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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 112 / 295Document ID.No.UKEPR-0002-162 Issue 04Continuous Combustion:The conditions in the containment above the SG compartment of loop 2 at the time of ignitionare:• Average atmospheric temperature: 64°C• Pressure: 1.3 bar• Amount of hydrogen: 91.2 kg• Amount of hydrogen in the dome: 72.5 kg• Average hydrogen concentration: 1.5% by volume• Average hydrogen concentration in the dome: 2.9% by volume• Average steam concentration in the 4% plume: 15% by volume• Average steam concentration in the plume in the dome: 16% by volume• Internal wall surface temperatures: 61 to 62°C• Containment shell surface temperature: 60°CThe average hydrogen concentration in the dome at the time of ignition is well below the ignitionlimit. However, above the SG of loop 2 the local hydrogen concentration is 6.4%.At the beginning of the combustion, the hydrogen release rate exceeds the hydrogen burningrate and therefore the amount of hydrogen present in the containment increases. The peakvalue of 200 kg is reached 800 seconds after ignition. This peak value can be compared with thetotal hydrogen release up to that time (760 kg) and with the amount of hydrogen present in thecontainment if hydrogen is removed by recombiners only (580 kg).As a consequence of this incomplete combustion, the pressure and atmospheric temperatureare far below AICC values. The maximum inner wall surface temperatures can be found close tothe combustion area with a peak value of 240°C about 1500 seconds after ignition, whichcorresponds to an increase of 178°C. The maximum surface temperatures of the other innerwalls are below 180°C.The peak values for the containment shell surface temperatures are reached later, around 2200seconds after ignition, with the highest value in the dome of 145°C corresponding to an increaseof 85°C.Instantaneous Combustion:The conditions in the containment above the SG compartment of loop 2 at the time of ignition,about 1500 seconds after the start of hydrogen release, are:• Average atmospheric temperature: 87°C• Pressure: 1.48 bar• Amount of hydrogen: 516 kg