between these two states in zero magnetic field is found to be in agreement with experiment. The significant oscillator strength of light-hole excitons, along with the observed doublet structure, are experimental confirmations that electron-hole attraction can transform a rather low barrier for light holes in a type-II structure into a quantum well with a parabolic “Coulomb” shape near its bottom, i.e., a “Coulomb well.” Fiz. Tekh. Poluprovodn. 31, 1109–1120 (September 1997)