SSG-67 Seismic Design for Nuclear Installations

Progress in the design of nuclear installations and in related research, as well as regulatory practices in States, considering the lessons identified from recent strong earthquakes that have affected nuclear installations;

Recent developments in regulatory practices on application of risk informed and performance based approaches for assessing the safety of nuclear installations;

The experience and results from seismic design for new nuclear installations in States;

A more coordinated treatment of the design of nuclear installations against seismically induced geological and geotechnical hazards and concomitant events.

Nuclear power plants;

Research reactors (including subcritical and critical assemblies) and any adjoining radioisotope production facilities;

Storage facilities for spent fuel;

Facilities for the enrichment of uranium;

Nuclear fuel fabrication facilities;

Conversion facilities;

Facilities for the reprocessing of spent fuel;

Facilities for the predisposal management of radioactive waste arising from nuclear fuel cycle facilities;

Nuclear fuel cycle related research and development facilities.

Protection against common cause failure of SSCs in the event of an earthquake affecting all units in a multiple unit site (seismic events can lead to serious challenges to the multiple layers of defence in depth, through common cause failures);

Minimization of seismic interaction effects;

Provision of adequate seismic margins and avoidance of cliff edge effects²;

Compliance with proven engineering design rules, as specified in relevant national and international codes and standards.

Defining the design basis earthquake.

Establishing the seismic categorization.

Selecting applicable design standards.

Providing seismically resistant structural systems in accordance with the layout and the functional requirements of the nuclear installation.

Evaluating the seismic demand.

Determining the preliminary design of structural elements based on codes and standards, and providing adequate reinforcement detailing.

Verifying that the seismic demand does not exceed the seismic capacity defined in the preliminary design and adjusting the design if necessary.

The assessment of seismic margins should use realistic and best estimate assessments, and should apply different procedures from the ones used for design purposes [5].

The specific seismic hazards at the site, particularly the vibratory ground motion hazards;

The detailed geological, geophysical and geotechnical characteristics of the site with the corresponding information on soil properties [11].

Static and dynamic soil properties, for example unit weight (γ) and/⁠or density (δ), strength capacity in drained and/⁠or undrained conditions, low‑strain shear wave (V_s) and primary wave (V_p) velocities, variation of shear modulus (G), and damping ratio as a function of shear strain levels. The data should include the variation of these properties with depth, together with an indication of the types of soil and rock encountered down to the bedrock level. A number of soil profiles should be developed to adequately represent the range of ground conditions and variations encountered at a given site. The profile is usually defined as a vertical section of horizontal layers of ground, with best estimate (mean) values of layer thickness, shear wave velocity and unit weight, and the shear modulus and damping ratio as a function of shear strain level. The use of horizontally layered soil profiles should be justified by the results of site investigations or sensitivity studies. The level or levels of the groundwater should be also determined.

The variability of the thicknesses and ground layer properties to determine the following:

The best estimate, upper bound and lower bound strain compatible soil profiles, taking into account the uncertainties in soil layer geometry and soil properties; or

The full probability distributions of the soil parameters, if the subsequent site response analysis is to be fully probabilistic.

Type 1 sites: V_s > 1100 m/s;⁴

Type 2 sites: 1100 m/s > V_s > 300 m/s;

Type 3 sites: V_s < 300 m/s

Low‑strain shear wave velocity (V_S, V_P);

Strain dependent shear modulus reduction and hysteretic damping properties;

Soil density;

Layer thickness.

The design should provide adequate seismic margins for those SSCs ultimately required to prevent core damage and to prevent an early radioactive release or a large radioactive release.

The design should provide adequate seismic margins to the safety classified SSCs credited for mitigatory measures for Level 4 of the defence in depth concept.

It should be demonstrated that cliff edge effects are avoided within the uncertainty associated with the definition of SL‑2.

Defining the beyond design basis earthquake level in terms of the SL‑2 level multiplied by a factor⁹ agreed by the regulatory body;

Defining the beyond design basis earthquake level on the basis of considerations derived from the probabilistic seismic hazard assessment;¹⁰

Defining the beyond design basis earthquake level on the basis of the maximum credible seismic hazard severity.

Seismic category 1;

Seismic category 2;

Seismic category 3.

