| 1. |
- Morosan, D. E., et al.
(författare)
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LOFAR tied-array imaging of Type III solar radio bursts
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 568, s. articl no. A67-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (
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| 2. |
- Morosan, D. E., et al.
(författare)
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LOFAR tied-array imaging and spectroscopy of solar S bursts
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous millisecond duration radio bursts have been reported, such as radio spikes or solar S bursts (where S stands for short). To date, these have neither been studied extensively nor imaged because of the instrumental limitations of previous radio telescopes. Aims. Here, LOw Frequency ARray (LOFAR) observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms. Methods. We used 170 simultaneous tied-array beams for spectroscopy and imaging of S bursts. Since S bursts have short timescales and fine frequency structures, high cadence (similar to 50 ms) tied-array images were used instead of standard interferometric imaging, that is currently limited to one image per second. Results. On 9 July 2013, over 3000 S bursts were observed over a time period of similar to 8 h. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (similar to 2.5 MHz) features, the majority drifting at similar to 3.5 MHz s(-1) and a wide range of circular polarisation degrees (2-8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere. Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however, they possess some of the characteristics of electron-cyclotron maser emission.
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| 3. |
- Lammer, H., et al.
(författare)
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Exoplanet status report: Observation, characterization and evolution of exoplanets and their host stars
- 2010
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Ingår i: Solar System Research. - 1608-3423 .- 0038-0946. ; 44:4, s. 290-310
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Tidskriftsartikel (refereegranskat)abstract
- After the discovery of more than 400 planets beyond our Solar System, the characterization of exoplanets as well as their host stars can be considered as one of the fastest growing fields in space science during the past decade. The characterization of exoplanets can only be carried out in a well coordinated interdisciplinary way which connects planetary science, solar/stellar physics and astrophysics. We present a status report on the characterization of exoplanets and their host stars by reviewing the relevant space- and ground-based projects. One finds that the previous strategy changed from space mission concepts which were designed to search, find and characterize Earth-like rocky exoplanets to: A statistical study of planetary objects in order to get information about their abundance, an identification of potential target and finally its analysis. Spectral analysis of exoplanets is mandatory, particularly to identify bio-signatures on Earth-like planets. Direct characterization of exoplanets should be done by spectroscopy, both in the visible and in the infrared spectral range. The way leading to the direct detection and characterization of exoplanets is then paved by several questions, either concerning the pre-required science or the associated observational strategy.
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| 4. |
- Bilous, A. V., et al.
(författare)
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Dual-frequency single-pulse study of PSR B0950+08
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Tidskriftsartikel (refereegranskat)abstract
- PSR B0950+08 is a bright nonrecycled pulsar whose single-pulse fluence variability is reportedly large. Based on observations at two widely separated frequencies, 55 MHz (NenuFAR) and 1.4 GHz (Westerbork Synthesis Radio Telescope), we review the properties of these single pulses. We conclude that they are more similar to ordinary pulses of radio emission than to a special kind of short and bright giant pulses, observed from only a handful of pulsars. We argue that a temporal variation of the properties of the interstellar medium along the line of sight to this nearby pulsar, namely the fluctuating size of the decorrelation bandwidth of diffractive scintillation makes an important contribution to the observed single-pulse fluence variability. We further present interesting structures in the low-frequency single-pulse spectra that resemble the "sad trombones"seen in fast radio bursts (FRBs); although for PSR B0950+08 the upward frequency drift is also routinely present. We explain these spectral features with radius-to-frequency mapping, similar to the model developed by Wang et al. (2019, ApJ, 876, L15) for FRBs. Finally, we speculate that μs-scale fluence variability of the general pulsar population remains poorly known, and that its further study may bring important clues about the nature of FRBs.
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| 5. |
- Hotta, A., et al.
(författare)
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Experimental and Analytical Investigation of Formation and Cooling Phenomena in High Temperature Debris Bed
- 2019
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Ingår i: Journal of Nuclear Science and Technology. - : Taylor and Francis Ltd.. - 0022-3131 .- 1881-1248.
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Tidskriftsartikel (refereegranskat)abstract
- Key phenomena in the cooling states of underwater debris beds were classified based on the premise that a target debris bed has a complicated geometry, nonhomogeneous porosity, and volumetric heat. These configurations may change due to the molten jet breakup, droplet agglomeration, anisotropic melt spreading, two-phase flow in a debris bed, particle self-leveling and penetration of molten metals into a particle bed. Based on these classifications, the modular code system THERMOS was designed for evaluating the cooling states of underwater debris beds. Three tests, DEFOR-A, PULiMS, and REMCOD were carried in six phases to extend the existing database for validating implemented models. Up to Phase-5, the main part of these tests has been completed and the test plan has been modified from the original one due to occurrences of unforeseeable phenomena and changes in test procedures. This paper summarizes the entire test plan and representative data trends prior to starting individual data analyses and validations of specific models that are planned to be performed in the later phases. Also, it tries to timely report research questions to be answered in future works, such as various scales of melt-coolant interactions observed in the shallow pool PULiMS tests.
