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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 142 / 295Document ID.No.UKEPR-0002-162 Issue 04As the core of the EPR is enclosed by a heavy reflector, which, due to its significant thermalcapacity, prevents early sideward melt-through, it is assumed that a large molten pool forms onthe lower support plate before the melt can penetrate the heavy reflector and discharge into thelower head [Ref]. Ultimately, two partially molten pools can coexist, one located in the lowerhead, the other above the core support plate, see Sub-section 16.2.2.4 - Figure 2a. In the furthercourse of the in-vessel melt-down, material may relocate from the upper into the lower pool,even after penetration of the lower head and start of MCCI in the pit.The prediction of the amount of melt in the lower head, which is initially released into the pit afterlocal failure of the RPV, does involve significant uncertainties. The approach followed here is toenvelop these uncertainties by a parametric variation of the amount of melt that is initiallyreleased into the pit and by considering the feedback between the MCCI in the pit and the heatupand failure of the RPV. This variation yields various initial melt masses in the residual RPVand pit.In considering the effective thermal interaction by radiant heat transfer between the lower headand lower support plate, see Sub-section 16.2.2.4 - Figure 2b, both masses are combined as asingle equivalent mass, thereafter called RPV-bottom, see Sub-section 16.2.2.4 - Figure 2c. TheRPV-bottom is assumed to be heated by thermal radiation emitted from the MCCI pool in the pitand by a fraction of the decay heat generated in the molten material remaining in-vessel.The RPV-bottom is assumed to detach from the RPV as soon as it reaches a predefined failuretemperature. Upon failure, the melt fraction remaining in-vessel, along with the RPV-bottom, isdischarged into the reactor pit and added to the MCCI pool. The temperature at which thisfailure takes place is taken as the temperature at which the ultimate strength of the steelapproaches zero. Data for high temperature tensile steel indicate that this will occur at ~1300°C.Most likely, the lower head will fail below 1300°C due to progressive, creep-induced reduction ofthe load-bearing cross-section.In-line with this phenomenology, the applied generalised melt release sequences involve twodistinct melt pours. The first pour initiates the MCCI in the pit and the second pour completes themelt release from the RPV. As the MCCI continues, the newly released second pour heats upand becomes mixed into the existing molten pool.The amount of the first pour varies between 40% and 80% of the total released mass (metalplus oxide) as obtained from MAAP-4 calculations, see section 2.1, sub-section 2.1.2.3.1, andSub-section 16.2.2.4 - Table 2. Furthermore, the time of the first pour is independently variedbetween 10,000 seconds (around 3 hours) and 86,400 seconds (1 day) to investigate theinfluence of the level of decay heat on the failure time of the RPV-bottom/melt plug and theretention time in the pit. The selected time interval yields a variation in the decay heat level of~30%. This variation is bounding for the different initiating scenarios, e.g. LB(LOCA) andSB(LOCA), and envelops the vast majority of possible melt release sequences.To describe the configuration within the MCCI pool, it is assumed that the immiscible metallicand oxidic melt fractions stratify into layers according to their density. The reasons for assumingstratified layers are:• The initially high density difference between the (heavier) oxide and metal of~2 te/m³• The expected formation of an oxidic crust at the oxide-metal interface caused by thefact that the temperature of the metal melt is initially about 500°C lower than that ofthe oxide