Items whose failure could directly or indirectly cause accident conditions;

Items that are necessary for shutting down a reactor and maintaining a reactor in a safe shutdown condition, including the removal of decay heat;

Items that are necessary to prevent or mitigate unintended radioactive releases, including SSCs in spent fuel storage pool structures and fuel racks;

Items that are necessary to mitigate the consequences of design extension conditions and whose failure would result in consequences of a high level of severity, as defined in para. 3.11 of IAEA Safety Standards Series No. SSG‑30, Safety Classification of Structures, Systems and Components in Nuclear Power Plants [12];

Items that are part of support, monitoring and actuating systems that are needed to fulfil the functions indicated in (b)–(d) above.

Items that might have spatial interactions (e.g. due to collapse, falling or dislodgement) or any other earthquake induced interactions (e.g. earthquake break of a pipe that is not important to safety resulting in spraying water onto electrical equipment that is important to safety) with items in seismic category 1, including effects on any safety related action by personnel at the installation;

Items not included in seismic category 1 that are necessary to mitigate design extension conditions;

Items relating to the infrastructure needed for the implementation of the emergency evacuation plan.

The centre of mass of all structures should be located at as low an elevation as practicable.

The centre of rigidity at the various elevations should be located as close as practicable to the centre of mass to minimize torsional effects.

Building plans and elevation layouts should be selected that are as simple and regular as practicable, with direct and clear paths for the transmission of seismic forces to the foundation.

Different embedment depths of adjacent buildings should be avoided, as far as practicable.

Buildings with large plan aspect ratios should be avoided. Plan aspect ratios should be as close to 1 as practicable, and large aspect ratios should be avoided.

Protruding sections (i.e. lack of symmetry) should be avoided, as far as practicable.

Rigid connections should be avoided between different building structures or between equipment of different seismic categories and dynamic behaviours.¹²

The diversity of SSCs belonging to redundant safety trains should be properly considered in order to reduce potential common cause failures.

Structures made of reinforced concrete shear walls that provide a lateral‑force‑resisting system;

Steel or reinforced concrete moment‑resisting frames specially designed to provide ductile behaviour;

Reinforced concrete slab or wall moment frames.

Ordinary moment‑resisting frame systems (i.e. no special design details to provide ductile behaviour);

Unreinforced concrete systems;

Precast concrete systems with gravity‑only bearing connections;

Unreinforced masonry systems;

Wooden structures.

In reinforced concrete structures, brittle failure in the shear and/or bond of rebars or in the compressive zones of concrete should be prevented.

For reinforcement, an appropriate minimum ratio of the ultimate tensile stress to the yield tensile strength should be defined to ensure a minimum ductility.

The lengths for reinforcing bar anchorage should generally be longer than the lengths for structures under static or non‑reversing loads.

In steel structures, brittle failure should be avoided.

Structural joints, particularly in reinforced concrete structures, should be designed to accommodate ductile displacements and rotations. This provision should be consistent with the acceptance criteria specified in the seismic categorization, and it should also take into account the need for adequate seismic behaviour in design extension conditions.

Sufficiently wide gaps should be provided between structures above ground level to avoid interaction (pounding) during seismic motion. Utilities crossing the gaps should be able to accommodate differential seismic displacements. However, if such interaction between structures could occur, the structural integrity should be confirmed.

Earth structures related to ultimate heat sinks: dams, dykes and embankments;

Site protection structures: dams, dykes, breakwaters, sea walls and revetments;

Site contour structures: retaining walls, natural slopes, cuts and fills.

Slope failure induced by design basis vibratory ground motions, including liquefaction;

Failure of buried piping or seepage through cracks induced by ground motions;

Overtopping of the structure due to tsunamis on coastal sites, seiches in reservoirs, earth slides or rock falls into reservoirs, or failure of spillway or outlet works;

Overturning of retaining walls.

Initial stiffness, as a function of frequency;

Post‑yield stiffness, as a function of frequency;

Damping provided by the isolation device, as a function of frequency and/⁠or of maximum displacement.

Ensuring uniformity of load and displacement. Ideally, all isolators should be of the same type, should be under the same gravity load and should sustain the same horizontal displacement during an earthquake.

Avoiding, or at least minimizing, uplift.

Avoiding ultimate deformations in isolators being exceeded during earthquakes more severe than the design basis earthquake.

Allowing in‑service inspection and replacement of each individual isolator.

Ensuring that the qualification conditions of isolators are consistent with the anticipated operating environmental conditions.

Ensuring that the environmental conditions do not present hazards such as fire at the level where isolators are located.

Avoiding detrimental effects to collocated SSCs that protect against other external hazards.

The full load path from the base of the equipment to the main structure should have sufficient capacity and stiffness so that the natural frequencies¹⁴ of the installed component are not significantly reduced.

