| 1. |
- Chernikova, Dina, 1982, et al.
(författare)
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The effect of capture gammas, photofission and photonuclear neutrons to the neutron-gamma Feynman variance-to-mean ratios (neutron, gamma and total)
- 2015
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Ingår i: Mathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015, Nashville, United States, 19-23 April 2015. - 9781510808041 ; 2, s. 1040-1051
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Konferensbidrag (refereegranskat)abstract
- Two versions of the neutron-gamma variance-to-mean (Feynman-alpha) formula for separate gamma detection and total neutron-gamma detection were recently derived and evaluated by Chernikova, et. al. [1]. However, the neutrons and gammas emitted in a photofission reaction or the release of gammas in certain thermal neutron capture reactions were not included in the theoretical models of Chernikova, et. al. [1]. In this paper, in order to evaluate the influence of these type of reactions to the values the neutron-gamma Feynman variance-to-mean ratios (neutron, gamma and total), we derive the enhanced Feynman-alpha formulae for separate neutron, gamma detection and total neutron-gamma detections. The theoretical derivation is based on the Chapman-Kolmogorov equation with inclusion of general reactions, photofission and capture gammas. The quantitative evaluation of the effect of capture gammas and photonuclear neutrons to the neutron-gamma Feynman variance-to-mean ratios (neutron, gamma and total) is done by using reaction intensities obtained from MCNPX simulations. The new enhanced formulas and their impact to the final values of different variance-to-mean ratios are the main subject of the discussion in the present paper.
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| 2. |
- Demaziere, Christophe, 1973, et al.
(författare)
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Modelling of stationary fluctuations in nuclear reactor cores in the frequency domain
- 2015
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Ingår i: Mathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015. - : American Nuclear Society. - 9781510808041 ; , s. 2406-2419
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Konferensbidrag (refereegranskat)abstract
- This paper presents the development of a numerical tool to simulate the effect of stationary fluctuations in Light Water Reactor cores. The originating fluctuations are defined for the variables describing the boundary conditions of the system, i.e. inlet velocity, inlet enthalpy, and outlet pressure. The tool calculates the three-dimensional space-frequency distributions within the core of the corresponding fluctuations in neutron flux, coolant density, coolant velocity, coolant enthalpy, and fuel temperature. The tool is thus based on the simultaneous modelling of neutron transport, fluid dynamics, and heat transfer in a truly integrated and fully coupled manner. The modelling of neutron transport relies on the two-group diffusion approximation and a spatial discretization based on finite differences. The modelling of fluid dynamics is performed using the homogeneous equilibrium model complemented with pre-computed static slip ratio. Heat conduction in the fuel pins is also accounted for, and the heat transfer between the fuel pins and the coolant is modelled also using a pre-computed distribution of the heat transfer coefficient. The spatial discretization of the fluid dynamic and heat transfer problems is carried out using finite volumes. The tool is currently entirely Matlab based with input data provided by an external static core simulator. The paper also presents the results of dynamic simulations performed for a typical pressurized water reactor and for a typical boiling water reactor, as illustrations of the capabilities of the tool.
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| 3. |
- Pazsit, Imre, 1948, et al.
(författare)
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Energy correlations of prompt fission neutrons in the laboratory frame
- 2015
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Ingår i: Mathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015, Nashville, United States, 19-23 April 2015. - 9781510808041 ; 2, s. 1158-1167
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Konferensbidrag (refereegranskat)abstract
- Correlations between the energies and emission angles of prompt fission neutrons are of significance for all methods which use the statistics of detection events for determining subcritical reactivity in reactor cores or for non-destructive assay of nuclear materials for safeguards purposes. There is no experimental knowledge available on the existence or properties of such correlations. Therefore, recently increasing attempts are made to determine these correlations from the properties of the fission process. One possible reason of such correlations between fission neutron energies and angles in the laboratory system is the fact that the prompt neutrons are emitted from the moving fission targets, even if their energies and emission angles are independent in the moving frame of the fission fragment. In this paper this concept is investigated analytically and through numerical simulations. It is shown that such correlations are due to the random properties (energy and direction of motion) of the fission fragments, and the magnitude of the covariance depends on the second order moments of the fission fragment parameters. Preliminary numerical simulations show that the correlations in energy, generated this way, are rather small.
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| 4. |
- Pazsit, Imre, 1948, et al.
(författare)
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The stability of boiling water reactors as a catastrophe phenomenon
- 2015
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Ingår i: Mathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015, Nashville, United States, 19-23 April 2015. - 9781510808041 ; 1, s. 763-774
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Konferensbidrag (refereegranskat)abstract
- The stability of boiling water reactors (BWRs) is revisited in terms of catastrophe theory. The hypothesis is proposed that the stability parameter (decay ratio) of a complex, many-variable non-linear system might obey a cusp catastrophe. The incentive for this surmise comes from indications in real measurements that in certain cases the decay ratio appears to behave discontinuously and might show a hysteresis as a function of the control parameters reactor power and coolant flow. Such observations can be explained by a phenomenological catastrophe model suggested in this article. Since a cusp-type behaviour implies that the decay ratio is many-valued in a certain region of the power-flow map, a mechanism is suggested how a Hopf bifurcation with multiplicative noise can lead to such a behaviour.
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