On the role of buoyancy in determining the course of PWR boron dilution transients

• PWR downcomer boron concentration is computed using a 3D-unsteady formulation.
• There are two stages: first, an establishment of concentration stratification.
• This is succeeded by a coherent downwards progression of low-borated water.
• However, large-scale mixing can occur at low values of the Richardson number (Ri).
• A critical Ri value/range might determine whether stratification or mixing occurs.

A computational investigation is undertaken into the role of buoyancy in a PWR boron dilution transient initiated by a postulated Small Break Loss of Coolant Accident (SB-LOCA). The scenario envisages a flow of de-borated and relatively high temperature water entering the annular downcomer from a single cold leg; flow rates are typical of natural circulation conditions. The study focuses on the temporal and spatial development of boron concentration distributions in the downcomer. The physical framework consists of a 3D-unsteady formulation of the mean flow and standard high-Reynolds-number k-ε turbulence model equations. It is found that the Richardson number (Ri = Gr/Re2) is the most important group in relation to the parameterization of the course of a concentration transient, and at Ri values characterizing a ‘baseline’ scenario, the present results indicate that there is a stable, circumferentially-uniform, descent through the downcomer of a stratified region of low-borated fluid. The same qualitative behaviour is found at a higher Richardson number; however, when Ri is reduced to approximately one-fifth of the baseline level there is evidence of large-scale mixing and a consequent absence of concentration stratification.