| 1. |
- Wolniewicz, Peter, 1978-, et al.
(författare)
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Reactivity changes in lead-cooled fast reactors due to bubbles in the coolant
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The formation of bubbles in the coolant of a Lead-Cooled Fast Reactor (LFR) may originate from a leaking heat-exchanger and is a potential safety hazard. Small bubbles can travel with the coolant without escaping to the cover gas, causing an increasing effective voiding of the coolant in a homogeneous manner. If the small bubbles coalesce into a larger bubble located at a stagnation zone, the reactor core may eventually be exposed to a transient bubble travelling axially through the core with a resulting change in the reactivity of the system. This study is focused on the reactivity changes caused by bubbles of various sizes and for different vertical positions as the bubble rises through the core. Three different sizes of LFR’s; 50 MWth, 300 MWth and 1200 MWth,respectively were user for the study. The 300 MWth reactor design is based on the Advanced LFR European Demonstrator (ALFRED) and the two other reactors are scaled up and scaled down versions of it and these were simulated in order study the sensitivity to void as a function of reactor size. We show that LFR’s may have a positive reactivity response to transient bubbles and that the sensitivity to changes in reactivity is larger the smaller the reactor. For sufficiently large bubbles all reactors may reach prompt criticality.
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| 2. |
- Alm Rosenblad, Magnus, 1957, et al.
(författare)
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The Signal Recognition Particle of the diplomonad Giardia lacks the Alu domain responsible for translational arrest
- 2008
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Ingår i: RNA Society Meeting 2008.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- One of the most conserved cellular processes is the co-translational targeting of secretory and membrane proteins to the Sec translocon by the Signal Recognition Particle (SRP). This ribonucleoprotein particle consists in most eukaryotes of one RNA molecule and six proteins, and may be divided into two domains with distinct functions: the "S domain", which is most conserved, binds to the nascent peptide chain as it emerges from the exit tunnel of the ribosome, and the "Alu domain" which has a translation-regulatory function and causes an elongation arrest of the peptide chain. Of the six proteins only two is part of the Alu domain: SRP9/14.
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| 3. |
- Anders, Alfjorden, et al.
(författare)
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Experimental challenge of Atlantic salmon (Salmo salar) with the diplomonad parasite Spironucleus salmonicida to characterize the infection cycle
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Experimental infections were performed of Atlantic salmon (Salmo salar) from the Baltic Sea region with the Diplomonad fish parasite Spironucleus salmonicida in order to define the infection cycle, specifically the time-line and putative routes of transmission. An oral infection protocol using axenic parasites was developed, as were new diagnostic tools using PCR and specific antibodies. We also produced firefly luciferase expressing S. salmonicida parasites that could be identified in the infected fish using in vivo and ex vivo imaging. The new tools made it possible to follow the S. salmonicida infection cycle in detail. Three different stages of the infection were identified: one initial intestinal stage, followed by a blood stage and a final tissue stage. Parasites intubated into the intestine attached to the intestinal surface and were identified in the blood after 1-3 weeks. Skin lesions and infections of the muscles, internal organs and eyes were seen 4-10 weeks after initiation of infection. Several morphologically different forms of S. salmonicida cells were detected in ex vivo cell-cultures of biopsies from skin lesions. By this infection trial we have been able to show that S. salmonicida may use several alternative routes of transmission. One alternative is the fecal-oral route, similar to other Diplomonad parasites but the parasites can also be excreted directly into the surrounding water from the mucous layer of the skin or from an ulcerated skin lesion. This information can be used to prevent the transmission of the parasite in fish farms.
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| 4. |
- Andersson, Jan O., et al.
