NB/T 20256-2013Seismic design code of nuclear safety related structure (English PDF)

Foreword

This document was issued on 8 June 2013 by the National Energy Administration of the PRC and takes effect on 1 October 2013.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.120.99, Chinese classification F 63.

This standard was drafted in accordance with the rules given in GB/T 1.1-2009.

This standard was proposed by the Technical Committee for Standardization of Nuclear Power of the Energy Industry.

This standard is under the centralized administration of the Research Institute of Standardization of the Nuclear Industry.

Drafting organization of this standard: China Nuclear Power Engineering Co., Ltd.

Principal drafters of this standard: Chen Mao, Zhang Chaoqi, Li Yumin, Lyu Fei.

The record number assigned to this standard is 41487-2013. It was issued by the National Energy Administration on 8 June 2013 and came into effect on 1 October 2013.

1 Scope

NB/T 20256-2013 is the Chinese seismic design code for nuclear safety related structures - the buildings and civil structures of a nuclear plant whose failure in an earthquake would compromise safety: the reactor and auxiliary buildings, the fuel building, the emergency power and cooling water structures, the control building, and the intake and outfall works that serve them. It defines the terms and the full set of symbols, then the general design requirements: the seismic classification of structures, the design earthquake levels and their return periods, the performance required at each level, the damping values to be used, and the load combinations that put seismic action together with dead, live, thermal and accident loads. Subsoil and foundation design follows, with the site response analysis, the assessment of liquefaction and slope stability, the bearing capacity and settlement under seismic loading, and the soil-structure interaction that on a stiff nuclear building governs the response. The code then sets out the analysis methods - response spectrum, time history, and equivalent static where it is permitted - the modelling of the structure, the generation of in-structure response spectra for equipment qualification, and the treatment of multiple support excitation. Design and detailing rules for reinforced concrete and steel members follow, together with base isolation where used, the separation between adjacent structures, and the anchorage of equipment to the structure. It applies to nuclear power plants and equivalent nuclear facilities in China.

This standard specifies the seismic analysis and design requirements for nuclear safety related structures under design basis earthquake conditions, including: design basis seismic input, response calculation of nuclear safety related structures, and seismic design.

This standard applies to the nuclear safety related structures of nuclear power plants and of other nuclear facilities having equivalent safety requirements.

2 Normative references

The following documents are indispensable for the application of this standard. For dated references, only the edition cited applies. For undated references, the latest edition (including all amendments) applies.

GB 50007 Code for design of building foundations (Code for design of subsoil and foundation of buildings).

EJ/T 926-1995 Design code for prestressed concrete containment of pressurized water reactor nuclear power plants.

EJ/T 925-1995 Design code for nuclear safety related concrete structures of pressurized water reactor nuclear power plants.

3 Terms and definitions

The following terms and definitions apply to this standard.

3.1 apparent wave propagation velocity: the propagation velocity of seismic waves along the ground surface, referred to the fixed local coordinate system of the analysis model.

3.2 competent soil: natural or man-made foundation soil having a shear wave velocity greater than or equal to 300 metres per second.

3.3 cut-off frequency: the highest frequency of the calculation model that has to be considered in soil-structure interaction analysis. It may be taken as 2 times the highest dominant vibration frequency of the calculation model, and shall not be less than 10 Hz.

3.4 design (or evaluation) ground acceleration: the acceleration value corresponding to the zero period of the design (or evaluation) response spectrum.

3.5 design (or evaluation) response spectrum: the smoothed response spectrum of the free-field input motion.

3.6 dominant frequency: the frequency of a vibration mode whose modal mass is greater than or equal to 20 percent of the total mass of the model.

3.7 finished grade: the outdoor ground elevation of the plant site.

3.8 free field ground surface: the ground surface of the site remote from the influence of structural vibration.

3.9 isolation system: the system composed of all individual isolation units, of the load-transferring structural members between the isolation units, and of the connecting members between them and the other structural members. The isolation system includes the wind-resisting restraint members of the isolated structure.

3.10 rigid: the characteristic of a structure or component whose fundamental frequency is greater than or equal to the rigid (ZPA) frequency.

