| 1. |
- Wallenius, Janne, 1968-, et al.
(author)
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Muonic atom deexcitation via formation of metastable molecular states in light of experimental verification
- 2001
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In: Hyperfine Interactions. - 0304-3843 .- 1572-9540. ; 138:04-jan, s. 285-288
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Journal article (peer-reviewed)abstract
- In a recent experiment performed at PSI, a peak in the time-of-flight distribution of pmu(1s) atoms could be identified with decay of ppmu* molecular ions situated below the 2s threshold, providing 900 eV of kinetic energy to the pmu atom. This finding may be interpreted in terms of the side path model which suggests that metastable muonic molecules may form with high probability in resonant collisions between muonic hydrogen in the 2s state and hydrogen molecules, e.g., pmu(2s)H-2-->[(ppmu*)(vJ)(pq) - pee(])v(K) --> [(ppmu*)(v'J')(p'q') - pe](+) + e(-). The Coulombic decay of the Auger stabilised ppmu* molecular ion then leads to the formation of highly energetic pmu(1s) atoms. In the present paper calculations of resonant formation rates in pure hydrogen are presented and compared to the quenching rate of pmu(2s) atoms measured at low hydrogen density.
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| 2. |
- Eriksson, Marcus, et al.
(author)
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Safety Analysis of Na and Pb-Bi Coolants in Response to Beam Instabilities
- 2003
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In: UTILISATION AND RELIABILITY OF HIGH POWER PROTON ACCELERATORS, WORKSHOP PROCEEDINGS. - 9264102116 ; , s. 227-236
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Conference paper (peer-reviewed)abstract
- A comparative safety study has been performed on sodium vs. lead/bismuth as coolant for accelerator-driven systems. Transient studies are performed for a beam overpower event. We examine a fuel type of recent interest in the research on minor actinide burners, i.e. uranium-free oxide fuel. A strong positive void coefficient is calculated for both sodium and lead/bismuth. This is attributed to the high fraction of americium in the fuel. It is shown that the lead/bismuth-cooled reactor features twice the grace time with respect to fuel or cladding damage compared to the sodium-cooled reactor of comparable core size and power rating. This accounts to the difference in void reactivity contribution and to the low boiling point of sodium. For improved safety features the general objective is to reduce the coolant void reactivity effect. An important safety issue is the high void worth that could possibly drive the system to prompt criticality.
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| 3. |
- Gudowski, Waclaw, et al.
(author)
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Review of the European project - Impact of Accelerator-Based Technologies on Nuclear Fission Safety (IABAT)
- 2001
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In: Progress in nuclear energy (New series). - 0149-1970 .- 1878-4224. ; 38:1-2, s. 135-151
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Journal article (peer-reviewed)abstract
- The IABAT project - Impact of Accelerator Based Technologies on Nuclear Fission Safety - started in 1996 in the frame of 4(th) Framework Programme of the European Union, R&D specific programme Nuclear fission safety 1994-1998, area A.2 Exploring innovative approaches/Fuel cycle concepts, as one of the first common European activities in ADS. The project was completed October 31, 1999. The overall objective of the IABAT project has been a preliminary assessment of the potential of Accelerator-Driven Systems (ADS) for transmutation of nuclear waste and for nuclear energy production with minimum waste generation. Moreover, more specific topics related to nuclear data and code development for ADS have been studied in more detail. Four ADSs have been studied for different fuel/coolant combinations: liquid metal coolant and solid fuel, liquid metal coolant and dispersed fuel, and fast and thermal molten salt systems. Target studies comprised multiple target solutions and radiation damage problems in a target environment. In a tool development part of the project a methodology of subcriticality monitoring has been developed based on Feynman-alpha and Rossi-alpha methods. Moreover, a new Monte-Carlo burnup code taking full advantage of continuous neutron cross-section data has been developed and benchmarked. Impact on the risk from high-level waste repositories fi om radiotoxicity reduction using ADS has been assessed giving no crystal-clear benefits of ADS for repository radiotoxicity reduction but concluding some important prerequisites for effective transmutation. In proliferation studies important differences between critical reactors and ADS have been underlined and non-proliferation measures have been proposed. In assessment of accelerator technology costing models have been created that allow the circular and linear accelerator options to be compared and the effect of parameter variations examined. The calculations reported show that cyclotron systems would be more economical, due mainly to the advantage of the cost of RF power supplies. However, the accelerator community regards with skepticism the possibility of transporting and extracting more than a 10mA beam current from a 1GeV cyclotron and therefore technical factors may limit the application of cyclotrons. Finally, this review summarizes development of nuclear data in the energy region between 20 Mev and 150 MeV. Neutron and proton transport data files for Fe, Ni, Pb, Th, U-238 and Pu-239 have been created. The high-energy part of the data files consists completely of results from model calculations, which are benchmarked against the available experimental data. Although there is obviously future work left regarding fine-tuning of several parts of the data files, the representation of nuclear reaction information up to 150 MeV is already better than can be attained with intranuclear cascade codes.