(författare)
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A genomic survey of the fish parasite Spironucleus salmonicida indicates genomic plasticity among diplomonads and significant lateral gene transfer in eukaryote genome evolution
- 2007
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Ingår i: BMC Genomics. - : Springer Science and Business Media LLC. - 1471-2164. ; 8, s. 51-
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Forskningsöversikt (refereegranskat)abstract
- Background: Comparative genomic studies of the mitochondrion-lacking protist group Diplomonadida (diplomonads) has been lacking, although Giardia lamblia has been intensively studied. We have performed a sequence survey project resulting in 2341 expressed sequence tags (EST) corresponding to 853 unique clones, 5275 genome survey sequences (GSS), and eleven finished contigs from the diplomonad fish parasite Spironucleus salmonicida (previously described as S. barkhanus). Results: The analyses revealed a compact genome with few, if any, introns and very short 3′ untranslated regions. Strikingly different patterns of codon usage were observed in genes corresponding to frequently sampled ESTs versus genes poorly sampled, indicating that translational selection is influencing the codon usage of highly expressed genes. Rigorous phylogenomic analyses identified 84 genes - mostly encoding metabolic proteins - that have been acquired by diplomonads or their relatively close ancestors via lateral gene transfer (LGT). Although most acquisitions were from prokaryotes, more than a dozen represent likely transfers of genes between eukaryotic lineages. Many genes that provide novel insights into the genetic basis of the biology and pathogenicity of this parasitic protist were identified including 149 that putatively encode variant-surface cysteine-rich proteins which are candidate virulence factors. A number of genomic properties that distinguish S. salmonicida from its human parasitic relative G. lamblia were identified such as nineteen putative lineage-specific gene acquisitions, distinct mutational biases and codon usage and distinct polyadenylation signals. Conclusion: Our results highlight the power of comparative genomic studies to yield insights into the biology of parasitic protists and the evolution of their genomes, and suggest that genetic exchange between distantly-related protist lineages may be occurring at an appreciable rate in eukaryote genome evolution.
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| 5. |
- Andersson, Jan O., et al.
(författare)
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The genome of Giardia and other diplomonads
- 2010
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Ingår i: Anaerobic Parasitic Protozoa: Genomics and Molecular Biology. - : Caister Academic Press. - 9781904455615 ; , s. 23-44
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 6. |
- Andersson, Peter, 1981-, et al.
(författare)
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Correction for dynamic bias error in transmission measurements of void fraction
- 2012
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 83:12, s. 125110-
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic bias errors occur in transmission measurements, such as X-ray, gamma, or neutron radiography or tomography. This is observed when the properties of the object are not stationary in time and its average properties are assessed. The nonlinear measurement response to changes in transmission within the time scale of the measurement implies a bias, which can be difficult to correct for. A typical example is the tomographic or radiographic mapping of void content in dynamic two-phase flow systems. In this work, the dynamic bias error is described and a method to make a first-order correction is derived. A prerequisite for this method is variance estimates of the system dynamics, which can be obtained using high-speed, time-resolved data acquisition. However, in the absence of such acquisition, a priori knowledge might be used to substitute the time resolved data. Using synthetic data, a void fraction measurement case study has been simulated to demonstrate the performance of the suggested method. The transmission length of the radiation in the object under study and the type of fluctuation of the void fraction have been varied. Significant decreases in the dynamic bias error were achieved to the expense of marginal decreases in precision.
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| 7. |
- Andersson, Peter, 1981-, et al.
(författare)
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Design and initial 1D radiography tests of the FANTOM mobile fast-neutron radiography and tomography system
- 2014
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 756, s. 82-93
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Tidskriftsartikel (refereegranskat)abstract
- The FANTOM system is a tabletop sized fast-neutron radiography and tomography system newly developed at the Applied Nuclear Physics Division of Uppsala University. The main purpose of the system is to provide time-averaged steam-and-water distribution measurement capability inside the metallic structures of two-phase test loops for Light Water Reactor thermal-hydraulic studies using a portable fusion neutron generator. The FANTOM system provides a set of 1D neutron transmission data, which may be inserted into tomographic reconstruction algorithms to achieve a 2D mapping of the steam-and-water distribution. In this paper, the selected design of FANTOM is described and motivated. The detector concept is based on plastic scintillator elements, separated for spatial resolution. Analysis of pulse heights on an event-to-event basis is used for energy discrimination. Although the concept allows for close stacking of a large number of detector elements, this demonstrator is equipped with only three elements in the detector and one additional element for monitoring the yield from the neutron generator. The first measured projections on test objects of known configurations are presented. These were collected using a Sodern Genie 16 neutron generator with an isotropic yield of about 1E8 neutrons per second, and allowed for characterization of the instrument’s capabilities. At an energy threshold of 10 MeV, the detector offered a count rate of about 500 cps per detector element. The performance in terms of spatial resolution was validated by fitting a Gaussian Line Spread Function to the experimental data, a procedure that revealed a spatial unsharpness in good agreement with the predicted FWHM of 0.5 mm.
