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Simulations and Models of Neutron Fluxes in BWRs Intended for Depletion Calculations of Withdrawn Control Rods
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
(Vattenfall Nuclear Fuel AB)
(Vattenfall AB)
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2011 (English)In: Nuclear science and engineering, ISSN 0029-5639, Vol. 177, no 3, 221-229 p.Article in journal (Refereed) Published
Abstract [en]

Models of the neutron flux shape in a withdrawn control rod in a boiling water reactor (BWR) bottom reflector have been constructed from simulations with the Monte Carlo code MCNP. These neutron flux models are intended for determining absorber depletion and fast fluence accumulation for withdrawn control rods with nodal codes. So-called G-factors are created for coupling the neutron flux models to a conventional nodal code via the core bottom neutron flux. The neutron flux models and G-factors are created for three different neutron energies, and their dependence on various parameters such as blanket enrichments, Hf and B(4)C control rod absorber, and depletion and reflector geometry is investigated. The neutron flux models and G-factors are found to be very insensitive; the neutron flux models predict the simulated neutron flux in the withdrawn control rod from MCNP over a variety of reflector configurations with an error < 3.0%. This implies that the neutron flux models constructed in this paper are generally applicable for BWR reflectors and control rods not fundamentally deviating from the designs investigated in this paper.

Place, publisher, year, edition, pages
2011. Vol. 177, no 3, 221-229 p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-132859ISI: 000288103600004OAI: oai:DiVA.org:uu-132859DiVA: diva2:359342
Available from: 2010-10-27 Created: 2010-10-27 Last updated: 2011-08-22Bibliographically approved
In thesis
1. Novel Diagnostics and Computational Methods of Neutron Fluxes in Boiling Water Reactors
Open this publication in new window or tab >>Novel Diagnostics and Computational Methods of Neutron Fluxes in Boiling Water Reactors
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The focus in this thesis is to improve knowledge of the BWR related uncertainties void, channel bow, and control rods. The presence of void determines the moderation of neutrons in BWRs. A high void fraction is less efficient in moderating neutrons than a low one. As a consequence, the ratio of thermal to fast neutrons is dependent on the surrounding void fraction. In this thesis, calculations with 2D/3D codes corroborate this dependence, the void correlation, to be linear and very robust to changes in different reactor parameters. The void fraction could be predicted from the ratio of simultaneously measured reaction rates from thermal and fast neutron detectors over the whole core with an uncertainty of ±1.5%. The only parameter found disturbing the void correlation significantly is channel bow. However, since channel bow is the only phenomenon found biasing the void correlation, it is found that the void prediction methodology can be used to indicate channel bow with a sensitivity of 4% per mm bow. Consequently, large channel bows could easily be detected. Increased knowledge of void fractions and channel bow could increase both safety and economy of nuclear power production.

This thesis also investigates how 2D/3D codes used in production perform in calculating detailed impact of control rods on pin powers and their ability to perform control rod depletion calculations in the reflector region. It is found that the axial resolution used in 3D nodal codes has very large impact on pin power gradients, i.e., using a standard nodal size of ~15 cm can cause underestimations of 50% in pin power gradients, which could lead to fuel damages. In addition, two methods for determining the neutron flux in the control rod when it is withdrawn from the core are presented. Both methods can be used in a 3D nodal code to reproduce the neutron flux in the reflector region with an uncertainty of ±3%.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 55 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 786
Keyword
Void, BWR, neutron fluxes, channel bow, MCNP, 2D/3D
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-133238 (URN)978-91-554-7943-5 (ISBN)
Public defence
2010-12-16, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Note
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 715Available from: 2010-11-23 Created: 2010-11-03 Last updated: 2011-03-21Bibliographically approved

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