3.11 rigid (ZPA) frequency: the lowest frequency at which the response spectrum acceleration becomes independent of damping (approximately equal to the maximum input acceleration).

3.12 synthetic time history: a time history generated by mathematical methods.

3.13 safety shutdown earthquake (SL-2 earthquake): when the plant is affected by the ground motion of the safety shutdown earthquake, the structures, systems and components of seismic Category I and seismic Category III shall meet the requirements of their design functions, and the structures, systems and components of seismic Category II shall meet the specified requirements.

3.14 zero-period acceleration (ZPA): the acceleration spectral value of the response spectrum in the rigid range above 33 Hz, equal to the maximum peak value of the acceleration time history.

4 Symbols

The following symbols apply to this standard.

alpha sub i, beta sub i: the proportional damping coefficients of part i of the structure.

alpha sub H: the horizontal seismic coefficient.

alpha sub g, alpha sub k: the wave velocity coefficients.

alpha bar sub i: the rigid response coefficient of mode i.

beta sub x, beta sub psi, beta sub z: constants, functions of the foundation dimension ratio L divided by B.

Gamma sub j: the participation factor of mode j.

gamma: the shear strain, or the unit weight of the soil.

gamma sub t: the total unit weight.

Delta sub max: the maximum relative nodal displacement.

Delta T: the time step.

epsilon sub ij: the correlation coefficient between mode i and mode j.

(epsilon sub a) sub max: the maximum axial strain.

theta sub max: the maximum nodal rotation.

lambda: the material damping ratio, as a percentage of critical damping.

lambda sub j: the damping ratio of mode j, expressed as a percentage of critical damping.

[lambda K]: the stiffness matrix, in the global coordinate system, of element i or subsystem i, multiplied by the damping ratio of element i expressed as a fraction of critical damping.

[lambda M]: the mass matrix, in the global coordinate system, of element i or subsystem i, multiplied by the damping ratio of element i expressed as a fraction of critical damping.

lambda sub w: the wavelength of the dominant seismic wave.

Lambda sub i: the modal mass ratio.

upsilon: Poisson ratio.

upsilon sub a: Poisson ratio of aluminium.

upsilon sub c: Poisson ratio of concrete.

upsilon sub s: Poisson ratio of steel.

rho: the mass density.

upsilon (lateral pressure symbol): the lateral dynamic earth pressure produced on the retaining wall by a horizontal seismic acceleration of 1 g.

tau: the shear stress.

{phi sub j}: the mode shape vector of mode j.

phi sub ji: the value of the mode shape vector of mode j at degree of freedom i.

[phi] sub i: the matrix of mode shape vectors of subsystem i (fixed base).

[phi]: the matrix of mode shape vectors.

phi sub max: the maximum curvature of the buried structure.

omega: the circular frequency (radians per second).

omega sub j: the circular frequency of mode j.

omega sub max: the highest significant circular frequency to be considered.

omega sub min: the lowest significant circular frequency to be considered.

a: the acceleration.

a sub max: the maximum ground acceleration.

A sub p: the cross-sectional area of the pipe.

B: the foundation width perpendicular to the direction of horizontal excitation.

c: the surface wave velocity.

[C]: the damping matrix of the whole system.

[C sub FB]: the fixed-base damping matrix of subsystem i.

[C sub H]: the effective damping force matrix due to the lag effect of fluid velocity.

[C] sub i: the damping matrix of subsystem or substructure i.

[C sub aa], [K sub aa]: the damping matrix and the elastic stiffness matrix associated with the active degrees of freedom, respectively.

[C sub ab], [K sub ab]: the damping matrix and the elastic stiffness matrix representing the coupling forces produced at the active degrees of freedom by the motion of the supports, respectively.

[C sub bb], [K sub bb]: the support forces produced by unit velocity and by unit displacement at the supports, respectively.

c sub v: the coefficient of variation.

c sub upsilon: the coefficient of the Poisson ratio function.

d: the displacement.

D: the hysteretic damping.

DOF: degree of freedom.

D sub upsilon: the coefficient of the Poisson ratio function.