The seismic demand at each support point should be computed from the in‑structure response spectra using the quasi‑static method or response spectrum method, with the level of damping accepted by the design standard for each particular equipment class. Simplified conservative approaches are acceptable, provided these are justified.

Nozzle loads should be considered when computing the seismic demand.

Prying action at base plates should be avoided by an appropriate positioning of fastenings (e.g. to avoid large eccentricities in the load path).

Any parts of the load path that are prone to brittle failure should be oversized to ensure ductile controlling failure modes (e.g. in cast‑in‑place bolts, the failure should take place at the bolt, not at the concrete).

Mixing different types of fastening for the anchorage of the same component (e.g. welding and expansion anchors) is not acceptable unless it can be shown that the stiffness of the different fastenings is similar.

The flexibility of base plates can significantly alter the distribution of anchor forces compared with the results computed with the common rigid plate assumption. This is especially relevant when brittle failure modes are involved (e.g. pull out of expansion anchors). In such cases, the design should give consideration to the base plate flexibility.

The preferred anchorage types are the following:

Expansion anchors other than the undercut type normally should not be used for rotating or vibrating equipment or for sustained tension supports.

Cast‑in‑place bolts or headed studs;

Welding to embedded plates;

Undercut type expansion anchors.

Calculation of seismic demand should take into account the flexibility of the tank shell and its influence on the natural frequencies of the tank.

A conservative freeboard should be provided to avoid damage to the roof due to sloshing of the fluid.

Unanchored tanks might have large uplifts and instability failures, which can rupture attached lines and cause a loss of contents of the tank. Consequently, unanchored tanks are not usually acceptable as seismic category 1 items.

The seismic capacity of the foundations of the tank should be appropriately verified, especially for ring type foundations. The assessment should be consistent with the capacity assessment of the tank shell and the anchorage.

The global stability of the tank in terms of the potential for overturning and sliding should be assessed.

The design of attached lines should allow for differential displacements between the tank and the first support, consistent with the design of the anchorage (i.e. the placing of supports very close to the tank should be avoided).

Pipe materials should be ductile at service temperatures (total elongation at rupture greater than 10%). Carbon steel and stainless steel are examples of ductile materials at the usual range of operating fluid temperatures in a nuclear installation; grey cast iron and PVC are examples of brittle materials.

Joints that rely only on friction should be avoided.

Vertical supports should not be excessively spaced. Guidelines from established national and/or international design codes should be followed.

Pipe supports should be able to withstand a seismic event without brittle failure and without loss of the restraint of the pipe.

When flexible joints (e.g. bellows) are used, the movement of the pipe at both sides of the joint should be restrained to keep relative end movements during a seismic event within vendor specified limits.

Piping should be sufficiently restrained in the lateral direction.

The full load path from the base of the equipment to the main structure should be considered.

The load path should have enough capacity and adequate stiffness.

Prying action at base plates should be avoided by an appropriate positioning of fastenings (e.g. avoiding large eccentricities in the load path).

The portions of the load path prone to brittle failure should be oversized to ensure ductile controlling failure modes (e.g. in cast‑in‑place bolts, the failure should take place at the bolt, not at the concrete).

The preferred anchorage types are the following:

For motor control centres, transformers, inverters, switchgear and control panels, the use of top bracing or lateral ties should be considered to limit the differential displacements imposed on cables, conduits and bus ducts.

Cast‑in‑place bolts or headed studs;

Welding to embedded plates;

Undercut type expansion anchors.

The lateral and transverse stiffness of the racks;

Overturning stability;

Anchorage to the rack supporting structure;

Adequacy of spacers between the batteries and use of shims at the ends of the battery rows.

Limit the span of cable trays;¹⁶

Limit the span of conduit;

For cantilever bracket‑supported raceways, fasten cable trays and conduits to their supports so that they cannot slide and fall off the supports;

Ensure that supports can withstand the design basis earthquake levels (SL‑1 or SL‑2, as applicable) with adequate margins against brittle failure.

Limit the span of duct supports.¹⁷

Fasten ducts to their supports (i.e. use duct tie downs) to preclude the possibility of displacing, falling or sliding off during a seismic event. The duct should be securely attached to the last hanger support at the terminal end of the duct run. Similarly, supports designed to limit the lateral movement of the duct system should also be attached to the duct.

