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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 150 / 295Document ID.No.UKEPR-0002-162 Issue 042.4.1.3.2. Modelling approachFor the postulated case of a local initial contact between melt and gate, bounding assumptionsare made for the size and location of the initial contact area as well as for the state of the melt,including its mass, temperature and the distribution of the immiscible metallic and oxidic phases.In addition, a variation of relevant assumptions is performed to demonstrate the large marginsinvolved in the design.2.4.1.3.3. ResultsThe expected case of a large initial contact area between the melt and the aluminium gate isexpected to lead to a quasi instantaneous melt-through of the gate and a fast release of theaccumulated melt inventory.Regarding outflow time, this case is bounded by the more conservative case of a limited initialcontact area. For the related small initial contact areas, it is shown [Ref] that, for the entirespectrum of considered cases, the melt release will be complete in less than 200 seconds. Thisis due to the predicted growth of the initial hole by heat transfer to the residual concrete of themelt plug that surrounds this initial hole. This heat transfer is highly effective in particular in thebest-estimate case of a layered molten system when the denser metal layer is located below thelighter oxide and thus is the first phase to be released through the hole.The final size of the opening at the end of the release process is typically on the order of 0.5 m²which results from the assumption of an average hole size during outflow of ~0.25 m²,corresponding to about 10% of the total gate area.2.4.1.4. Assessment of melt spreadingAfter penetration of the gate the melt enters the melt discharge channel which guides it into thecore catcher. Due to its large cross-section of more than 1 m² and the fact that the walls arecovered with a thermally insulating ceramic layer, the transfer channel is not expected to have asignificant effect on the flow, especially when the failure cross-section of the gate is less thanthat of the channel.The inlet level of the melt discharge channel into the core catcher is above the maximum meltlevel. Therefore, once the melt has covered the bottom of the core catcher any melt arriving laterdoes not have to physically move from the inlet to its final position. Instead, the melt will find itsown level and rise uniformly across the entire core catcher.Within the core catcher, the melt spreads under practically dry conditions, as the spreadingcompartment is a dead-end room and isolated from the rest of the containment. There is noinflow of water from sprays or leaks and only a limited amount of condensate may form insidethe room. Though dry conditions are not required for a successful spreading, they make thedistribution of the melt more predictable because fuel coolant interactions are avoided.