FUNDAMENTAL SAFETY OVERVIEW VOLUME 2: DESIGN AND SAFETY CHAPTER C: DESIGN BASIS AND GENERAL LAYOUT

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1 PAGE : 1 / 9 4. OTHER STRUCTURES CLASSIFIED AT SEISMIC CATEGORY I 4.0. SAFETY REQUIREMENTS Chapter C.5.0 includes the list of Seismic Category I safety classified civil structures and their associated safety requirements DESRIPTION OF THE CIVIL-ENGINEERING WORKS General remarks Chapter C.5.5 relates in particular to the foundation raft. Chapter C.5.1 relates in particular to the inner containment. This section is specifically concerned with the design of those buildings in the nuclear island classed as SC1: the outer containment, the Fuel Building, the Safeguard Buildings, the Nuclear Auxiliary Building, the Diesel-Generator Buildings, the Access Tower, tunnels, the Pumping Station and the Effluent-treatment Building. The buildings resting on the common foundation raft (Reactor Building, Safeguard Building and Fuel Building) and directly adjacent independent structures (Nuclear Auxiliary Building and Access Tower) are separated by construction joints. Similarly, one construction joint separates the Effluent-treatment Building from the Nuclear Auxiliary Building, and another separates the storage and treatment areas within the Effluent-treatment Building. The external walls in contact with the water table have a protective membrane or additional waterproofing appropriate to the structures function. Structures within the nuclear island have a restricted number of outward openings. Flat roofs are covered with both thermal insulation and a protective layer. The walls and slabs in the nuclear buildings have coatings that can be decontaminated as required in the current specifications. In the nuclear island, only some of the passages are sloping, with sand / cement screed; the other areas have no slopes and the screed is monolithic. Doors and steel structures have appropriate surface protection The outer containment and protective shield building The outer containment of the Reactor Building The geometric form of the outer containment is broadly similar to that of the inner containment (see Chapter C.5.1.) It is constructed of reinforced concrete and, from bottom to top, consists of:

2 PAGE : 2 / 9 - Cylindrical part or outer containment wall, - Torispherical dome. The exposed part of the outer containment is also intended to protect against aircraft crashes (outer shield building). The outer containment is penetrated for various facilities including: penetration lines (or metal apertures) which allow the passage of electric cables, metal sleeves for pipework and access ways (personnel airlocks and equipment access hatch). Aircraft crashes The installations required to achieve a safe shutdown condition and withstand an event whilst remaining operable are protected against aircraft crashes. As described in Chapter C.3, there are two types of protection: - An outer shield building, consisting of a bunker built along the north/south axis (designed to withstand aircraft crash load scenarios), - Geographical separation of redundant systems. Outer shield building The specific function of the outer shield building is to protect against aircraft crashes. It is made of reinforced concrete and covers the following buildings: - Reactor Building - Two Divisions of the Safeguard Building, - Fuel Building, Safeguard Buildings General remarks The Electrical and Safeguard Buildings are constructed of reinforced concrete. The structure has two zones: - The mechanical zone or Safeguard Building, - The electrical zone or Electrical Building. The four Divisions of the Electrical and the Safeguard Building adjoin the Reactor Building. They have nine main levels (excluding the roof). The mechanical rooms are sited below the electrical rooms.

3 PAGE : 3 / 9 Access The mechanical rooms are separated vertically into two zones, one radiological controlled and the other uncontrolled. The controlled zone houses the rooms containing radioactive materials. Screening walls and slabs separate the controlled and uncontrolled radiological zones. Additional screening protects operational staff within the controlled zone from sources of radiation. All the floors in the Safeguard Buildings are connected and accessible by stairways. Each building has a lift in the controlled and uncontrolled zones. Underground links between buildings with no mechanical continuity have watertight joints resistant to differential settlement. Only the control room and its associated rooms are intended for continuous occupation by staff. Main steam-valve and feedwater compartments The main steam and feedwater valve compartments adjoin the Reactor Building. The compartments contain the isolation and safety relief valves for the steam and feedwater circuits Fuel Building The Fuel Building adjoins the Reactor Building, and is constructed of reinforced concrete. Structures inside the building are protected by the outer shield building. Access The building is part of the radiological controlled zone, and houses the rooms containing radioactive materials (hot zone). Protective walls and slabs are used as screens near the radioactive sources. All the levels of the Fuel Building are accessible by stairway (two stairwells next to the Reactor Building). The building also has two freight elevators Nuclear Auxiliary Building The Nuclear Auxiliary Building is a reinforced concrete structure which adjoins the Fuel Building and Safeguard Building. The Nuclear Auxiliary Building is rectangular in form, with a truncated corner on the Reactor Building side. Functions The Nuclear Auxiliary Building is intended to house systems secondary to the reactor cooling system, and also contains maintenance areas. It houses the following main systems: - Primary Effluent Treatment system (TEP) [CSTS],