Ensure a positive connection at joints.¹⁸

Ensure positive attachment of appurtenances: accessories attached to heating, ventilation or air‑conditioning ducts, such as dampers, turning vanes, registers, access doors, filters or air diffusers, should be positively attached to the duct by means of screws or rivets.

Ensure against brittle failure of supports: supports should be able to withstand the design basis earthquake levels (SL‑1 or SL‑2, as applicable) with adequate margins against brittle failure.

The seismic input should be defined either by design response spectra or by acceleration time histories that are compatible with response spectra.

The analysis model should adequately represent the behaviour of the structure under the seismic action and consider a realistic distribution of the mass, stiffness and damping properties of the structure.

The soil–structure interaction should be considered for all safety related nuclear structures not supported by a rock or rock‑like soil foundation, taking into account uncertainties in ground properties.

The structural response should be obtained for the three orthogonal components of seismic motion (one vertical and two horizontal).

Potential second order effects, if relevant, should be considered for all vertical load path elements (P‑Delta effects). In particular, all vertical load path elements should be designed to withstand the lateral displacements induced by seismic loads.

Hydrodynamic effects should be considered for SSCs containing large volumes of water, for example fuel pools and service pools.

The purpose of the soil–structure interaction analysis and the intended use of the results (e.g. as input for determining the seismic response of the SSCs);

Relevant phenomena that need to be simulated (e.g. seismic wave fields; linear, equivalent linear and non‑linear soil behaviour; linear and non‑linear simulation of soil–foundation contact; wave incoherence);

The methodology and software to be used, based on (a) and (b).

Developing the soil–foundation–structure model, normally using a finite element modelling method;

Locating the bottom and lateral boundaries of the model and assigning appropriate boundary conditions;

Defining the input motion to be applied at the boundaries, compatible with the site response analysis (Section 3);

Performing the analyses and obtaining the necessary response parameters.

Conducting site response analysis (see Section 3).

Developing the model for the structure, normally using finite elements.

For rigid boundary methods, obtaining the foundation input motion (kinematic interaction or ‘wave scattering problem’). ‘Rigid boundary’ refers to the interface between the foundation and the soil being rigid. The validity of the rigid base assumption, wherever it is employed, should be verified by sensitivity analysis.

Obtaining the foundation impedances using continuum mechanics methods, finite element methods or impedance handbooks.

Analysing the coupled soil–structure system and obtaining the necessary response parameters.

Layout of the installation and separation between independent structures.

Soil stiffness and damping.

Differences in footprint and total mass among adjacent buildings. Smaller buildings located close to larger, heavy buildings or underground structures (e.g. tunnels) are of particular concern.

L1: loads during normal operation;

L2: additional loads during anticipated operational occurrences;

L3: additional loads during accident conditions.

For items in seismic category 1:

For items in seismic category 2 that have been identified to interact with items in seismic category 1, the same combinations as for seismic category 1 should be applied, possibly associated with different acceptance criteria.

For items in seismic category 3, combinations that are consistent with national practice should be applied to the relevant design basis loads.

The mass of snow should also be considered for sites where design snow load is relevant (e.g. larger than 1.5 kN/m²).

L1 loads should be combined with the demand from the design basis earthquake.

L1 and L2 or L3 loads should be combined with the demand from the design basis earthquake if the L2 or L3 loads are caused by the earthquake and/or have a high probability of being coincident with the earthquake loads (which may be the case, for example, for L2 loads that occur sufficiently frequently, independently of an earthquake).

Analysis;

Testing;

A combination of analysis and testing;

Indirect methods (e.g. similarity).

The input to the SSC should be defined by design spectra, by in‑structure time histories or by acceleration time histories that are compatible with response spectra. If design spectra (or related time histories) are used, these need to be shown to either envelop or be conservative with respect to the in‑structure loading conditions at the location of the SSC.

The computational model should conservatively represent the behaviour of the candidate item under the seismic action (e.g. mass distribution, stiffness and damping characteristics).

The important natural frequencies of the SSC should be estimated; alternatively, the peak of the design response spectrum multiplied by an appropriate factor greater than 1 should be used as input. Multimode effects should also be considered.

A load path evaluation for seismic induced inertial forces should be performed. A continuous load path, with adequate strength and stiffness, should be provided to transfer all inertial forces from the point of application to the main structure housing the item. The seismic demand for all the links of this path should be computed.

The seismic demand should be obtained for the three orthogonal components of seismic motion (one vertical and two horizontal).

