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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 161 / 295Document ID.No.UKEPR-0002-162 Issue 04Initial melt floodingThis phase starts with the first pour of water onto the melt and ends with the complete floodingof the entire melt surface. During this phase, as the total heat transport to the surface exceedsthe power needed for the evaporation of all added water, the rate of steam generation is limitedby the flooding rate. As a bounding estimate, the steaming rate over the entire period of meltflooding is taken equal to the rate of water addition of ~100 kg/s.The duration of the flooding period depends on the assumption about what fraction of the energyinitially stored in the melt can be released to the water by the two main mechanisms of bulkcooling and superficial fragmentation. The initial quenching phase ends when this amount ofheat plus all newly generated decay heat is used up in the generation of steam.For the quenching process, only the upper oxidic part of the melt is considered because, afterthe preceding MCCI, the temperature of the metallic melt below is lower than that of the oxide.Thus, no energy can be transferred from the metal to the oxide. Neglecting the metal istherefore conservative as in reality it will serve as an additional heat sink for the oxide.The energy release from bulk cooling and quenching is calculated conservatively using thefollowing assumptions:• The total fraction of the fragmented oxidic corium is 20% by weight• The quenched debris cools to the saturation temperature of the surrounding water• The temperature after bulk-cooling is the corium immobilisation temperature• The decay heat power is kept constantThe immobilisation temperature of the oxidic corium is the temperature at which the solidfraction reaches 50% by volume.With these assumptions, the duration of the initial flooding period can be calculated. It is typicallyin the range of 1000 to 2000 seconds. The generated steam is released into the containment atsaturation temperature.This conservative model concentrates the energy release from the melt into the shortestpossible time and is thus conservative with respect to the pressure build-up in the containment.Water fill-up phaseAccording to the conservative approach taken, convection in the melt ceases after the initialflooding phase so further heat transport to the water is very low. All the decay heat is initiallyused up in the reheating of the melt pool. This results in the accumulation of a water pool on topof the crust. The water inflow rate remains relatively constant as it is defined by the fixed heightof the circumferential overflow.The bounding case is established by assuming that the heat flux from the bulk melt to the waterremains zero until the average melt temperature has reached the pre-quench level andadequate convective heat transport is re-established. The only heat entering the water in thisperiod is the decay heat produced in the fragmented melt.