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| 8. |
- Andersson, Peter, 1981-, et al.
(författare)
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Effects of proton escape on detection efficiency in thin scintillator elements and its consequences for optimization of fast-neutron imaging
- 2011
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 651:1, s. 110-116
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Tidskriftsartikel (refereegranskat)abstract
- Plastic scintillators are commonly used for neutron detection in the MeV energy range, based on n–p scattering and the subsequent deposition of recoil proton's kinetic energy in the detector material. This detection procedure gives a quasi-rectangular energy deposition distribution for mono-energetic neutrons, extending from zero to the neutron energy. However, if the detector sensitive element (DSE) is small, the energy deposition may be incomplete due to the recoil proton escape.In the application of neutron imaging, here exemplified by fast-neutron tomography, two conflicting requirements have been identified: (1) thin DSEs are required to obtain high spatial resolution and (2) energy discrimination may be required to reduce the influence of neutrons being scattered into the DSEs, which generally occurs at lower energies. However, at small DSE widths, the reduction of energy deposition due to recoil proton escape may cause a significant decrease in detection efficiency when energy discrimination is applied.In this work, energy deposition distributions in small-size DSEs have been simulated for Deuterium–Deuterium (DD; 2.5 MeV) and Deuterium–Tritium (DT; 14.1 MeV) fusion neutrons. The intrinsic efficiency has been analyzed as a function of energy discrimination level for various detector widths. The investigations show that proton recoil escape causes a significant drop in intrinsic detection efficiency for thin DSEs. For DT neutrons, the drop is 10% at a width of 3.2 mm and 50% at a width of 0.6 mm, assuming an energy threshold at half the incident neutron energy. The corresponding widths for a DD detector are 0.17 and 0.03 mm, respectively.Finally, implications of the proton escape effect on the design of a fast-neutron tomography device for void distribution measurements at Uppsala University are presented. It is shown that the selection of DSE width strongly affects the instrument design when optimizing for image unsharpness.
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| 9. |
- Andersson, Peter, 1981-
(författare)
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Fast-Neutron Tomography using a Mobile Neutron Generator for Assessment of Steam-Water Distributions in Two-Phase Flows
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions.By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection.The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced.Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements.Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations.
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| 10. |
- Andersson, Peter, 1981-, et al.
(författare)
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Neutron tomography for void distribution measurements
- 2010
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Ingår i: ENC 2010 Transactions. - 9789295064096 ; , s. 40-45
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Neutron tomography has previously been performed using large, stationary neutron sources such as reactors and spallation sources for applications where the object under study can be transported to the source. This paper accounts for the challenges met when applying neutron tomography using a portable accelerator driven neutron generator, which is required when studying non-transportable objects. In general, portable sources offer significantly lower neutron yields than stationary sources, implying the need for either longer measurement times or highly efficient measurement and/or analysis procedures.The particular application investigated here is the mapping of steam distributions in water (void distribution), which is of high importance for the performance of nuclear fuel assemblies in boiling water reactors (BWR). The void distribution cannot be measured directly in a reactor core, so instead various electrically-heated thermal-hydraulic test loops are used. In these loops, void correlations can be determined in full-size fuel-assembly models, such as FRIGG in Sweden and DESIRE in Holland, but measurements are also performed in smaller, less complicated geometries. Previously, gamma tomography has been used to measure the void distribution in the FRIGG loop. However, improved capabilities to map the void distribution can be expected using neutrons because of their higher sensitivity to water relative to metal structures, as compared to gamma rays. At the same time, neutrons as probe also give rise to some challenges, such as high background from scattering.This paper investigates the possibility to use neutron tomography at axially symmetric objects such as the HWAT test loop in Sweden, where an annular two-phase flow of water/void is confined and heated by a steel cylinder. Monte Carlo simulations of the HWAT geometry and a suggested measurement setup have been carried out, using the particle transport code MCNPX. A reconstruction technique which exploits the symmetries in the test loop has been developed, making it possible to reconstruct the internal void distribution from one single projection. A reconstruction is presented, which is based on simulated data corresponding to a 13-min measurement using a DT source emitting 2∙109 neutrons/s. The reconstruction offers a radial view of the local void fraction in 10 annular sections of HWAT, with uncertainties between 2 and 5 void percent units.
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