Mixed convective flow penetration in vertical channels

The primary purpose of the study is to investigate the factors relevant to the decay heat removal system in pool-type liquid metal reactors which are designed to remove decay heat in a passive way utilizing natural circulation. The reactor geometry is simulated by a vertical rod bundle channel connected to an upper plenum. Penetration of cold fluid from the upper plenum into the rod bundle channel is investigated experimentally and analytically with water as a working fluid. Three correlations to predict the onset of penetration, the penetration depth, and the ratio of penetrating to forced flowrate were developed. The correlations were found to agree well with experimental results for the range of Reynolds number in which experimental data were obtained.     

Oxide thickness-dependent transient cladding hoop stress

A few thrice-burned Zry-4 fuel assemblies which were loaded in one of the PWRs operating in Korea were found to be failed due to PCI during a power ramp following a rector trip, while thrice-burned Zr–Nb fuel assemblies and twice-burned Zry-4 ones were intact. To investigate the effect of fuel rod oxide thickness on power ramp-induced cladding hoop stress, three intact fuel rods were selected, which include an intact twice-burned Zry-4 fuel rod, an intact thrice-burned Zr-4 fuel rod and an intact thrice-burned Zr–Nb fuel rod. With the use of a fuel performance analysis code, burnup-dependent steady-state cladding stress and ramp power-dependent cladding stresses at the power-ramped burnup were predicted for the three intact fuel rods. It was found that the cladding oxide thickness has a considerable effect on the ramp power-dependent cladding hoop stresses. In addition, the cladding maximum stress of the thrice-burned Zry-4 fuel rod with 125 μm oxide thickness exceeded an ultimate cladding tensile strength of the Zry-4 cladding when the pellet–clad friction coefficient-dependent cladding stress concentration ratio was considered. However, the thrice-burned Zr–Nb fuel rod with 50 μm oxide thickness was evaluated to have a considerable margin against the power ramp-induced PCI failure.     

Development of a maintenance effectiveness monitoring program for CANDU reactors

A procedural program to monitor the effectiveness of maintenance activities was developed for CANDU reactors and, to confirm its applicability, was tested on a CANDU power plant being operated by the Korea Hydro & Nuclear Power Co. The monitoring program is based on a methodology utilizing probabilistic risk information to meet US regulation 10CFR50.65, which is known as the Maintenance Rule. There are many cases in which the Maintenance Rule is applied to Light Water Reactor systems, including PWRs and BWRs. However, it has not been applied to a CANDU Reactor System thus far. In this paper, a procedure to set up a maintenance effectiveness monitoring program is presented with an emphasis on its application to the CANDU system. Relevant solutions to problems that were encountered are introduced to make the program more suitable for the characteristics of CANDU systems. In the end, an application of the program to an operating CANDU power plant is discussed to evaluate the performance status of the plant.     

Graphitic carbon grown on fluorides by molecular beam epitaxy

We study the growth mechanism of carbon molecules supplied by molecular beam epitaxy on fluoride substrates (MgF₂, CaF₂, and BaF₂). All the carbon layers form graphitic carbon with different crystallinities depending on the cation. Especially, the growth on MgF₂ results in the formation of nanocrystalline graphite (NCG). Such dependence on the cation is a new observation and calls for further systematic studies with other series of substrates. At the same growth temperature, the NCG on MgF₂ has larger clusters than those on oxides. This is contrary to the general expectation because the bond strength of the carbon-fluorine bond is larger than that of the carbon-oxygen bond. Our results show that the growth of graphitic carbon does not simply depend on the chemical bonding between the carbon and the anion in the substrate.     

