Seismic Design and Verification of Nuclear Power Plants
(Revision of 1996)
This guide was implemented on May 13, 1996.
This guide is interpreted by the National Nuclear Safety Administration.
1 Foreword
This safety guide is an explanation and supplement of the relevant clauses in the "Safety Guide on Design of Nuclear Power Plants" HAF102. This guide is a guidance document. Methods and projects different from those specified in this guide are also available, but they must be proved to the National Nuclear Safety Administration that they can anyway guarantee the normal operation of the nuclear power plants in like manner and will not bring in improper risks to the personnel and public in the nuclear power plants.
1.1 Purpose
This safety guide aims to provide universal and detailed methods for the design of the nuclear power plants in order that the relevant earthquakes occurred in the plant site which are determined according to the safety guide HAD101/01 will not endanger the safety of the plant. It also aims to provide guidance for analyzing, testing and verifying the structures and equipments in the aspects of methods and procedures, enabling them to achieve the design scheduled purpose. This guide is available for the designers and safety review personnel of the nuclear power plants as well as the supervisors of the nuclear safety departments.
1.2 Scope
This guide is applicable to the seismic design of the nuclear power plants, non-relevant to the ground motion intensity or the material risks in the plant. It allows some proposed design and verification methods to have simplified programs, which shall be determined according to various conditions with regard to the safety suitability. It also allows that the problems may be solved by more than one engineering method and there may be large differences between one nuclear power plant and another which is designed in a different method.
1.3 Structures
1.3.1 Chapter 2 specifies that the materials used in the nuclear power plant are classified into two or more classes according to their safety significances, and proposes different guidance and proposals accordingly. In addition, this Chapter also discusses the load combinations, allowable stress or allowable deformation. Chapter 3 describes the main design principles for seismic design. Chapter 4 describes the analysis method and guidance for the structures, machinery or electrical equipment, and proposes some recommendations for verification and testing practices. Chapter 5 describes the recommended seismic instruments as well as their arrangements and applications.
1.3.2 The annexes and appendixes of this guide set forth some special problems such as seismic classification, analysis and simulation technology of the materials.
2 General Provisions
This chapter aims to classify the structures, systems and parts of the nuclear power plant according to their safety significances when a design basis earthquake happens, bring up guidance and proposals for the seismic design, analysis and test in order to ensure the safety.
2.1 Seismic Level
2.1.1 In terms of the safety design, SL-2 seismic level① must be adopted. Herein, SL-2 is the ultimate ground motion level and it must have an extremely low exceeding probability② during the lifespan of the nuclear power plant. The lowest peak ground acceleration is recommended to be 0.1g.
2.1.2 As for such conditions as several combinations of the events, inspection after the accidents, national examination and approval requirements as well as the economical factor, SL-1 seismic level②③ may be taken into account in the design. This level is a lower-intensity and more likely earthquake load seismic load condition which distinguishes from SL-2 level in the safety sense.
① This level is equivalent to the seismic level of the commonly-used SSE (safe shutdown earthquake).
② Chapter 5 of the safety guide HAD101/01 determines the background materials for SL-2 and SL-1 level.
③ This level is equivalent to the seismic level of the commonly-used OBE (operational basis earthquake).
2.2 Seismic Classifications of Structures, Systems and Parts
2.2.1 Structures, systems and parts (hereinafter referred to as "materials") may be classified into two or more classes according to their safety significances in the earthquake. This is in favor of the public and environmental protection against the radioactive release.
2.2.2 ClassⅠ seismic materials of the nuclear power plant must be specified. They must be designed and demonstrated according to SL-2 seismic level ground motion. The following materials must be included①:
(1) Materials which may directly or indirectly contribute to accidental conditions if they are damaged;
(2) Materials needed for reactor safe shutdown, monitoring the critical parameter, keeping the reactor off-state and discharging the residual heat over a long period.
(3) Materials (such as the containment system) needed for preventing the radioactive materials from releasing or keeping the released substances to a maximum of the limitation specified by the national nuclear safety departments for the accidental conditions.
2.2.3 Although the primary loop main pressure boundary is designed to be able to bear the seismic load, it is a conservative measure, so materials are still installed in order to alleviate the consequences of some design basis accidents which are assumed to be likely to happen at the primary loop pressure boundary. Such materials also shall be included in the class Ⅰ seismic materials.
2.2.4 ClassⅡ seismic materials of the nuclear power plant may be specified. They must be designed according to the following seismic level ground motion specified in 2.2.5, including:
(1) Materials not belonging to the classⅠ seismic materials and needed for preventing the radioactive materials from spillovers and exceeding the normal operation limitation;
(2) Materials not belonging to the classⅠ seismic materials and needed for alleviating the consequences of some accidental conditions. These conditions may endure for quite a long period when the scheduled-intensity earthquakes are possible to happen.
