Transient two-phase flow and application to severe nuclear reactor accident.

摘要

The present research is to study the transient two-phase flow in the postulated Direct Containment Heating reactor accident. The corium dispersion phenomenon in the accident scenario is investigated both experimentally and analytically. In addition, a non-linear stability criterion is developed for the separated two-phase flow in a horizontal channel, resulting in a more accurate flow regime transition criterion for the flow in the reactor cavity. For the DCH simulation experiments, a capacitance probe for liquid metal film thickness measurement is developed. It is a plate type electrode coated with insulating paint, and the film thickness is measured via the capacitance of the coating immersed in the film flow. A test facility with 1:10 scale of the Zion reactor geometry is designed and constructed for the DCH separate effect experiments. The tests are conducted under 1.4 MPa to 14.2 MPa vessel pressure with water or woods metal to simulate the corium. The parameters for theoretical analysis are obtained with various instruments. These include the size and size distribution of the dispersed droplets, the liquid film thickness and velocity in the cavity, and the pressure transient at different locations. Based on experimental results, a mechanistic model is developed for the corium dispersion process. In this model, the accident transient is divided into four stages: (1) liquid and gas blow-down, (2) liquid film flow in the cavity before gas discharge, (3) liquid and gas flow in the cavity, and (4) droplet transport and re-entrainment in the subcompartment. In each stage, the flow is modeled with the dominant driving mechanism. Consequently, the droplet dispersion and the mean droplet size in the containment are predicted. Compared to the experimental data, fairly good agreement is achieved. To help the modeling, the interfacial wave stability problem is investigated with wave front perturbation method. A wave stability criterion is obtained with weakly non-linear effects. Moreover, the types of the unstable waves, under-cut wave and roll wave, are predicted from the non-linear term.