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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 154 / 295Document ID.No.UKEPR-0002-162 Issue 042.4.1.4.3. Results2.4.1.4.3.1. CORFLOW codeCases 1a, 2 and 3 simulate the spreading of various kinds of corium with various initialtemperatures onto cold concrete [Ref]. Case 1b describes the spreading of oxidic corium,including slag, onto an already spread, hot metallic corium. Sub-section 16.2.2.4 - Table 6summarises selected results. In all the cases considered, the melt reached all parts of thespreading area within about 10 seconds. An almost homogeneous distribution is generallyachieved in less than one minute.For the standard cases, Sub-section 16.2.2.4 - Table 7 lists selected results after one minuteand summarises information on corium heat transfer. Due to the applied symmetry, the heatsource and sink terms relate to only one half of the real geometry (~85 m²).Due to its high heat conductivity and low emissivity, heat conduction to the substratum is themajor heat sink for the spreading metal in case 1a. For all other standard cases, radiation to thesurroundings is the dominant heat removal process. The comparison between heat loss andheat generation terms indicates that, over the period of one minute, the total decay heatproduction is one order of magnitude lower than the heat losses.Variants of the standard cases were assessed with more conservative conditions. Theseinclude: initial temperatures that were 200°C lower than for the reference case, a reduction ofthe release cross-section of the gate, and the consideration of the non-Newtonian rheology ofthe oxidic melt. It was found that these variations have only minor influence on melt propagation,see Sub-section 16.2.2.4 - Table 8.The CORFLOW calculations demonstrate that, for a variation of the open cross-section between10% and 100% of the total gate area of 2.4 m², outflow times range from 100 seconds (0.24 m²)to 10 seconds (2.4 m²). If smaller gate failure cross-sections are assumed, the outflow timethrough the hole will be correspondingly extended. The CORFLOW analysis demonstrates thatmelt spreading is sufficiently fast and complete even for the most conservative case consideredof a core oxide melt without the preceding release of liquid metal. For this case, melttemperatures are much higher than in the other cases because the oxidic corium has not yetincorporated the slag layer.The calculated evolution of height profiles of the melt along the centreline within the transferchannel and spreading compartment shows that, even for the most conservative assumed case,the release and spreading of the melt will be complete in less than 200 seconds. The additionalconsideration of metal/slag layers above the melt in the pit would further increase the outflowvelocity for the oxide due the increased hydrostatic pressure.2.4.1.4.3.2. RIT modelFor all selected cases, it was found that even the one-dimensional spreading model predicts anearly complete spread (A theo /A EPR = 0.85 to 1), which, for a width of the channel equal to that ofthe melt discharge channel of the EPR, corresponds to a 1D spreading length of >100 m [Ref].In comparison, the spreading-into-open-area model predicts ratios of A theo /A EPR >> 1. The higherratio in the 2D-case relates to the easier spreading, caused by the fact that even if the meltbecomes locally immobilised, the obstacle can be by-passed, in contrast with the situation in achannel, where melt flow is constrained by the channel sidewalls.