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| 4. |
- 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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| 5. |
- Lindroth, E., et al.
(author)
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Decay rates of excited muonic molecular ions
- 2003
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In: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 68:3
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Journal article (peer-reviewed)abstract
- Muonic molecular ions in excited states have been predicted to form in collisions between excited muonic atoms and hydrogen molecules. We have calculated radiative and Coulombic decay rates for ppmu(*) and ddmu(*) molecular states located below the 2s threshold, using the complex rotation method. The x-ray spectrum from the radiative decay is shown to exhibit several maxima, corresponding to the vibrational motion of the decaying molecule. The branching ratio of the radiative decay mode was calculated to be less than 15% for ppmu(*), while a radiative yield of more than 80% is predicted for the decay of ddmu(*). Our results have a significant impact on the analysis of the muon catalyzed fusion cycle as well as on the interpretation of exotic hydrogen spectroscopy.
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| 6. |
- Malerba, L., et al.
(author)
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Molecular dynamics simulation of displacement cascades in Fe-Cr alloys
- 2004
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 329-33, s. 1156-1160
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Journal article (peer-reviewed)abstract
- An embedded atom method (EAM) empirical potential recently fitted and validated for Fe-Cr systems is used to simulate displacement cascades up to 15 keV in Fe and Fe-10%Cr. The evolution of these cascades up to thermalisation of the primary damage state is followed and quantitatively analysed. Particular attention is devoted to assessing the effect of Cr atoms on the defect distribution versus pure Fe. Using the Wigner-Seitz cell criterion to identify point defects, first results show that the main effect of the presence of Cr in the system is the preferential formation of mixed Fe-Cr dumbbells and mixed interstitial clusters, with expected lower mobility than in pure Fe.
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| 7. |
- Olsson, Pär, et al.
(author)
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Ab initio formation energies of Fe-Cr alloys
- 2003
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 321:1, s. 84-90
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Journal article (peer-reviewed)abstract
- We have calculated ab initio lattice parameters, formation energies, bulk moduli and magnetic moments of Fe-Cr alloys. The results agree well with available experimental data. In addition to body centered cubic (bcc) alloys, which are representative of ferritic steels used in fast neutron reactors, face centered cubic (fcc) and hexagonal close packed (hcp) phases were considered in order to complete a theoretical database of thermodynamic properties. Calculations were done for the ferromagnetic phase, as well as for a phase with local moment disorder, simulating the magnetic structure at high temperatures. For the latter case, the formation energy of the alloy is strictly positive smooth function of chromium concentration, in agreement with experiments performed at high temperature. In the ferromagnetic case, a negative mixing enthalpy is found for chromium concentrations below 6 %. Our observation is consistent with the experimentally observed inversion of the ordering trend, as well as with formation of the chromium rich alpha phase at Cr-concentrations above 9%, occurring at T < 900 K.