[D]: the diagonal matrix whose terms are D sub kk equal to 2 lambda sub k M star sub k omega sub k.

E sub a: the modulus of elasticity of aluminium.

E sub c: the modulus of elasticity of concrete.

E sub j: the strain energy associated with mode j of the system.

E sub m: the strain energy lost by the system.

E sub s: the modulus of elasticity of steel.

E sub sct: the secant modulus of elasticity.

f: the friction force per unit length.

f prime sub c: the specified compressive strength of concrete.

f sub max: the maximum friction force per unit length.

f sub pi: the frequency of mode i of the primary system.

f sub zpa: the rigid body (ZPA) frequency.

f sub s: the frequency of the subsystem.

F sub a: the axial force of the buried structure.

F sub b: the reaction at the supports of the system.

F sub r: the resultant of the dynamic earth pressure acting on the retaining wall.

g: the acceleration of gravity.

G: the shear modulus.

h: the thickness of the shell.

H: the storey height; the embedment depth.

I sub o: the total mass moment of inertia of the structure and of the base slab about the foundation axis.

[I]: the unit diagonal matrix.

[K]: the stiffness matrix.

[K sub i]: the stiffness matrix of part i of the structure.

K sub p: the torsional stiffness.

K sub xi, K sub yi: the stiffnesses of wall or column i in the x and y directions respectively, assuming a rigid connection with the floor slab.

l sub c: twice the distance from the top of the fluid to the centre of the sloshing fluid mass.

l sub I: twice the distance from the bottom of the pool to the centre of the sloshing fluid mass.

L: the length of the base slab; one half of the wavelength; the distance between the flexible joints of a long straight buried structure.

m: the number of vibration modes considered.

[M]: the mass matrix.

[M star]: the floating mass matrix.

[M sub a], [M sub b]: the mass matrices associated with the active degrees of freedom and with the support points, respectively.

[M sub ab]: the mass matrix representing the coupling between U sub a and the support degrees of freedom.

M sub c: the sloshing fluid mass.

[M sub H]: the effective (or added) mass matrix produced by the hydrodynamic effect, of order n by n.

[M sub i]: the mass matrix of part i of the structure.

M sub li: the impulsive fluid mass at node i.

M star sub k: the generalized mass of mode k of subsystem i normalized to 1.

M sub pi: the modal mass of mode i of the primary structure.

M sub r: the bending moment reaction of the pressure distribution load about the base of the retaining wall.

M sub s: the mass of the subsystem or substructure.

M sub si: the mass of the pool structure at node i.

n: the number of dynamic degrees of freedom or the number of elements considered.

N: the number of structural parts considered.

NF: the number of natural frequencies of the subsystem.

NS: the number of integrated substructures.

P sub u: the hydrodynamic pressure produced by vertical motion.

r sub ia: the modal mass ratio between mode i of the primary system and mode alpha of the secondary system.

R: the combined response of the 3 orthogonal earthquake components, or the radius of a circular foundation.

R sub I: the response to component I of the motion.

R sub Ii, R sub Ij: the maximum probable responses obtained from the response spectrum analysis for mode i (or j) under excitation in direction I (I equal to 1, 2, 3).

S sub a: the spectral acceleration value.

S sub aj: the spectral acceleration value of mode j.

S sub av: the vertical spectral acceleration value.

S sub d: the spectral displacement value.

S(f, lambda): the response spectrum value (a function of frequency and damping).

S sub v: the spectral velocity value.

t: the time.

t sub d: the decaying phase of the ground motion.

t sub m: the strong-motion phase of the ground motion.

t sub r: the rising phase of the ground motion.

T(omega): the transfer function of the structure at circular frequency omega.

[T sub r]: the connection matrix between the rigid body motion of the base coordinates and the unconstrained degrees of freedom of the subsystem.

u double dot sub g: the value of the ground or base acceleration.

u double dot sub g(omega): the Fourier transform of the ground acceleration time history u double dot sub g(t).

{u double dot sub g}: the column vector of ground acceleration values.

U sub a: the translational displacement degree of freedom.

{U sub b}: the vector indicating the direction of the ground acceleration relative to the global coordinate system.

