- 82 - The containments are of two types: the 900 MWe reactor containments, consisting of a single wall of pre-stressed concrete (concrete containing steel cables tensioned to ensure compression of the structure). This wall provides mechanical resistance to the most severe design accident pressure and structural integrity against external hazards. Leaktightness is assured by a metal liner on the inside of the concrete wall; the 1,300 MWe and 1,450 MWe reactor containments, comprising two walls, an inner wall made of prestressed concrete and an outer wall made of reinforced concrete. Leaktightness is provided by the inner wall and the ventilation system (EDE or AVS) which, in the annular space between the walls, channels any radioactive fluids and fission products that could come from inside the containment as a result of an accident. Resistance to external hazards is mainly provided by the outer wall. The photo below shows a view of the exterior concrete of a 900 MWe reactor building: The main auxiliary and safeguard systems In normal operation or during normal shutdown of the reactor, the role of the auxiliary systems is to provide basic safety functions: control of neutron reactivity, removal of heat from the primary system and fuel residual heat, containment of radioactive materials. This chiefly involves the chemical and volume control system (RCV or CVCS) and the residual heat removal system (RRA or RHRS). The purpose of the safeguard systems is to control incidents and accidents and mitigate their consequences. This primarily concerns the safety injection system (RIS or SIS), the reactor building containment spray system (EAS or CSS) and the steam generator auxiliary feedwater system (ASG or EFWS). The other systems important for safety The other systems necessary for reactor operation and important for safety include: the component cooling system (RRI or CCWS), which cools equipment; this system operates in a closed loop between the auxiliary and safeguard systems , and the essential service water system (SEC or ESWS), which uses the heat sink to cool the RRI system; the reactor cavity and spent fuel pool cooling and treatment system (PTR or FPC(P)S), used notably to remove residual heat from irradiated fuel elements stored in the spent fuel pool; the ventilation systems, which play a vital role in containing radioactive materials by depressurising the environment and filtering all discharges;
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