Regional Basic Professional Training Course in Korea

❙ 519 ❙
8-B. Deterministic Accident Analysis (LOCA)
The rate of depressurization decreases as the difference between vessels and containment
pressures decreases. The rate of steam generation due to flashing decreases, the rate of
liquid dumping into the lower plenum increases, and the sweepout rate decreases. Very
little ECC injection is bypassed out the break; most of it flows to fill the downcomer and
lower plenum.
(3) Reflood phase
The core begins to reflood with ECC water after the lower plenum is filled. Initially, core
refill is quite rapid because the downcomer is maintained full by the continued ECC
injection and there is very little resistance in the intact loop because there is no steam
flow and no liquid to vaporize in the steam generator.
Due to high temperatures of fuel rods, the thermal regimes in the core during reflood
encompass the entire spectrum starting with single liquid and progressing with nucleate
boiling, transition boiling, film boiling, churn twe‐phase flow, dispersed droplet flow and
single phase steam flow as shown in Figure 8.7‐4. Due to droplet deentrainment at the tie
plate and grid spacers, top‐quenching and local quenching occur in addition to the bottom
one. Higher vapor velocities and liquid entrainment occur in the central region of the core
due to higher powered fuel rods. The entrained liquid has a beneficial cooling effect.
Some of the entrained liquid is deentrained at the upper tie plate. The remainder is carried
into the upper plenum, where it is deentrained, forming a two‐phase pool. Liquid from the
pool can reenter the low power regions of the core because of the lower vapor velocities.
As a result, a 3‐D flow pattern is established. Liquid from the two‐phase pool in upper
plenum may be entrained by vapor and carried over to hot legs. In traversing the upper
plenum it can be deentrained on upper plenum structure.
As the bottom quench progresses upward through he core, more liquid is carried over to