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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 12 / 295Document ID.No.UKEPR-0002-162 Issue 041.3.1.1.1.1. Premixing PhaseIn the premixing phase, the molten jet breaks up and a coarsely mixed region of moltencorium and coolant water is formed. A vapour film separates the melt particles and the water,so the heat transfer between the melt and water is relatively low. The premixing phase ischaracterised by a relatively low steam production rate resulting in a slow pressurisation ofthe system or, if condensation is able to balance vaporisation, no pressurisation at all. Thesystem can remain in this meta-stable state until the melt is quenched or, if the conditions areappropriate, until a steam explosion is triggered. The timescale of the premixing process is inthe range of seconds, and the length of the melt particles is measured in centimetres.1.3.1.1.1.2. Triggering PhaseIn the triggering phase, the steam explosion is triggered. The triggering event is adisturbance, which destabilises the vapour film around a melt particle allowing liquid-liquidcontact and leads to locally enhanced heat transfer, pressurisation and local finefragmentation. There are many reasons for vapour film destabilisation, including pressurepulses resulting from different impacts (in experiments interactions are often triggered whenthe melt reaches the bottom of the tank), transition from film boiling to nucleate boiling, andwater entrapment at melt-structure contact.1.3.1.1.1.3. Propagation PhaseDuring the propagation phase, an escalation process takes place resulting from the couplingbetween pressure wave propagation, fine fragmentation, and heat transfer initiated by thetriggering event. The pressurisation induced by the triggering event destabilises the vapourfilms of surrounding melt particles, leading to local liquid-liquid contacts between meltparticles and coolant. Locally, some coolant is rapidly heated and pressurised and thiscauses some fine fragmentation of surrounding melt particles. This type of fragmentation isoften called thermal fragmentation. Later, when the pressure is already high, the finefragmentation is believed to be of a hydrodynamic nature owing to the relative motionbetween the melt and the coolant induced by their different densities and compressibilities.The fine fragmentation propagates at a velocity which depends on the conditions in thepremixing region. It can be governed by timescales corresponding to the propagation ofdisturbances in the premixing region, resulting in sequential “ignition” of the mixture. Typicalvelocities in this case are in the order of some tens of meters per second. In this case, thepressurisation of the system is relatively limited, slow and uniform, without generation ofshock waves.However, the fine fragmentation can escalate up to supersonic velocities in the pre-mixtureand quasi steady state propagation. Depending on the conditions, the pre-mixture can “burn”more or less completely before the system can expand, thus creating a zone of highpressure. During the propagation phase, the thermal energy of the melt is converted intothermal energy of the coolant. The timescale of the explosion propagation process is in therange of milliseconds, and the length of the fine fragmented particles is measured inhundreds of micrometers.