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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 147 / 295Document ID.No.UKEPR-0002-162 Issue 04Sub-section 16.2.2.4 - Figure 10 indicates that the predicted temperature of the oxidic melt atthe time of spreading is always approximately 2050°C. Compared to the initial temperature ofthe melt during release from the RPV of about 2500°C, this temperature reduction of about500°C is caused by the change in melt chemistry and by the admixture of concretedecomposition products. The lower temperature helps to reduce the radiant heat losses at theupper surface and the conduction heat losses into the bottom during spreading.The calculated sub-cooling of the oxidic melt at the end of the MCCI, i.e. the temperaturedifference between liquidus and melt temperature, is generally only ~25°C. This must becompared to a difference of several hundred degrees between the liquid and the immobilisationtemperature of the melt. The latter is characterised as the temperature at which the volumetricsolid fraction reaches about 50%.The low volumetric solid fraction in the oxidic melt (see Sub-section 16.2.2.4 - Figure 11)confirms the ability of the melt to spread. Essentially, the values vary between 0.03 and 0.05 forall cases, despite substantial differences in the preceding course of temporary melt retention.These values are an order of magnitude below the value at which immobilisation occurs.In the mixed mode, the volumetric solid fraction is initially very high, due to the postulatedaddition of "cold" metal to the oxidic melt, but steadily decreases due to melt reheating. Thesame effect arises after a mixture of oxidic and metallic melt is added to the MCCI pool afterfailure of the RPV-bottom.A low volumetric solid fraction relates to low viscosities [Ref]. The calculated low values ofviscosity (see Sub-section 16.2.2.4 - Table 5) and the low sub-cooling at the time of gate contactestablish very favourable conditions for melt spreading. Due to the unifying characteristic of thetemporary melt retention, this result holds true independently of the underlying scenario.2.4.1.2.3.3. Mass and energy release by the MCCI in the reactor pitThe prediction of mass and energy release by the MCCI in the reactor pit is based on initial andboundary conditions that yield conservative results regarding the containment response.Such conditions are provided by a surge line break scenario, which leads to vessel failure andmelt release into the reactor pit as early as 2 hours 45 minutes after scram and thus involves ahigh decay power level in the melt. At the same time, the duration of melt release for thisscenario is relatively short. Relative to other scenarios, the selected scenario therefore yieldshigh concrete ablation rates, which translate into conservative off-gas rates. The integral gasrelease is predominately determined by the amount of available concrete, which is determinedby the thickness of the sacrificial layer in combination with the presence of the refractoryshielding and thus is almost independent of the scenario and the related melt release sequence.Sub-section 16.2.2.4 - Figures 12 to 15 show the release of H 2 O, H 2 , CO 2 and CO as a functionof time, while Sub-section 16.2.2.4 - Figure 16 gives the corresponding release temperature.The release temperature is assumed to correspond to the temperature of the particular meltphase, which forms the upper layer of the MCCI pool. Hence, it corresponds to the temperatureof the slag layer before layer inversion and of the oxide melt afterwards. As the oxide meltexhibits the highest temperature in the MCCI system, the release temperature increases by~310°C as a consequence of the layer inversion at 16120 seconds. Furthermore, withsuccessive layer inversions the gas release rates strongly increase. This effect results from thehigh transient ablation rates, mainly in the downward direction due to the interaction of theconcrete with superheated metal, which prevails until the end of the retention phase in the pit.