U bar sub b: the horizontal displacement input at the support points.

v: the velocity.

v sub max: the maximum ground velocity.

w sub c: the unit weight of concrete.

W: the actual width of the flange.

W sub e: the effective width of the flange.

{X}: the relative displacement vector.

{X dot}: the relative velocity vector.

{X double dot}: the relative acceleration vector.

X bar sub i, Y bar sub i: the coordinates of wall or column element i.

X sub cr, Y sub cr: the coordinates of the centre of rigidity.

y: the depth measured downwards from the top of the fluid.

Y: the distance measured upwards from the base of the retaining wall.

{Y}: the unit vector, or generalized vector, of coordinates (m by 1).

Y sub j: the generalized coordinate of mode j.

5 General design requirements

5.1 General safety requirements.

5.1.1 The seismic design of nuclear safety related structures shall meet the design requirements of the nuclear facility as a whole.

5.1.2 Under the action of the design earthquake, nuclear safety related structures shall have their structural integrity and their safety function during the operation of the nuclear power plant ensured.

5.1.3 Where methods not specified in this standard are partly adopted, or where data and methods obtained from previous earthquakes, from special analyses or from experiments are used, there shall be sufficient justification to ensure that the overall safety degree is consistent with the overall safety degree specified in this standard.

5.2 Seismic classification of the nuclear safety related structures of nuclear power plants.

5.2.1 The seismic classification of nuclear safety related structures shall meet the general requirements of the safety classification of the nuclear power plant and shall satisfy the following provisions: a) the classification shall be based on the safety class of the systems and components supported by the plant building; b) according to the importance for safety during and after the occurrence of the earthquake, structures, systems and components shall be divided into 4 or more classes; c) the seismic classification in this standard shall be consistent with the external event classification specified for the nuclear power plant. Where no overall classification for external events exists, the classification shall be established anew according to the particular features of seismic design.

5.2.2 Nuclear safety related structures shall be seismically classified according to the following requirements: a) the seismic classification of nuclear safety related structures shall be consistent with the overall seismic classification of the structures, systems and components of the nuclear power plant, and a detailed list of nuclear safety related structures associated with the design and with the acceptance criteria shall be established during design.

5.2.2 b) The plant building structures in a nuclear power plant that are nuclear safety related are generally divided into three classes: seismic Category I, seismic Category II and seismic Category III.

5.2.2 c) Seismic Category I structures. The design of the plant building structures belonging to this class shall withstand the action of the ground motion of the SL-2 earthquake. Seismic Category I is normally consistent with the highest determined safety class, and shall include the plant building structures that act as, or support, the following items: 1) items whose failure under SL-2 earthquake conditions would directly or indirectly cause the occurrence of an accident condition; 2) items used to shut down the reactor, to maintain the reactor in the shutdown state, to remove residual heat within the required time, and to monitor the parameters of these functions; 3) items used, for any postulated initiating event considered in the design and irrespective of its probability, to prevent or mitigate an unacceptable radioactive release (beyond the limit values required by the regulations).

5.2.2 d) Seismic Category II structures. Among all the items of the plant that include items not related to nuclear safety, seismic Category II includes: 1) items that may have a spatial interaction with seismic Category I and seismic Category III items (for instance due to collapse, falling or displacement) or other interactions (brought about by the release of hazardous substances, by fire, by flooding or by interaction produced by the earthquake). It shall be demonstrated that the failure of seismic Category II items and the potential effects of such failure do not affect the safety related functions of any seismic Category I and seismic Category III item, nor any safety related operation by the operators; 2) items not included in seismic Category I but which are nevertheless needed, within a time in which an SL-2 earthquake may reasonably occur, to prevent or mitigate accident conditions of the plant (arising from a postulated initiating event and not produced by the earthquake); 3) items related to the on-site access routes and items required for the implementation of the emergency evacuation plan.

5.2.2 e) Seismic Category III structures. Seismic Category III structures shall include the plant buildings that support or house items unrelated to the reactor that may give rise to radiological hazards (such as the spent fuel building and the radioactive waste building).

5.2.2 f) Seismic Category III structures may be classified in a simplified manner as seismic Category I structures.

