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1.1 Problems in Modeling of Small Break LOCA

DOI: 10.1615/ICHMT.1982.AdvCourHeatTransfNucReactSaf.20
pages 3-48

Novak Zuber
Division of Reactor Safety Research, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, USA


This report considers two-phase flow phenomena which may occur in horizontal pipes during a small break LOCA. Specifically, it deals with:

  1. Two-phase flow regime transitions,
  2. Liquid entrainment in break flow,
  3. Vapor pull-through,
  4. Counter-current flow limitation (CCFL).
The first three processes influence the mass flow rate through the break, whereas the fourth one imposes a limit on the liquid flow from the steam generator through the hot leg back into the core.
The report presents some of the results and correlations available in the literature which can be used to estimate conditions in a duct or at a break, that can lead to a two-phase mixture reaching the break. These correlations are then applied to a hot leg of a PWR, LOFT and Semiscale for quantitative estimates. Secondly, the report deals with rules that scale the four processes noted above. These rules are then applied again, to a hot leg of a PWR, LOFT and Semiscale, to determine the scale distortion in the latter two facilities.
Calculations indicate that conditions which may lead to the occurrence of these four processes in a PWR, are bracketed by those scaled to LOFT and to Semiscale.
It should be stressed that data and correlations available in the literature, which are summarized and used in this report, often do not correspond exactly to conditions that one could expect in a PWR. Consequently, the results presented in this report must not be used for definitive quantitative statements. However, they may be useful for making estimates as well as for guiding experiments and interpreting their results.

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