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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 144 / 295Document ID.No.UKEPR-0002-162 Issue 04The compliance of the concept with these targets is demonstrated in the following sections.2.4.1.2.3.1. Melt accumulationGeneralised release casesRegarding the first criterion, it is necessary to track melt front progression into the sacrificialconcrete in the axial direction (downward) and to relate it to the failure history of the RPVbottom,in particular to the time of the final discharge of the melt inventory into the reactor pit.In Sub-section 16.2.2.4 - Table 3 and in this section the different cases considered areannotated by a percentage and two letters (e.g. 80%E-M). The percentage refers to the initialmelt mass as a percentage of that predicted by MAAP; the first letter to the time of melt release(Early or Late); and the second letter to the mode (Layered/Mixed).In this respect, Sub-section 16.2.2.4 - Figure 3 shows the calculated course of axial ablation forthe cases analysed with the "layered mode" assumption. To evaluate the influence of the“layered mode” assumption, the figure also includes two cases (80%E-M and 80%L-M) analysedwith the “mixed mode” assumption. The figure demonstrates that, for all cases, the failure of theRPV-bottom will occur long before the melt reaches the gate, at a time when less than half ofthe sacrificial concrete is ablated. This holds true despite a substantial variation in the calculatedtotal melt-concrete interaction time, which results from the variations in the decay heat level andthe initial mass of melt in the pit.The calculated duration of the MCCI in the pit for a typical low-power scenario is ~35% greaterthan for a high power scenario with an equal amount of initial melt. This correlates well with thedecrease in decay power of ~34% resulting from the corresponding shift in the onset of MCCIfrom 10,000 seconds to 1 day and thus confirms the statement made in section 2.4.1.1 of thissub-chapter about the self-adjusting characteristic of the MCCI in the pit, which comes aboutbecause a given amount of energy (integral decay power) is needed to ablate a concrete-layerof a given thickness.The initial amount of melt affects the duration of temporary melt retention because an increasedmass not only results in a higher absolute decay power, but also in a lower surface-to-volumeratio. This leads to higher heat flux densities at the melt-concrete interface and higher erosionrates. Furthermore, at the same time, there is an increase in the heat flux to the upper surface ofthe pool, which faces the RPV lower head. The higher thermal radiation from this surfaceaccelerates the heat-up of the RPV-bottom in the same proportion as the MCCI rate.The latter becomes obvious from a comparison of the times needed for the heat-up of the RPVbottomfor the two cases 80%E-L and 40%E-L. Though the masses of melt initially released intothe pit differ by a factor of two, the obtained failure times of the RPV-bottom of 3100 secondsand 4400 seconds are much closer. The result illustrates the self-adjusting effect of the thermalcoupling between ex-vessel melt and RPV-bottom, which ensures a sufficiently fast failure of thelower head.The cases conducted using the “mixed mode” assumption are characterised by an initially lowerosion rate. This is caused by the large fraction (> 90%) of solid oxide, which results from thepostulated mixing of the “cold” metallic melt into the oxidic melt at the beginning of the MCCI.Consequently, a high fraction of the oxidic melt first solidifies and must be re-melted beforeconcrete ablation can become effective. This prolongs the MCCI as compared to the “layeredmode” cases.