This paper describes the work done in collaboration with the Spanish Nuclear Regulatory Commission (CSN), and it is related with scaling analysis in multiphase systems like pressure water reactors (PWR) during accidental conditions. A method was developed in order to identify possible deviations from thermal-hydraulic similarity, or scale distortion, between two multiphase systems; and it consists of the following steps. First governing conservation equations are derived. Then, the equations are nondimensionalized with reference parameters. And an order of magnitude analysis is performed based on the numerical values of the nondimensionalized coefficients with only the large order terms being retained, which are the terms that impact on system response. So that, equations are used to identify the leading processes and quantify the scale distortions in both systems. The scaling criteria are expressed through π-Groups. This technique has been used to assess the capability of a test facility to simulate the global system response of a typical PWR during a small break lossof-coolant accident (SBLOCA). After developing governing conservation equations, it obtained a simple multi-phase pressure rate equation useful for the analysis of the depressurization of a system containing subcooled liquid and saturated fluid, like the vessel or the hot-legs in a power plant. Then the equation is nondimensionalized. And it is found that the same nondimensional groups are important for both systems. Indicating that, although there are some distortions in scaling, the behaviour of both systems is very similar.
In this paper, a time dependent model for studying BWR in phase instabilities in the nonlinear regime is developed. This model is based on solving the mass energy and momentum conservation equations, for the single phase region and for both phases in the sub-cooled boiling and the bulk boiling regions. The model has been implemented in a code that also integrates: the recirculation loop dynamics equation; the heat transfer dynamic equations between the fuel and the channel; and the neutron kinetics equations. Special attention has been given in this paper to the sub-cooled boiling region and to the consequences of the degree of sub-cooling on the bubble dynamics. Also the direct heating of the water channel by neutrons and gamma rays has been considered. The result is a code called DYNAMICS that is able to perform in the time domain quantitative analysis of all the processes that affect the reactor stability. Keywords: density wave oscillations, two phase flow channels, in phase oscillations in BWR, sub-cooled boiling.
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