2.2.5 The design seismic level of the classⅡ seismic materials must be determined according to the following basics. The additional work done for protecting the materials against this seismic level must be commensurate to alleviating the risks caused by an earthquake to the personnel or the public in the plant. In addition, this seismic level must comply with the national specified limitation available for the radioactive material release. Therefore, SL-1 level may be reasonably adopted as the design reference in many conditions. This method may reduce the necessity of the shutdown, inspection and re-examination/approval of the plant so as to guarantee the continuous operation of the nuclear power plant.
2.2.6 Materials, not belonging to the class Ⅰ or Class Ⅱ seismic materials, shall be designed according to the national routine methods for non-nuclear purpose materials.
2.2.7 If one material is anticipated to have destruction, falling, displacement or any other space response through analysis, tests or experiments and is likely to endanger the functions of high-class materials, then one of the following measures shall be adopted:
(1) Materials of this class must come under the same class as the endangered materials;
(2) Low-class materials must be proved to be able to stand and function under the reference ground motion actions;
(3) Appropriate protection must be afforded for the endangered materials in case they are endangered.
As for those materials which are reclassified due to their possibility of imperiling the high-class materials, inexact seismic design may be adopted as long as their structural integrity is guaranteed.
① Appendix 1 describes the representative classifications of the parts and systems as well as the classifications adopted in some foreign countries.
2.2.8 To determine a certain material is grouped under the classⅠ or the classⅡ seismic material must be based on a clear understanding of the function requirements for its safety during or after an earthquake, or after the accidents (see 2.2.4 (2)) which are caused by non-seismic ground motion. For instance, such problems (see 2.4) must be considered as the airtightness, damage (fatigue, abrasion and cracking, etc.) extent, machinery and electrical function capacities, maximum displacement, permanent deformation degree and the geometric size maintenance. Different parts of the same system may belong to different classes according to their different functions.
2.3 Combinations of Seismic Loads and Other Operational Loads of the Nuclear Power Plant
2.3.1 The operational loads of the nuclear power plant are classified as follows:
L1: loads resulting from the normal operation;
L2: additional loads resulting from the anticipated operation accidents;
L3: additional loads resulting from the accidental conditions.
2.3.2 The seismic loads shall be calculated according to the specific position of the considered material. And the soil/plant structure characteristics, including the mass and rigidity as well as the distribution of the in-plant equipments, shall be taken into account in the calculation. It shall be ensured that the controlling load combination has been taken into account in order to determine and contain the maximum stress of the structural members.
2.3.3 As for the seismic design, the following combinations are recommended for the seismic loads and the operational loads of the nuclear power plant:
(1) As For the classⅠ seismic materials, L1 and SL-2 loads are recommended to be combined with each other;
(2) As for the class Ⅱ seismic materials and non-seismic materials, L1 load shall be combined respectively with the seismic loads described in 2.2.5 and 2.2.6;
(3) As for all the materials, if L2 or L3 load is caused by an earthquake and have the same high probability as the seismic loads, or L2 load is irrelevant to the earthquake but happens quite frequently, then L2 or L3 load shall be combined respectively with the seismic loads①;
(4) As for the classⅠ seismic materials, the relevant L3 load shall be combined with the SL - 2 load, except that it is unrelated to the SL-2 load.
It shall be noted that, as for one group of materials (such as the reactor coolant system), they may belong to the L3 loads; while for another group of materials (such as the containment system or the safety injection system), they may belong to the L1 loads.
2.3.4 As for the seismic design of equipments and civil structures, the surrounding environment and other natural phenomena such as flood or fire, if caused by an earthquake, must be considered. And they can be determined according to the probability study [1].
① The typical L2 loads caused by the seismic loads can be the ones generated by the shutdown of the reactor or the primary loop system pressure peak caused by the steam turbine load rejection in the boiling water reactor (BWR) making little steam enter the condenser.
2.4 Allowable Limit of Stress, Strain and Deformation
2.4.1 As for the limitation of the load combination characteristics (such as the stress, strain and deformation) of each group of materials, it shall be the same as the limitation for L3 (therein, the load effect combinations include the effect between SL-2 and L1 or L2 and the effect between SL-1 and SL-2 L3). In the load factor design, the characteristic limitation shall be determined by different load factors according to the stress, strain or deformation limitation. This is opposite to the working stress design, in which different limitations are adopted for a group of fixed load, stress, strain or deformation. To increase the allowable stress, strain or deformation limitation has the similar effect as to reduce the load factors.
2.4.2 As for those materials (such as the control rod drive mechanism) which require much stricter characteristic limitation due to their capable function, they shall be designed according to the load combinations of SL-2 seismic loads in a proper limitation ensuring their operability during or after an earthquake.
2.4.3 If SL-1 is taken as the design reference, as for the combination of L1/L2 and SL-1 seismic loads, its design margin shall be larger than the design margin specified in 2.4.1. As for the incapable parts (such as the pipes and supporting members) and the capable parts (such as the valves, pumps and control rod drive mechanisms important for the operation safety), all the characteristics of them caused by applicable load combinations shall be remained within the proper limitation.
