| 1. |
- Hotta, A., et al.
(author)
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Experimental and Analytical Investigation of Formation and Cooling Phenomena in High Temperature Debris Bed
- 2019
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In: Journal of Nuclear Science and Technology. - : Taylor and Francis Ltd.. - 0022-3131 .- 1881-1248.
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Journal article (peer-reviewed)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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| 2. |
- Johnson, Kyle, et al.
(author)
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Oxidation of accident tolerant fuel candidates
- 2017
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In: Journal of Nuclear Science and Technology. - : Taylor & Francis. - 0022-3131 .- 1881-1248. ; 54:3, s. 280-286
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Journal article (peer-reviewed)abstract
- In this study, the oxidation of various accident tolerant fuel candidates produced under different conditions have been evaluated and compared relative to the reference standard–UO2. The candidates considered in this study were UN, U3Si2, U3Si5, and a composite material composed of UN–U3Si2. With the spark plasma sintering (SPS) method, it was possible to fabricate samples of UN with varying porosity, as well as a high-density composite of UN–U3Si2 (10%). Using thermogravimetry in air, the oxidation behaviors of each material and the various microstructures of UN were assessed. These results reveal that it is possible to fabricate UN to very high densities using the SPS method, such that its resistance to oxidation can be improved compared to U3Si5 and UO2, and compete favorably with the principal ATF candidates, U3Si2, which shows a particularly violent reaction under the conditions of this study, and the UN–U3Si2 (10%) composite.
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| 3. |
- Jolkkonen, Mikael, et al.
(author)
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Thermo-chemical modelling of uranium-free nitride fuels
- 2004
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In: Journal of Nuclear Science and Technology. - : Informa UK Limited. - 0022-3131 .- 1881-1248. ; 41:4, s. 457-465
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Journal article (peer-reviewed)abstract
- A production process for americium-bearing, uranium-free nitride fuels was modelled using the newly developed ALCHYMY thermochemical database. The results suggested that the practical difficulties with yield and purity are of a kinetic rather than a thermodynamical nature. We predict that the immediate product of the typical decarburisation step is not methane, but hydrogen cyanide. HCN may then undergo further reactions upon cooling, explaining the difficulty in observing any carbophoric molecules in the gaseous off stream. The thermal stability of nitride fuels in different environments was also estimated. We show that sintering of nitride compounds containing americium should be performed under nitrogen atmosphere in order to the avoid the excessive losses of americium reported from sintering under inert gas. Addition of nitrogen in small amounts to fuel pin filling gas also appears to significantly improve the in-pile stability of transuranium nitride fuels.
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| 4. |
- Jolkkonen, Mikael, et al.
(author)
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Uranium nitride fuels in superheated steam
- 2017
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In: Journal of Nuclear Science and Technology. - : Informa UK Limited. - 0022-3131 .- 1881-1248.
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Journal article (peer-reviewed)abstract
- Uranium mononitride (UN) pellets of different densities were subjected to a superheatedsteam/argon mixture at atmospheric pressure to evaluate their resistance to hydrolysis. Completedegradation of pure UN pellets was obtained within 1 hour in 0.50 bar steam at 500 °C. Theidentified reaction products were uranium dioxide, ammonia and hydrogen gas, with no detectableamounts of nitrogen oxides formed. However, the reaction could not be carried to completion, andthe presence of uranium sesquinitride and higher uranium oxides or uranium oxynitrides in the solidresidue is indicated. Evolution of elemental nitrogen was seen in connection with very high reactionrates. The porosity of the pellets was identified as the most important factor determining reactionrates at 400 – 425 °C, and it is suggested that in dense pellets, cracking due to internal volumeincrease initiates a transition from slow surface corrosion to pellet disintegration. The implicationsfor the use of nitride fuels in light water reactors are discussed, with some observations concerninghydrolysis as a method for 15N recovery from isotopically enriched spent nitride fuel.
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| 5. |
- Kajan, Ivan, 1984, et al.