Energy dissipation should be taken into account in a conservative manner (considering the uncertainties associated with dissipation mechanisms) and can be modelled for SSCs in a number of ways. If a modal analysis is being performed, modal damping values can be used for common components and materials recommended by applicable nuclear design codes.

Acceptance test (proof test);

Low impedance test (dynamic characterization test).

Applicable seismic test standards;

Acceptance criteria;

Input motion;

Functional requirements;

Boundary (support) conditions;

Number of repetitions of testing or cycles of loading per test;

Environmental conditions (e.g. pressure, temperature);

Operational conditions, if functional capability has to be assessed.

Functional tests intended to verify the performance of the required safety function of the component;

Integrity tests aimed at proving the mechanical strength of the component.

To justify the extrapolation of qualification by testing to more complex assemblies (e.g. multicabinet assemblies);

To help define the testing programme by obtaining a better understanding of the dynamic behaviour of complex systems;

To investigate and explain unexpected behaviour during a test;

To obtain a first estimate of the response before performing tests on complex systems;

To develop an analytical model with modal frequencies and damping, verified by the testing of a typical component, which enables the effects of component configuration variations to be simulated analytically.

To provide triggering mechanisms for the automatic shutdown of the nuclear installation if the earthquake level exceeds a defined threshold;

To provide alarms to alert operating personnel of the occurrence of the earthquake and to provide information for the decision making process defined by the operating procedures for the installation;

To collect data on the dynamic behaviour of SSCs during an earthquake, to obtain realistic data on the structural response and to assess the degree of validity of the analytical methods used in the seismic design and qualification of the buildings and equipment.

At all nuclear installations: one triaxial strong motion recorder installed to register the free field vibratory ground motion.

At nuclear power plants:

At nuclear installations other than nuclear power plants: two triaxial strong motion recorders installed in the basemat of the building or structure with the largest inventory of radioactive material.

Three triaxial strong motion recorders installed to register the vibratory motion of the basemat of the reactor building;³¹

Two triaxial strong motion recorders installed on the most representative floors of the reactor building.

An experience based approach for determining the real damage potential of felt and significant earthquakes (see paras 8.13–8.15);

A systematic methodology for assessing the need for shutdown of the installation and for assessing the readiness for restart (if the installation has been shut down), based on physical inspections and tests;

Criteria for ensuring the long term integrity of the installation.

Planning: steps taken before an earthquake occurs to prepare an appropriate post‑earthquake action plan. Many of these activities will be performed at the design stage.

Response: implementation of the post‑earthquake action plan — on the basis of the earthquake felt or the vibratory ground motion recorded at the site and the observed consequences to the installation — as part of the operational response.

The post‑earthquake actions will facilitate timely decision making concerning the present or future state of the nuclear power plant, for example the need to shut down, to continue operation or to restart.

Communication to all stakeholders will be timely and transparent with regard to the status of the installation, actions taken and actions to be taken.

A tiered approach will be employed starting with overall evaluations and proceeding to very detailed evaluations only when required by the situation.

The amount, type and status of the radioactive inventory (e.g. solid, liquid or gaseous; processed or stored);

The intrinsic hazard (e.g. criticality) associated with the physical processes and chemical processes that take place at the installation;

The thermal power of the nuclear installation, if applicable;

The configuration of the installation for activities of different kinds;

The distribution of radioactive sources within the installation (e.g. in research reactors, most of the radioactive inventory will be in the reactor core and fuel storage pool, while in processing and storage facilities it may be distributed throughout the facility);

The changing nature of the configuration and layout of installations designed for experiments;

The engineered safety features necessary for preventing accidents and for mitigating the consequences of accidents, including the need for active safety systems and/or operator actions to prevent accidents and to mitigate the consequences of postulated accidents;

The characteristics of the structures of the nuclear installations and the means of confinement of radioactive material;

Any characteristics of the process or of the engineered safety features that might lead to a cliff edge effect in the event of an accident;

The potential for on‑site and off‑site contamination.

Seismic design category 1: high hazard nuclear installations;

Seismic design category 2: medium hazard nuclear installations;

Seismic design category 3: low hazard nuclear installations;

Seismic design category 4: conventional installations.

The seismic design category of the nuclear installation and the need to perform in the event of an SL‑2 level hazard.

The appropriate limit state³² in the event of an SL‑2 level hazard (specifying the analysis methodology, design procedures and acceptance criteria).

SSCs whose seismic failures do not have any interactions with the performance of safety functions; these should correspond to seismic category 3. National codes and standards for seismic design of conventional installations apply (see Table 3).

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