2.4.4 As for the characteristic limitation of the seismic materials, SL-1 ground motion must be assumed to happen more than once in the lifespan of the nuclear power plant. If SL-1 is taken as the design reference for fatigue analysis, then 5 SL-1 earthquakes and 1 SL-2 earthquake shall be taken into account in the design and every earthquake is supposed to have 10 peak value circulations [2, 3].
2.4.5 As for those non-seismic materials, their characteristic limitation shall comply with applicable national norms.
3 Seismic Designs
3.1 General Methods of Seismic Design
3.1.1 The input ground motion is usually set at the free fields of the following places: (a) ground surface; (b) elevation of the foundation bottom; (c) bedrock (see 5.2.7 of the safety guide HAD101/01). The input ground motion may be further defined as "control motion" and "control point" according to their application in the design (see 3.5 of the safety guide HAD101/12).
3.1.2 In the early design stage, the skeleton layout of the major equipment shall be set forth and studied continuously in order to get the most favorable seismic design solution. The whole seismic design process must be strictly based on a clear understanding of the consequences of the destructive earthquake and such understanding shall be practically applied to this preliminary work. In order to alleviate the seismic effects on the structures and parts, the following principle (see 3.1.5) shall be taken into account.
3.1.3 Generally, the seismic effect① can be reduced by adopting the following methods:
(1) Fix the gravity center of all the structures as low as possible;
(2) Select the surfaces and elevations as simple and regular as possible;
(3) Avoid the projecting parts (which are asymmetric) as much as possible;
(4) Make the rigidity center of each layer near to the gravity center as much as possible;
(5) adopt seismic systems and devices (for instance, the foundation vibration isolators)②.
3.1.4 In order to reduce the differential motion among the structures, these structures must be installed on the same foundation in practical scope. As for the site orientation, large characteristic differences of the soils under the foundation shall be avoided. If extended foundations are adopted, then the necessity of these foundations connecting with the adjacent foundation structures shall be studied. And the design load of these connecting members shall be determined according to the routine method.
3.1.5 To adopt very simple arrangement and structure coupling is convenient for the seismic analysis and improving the pipe and equipment seismic properties of the building. It must be noted that damages or destructions caused by the differential motions must be avoided at the structure boundaries (such as the expansion joints and the construction joints), the structure coupling places or to the equipments in and out of the building. All of the equipments must be reliably anchored unless other correct methods are proved.
3.2 Civil Engineering Structure
The following points must be paid special attention to in the processes of structure design and design review:
(1) Suitability of the lower part of the soil (see the safety guide HAD101/12);
(2) Suitability of the foundation supporting type or different foundation types under the coupling structures (for instance, that one part of the building foundation support is installed on the stakes or rocks, while the other part is directly installed on the soil shall be avoided);
(3) Balanced and symmetric arrangement of the structure frame and shear wall in order to obtain the optimal rigidity, load and weight distribution as well as the minimum twisting effect;
(4) Prevent the adjacent buildings from colliding (such phenomena are also common in the weak coupling structures) with each other because of the dynamic deformation;
(5) Appropriate coupling between the annexes/appendage and the main structure (see (4) too);
① (1)-(4) points are in favor of reducing the seismic force, harmful torsion and swing effect.
② Foundation vibration isolator shall be adopted cautiously so as to avoid increasing the relative movement or displacement aforesaid in 3.1.4 and 3.1.5.
(6) The main structure members must be guaranteed to have adequate resistance, in particular, the anti-transverse shearing force;
(7) Adequate ductility shall be ensured and brittle failure caused by the shearing force or pressure shall also be avoided. For instance, there shall be adequate steel bars, especially there shall be adequate stirrup bindings (for full restriction);
(8) Arrangement and distribution of the steel bars: it the steel bars are highly centralized, this may contribute the concrete to cracking along the steel bars;
(9) It is necessary to design the material anchor of the joints and castings of the structural members in the concrete. Ductile fracture mode shall be ensured (for instance, the anchor length must be long enough in case being pulled out and adequate transverse bars shall be provided). Meanwhile, the structure coupling shall be done with much higher intensity and ductility than the members being connected;
(10) If large deformation is allowed, then the bending moment generated by transverse deformation shall be calculated. Therein, the transverse deformation is caused by the P-△ effect of the structure formed by the vertical force and the seismic action;
(11) Additive effect of the launching buoyancy on the foundation;
(12) Possibility of the structure’s transverse slide on the water proofer during an earthquake (such a phenomenon is more likely to happen in a wet condition);
(13) The effect of the "non-structure" members (such as the partitions) on the structure members: due to the force in the partition, there may be cracks at the beam-column. This phenomenon is much likely to happen at the highest layer, for where the "non-structure" members reduce their favorable actions for bearing the vertical loads;
(14) As for those structures which are designed to be large-scale and complete due to the seismic differential motion, their construction joints and heat stress shall be respectively designed and calculated carefully.
(15) If the containment rigidity is larger than the surrounding concrete structure rigidity, they are coupled together, or the seismic loads of the concrete structure may pass to the containment, then the force transmission effect shall be considered. If this kind of force is difficult to be calculated due to the interactional structures' complexity, then these structures above the foundation elevation are recommended to be separated.