5.2.3 Where the interaction produced by the earthquake would lead seismic Category II structures to damage the function of seismic Category I and seismic Category III items (including the actions of the operators), the following measures shall be taken: a) the item in question of seismic Category II shall be reclassified as seismic Category I or seismic Category III and designed accordingly; b) the item in question of seismic Category II shall be verified against the SL-2 earthquake so as to ensure that it produces no adverse effect on seismic Category I and seismic Category III items; c) the threatened seismic Category I and seismic Category III items shall be suitably protected, so as to ensure that the function of these items is not damaged by the interaction of seismic Category II items.

5.3 Calculation of design seismic effects.

5.3.1 The seismic classification safety factor H distinguishes seismic Category I and seismic Category III within nuclear safety related structures on the basis of the nuclear safety analysis: for seismic Category I, H is equal to 1; for seismic Category III, H is generally less than or equal to 1; for seismic Category II, H is less than 1. During the feasibility study stage, H shall be determined by a dedicated study.

5.3.2 The design loads combined with the seismic load include the following loads: L1, the normal operating loads; L2, the additional loads expected to occur during operation; L3, the additional loads under accident conditions.

5.3.3 Load combinations are divided into strength-based acceptance criterion load combinations and displacement-based acceptance criterion load combinations. The load combinations based on the strength acceptance criterion shall take into account the inelastic energy absorption factor F sub mu. The load combination is the combination of the non-seismic design effect D sub NS and of the seismic design effect D sub S. The non-seismic design effect D sub NS includes the design effects of the following load combinations: combination 1, LC1 equal to L1, formula (1); combination 2, LC2 equal to L1 plus L2, formula (2); combination 3, LC3 equal to L1 plus L3, formula (3).

5.3.4 The load combinations based on the strength acceptance requirements shall meet the requirements of elastic analysis. For each of the following load combinations, the total design effect acting on an individual member shall be taken as the sum of the non-seismic design effect D sub NS and of the seismic design effect D sub S, as follows: for bending moment, in-plane shear force and axial force in diagonal bracing, formula (4) applies, D equal to D sub NS plus D sub S divided by F sub mu S; for other axial loads, other shear loads and torsional loads, formula (5) applies, D equal to D sub NS plus D sub S divided by 1.0.

5.3.4 Where: D is the total design effect acting on the individual member; D sub NS is the non-seismic design effect acting on the individual member, which shall include the mean effect values of dead load, live load, equipment load, fluid load, snow load and lateral static earth pressure; D sub S is the seismic response under the DBE calculated by the elastic methods required in this standard (response spectrum method or time history analysis method) and with the damping values; F sub mu S is the system inelastic energy absorption factor of the structural member, calculated in accordance with the requirements of Annex D.

5.3.5 When calculating seismic design effects such as forces, bending moments or stresses, the inelastic energy absorption factor may be reduced starting from the inelastic energy absorption factors F sub mu given in Table 1 according to the limit state considered. For the limit state D, F sub mu shall be taken as 1. For the out-of-plane behaviour of concrete wall-slab systems that satisfy the detailing requirements, the value of F sub mu for concrete moment-resisting frames shall be adopted. For intermediate values of l divided by h, f sub y, P and e in Table 1, F sub mu may be determined by linear interpolation. For compression and shear in columns, F sub mu is equal to 1.

5.3.6 The load combinations based on the displacement acceptance criterion are used to calculate the deformations of nonlinear seismic analysis and of linear seismic analysis. The total design effect D acting on the member shall be taken, with the following load combination, as the sum of the seismic design effect D sub S and of the non-seismic design effect D sub NS, that is, formula (6), D equal to D sub NS plus D sub S.

5.4 Acceptance criteria.

5.4.1 Linear and nonlinear seismic analyses shall meet the following requirements: a) linear analysis shall satisfy the strength acceptance criteria of 5.4 for the load combinations according to formula (4) and formula (5), and the displacement acceptance criteria of 5.4.3 for the load combination according to formula (6); b) nonlinear analysis shall satisfy the displacement acceptance criteria of 5.4.3. For nonlinear analysis, the load-bearing capacity of the yielding members shall satisfy the strength acceptance criteria listed in item b) of 5.4.2. The load combination according to formula (6) shall be used in conjunction with the nonlinear acceptance criteria; c) all structures shall satisfy the ductility detailing requirements.