4 PAGE : 4 / 9 Access - Part of the Pool water Treatment and Cooling System (PTR) [FPPS/FPCS], - Gaseous Effluent Treatment system (TEG) [GWPS], - Part of the Steam Generator Blowdown System (APG) [SGBS] - Systems for producing and distributing chilled water (DER), and for ventilating the BAN (DVN). All of these rooms are within the controlled zone (ZC), except for rooms housing the DER units, which are outside both the controlled and monitored zones (HZC and HZS). Protective walls and slabs are used as screens near radioactive sources. All the connections with external structures, within the groundwater table zone, are designed to be watertight Diesel Generator Buildings The form and structure of these buildings are virtually identical. They are respectively known as the Division 1-2 and the Division 3-4 Diesel Generator Buildings. Functions The emergency electricity supply is provided by diesel generators, housed in the Diesel Generator Buildings. Each building contains two redundant main diesel generators, a backup generator, generators fuel tanks and miscellaneous related equipment Tunnels The following tunnels are classified as Seismic Category I: - The tunnel between the Unit 4 Nuclear Auxiliary Building and the Effluent Treatment Building. The tunnel entrance features include: - Waterproofing against ground water, - Protection against seismic loads, - Protection against the effects of radiation. - The SEC [ESWS] tunnels: these tunnels link the Pumping Station to the Safeguard Buildings. Although tunnels SEC1 and SEC2 are connected to the same structure, there is no communication between the two tunnels as they are separated by a concrete protective wall. Tunnels SEC3 and SEC4 are built on the same principle. Tunnels SEC 1/2 and SEC 3/4 are geographically separated to protect against the external risk of an aircraft crash. Each tunnel essentially contains the SEC [ESWS] intake and discharge pipework. Tunnels SEC1 and SEC4 also contain the SRU intake and discharge pipework.

5 PAGE : 5 / Pumping Station The Pumping Station is a substantial structure located adjacent to the feedwater channel. The building has four separate intake channels: two central channels, each with four waterways (narrow channels), and two side channels each with a single waterway. The four SEC [ESWS] trains are independent and geographically separate EFFLUENT TREATMENT BUILDING The Effluent Treatment Building is constructed of reinforced concrete. It has 5 levels, two below ground, and is sub-divided into 2 parts; Functions - Storage area (HQA), - Effluent treatment area (HQB). The storage area is used to package resins and low-activity waste, to store drums and concrete shells containing APG [SGBS] resins, and to check the drums and concrete shells before removal. There is a temporary storage area for stopped up concrete shells awaiting sealing just after the encapsulation bay. These stored concrete shells are not kept next to unconditioned concrete shells. The effluent treatment area is used to treat and encapsulate waste. Access The treatment area comprises a controlled zone and an uncontrolled zone. The uncontrolled zone is sited to the north-east of the building and has two staff access points to the outside There is no direct communication between the controlled and uncontrolled zones DESIGN BASIS Sizing rules Design scenarios The ETC-C is used to size the concrete structures. For each design scenario, the following are checked: - Static equilibrium, - Ultimate limit state for both fundamental and accidental combinations, - Serviceability limit state.

