Avalanche multiplication in a compact bipolar transistor model forcircuit simulation

A simple weak avalanche model valid in a wide range of voltages and currents, is presented. The proposed model is derived by using the base-collector depletion capacitance for predicting the avalanche current. The model needs only one additional transistor parameter; the extraction method and temperature dependence of this parameter are discussed. The decrease in avalanche current for high collector current densities may originate from internal device heating, a voltage drop in the epilayer, or mobile carriers in the depleted part. From experimental results it is concluded that, below a critical hot-carrier current, the decrease in avalanche current due to mobile carriers is negligible     

Investigation of bounds on particle packing in pebble-bed high temperature reactors

Models and methods are presented for determining practical limits of the packing density of TRISO particles in fuel pebbles for a pebble-bed reactor (PBR). These models are devised for designing and interpreting fuel testing experiments. Two processes for particle failure are accounted for: failure of touching particles at the pressing stage in the pebble manufacturing process and failure due to inner pressure buildup during irradiation. The second process gains importance with increasing fuel temperature, which limits the particle packing density and the corresponding fuel enrichment. Suggestions for improvements to the models are presented.     

HTGR reactor physics and fuel cycle studies

The high-temperature gas-cooled reactor (HTGR) appears as a good candidate for the next generation of nuclear power plants. In the “HTR-N” project of the European Union Fifth Framework Program, analyses have been performed on a number of conceptual HTGR designs, derived from reference pebble-bed and hexagonal block-type HTGR types. It is shown that several HTGR concepts are quite promising as systems for the incineration of plutonium and possibly minor actinides.These studies were mainly concerned with the investigation and intercomparison of the plutonium and actinide burning capabilities of a number of HTGR concepts and associated fuel cycles, with emphasis on the use of civil plutonium from spent LWR uranium fuel (first generation Pu) and from spent LWR MOX fuel (second generation Pu). Besides, the “HTR-N” project also included activities concerning the validation of computational tools and the qualification of models. Indeed, it is essential that validated analytical tools are available in the European nuclear community to perform conceptual design studies, industrial calculations (reload calculations and the associated core follow), safety analyses for licensing, etc., for new fuel cycles aiming at plutonium and minor actinide (MA) incineration/transmutation without multi-reprocessing of the discharged fuel.These validation and qualification activities have been centred round the two HTGR systems currently in operation, viz. the HTR-10 and the HTTR. The re-calculation of the HTTR first criticality with a Monte Carlo neutron transport code now yields acceptable correspondence with experimental data. Also calculations by 3D diffusion theory codes yield acceptable results. Special attention, however, has to be given to the modelling of neutron streaming effects. For the HTR-10 the analyses focused on first criticality, temperature coefficients and control rod worth. Also in these studies a good correspondence between calculation and experiment is observed for the 3D diffusion theory codes.     

New breeding gain definitions and their application to the optimization of a molten salt reactor design

The molten salt reactor (MSR) is an attractive breeder reactor. A graphite-moderated MSR can reach breeding because of the online salt processing and refueling. These features give difficulties when the breeding gain (BG) of the MSR is evaluated. The inventory of the core and external stockpiles have to be treated separately in order to quantify the breeding performance of the reactor. In this paper, an improved BG definition is given and it is compared with definitions used earlier. The improved definition was used in an optimization study of the graphite – salt lattice of the core. The aim of the optimization is a passively safe, self-breeder reactor. The fuel channel diameter, graphite volume and thorium concentration were varied while the temperature feedback coefficient of the core, BG – as defined in the paper – and the lifetime of the graphite were calculated. There is a small range of lattices which provide both negative temperature feedback and breeding. Furthermore, breeding is possible only at low power densities in case of the salt processing efficiencies set in this study. In this range of power the lifetime of the graphite is between 12 and 20 years.     

Low molecular weight Cooperia oncophora antigens. Potential to discriminate between susceptible and resistant calves after infection

The recognition of low molecular weight proteins by sera obtained during a single oral (primary) infection with 100,000 3rd-stage Cooperia oncophora larvae was studied in calves. Three groups of 6 or 7 calves were selected based on different egg excretion patterns. SDS-gel electrophoresis of adult Cooperia antigen under reducing conditions, followed by Western blotting, revealed that resistance of individual calves to C. oncophora might be related with antibody responses (42 days post infection) against at least 2 protein fragments (14-16 kDa and 27 kDa). The 14-16-kDa protein complex was bound, to some extent, by individual sera from all calves. The intensity of staining was negatively correlated with egg excretion on Day 42 p.i. Calves with high egg counts on Day 21 p.i. either did not or only weakly recognized the 27-kDa band. It has to be established whether the 14-16 kDa (or recombinant 14.2 kDa) provides a tool for immunodiagnostics and whether the 27-kDa fragment can help further unravel immune-mediated resistance to Cooperia.     

Applying burnable poison particles to reduce the reactivity swing in high temperature reactors with batch-wise fuel loading

Burnup calculations have been performed on a standard HTR fuel pebble with a radius of 3 cm containing 9 g of 8% enriched uranium and burnable poison particles (BPP) made of B₄C highly enriched in ¹⁰B. The radius of the BPP and the number of particles per fuel pebble have been varied to find the flattest reactivity-to-time curve. It was found that for a k∞ of 1.1, a reactivity swing as low as 2% can be obtained when each fuel pebble contains about 1070 BPP with a radius of 75 μm. For coated BPP that consist of a graphite kernel with a radius of 300 μm covered with a B₄C burnable poison layer, a similar value for the reactivity swing can be obtained. Cylindrical particles seem to perform worse. In general, the modification of the geometry of BPP is an effective means to tailor the reactivity curve of HTRs.     

Recent Activities in the Field of Thermal Barrier Coatings Including Burner Rig Testing in the European Union

Although thermal barrier coatings are used in industry for several decades there is still considerable demand for a further improvement of this coating system. Present research projects are for example dealing with the improvement of temperature capability and lifetime, developing of lifetime models, new processing technologies, or incorporating of additional functional properties in the coating. The present paper tries to give an overview on these recent research activities. In addition, it will also try to describe the different burner rig test facilities in Europe, which serve as an important test bed of thermal barrier coating systems.     

Just for Starters: Commercial Gentrification by Immigrant Entrepreneurs in Amsterdam and Rotterdam Neighbourhoods

Many large European cities are now displaying clear social, ethnic and spatial divisions. These different types of cleavages tend to overlap. Governments try to chase away this spectre of an increasingly divided city by embarking on various policies. These policies generally neglect the (potential) role of immigrant entrepreneurs in improving neighbourhoods. In this contribution, we have focused on the immigrant business start-ups in Amsterdam and Rotterdam. Neighbourhoods with high shares of immigrants indeed turn out to show relatively higher rates of immigrant businesses than other neighbourhoods in these two cities. Immigrant entrepreneurs, may, therefore, strengthen the local economy of these neighbourhoods and offer not only specific goods and services but also jobs, nodes of information and role models. Urban policies should, hence, explicitly target their policies to this kind of immigrant-driven process of commercial gentrification by creating cheap commercial properties in these neighbourhoods.