(author)
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Interaction of ruthenium tetroxide with iodine-covered surfaces of materials in nuclear reactor containment building
- 2016
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In: Journal of Nuclear Science and Technology. - : Informa UK Limited. - 0022-3131 .- 1881-1248. ; 53:11, s. 1889-1898
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Journal article (peer-reviewed)abstract
- Volatile iodine and ruthenium species are likely to be released from the fuel during a severe nuclear accident. Both iodine and ruthenium are expected to deposit on the surfaces in the containment building of the nuclear power plant. It is assumed that, due to the different release times from the fuel, ruthenium will reach the containment at the time when surfaces are already deposited with iodine species. The influence of ruthenium tetroxide on elemental iodine-covered surfaces in the containment of nuclear power plants was studied in this work. The ability of ruthenium tetroxide to oxidize iodine deposits on zinc, aluminum, copper and epoxy paint at high humidity conditions was evaluated. Quantification of both iodine and ruthenium deposits was done by the means of gamma spectroscopy. The chemical speciation of deposited elements was observed with SEM, XPS and EDX techniques. Experiments showed that ruthenium tetroxide oxidized iodine deposits into the volatile forms of iodine on zinc and aluminum samples and higher iodine oxides in the case of copper and epoxy paint samples. A major increase of ruthenium uptake on iodine-exposed surfaces in comparison to clean surfaces was observed.
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| 6. |
- Kajan, Ivan, 1984, et al.
(author)
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Interaction of ruthenium tetroxide with surfaces of nuclear reactor containment building
- 2016
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In: Journal of Nuclear Science and Technology. - : Informa UK Limited. - 0022-3131 .- 1881-1248. ; 53:9, s. 1397-1408
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Journal article (peer-reviewed)abstract
- © 2016 Atomic Energy Society of Japan. All rights reserved. During a severe nuclear accident, different fission products will be released from the nuclear fuel and some of them may eventually reach the containment building. Ruthenium is considered to be an important fission product due to the possible formation of volatile oxides. Radiotoxicity and chemical toxicity of the volatile ruthenium compounds present a considerable hazard during a severe nuclear accident. In this work, experiments regarding behavior of ruthenium tetroxide in the reactor containment were performed. The interactions of ruthenium tetroxide (RuO4) with zinc, copper, aluminum and epoxy paint in dry and humid atmosphere were examined. SEM/EDX (scanning electron microscope/energy-dispersive X-ray spectroscopy), XPS (X-ray photoelectron spectroscopy) and EXAFS (extended X-ray absorption fine structure) techniques were used to identify the chemical composition of the deposits formed after the interaction of RuO4 with the different materials. Additionally, distribution of ruthenium between different metals was examined. Interaction of RuO4 with the studied samples led to formation of dark, ruthenium-rich deposits. Examination of these deposits showed different chemical speciation of ruthenium on the surface when compared to the deeper layers of deposits. Interaction of RuO4 with zinc, copper and aluminum resulted to different amounts of the deposited ruthenium on the metals.
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| 7. |
- Lopes, Denise Adorno, et al.
(author)
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Degradation of UN and UN-U3Si2 pellets in steam environment
- 2017
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In: Journal of Nuclear Science and Technology. - : Taylor & Francis. - 0022-3131 .- 1881-1248. ; 54:4, s. 405-413
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Journal article (peer-reviewed)abstract
- In this work, a systematic study of the degradation of UN pellets (density range 96%-99.9% and grain size of 6-24 mu m) and UN-10%U3Si2 (wt%) composite in a steam environment is presented. Static steam autoclave tests were performed at 300 degrees C and 9 MPa for period of 0.5-1.5 hours. Microstructural analyses of UN pellets show that, in a high-pressure atmosphere, the fuel collapses principally by intergranular cracking generated by the precipitation of an oxide phase in the grain boundaries. This mechanism leads to a premature mechanical collapse of the fuel pellet, exposing fresh surfaces to steam, and ultimately accelerating the oxidation process. Increasing density (specifically eliminating open porosity) was found to delay the oxidation process, while increasing grain size was found to accelerate the degradation process due to a greater susceptibility to mechanical fracture by way of intergranular oxidation. The performance of the UN-10%U3Si2 composite proved to be better when compared to UN. The U3Si2 phase served to stabilize the UN grain boundary interface and reacted preferentially with the steam, thereby altering the failure mechanism. In this composite material, the cracking was predominantly intra-granular and the exposure of fresh surfaces was limited, resulting in a slower degradation process.