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| 6. |
- Bandaru, S V Ravikumar, et al.
(författare)
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Upward-facing multi-nozzle spray cooling experiments for external cooling of reactor pressure vessels
- 2020
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 163
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Tidskriftsartikel (refereegranskat)abstract
- Cooling by water spray is a well-known technology that can reach significantly higher Critical Heat Flux (CHF) compared to other cooling methods. For the light water reactor safety, the in-vessel retention (IVR) by external reactor vessel cooling (ERVC) is a comprehensive severe accident management strategy to arrest and confine the corium in the lower head of the reactor pressure vessel. Heat fluxes up to 1.5 MW/m2 have already been assumed attainable in low-power nuclear reactors while cooling required in high-power reactors is expected to reach 2.5 MW/m2. Instead of reactor lower head flooding and relying on cooling due to natural convection, a viable and more efficient alternative is to spray the external surface of the vessel. Given all the advantages of spray cooling reported in the literature, a lab-scale experimental facility was built to validate the efficiency of multi-nozzle spray cooling of a downward-facing heated surface inclined at different angles up to 90o. The facility employed a 2×3 matrix of spray nozzles to cool the FeCrAl alloy foil with an effectively heated surface area of 96 cm2 using water as the coolant. Heat loads and surface inclinations were varied parameters in the test matrix. The results show that no significant variations in spray cooling performance concerning the inclination of the heated surface. A surface heat flux of 2.5 MW/m2 was achieved at every inclination of the downward-facing surface. The results also indicate that more uniform liquid film distribution could be obtained for some inclinations, which in turn leads to maintaining low surface temperature. The obtained surface heat flux margin by spray cooling indicates that it is feasible to adopt IVR-ERVC strategy for a large power reactor.
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| 7. |
- Hoseyni, Seyed Mohsen, et al.
(författare)
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Metallic melt infiltration in preheated debris bed and the effect of solidification
- 2021
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Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 379, s. 111229-
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Tidskriftsartikel (refereegranskat)abstract
- The re-melting of multi-component debris is important for both in-vessel and ex-vessel phases of severe accident progression in nuclear power plants. However, current knowledge is limited with respect to understanding the associated complex phenomena and their interactions. In this paper, the phenomenon of melt infiltration through a porous debris bed with and without solidification is examined by synthesizing the data obtained from ongoing experimental research (REMCOD facility). In this regard, results obtained from 12 experiments are analyzed. Eight tests were conducted for melt infiltration through debris at temperatures above solidification. At this condition, two flow regimes are identified for the melt flow inside the hot porous debris, which is initially dominated by capillary forces and hydrostatic head and then later by the gravity forces. In addition, 4 tests were performed for melt penetration into cold debris where melt infiltration is limited by solidification. It was found that the depth of penetration is correlated with the difference between "sensible heat of melt" and "the amount of heat required to heat the bed up to the melting point of specific melt composition."
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| 8. |
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| 9. |
- Xiang, Yan, et al.
(författare)
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Pre-test simulation and a scoping test for dryout and remelting phenomena of an in-vessel debris bed
- 2023
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Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 403
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Tidskriftsartikel (refereegranskat)abstract
- The present study is intended to investigate the dryout and remelting phenomena of a debris bed during the late phase of an in-vessel severe accident progression. The SIMECO-2 facility at KTH is adapted to conduct the experimental investigation. For selection of an appropriate debris bed in the facility, pre-test simulations are performed by using the COCOMO code to determine: (i) simulant materials of debris particles; (ii) debris bed particle diameters; (iii) configuration and geometry of the debris bed (e.g., shape, layers, dimensions). Candidate particulate beds packed with different mixtures of particles are identified and simulated to obtain their thermal hydraulics in the hemispherical slice test section with radius of 500 mm and width of 120 mm. Based on the simulation results, a particulate bed is chosen and loaded in the SIMECO-2 facility for a scoping investigation. FBG probes with multiple measurement points of each probe are employed to acquire the temperature field of the particulate bed inductively heated. A video recording is applied to detect the dryout and remelting phenomena. In the scoping test, the dryout phenomenon occur first at the elevation of 5 cm from the bed surface under the induction heating power of 14.8 kW, which are comparable with the data predicted by the COCOMO code (6 cm from the bed surface under the heating power of 13.8 kW) in the pre-test simulations.
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| 10. |
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