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| 8. |
- Seltborg, Per, et al.
(author)
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Definition and application of proton source efficiency in accelerator driven systems
- 2003
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In: Nuclear science and engineering. - 0029-5639 .- 1943-748X. ; 145:3, s. 390-399
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Journal article (peer-reviewed)abstract
- In order to study the beam power amplification of an accelerator-driven system (ADS), a new parameter, the proton source efficiency psi* is introduced. psi* represents the average importance of the external proton source, relative to the average importance of the eigenmode production, and is closely related to the neutron source efficiency rho*, which is frequently used in the ADS field. rho* is commonly used in the physics of subcritical systems driven by any external source (spallation source, (d,d), (d, t), Cf-252 spontaneous fissions, etc.). On the contrary, psi* has been defined in this paper exclusively for ADS studies where the system is driven by a spallation source. The main advantage with using psi* instead of rho* for ADS is that the way of defining the external source is unique and that it is proportional to the core power divided by the proton beam power, independent of the neutron source distribution. Numerical simulations have been performed with the Monte Carlo code MCNPX in order to study psi* as a function of different design parameters. It was found that, in order to maximize psi* and therefore minimize the proton current needs, a target radius as small as possible should be chosen. For target radii smaller than similar to30 cm, lead-bismuth is a better choice of coolant material than sodium, regarding the proton source efficiency, while for larger target radii the two materials are equally good. The optimal axial proton beam impact was found to be located similar to 20 cm above the core center. Varying the proton energy, psi*/E-p was found to have a maximum for proton energies between 1200 and 1400 MeV Increasing the americium content in the fuel decreases psi* considerably, in particular when the target radius is large.
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| 9. |
- Seltborg, Per, et al.
(author)
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Impact of heterogeneous Cm-distribution on proton source efficiency in accelerator-driven systems
- 2004
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In: Proceedings of the PHYSOR 2004. - 0894486837 ; , s. 489-497
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Conference paper (peer-reviewed)abstract
- The proton source efficiency (ψ*) was studied for homogeneous and heterogeneous distributions of minor actinides in a nitride-fuelled and lead-bismuth-cooled accelerator-driven system. The findings from the MCNPX simulations indicate that, compared to a homogeneous configuration, a gain in ψ* by up to 16% can be obtained by distributing the minor actinides heterogeneously, Cm being placed in the inner zone of the active core and Am in the outer zone. The reason for this is the higher fission probability for neutrons for Cm than for Am in the energy range below 1.0 MeV. Moreover, a comparative study of two different physics packages available in MCNPX, the Bertini and the CEM models, has been performed, focusing on the production of neutrons in the spallation target and on the proton source efficiency. The Bertini model was found to produce a higher number of neutrons in the low-energy range (below ∼15 MeV) than the CEM model. Consequently, the Bertini model also over-estimates ψ* by about 10%, compared to the CEM model.
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| 10. |
- Tucek, Kamil, et al.
(author)
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Coolant void worth in fast breeder reactors and accelerator-driven transuranium and minor-actinide burners
- 2004
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In: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 31:15, s. 1783-1801
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Journal article (peer-reviewed)abstract
- Liquid metal coolant void worth have been calculated as a function of fuel composition and core geometry for several model fast breeder reactors and accelerator-driven systems (ADSs). The Monte Carlo transport code MCNP with continuous energy cross-section libraries was used for this study. With respect to the core void worth, lead/bismuth cooled FBR, appear to be inferior to those employing sodium for pitch-to-diameter ratios exceeding 1.4. It is shown that in reactor systems cooled by lead/bismuth eutectic radial steel pin reflector significantly lowers the void worth. The void worth proves to be a strong function of the fuel composition, reactor cores with high content of minor actinides in fuel exhibiting larger void reactivities than systems with plutonium based fuel. Enlarging the lattice pitch in ADS burners operating on Pu rich fuel decreases the void worth while the opposite fact is true for ADSs employing americium based fuels.
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