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

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SUB-CHAPTER : <strong>16.2</strong>PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 195 / 295Document ID.No.UKEPR-0002-162 Issue 04For all scenarios a similar course of the ex-vessel phase is assumed:• After vessel failure the core melt pours into the reactor pit and MCCI occurs.After consumption of the sacrificial concrete, the melt gate fails and the liquidcore melt/concrete mixture flows down to the core catcher. Steam, hydrogen,CO 2 and CO are produced during MCCI in the reactor pit and released into thecontainment. Because of the high temperatures in the pit, an instantaneouscombustion of the generated hydrogen and CO in a standing flame is assumed.• Hydrogen, CO 2 and CO are produced during MCCI in the core catcher steamand released into the containment. Again, because of the high temperatures, aninstantaneous combustion of the generated hydrogen and CO in a standingflame is assumed.• Flooding of the core melt is triggered when the core melt arrives in the corecatcher. After a time delay of 300 seconds, the quenching of the core melt startsusing sub-cooled water from the IRWST. As a ”best estimate” value afragmentation of 20% of the over laying oxidic core melt layer is assumed.Because of the high heat fluxes in this phase, initially the whole water inflow of100 kg/s is evaporated. This continues until the fragmented core melt has cooleddown. Once this has happened, steam production ceases until the core catcheris filled with 450 x 10 3 kg of sub-cooled water which is heated up by the decayheat of the core melt. When this water reaches saturation, boiling starts andsteam production occurs at a rate determined by the decay heat of the entirecore melt. The water released as steam will be replaced by sub-cooled waterfrom the IRWST.In the long term, decay heat is used as an input for COCOSYS and the steaming rate iscalculated by the code taking any time dependant properties into account, such as:• IRWST water temperature,• saturation temperature, (which is a function of the containment pressure),• effective decay heat in the melt (accounting for fission product release from themelt).The ex-vessel calculation is based on the assumption that MCCI and fragmentation in thespreading compartment occur simultaneously. Quenching proceeds from the sides of thecore catcher to the centre. The rate of quenching is such that there is still no water on thesurface of the melt in the centre at the end of MCCI.The decay heat refers to a thermal power of 4500 MW(th) and is calculated using ORIGENbest-estimate values.2.5.2. Containment data2.5.2.1. Containment model for COCOSYSThe principle of the ”lumped parameter” concept used in COCOSYS (Appendix 16A) consistsof the subdividing a real building into a number of well defined zones connected by ventopenings.
SUB-CHAPTER : <strong>16.2</strong>PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 196 / 295Document ID.No.UKEPR-0002-162 Issue 04The basic assumptions are:• Inside a zone there are no differences in the gas pressure, gas temperature andgas composition used for the determination of the thermodynamic state. Staticpressure differences between zones are used to determine the mass flowthrough vent openings. All gases entering a zone become immediatelyhomogeneously mixed.• No counter-flow is possible through a vent opening.• The governing flow paths must be modelled by a sufficient number of carefullyarranged vent openings. Otherwise some flow paths existing in the real buildingcould be suppressed, resulting in an artificial distortion of the flow field in themodel.• The boundaries of a zone are usually solid structures but may also be subdivisionswithin the physical gas volume.The ”lumped parameter” concept allows the transient evolution of gas pressure, gastemperature and gas composition to be determined across the whole containment fordifferent scenarios with reasonable accuracy in a reasonable time.The key points related to the ”lumped parameter” containment model are:• use of best estimate data for volumes, structure surfaces, vent openings andmaterial properties,• limitation of the number of zones to that necessary to determine pressure andtemperature in the important parts of the containment with the required accuracy,• assignment of the zones to both the accessible and inaccessible parts of thecontainment (“two-room concept”),• assignment of the zones of the SG and pump rooms such that each zonebelongs to only one of the 4 loops to represent the effect of different breaklocations,• where a physical volume is divided into several zones, the centre points ofhorizontally adjacent zones are at the same height level to avoid artificial gravitydriven flows which can be an artefact of the lumped parameter model,• important, well-defined rooms such as the reactor pit, cavity, IRWST and corecatcher are explicitly modelled,• modelling of the IRWST in two horizontal layers.For the containment the ”two-room concept” is used. The characteristic feature of thisconcept is that a large part of the containment is accessible to personnel during plantoperation. This is an important advantage with respect to availability of the plant becausemaintenance measures inside the containment can be carried out at any time, without theneed for a shutdown.
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16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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