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| 8. |
- Manngård, Tero, et al.
(author)
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Modelling and Simulation of Reactor Fuel Cladding under Loss-of-Coolant Accident Conditions
- 2011
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In: Journal of Nuclear Science and Technology. - : Atomic Energy Society of Japan. - 0022-3131 .- 1881-1248. ; 48:1, s. 39-49
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Journal article (peer-reviewed)abstract
- We present a unified model for calculation of zirconium alloy fuel cladding rupture during a postulated loss-of-coolant accident in light water reactors. The model treats the Zr alloy solid-to-solid phase transformation kinetics, cladding creep deformation, oxidation, and rupture as functions of temperature and time in an integrated fashion during the transient. The fuel cladding material considered here is Zircaloy-4, for which material property data (model parameters) are taken from the literature. We have modelled and simulated single-rod transient burst tests in which the rod internal pressure and the heating rate were kept constant during each test. The results are compared with experimental data on cladding rupture strain, temperature, and pressure. The agreement between computations and measurements in general is satisfactory. The effects of heating rate and rod internal pressure on the rupture strain are evaluated on the basis of systematic parameter variations of these quantities. In the α-phase of Zr, the burst strain decreases with increasing heating rate, whereas in the two-phase coexistence (α+β) domain and β-phase, the situation is more complex. Also, the mechanism for creep deformation in the (α+β) domain is not well understood; hence, its mechanistic constitutive relation is presently unknown.
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| 9. |
- Massih, Ali
(author)
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High-temperature creep and superplasticity in zirconium alloys
- 2013
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In: Journal of Nuclear Science and Technology. - : Taylor & Francis. - 0022-3131 .- 1881-1248. ; 50:1, s. 21-34
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Journal article (peer-reviewed)abstract
- High-temperature (≈ 900−1400 K) steady-state creep test data on as-received zirconium alloys, Zr-1wt%Nb and Zircaloy-4 used as fuel cladding materials in light water reactors are evaluated by employing two sets of models. In particular, the focus of the paper is on the former alloy and in the two-phase coexistence region, i.e. the (α+β)-domain of the alloy. In one modeling approach, the constitutive relations for the two single phase regions (α and β) are combined through a phase transition kinetic model and a phase mixing rule; in another, a superplasticity model is used directly to calculate the creep deformation rate as a function of stress and temperature in the (α+β)-domain. The results show that the former approach is inadequate in retrodicting the experimental data, while the latter one gives a fair overall agreement. The paper describes the details of the models, the data, and derivations of the constitutive laws.
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| 10. |
- Pazsit, Imre, 1948, et al.
(author)
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The role of the eigenvalue separation in reactor dynamics and neutron noise theory
- 2018
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In: Journal of Nuclear Science and Technology. - : Informa UK Limited. - 0022-3131 .- 1881-1248. ; 55:5, s. 484-495
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Journal article (peer-reviewed)abstract
- The concept of eigenvalue separation (ES) was introduced in the past for the characterisation of the space-time kinetics of reactor transients, and the stability properties of large loosely coupled cores. However, most of the investigations reported so far concern the determination of the ES itself either from static calculations, or from measurements of the flux tilt or neutron noise cross-correlations. Conclusions on system behaviour were only drawn from the properties of the static eigenfunctions, comparing non-perturbed and perturbed systems, without explicitly solving the time- or frequency-dependent problem. In this paper, we explore the role of the ES on the neutronic response of a critical core to small stochastic perturbations (neutron noise); in particular, the spatial and frequency characteristics of the arising neutron noise as a function of the ES, as well as the spatial structure of the perturbation. It is shown that for systems with small ES and non-uniform perturbations, point kinetics will not dominate even for very low frequencies. The results lend some further insight into the origin and properties of the various types of boiling water reactor instabilities.
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