5.4.2 The strength acceptance criteria shall satisfy the following strength acceptance criteria for linear analysis and nonlinear analysis: a) for linear analysis, the total design effect D acting on a member shall be less than or equal to the nominal load-bearing capacity of the member, formula (7), H times D not greater than C. Where: the nominal load-bearing capacity C is calculated according to Clause 15, and the design effect D is calculated according to the seismic design effect formulae (4) and (5) of 5.4.

5.4.2 b) For nonlinear analysis, the load-bearing capacity of all members, including the yielding members, shall be less than the nominal load-bearing capacity. The nominal load-bearing capacity is taken as phi times M sub n for bending moment, phi times V sub n for shear force and phi times P sub n for axial force, where M sub n, V sub n and P sub n are respectively the nominal values of the load-bearing capacity in bending, in shear and in axial force, and phi is a reduction factor whose value shall be sufficiently conservative.

5.4.3 Deformations may be evaluated in accordance with the translational and rotational acceptance criteria of Annex E.

6 Subsoil and foundation

6.1 General requirements.

6.1.1 The subsoil and foundation include the foundation structure supporting the superstructure and the competent soil. The foundation structure shall have sufficient strength and stiffness and shall form one or more continuous load transfer paths that transmit all the loads to the supporting competent soil or to the piles. The foundation structure shall be able to resist the forces produced in the foundation members by the design ground motion and to sustain the motion produced in the structure. The foundation structural members and the joints between them shall be ductile. In the design of foundation structures subjected to the combination of seismic and non-seismic loads, the static soil bearing capacity may be increased by one third.

6.1.2 The selection of the subsoil should avoid marked differences in the properties of the soil beneath the foundation.

6.1.3 The design and construction of the subsoil and foundation shall additionally satisfy the corresponding detailing requirements for subsoil and foundations given in GB 50007.

6.2 Linear analysis.

6.2.1 The seismic loads sustained by the competent soil or by the pile foundation, and by the connections between pile foundation caissons and the structure, shall be calculated using the loads specified for the seismic analysis in this standard, in accordance with formula (8), D equal to D sub NS plus D sub S.

6.2.2 The maximum load sustained by the subsoil and foundation need not exceed the force that the superstructure is able to transmit to the subsoil and foundation.

6.3 Nonlinear analysis.

6.3.1 The seismic effects sustained by the subsoil and foundation, as well as by the connections of the piles and caissons to the structure, shall be calculated in accordance with the requirements of Clause 9 of this standard.

6.3.2 In calculating the force-displacement curve of the subsoil and foundation, the stiffness and strength adopted shall be calculated by mature soil mechanics calculation methods.

6.3.3 In order to ensure that the mean expected value of the design effects of the subsoil and foundation is obtained, the variation of stiffness and strength shall be considered when calculating the design effects of the subsoil and foundation.

6.4 Prevention of potential liquefaction and loss of soil strength.

6.4.1 The design shall take into account the potential consequences produced by any liquefaction and loss of soil strength, including differential settlement, lateral displacement and reduction of the bearing capacity of the subsoil. For the above situations, measures may be taken such as ground improvement, selection of a structural system able to sustain the expected displacements, consideration of the loss of the soil restraint at the supports, or a combination of these measures.

6.4.2 The evaluation of potential liquefaction and of the loss of soil strength shall use the site peak ground acceleration, the magnitude and the source characteristics consistent with the design ground motion.

6.5 Sliding of buildings.

6.5.1 The stability of buildings against sliding may be checked by the static method according to formula (9), V sub R not less than 1.1 times V sub BS.

6.5.2 The lateral sliding shear force at the base calculated by linear analysis is determined by the method of formula (5), taking into account the inelastic energy absorption factor. For the foundation, the static lateral resisting force is calculated by the following expression, formula (10), V sub R equal to C plus N times mu plus P sub u.