| 1. |
- De Geer, Lars-Erik, et al.
(författare)
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A Nuclear Jet at Chernobyl Around 21:23:45 UTC on April 25, 1986
- 2018
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Ingår i: Nuclear Technology. - : Informa UK Limited. - 0029-5450 .- 1943-7471. ; 201:1, s. 11-22
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Tidskriftsartikel (refereegranskat)abstract
- The nature of two explosions that were witnessed within 3 s at the Chernobyl-4 reactor less than a minute after 21:23:00 UTC on April 25, 1986, have since then been the subject of sprawling interpretations. This paper renders the following hypothesis. The first explosion consisted of thermal neutron mediated nuclear explosions in one or rather a few fuel channels, which caused a jet of debris that reached an altitude of some 2500 to 3000 m. The second explosion would then have been the steam explosion most experts believe was the first one. The solid support for this new scenario rests on two pillars and three pieces of corroborating evidence. The first pillar is that a group at the V. G. Khlopin Radium Institute in then Leningrad on April 29, 1986, detected newly produced, or fresh, xenon fission products at Cherepovets, 370 km north of Moscow and far away from the major track of Chernobyl debris ejected by the steam explosion and subsequent fires. The second pillar is built on state-of-the-art meteorological dispersion calculations, which show that the fresh xenon signature observed at Cherepovets was only possible if the injection altitude of the fresh debris was considerably higher than that of the bulk reactor core releases that turned toward Scandinavia and central Europe. These two strong pieces of evidence are corroborated by what were manifest physical effects of a downward jet in the southeastern part of the reactor, by seismic measurements some 100 km west of the reactor, and by observations of a blue flash above the reactor a few seconds after the first explosion.
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| 2. |
- Dykin, Victor, 1985, et al.
(författare)
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The Molten Salt Reactor Point-Kinetic Component of Neutron Noise in Two-Group Diffusion Theory
- 2016
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Ingår i: Nuclear Technology. - 0029-5450 .- 1943-7471. ; 193:3, s. 404-415
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Tidskriftsartikel (refereegranskat)abstract
- The derivation of the point-kinetic component of the neutron noise in two-group diffusion theory in molten salt reactors (MSRs), based on different techniques, is discussed. First, the point-kinetic component is calculated by projecting the corresponding full space-frequency-dependent solution onto the static adjoint. Then, following the standard procedure in reactor physics, the point-kinetic solution is determined by solving the linearized point-kinetic equations. Both results are thereafter analyzed and compared quantitatively. Such a comparison clearly indicates that the solution obtained by the conventional derivation, i.e., from the point-kinetic equations, significantly differs from the exact one and is not able to reproduce certain features of the latter. Similar discrepancies between the two methods were also pointed out and confirmed earlier in one-group MSR calculations.
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| 3. |
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| 4. |
- Jeltsov, Marti, 1985-, et al.
(författare)
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Parametric study of sloshing effects in the primary system of an isolated lead-cooled fast reactor
- 2015
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Ingår i: Nuclear Technology. - 0029-5450 .- 1943-7471. ; 190:1, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Risks related to sloshing of liquid metal coolant due to seismic excitation need to be investigated. Sloshing effects on reactor performance include first, fluid-structure interaction and second, gas entrapment in the coolant with subsequent transport of void to the core region. While the first can hypothetically lead to structural damage or coolant spill, the second increases the risk of a reactivity insertion accident and/or local dryout of the fuel. A two-dimensional computational fluid dynamics study is carried out in order to obtain insights into the modes of sloshing depending on the parameters of seismic excitation. The applicability and performance of the numerical mesh and the Eulerian volume of fluid method used to track the free surface are evaluated by modeling a simple dam break experiment. Sloshing in the cold plenum free surface region of the European Lead-cooled SYstem (ELSY) conceptual pool-type lead-cooled fast reactor (LFR) is studied. Various sinusoidal excitations are used to imitate the seismic response at the reactor level. The goal is to identify the domain of frequencies and magnitudes of the seismic response that can lead to loads threatening the structural integrity and possible core voiding due to sloshing. A map of sloshing modes has been developed to characterize the sloshing response as a function of excitation parameters. Pressure forces on vertical walls and the lid have been calculated. Finally, insight into coolant voiding has been provided.
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| 5. |
- Mickus, Ignas, et al.
(författare)
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PERFORMANCE OF THE EXPLICIT EULER AND PREDICTOR-CORRECTOR-BASED COUPLING SCHEMES IN MONTE CARLO BURNUP CALCULATIONS OF FAST REACTORS
- 2015
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Ingår i: Nuclear Technology. - 0029-5450 .- 1943-7471. ; 191:2, s. 193-198
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Tidskriftsartikel (refereegranskat)abstract
- We present a stability test of the explicit Euler and predictor-corrector based coupling schemes in Monte Carlo burnup calculations of the gas fast reactor fuel assembly. Previous studies have identified numerical instabilities of these coupling schemes in Monte Carlo burnup calculations of thermal-spectrum reactors due to spatial feedback induced neutron flux and nuclide density oscillations, where only sufficiently small time steps could guarantee acceptable precision. New results suggest that these instabilities are insignificant in fast-spectrum assembly burnup calculations, and the considered coupling schemes can therefore perform well in fast-spectrum reactor burnup calculations even with relatively large time steps. Note: Some figures in this technical note may be in color only in the electronic version.
