16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 162 / 295Document ID.No.UKEPR-0002-162 Issue 04Consequently, the accumulating water pool on top of the melt will stay sub-cooled and steamdischarge into the containment is completely stopped. After the melt is again at hightemperature, the decay heat generated in the bulk contributes to the heating of the water. Thetotal decay power is sufficient not only to bring the newly arriving water to saturation conditionsbut also to heat the accumulated pool. As soon as this pool becomes saturated, steamdischarge into the containment restarts.2.4.1.5.3. Results2.4.1.5.3.1. MCCIDuring the MCCI, the density of the oxidic melt is reduced by the incorporation of concretedecomposition products. Due to this, the melt is expected to form a stable stratified system withthe lighter oxide layer on top of the heavier metal. Therefore, the sacrificial concrete on top ofthe bottom cooling structure interacts exclusively with the metallic melt. At the beginning of thisprocess, the cooldown of the initially superheated metal can result in high rates of concreteablation and gas release.When the metal layer temperature approaches the solidification temperature, erosion ratesdecline. The further course of the MCCI is dominated by the energy transport from the decayheatedoxidic melt.This behaviour is illustrated by the calculation for the LB(LOCA) case (seeSub-section 16.2.2.4 - Figure 20), which shows two distinctly different phases. In the secondphase, the ablation speed and the rate of release of concrete decomposition gas aresignificantly lower than in the preceding transient phase. The transient cooldown phase hasablation rates of the order of 1x10 -4 m/s. After the end of the transient phase ablation hasreached a depth of ~6 cm and further concrete erosion is controlled by decay heat alone andproceeds at rates of the order of 3x10 -5 m/s. In the figure the time at which the bottom coolingstructure is completely flooded is also marked.The resulting temperatures in the melt are depicted in Sub-section 16.2.2.4 - Figure 21. Thisconfirms that the initially superheated metallic melt quickly cools to its solidification temperature,where it remains for the remainder of the MCCI.Whilst the metallic melt undergoes a rapid cooldown during MCCI, the temperature of the oxideonly declines moderately. This is attributed to the fact that the liquidus temperature does notdecline significantly with the incorporation of siliceous concrete. As a result, the oxidic melttemperature, which follows the evolution of the liquidus temperature, is still high at about 2050°Cat the end of the MCCI phase.This analysis does not consider the possible impact of the flooding process and the cooldown ofthe bulk melt by the incorporation of crust particles (bulk cooling). It thus yields bounding meltconditions, and therefore bounding thermal loads at the bottom of the cooling structure. Thedensity difference between the oxidic and metallic melts at the end of the MCCI is predicted tobe about 2000 kg/m³, which confirms the assumed stable layering.2.4.1.5.3.2. Melt flooding and quenchingWith the model assumption given above, the course of steam release into the containment hasbeen calculated for a typical LB(LOCA) sequence. The initial conditions are taken from theCOSACO analysis in sub-section 2.4.1.5.2.1.