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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 197 / 295Document ID.No.UKEPR-0002-162 Issue 04The accessible part of the containment includes zones Ac1 – Ac12 with a total volume of63,800 m 3 (Sub-section 16.2.2.5 - Table 1) and marked in green in Sub-section 16.2.2.5 -Figure 1. The inaccessible part of the containment includes zones Eq1 – Eq18 (Sub-section16.2.2.5 - Table 2) with a total volume of 16,200 m 3 and marked in orange in Figure 1.The “two-room concept” imposes specific requirements on the ventilation system to maintainseparation between accessible and inaccessible areas. These are realised by:• permanent closure of the vents between SG compartments and annularcompartments,• closure of 4 vents between the accessible and inaccessible parts during normalplant operation by a system of rupture and convection foils and mixing dampers,• provision of a combustible gas control system.These vents open in the case of steam release from the leak or after quench tank diskrupture. The combustible gas control system and the opening conditions are described insection 5 of Sub-chapter 6.2. The rupture and convection foils are located between the upperSG rooms and the dome (Eq7-Ac7, Eq14-Ac7). The mixing dampers are located betweenIRWST and the lower annular rooms (Eq2-Ac1, Eq2-Ac2).2.5.2.2. Boundary conditionsThe total surface areas of the containment structures are taken to be [Ref]:• Concrete structures: 43,300 m 2• Steel structures: 24,600 m 2 .The steel liner covers the entire containment wall of 9,200 m 2 . No gap between the liner andconcrete is assumed. The results obtained remain valid even if a gap exists in a small part ofthe containment wall area (e.g. up to about 15%).All structural surfaces, whether steel or concrete which are in contact with the containmentatmosphere are protected by a paint layer of thickness 0.2 mm.Typical material properties of the containment structures are listed in Sub-section 16.2.2.5 -Table 3.The rupture and convection foils open completely in the case of a LB(LOCA) soon after thebeginning of a steam release into the containment. In the case of SB(LOCA) and LOOPscenarios only the convection foils above the SG compartments open. The mixing damperstowards the IRWST open in all scenarios.The arrangement of the recombiners in the containment is chosen in accordance with thehydrogen risk analyses performed using the CFD code GASFLOW, described in section 5 ofSub-chapter 6.2. The locations of the 41 large recombiners and 6 small recombiners areindicated in Sub-section 16.2.2.5 - Figure 1 by the symbol ”Reco”. The COCOSYSrecombiner model takes into consideration hydrogen, CO and oxygen consumption and theconnected heat, steam and CO 2 release to the containment. The recombination processstarts automatically when the H 2 concentration rises above 2% by volume and stops whenthe H 2 concentration decreases below 0.5%.