Dryout model

Models available to explain the CHF phenomenon are the hydrodynamic instability model and the macrolayer dryout model. The former postulates that the increase in vapor generation from the heater surface causes a limit of the steady-state vapor escape flow when CHF occurs. The latter postulates that a liquid sublayer (macrolayer) formed on the heating surface (see Secs. 2.2.5.5 and 2.4.1.2)... 

The CHF in high-quality annular flow uses a liquid film dryout model, i.e., the concept of liquid film dryout (zero film flow rate) in annular flow. Sugawara (1990) reported an analytical prediction of CHF using FIDAS computer code based on a three-fluid and film dryout model. 

Sudo, Y., T. Usui, and M. Kaminaga, 1991, Experimental Study of Falling Water Limitation under a Counter-current Flow in a Vertical Rectangular Channel, JSMEInt. J. Ser. H 34 169-174. (5) Sugawara, S., 1990, Analytical Prediction of CHF by FIDAS Code Based on Three-Fluid and Film-Dryout Model, J. Nuclear Sci. Technol. 27. 12 29. (5)... 

A physical model of ONB for the explosive boiling and dryout, was suggested. In order to understand why dryout occurred even at a low value of vapor quality x, it is important to keep in mind that the liquid film does not cover the entire heated surface of the micro-channel, and two-phase flow is characterized by an unsteady cyclic behavior. The following assumptions are made in the development of the model ... 

Dispersed flow model. To calculate the actual quality, vapor temperature, and wall temperature, or heat flux, as functions of axial position beyond dryout... 

An improved CHF model for low-quality flow The Weisman-Pei model was later improved by employing a mechanistic CHF model developed by Lee and Mu-dawwar (1988) based on the Helmholtz instability at the microlayer-vapor interface as a trigger condition for microlayer dryout (Fig. 5.21). The CHF can be expressed by the following equation due to the energy conservation of the microlayer (Lin et al. 1989) ... 

Levy, Healzer, and Abdollahian (1980) predicted the dryout flux in vertical pipes by a semiempirical adiabatic model (Levy and Healzer, 1980) for liquid film flow and entrainment. It starts with a heat balance along the flow direction and a mass balance perpendicular to the flow direction ... 

The assumption that the fraction (1-Hce) of external catalyst surface is dry, as used by some other investigators (11), results in a very large B1q which cannot explain or even match the observed experimental results. Dryout of a catalyst surface appears possible only when much larger temperature gradients are present. On the other hand the assumption of riCE= 1 everywhere leads to unrealistic dependence of mass transfer coefficients on liquid velocity. Matching the data with a single parameter model (an overall mass transfer coefficient) results in too high an effect of velocity on such a parameter and in the loss of model predictive ability for different solvents. 

In the following, we examine the experimental results [127], that support these two asymptotic behaviors, that is. Bo > 1, the high-permeability asymptote, and Bo 1, the low-permeability asymptote. Then, we discuss the low-permeability asymptote using a onedimensional model [109,125,128], The one-dimensional model is also capable of predicting qa, that is, the onset of dryout. We note that the hysteresis observed in isothermal two-phase flow in porous media is also found in evaporation-condensation and that the q versus T0 - Ts curve shows a decreasing q (or T0 - Ts) and an increasing q (or T0 - Ts) branch. 

Drop-wall interaction. Hie earlier models ignored the contribution of drop-wall interaction. The direct evaporation of droplets at the hot channel wall can play an important role when the wall temperatures are relatively low just upstream of the dryout point. Evans et al. [344] made measurements of vapor superheat just downstream of the dryout point and found that for approximately one-third of a meter downstream, the vapor remained at its saturation temperature, indicating that, in this region, the heat flux was being absorbed by... 

The second functional event AC-0.5HR in the station blackout event tree is related to the restoration of electrical power to the plant safety buses before the dryout of the coolant on the secondary side of the steam generator, assessed from the wide range level, and start of the core heat up. Period of 30 min is assumed to be available for restoration of power in the PSA model for this functional event (Volkanovski and Prosek, 2013, Bertucio et al., 1990). The successful mitigation of the transient is related with power restoration within first 30 min results in successful mitigation of the transient, regardless of other failures, except criticality, in that time period. 

The flow distribution problem within the sub-channels of the rod cluster fuel elements is solved by the HAMBO code using a power distribution derived from the reactor physios calculations. The effects of flow mixing between sub-channels and the influence of grids on pressure drop are included through experimentally determined parameters. Dryout limits to performance are determined by the use of special empirical correlations which describe the performance of the individual sub-channels, HAMBO is used to predict the overall channel properties for use in simpler one-dimensional models and to indicate the dryout limits on channel output. 

R. Lipinski, "Debris Bed Dryout," Appendix 3.2-B of Thermal-Hydraulic Process Modeling in Risk Analysis An Assessment of the Releyant Systems. Structures, and Phenomena. NUREG/CR-3986, SAND84-1219, Sandia National Laboratories, Albuquerque, NM, August 1984. 

Pool scrubbing is produced in the secondary side of the broken steam generator, starting with the gap release, at 8276 s, and ending the dryout of the failed steam generator at 16401 s. During this time, 7.5 Kg of materials, most of it Cs(OH), are retained, as given in table 4.3.2.3. Nevertheless, it is considered that the SPARC model in MELCOR does not properly represent the complicated phenomena taken place under such accidental conditions. 

Pool scrubbing may be particularly important in the secondary side of a broken steam generator, mainly if feed water is added to prevent its dryout. Nevertheless, the SPARC models included in MELCOR may not represent correctly the complicated phenomena taking place under such accidental circunstances. 

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