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| 6. |
- Neretnieks, Ivars, et al.
(författare)
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Density-Driven Mass Transfer in Repositories for Nuclear Waste
- 2019
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Ingår i: Nuclear Technology. - : Taylor & Francis. - 0029-5450 .- 1943-7471. ; 205:6, s. 819-829
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Tidskriftsartikel (refereegranskat)abstract
- In geologic repositories for nuclear waste located in crystalline rocks, the waste is surrounded by a bentonite buffer that in practice is not permeable to water flow. The nuclides must escape by molecular diffusion to enter the seeping water in the fractures of the rock. At high water-seepage rates, the nuclides can be carried away rapidly. The seepage rate of the water can be driven by the regional hydraulic gradient as well as by buoyancy-driven flow. The latter is induced by thermal circulation of the water by the heat produced by radionuclide decay. The circulation may also be induced by salt exchange between buffer and water in the fractures. The main aim of this paper is to explore how salt exchange between the backfill and mobile water in fractures, by buoyancy effects, can increase the escape rate of radionuclides from a repository. A simple analytical model has been developed to describe the mass transfer rate induced by buoyancy. Numerical simulations support the simple solution. A comparison is made with the regional gradient-driven flow model. It is shown that buoyancy-driven flow can noticeably increase the release rate.
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| 7. |
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| 8. |
- Winberg-Wang, Helen, et al.
(författare)
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A Note on the Use of Uranine Tracer to Visualize Radionuclide Migration Experiments : Some Observations and Problems
- 2019
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Ingår i: Nuclear Technology. - : Taylor & Francis. - 0029-5450 .- 1943-7471. ; 205:7, s. 964-977
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Tidskriftsartikel (refereegranskat)abstract
- Uranine is a dye commonly used in tracer experiments; it is chosen for its high visibility even at low concentrations. Uranine solutions are slightly denser than water at the same temperature. However, in laboratory experiments uranine solutions have been known to occasionally show unpredictable flow behaviors. This paper investigates the possible effect of light-induced density change to explain some of these behaviors. Uranine has a wide light absorption spectrum for visible light, which can heat the dye solution and lower its density to below that of the surrounding water, which induces buoyancy-driven flow. Simulations are made in both one dimension and two dimensions to determine the extent of the effect. The results are then compared to different experiments with unanticipated flow patterns.
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| 9. |
- Winberg-Wang, Helen
(författare)
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Diffusion in a Variable Aperture Slot : Impact on Radionuclide Release from a Repository for Spent Fuel
- 2018
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Ingår i: Nuclear Technology. - : TAYLOR & FRANCIS INC. - 0029-5450 .- 1943-7471. ; 204:2, s. 184-194
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Tidskriftsartikel (refereegranskat)abstract
- Diffusion experiments under stagnant conditions in a constant aperture and a variable aperture slot were made to obtain data for simulation of simultaneous flow and diffusion in fractures. This approach was necessitated by the need to avoid buoyancy-induced flow caused by density differences generated by the presence of a tracer. For this purpose, to avoid flow but negligibly influence diffusion the slots were filled with agar, which generates a 99% porous matrix, which negligibly affects diffusion but essentially stops flow. A simple photographic technique was used to follow diffusion and to determine the aperture distribution on the variable aperture slot. With the obtained data, numerical simulations were performed to illustrate how a solute diffuses from a source into the water seeping past. The results support the simple analytical solution that has been used to determine the escape of radionuclides from a damaged canister containing spent nuclear fuel in a geologic repository in fractured rock.
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| 10. |
- Fischer, Manfred, et al.
(författare)
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Core Melt Stabilization Concepts for Existing and Future LWRs and Associated Research and Development Needs
- 2016
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Ingår i: NUCLEAR TECHNOLOGY. - : American Nuclear Society. - 0029-5450. ; 196:3, s. 524-537
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Tidskriftsartikel (refereegranskat)abstract
- In the event of a severe accident in a nuclear power plant with the core melting, the stabilization of the molten corium is an important mitigation issue, as it can avoid late containment failure caused by basemat penetration, overpressure, or severe damage to internal structures. The related failure modes may result in significant long-term radiological consequences and related high costs. Because of this, the licensing frameworks of several countries now include a requirement to implement mitigative core melt stabilization measures. This applies not only to new builds but also to existing light water reactors. The paper gives an overview of the ex-vessel core melt stabilization strategies developed during the last decades. These strategies are based on a variety of physical principles, like melt fragmentation in a deep water pool or during the molten core-concrete interaction with top flooding, water injection from the bottom (COMET), and retention in an outside-cooled crucible structure. This overview covers the physical background and functional principles of these concepts, as well as their validation status and, if applicable, the remaining open issues and research and development needs. For the concepts based on melt retention inside a cooled crucible that have reached sufficient maturity to be implemented in current Generation III+ designs, like the VVER-1000/1200 and the European Pressurized Water Reactor, more detailed descriptions are provided, which include key aspects of the related technical realization. The paper is compiled using contributions from the main developers of the individual concepts.
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