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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 75 / 295Document ID.No.UKEPR-0002-162 Issue 04A general view of the primary depressurisation system is shown in Section 16.2.2.2 –Figure 1.2.2.2. Characteristics of the Considered ScenariosThe depressurisation capacity of the primary system and the time required to open the reliefvalves in the case of a severe accident have a significant impact on the core degradationkinetics. They affect all the severe accident in-vessel source terms (depressurisation rate andpressure peak, hydrogen release, corium release from the reactor vessel).The analysis to justify the effectiveness of the RCP [RCS] discharge capability is performedbased on the capacity of the severe accident relief valves with a mass flow rate of900 te/hour. In fact, in many core melt scenarios, the pressuriser relief valves will also beavailable and may provide a similar additional discharge capacity of 3 x 300 te/hour.Taking into account the selection of the most relevant scenarios, the specific detaileddescription of two bounding scenarios considered for the analysis of the discharge capacitythrough the PDS is provided in the following two sections.2.2.2.1. Station Blackout Sequence [Ref]This sequence is initiated by loss of off-site power (LOOP) with loss of the emergency dieselgenerators (EDG) and loss of emergency feedwater system (ASG [EFWS]). It is consideredas the basic high pressure scenario. During this sequence, the liquid level in the steamgenerator decreases causing the primary system to heat-up and pressurise until the openingsetpoint pressure of the pressuriser safety valves is reached 1 hour 20 minutes after accidentinitiation. The safety valves start to cycle to control the pressure. The decay heat starts to beremoved by loss of RCP [RCS] water. About 2 hours after accident initiation, the steamgenerators become completely dry while the pressuriser safety valves discharge into thePRT. At this time, the differential pressure limit of the rupture disk will be exceeded and theRCP [RCS] water will immediately start to discharge into the containment.As the energy released with the water flowing through the pressuriser safety valves is smallerthan the decay heat produced in the core, the RCP [RCS] average water temperaturecontinues to increase until it reaches the saturation temperature at the opening pressure ofthe pressuriser safety valves. This occurs at about 2 hours 30 minutes after accidentinitiation. After this time, the fluid discharged through the pressuriser safety valves is a twophasewater and steam mixture.About 3 hours after accident initiation, the core uncovers and the accident progression willdepend upon the opening time of the PDS valves.2.2.2.2. Total Loss of Feed Water Sequence [Ref]Total loss of feedwater sequence (TLOFW) is initiated by the failure of the main feedwatersystem (ARE [MFWS]) and emergency feedwater system (ASG [EFWS]).The TLOFW scenario differs from the LOOP scenario because of the following two events:• reactor automatic shutdown is activated by the signal of low steam generatorwater level about 30 seconds after loss of main feedwater and emergencyfeedwater (time t = 0 seconds).