6 PAGE : 6 / 9 When sizing the concrete structures, the following design scenarios are considered: - Conditions during construction, - Conditions during normal operation, - Accident conditions. Properties of the materials The properties of the materials comply with the ETC-C Loads and load combinations General remarks The loads and load combinations are defined in the ETC-C and listed in Chapter C.5.0. The seismic design for the standard civil engineering structures is "Classed as Seismic Category I" (e.g. buildings within the nuclear island constructed of reinforced concrete and reinforcing bars) For French sites the acceleration in the vertical direction is assumed to be 2/3 of the horizontal value. This assumption will be adjusted appropriately for a UK sited EPR PRELIMINARY STRUCTURAL ANALYSIS The following data originates from the preliminary analysis carried out The Fuel Building and Division Safeguard Buildings As a result of the buildings being linked to other structures on the common foundation raft, part of the structural analysis considers the overall site, including the entire nuclear island. Initial values for the main factors The sizing of the structures complies with the ETC-C. The structures are designed for the UK site specific Design Basis Earthquake accelerations. The structural calculations under seismic load use the equivalent horizontal and vertical static accelerations. Preliminary structural analysis with global loads Checks are carried out for the structures that contribute the most significant stiffening, i.e.: - External walls - Stairwell walls - Internal shear walls

7 PAGE : 7 / 9 - Main floors The behaviour of the structures, including overall load transmission, is established for the Design Basis Earthquake and defined load scenarios. The largest amount of reinforcement is used in the internal walls of the Fuel Building and in the transit area between the stairs and the outer containment Nuclear Auxiliary Building Building layout Details of the Nuclear Auxiliary Buildings layout are given in the general arrangement drawings. The building design ensures that the load is properly transferred to the foundations, mainly via internal and external shear walls and short-span slabs. Due to the buildings complexity, the structure is analysed using 3-D finite element modelling. The two-dimensional finite elements take into account the bending loads and membrane forces. Initial values for the main factors The structural analysis carried out complies with the requirements of the ETC-C. The structures are sized for the Design-basis Earthquake. The structural analysis for seismic loading uses the equivalent horizontal and vertical static accelerations. Structural analysis with global loads Global stability is checked by calculating the maximum displacements and the areas where the foundation raft could lose contact with the ground bearing strata. The stability of the Nuclear Auxiliary Building is checked for all combinations of defined load Diesel-generator Buildings Building layout The Diesel-generator Buildings 1/2 and 3/4 are identical. As a result, the structural analysis is limited to a single structure. The structure is analysed using 3-D using finite-element modelling. The two-dimensional finite elements take into account the bending loads and the membrane forces. Seismic factors The structures are sized in relation to the Design Basis Earthquake. Structural analysis for global loads The building design ensures that the load is properly transferred to the foundations, mainly via internal and external shear walls and short-span slabs. The stability of the Diesel Generator Buildings is established for all combinations of defined load.

8 PAGE : 8 / Tunnels The effect of seismic stresses on a large buried structure may be summarized as the deformation the structure undergoes either because the ground is displaced, or because there is a relative displacement between the ground and the buildings connected to the structure. The impact of these externally-imposed displacements is simple to calculate if we assume that the structure follows the movement of the ground. There are therefore three stages in carrying out the tunnel calculations: - Determining the induced stresses in the structure when it follows the movement of the ground. This requires only a straightforward calculation; - Checking, using a simple model, that the structure follows the ground movement; if this proves to be so, the sizing of the structure is checked based on the stresses calculated previously; - Otherwise, the joints and the connections must be able to absorb the differential displacement (and in particular axial displacement) resulting from a loss of bonding between the structure and the ground. The upper limit for this displacement is calculated using simple methods Pumping Station The Pumping Station is a substantial structure, well braced by numerous transverse and longitudinal supporting walls. It is therefore generally considered a very robust structure. This size of the reinforced concrete structure is determined mainly by: - Bearing pressure created by the water load on the overall volume of concrete (nonfloating criterion), - Pressure from the soil and the water table on the external protective walls, - Water pressure on the internal walls, - Equipment and earthquakes loads on the rafts. The buildings general stability is ensured by: - Large quantity of concrete used, particularly in the foundation raft, to meet the nonfloating criterion, - The fact that it is largely buried and that most of the seismic energy is dissipated in the ground, so that the buildings overall stability is ensured even for seismic loads Main conclusions from the preliminary analysis At the end of the preliminary analysis, the Safeguard Buildings, the Fuel Building, the Nuclear Auxiliary Building and the 1/2 and 3/4 Diesel Generator Buildings were found to behave generally in accordance with the design for all the loads and load combinations considered.

9 PAGE : 9 / 9 The stability of the buildings structures is ensured by the sizing of the concrete structures (internal and external shear walls and slabs, columns and beams) and by using reinforcement in quantities that comply with regulations EC2 and ETC-C.