The impact of device type and sizing on phase noise mechanisms

Phase noise mechanisms in integrated LC voltage-controlled oscillators (VCOs) using MOS transistors are investigated. The degradation in phase noise due to low-frequency bias noise is shown to be a function of AM-PM conversion in the MOS switching transistors. By exploiting this dependence, bias noise contributions to phase noise are minimized through MOS device sizing rather than through filtering. NMOS and PMOS VCO designs are compared in terms of thermal noise. Short-channel MOS considerations explain why 0.18-/spl mu/m PMOS devices can attain better phase noise than 0.18-/spl mu/m NMOS devices in the 1/f/sup 2/ region. Phase noise in the 1/f/sup 3/ region is primarily dependent upon the upconversion of flicker noise from the MOS switching transistors rather than from the bias circuit, and can be improved by decreasing MOS switching device size. Measured results on an experimental set of VCOs confirm the dependencies predicted by analysis. A 5.3-GHz all-PMOS VCO topology demonstrates measured phase noise of -124 dBc/Hz at 1-MHz offset and -100dBc/Hz at 100-kHz offset while dissipating 13.5 mW from a 1.8-V supply using a 0.18-/spl mu/m SiGe BiCMOS process.     

Graphitic carbon growth on crystalline and amorphous oxide substrates using molecular beam epitaxy

We report graphitic carbon growth on crystalline and amorphous oxide substrates by using carbon molecular beam epitaxy. The films are characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The formations of nanocrystalline graphite are observed on silicon dioxide and glass, while mainly sp² amorphous carbons are formed on strontium titanate and yttria-stabilized zirconia. Interestingly, flat carbon layers with high degree of graphitization are formed even on amorphous oxides. Our results provide a progress toward direct graphene growth on oxide materials.PACS: 81.05.uf; 81.15.Hi; 78.30.Ly.     

Quantification of creep stresses within HAZ in welded branch junctions

This paper quantifies the mismatch effect in creep properties on creep stresses in the heat‐affected zone of welded branches using systematic elastic‐creep finite element analysis. It is found that the section‐averaged normalized stresses in the heat‐affected zone can be uniquely characterized by the mismatch factor in creep. The relationship is almost linear and is not so sensitive to the loading condition. Implication to practical creep life assessment of welded branch components is discussed in the context of the R5 procedure.     

Limit loads and fracture mechanics parameters for thick-walled pipes

In this paper, information on plastic limit loads and both elastic and elastic-plastic fracture mechanics parameters is given for cracked thick-walled pipes with mean radius-to-thickness ratios ranging from two to five. It is found that existing limit load expressions for thin-walled pipes can be applied to thick-walled pipes, provided that they are normalized with respect to the corresponding un-cracked thick-walled pipe values. For elastic fracture mechanics parameters, FE values of the influence functions for the stress intensity factor and the crack opening displacement are tabulated. For elastic-plastic J, it is shown that existing reference stress based J estimates can be applied, provided that a proper limit load for thick-walled pipes is used.     

A Wideband ΔΣ Digital-RF Modulator for High Data Rate Transmitters

A wideband software-defined digital-RF modulator targeting Gb/s data rates is presented. The modulator consists of a 2.625-GS/s digital DeltaSigma modulator, a 5.25-GHz direct digital-RF converter, and a fourth-order auto-tuned passive LC RF bandpass filter. The architecture removes high dynamic range analog circuits from the baseband signal path, replacing them with high-speed digital circuits to take advantage of digital CMOS scaling. The integration of the digital-RF converter with an RF bandpass reconstruction filter eliminates spurious signals and noise associated with direct digital-RF conversion. An efficient passgate adder circuit lowers the power consumption of the high-speed digital processing and a quadrature digital-IF approach is employed to reduce LO feedthrough and image spurs. The digital-RF modulator is software programmable to support variable bandwidths, adaptive modulation schemes, and multi-channel operation within a frequency band. A prototype IC built in 0.13-mum CMOS demonstrates a data rate of 1.2 Gb/s using OFDM modulation in a bandwidth of 200 MHz centered at 5.25 GHz. In-band LO and image spurs are less than -59 dBc without requiring calibration. The modulator consumes 187 mW and occupies a die area of 0.72 mm².