1 Introduction
1.1 Purpose
1.1.1 This guide is to explain and refine the relevant provisions of the Regulations on Safety of nuclear power plants: Design (HAF 102, hereinafter referred to as the Regulations) for the purpose of providing acceptable general methods for nuclear safety supervision and management departments, nuclear power plant designers and operating units on design and qualification of nuclear power plants, so that the ground motion at the site will not endanger the safety of nuclear power plants, and giving guidance on the consistency of methods and procedures used for analysis and test qualification of structures and equipment, making them meet the requirements of the Regulations.
1.1.2 Annex I has the equal force as the text.
1.2 Scope
1.2.1 This guide is applicable to the design of land-based fixed water-cooled reactor nuclear power plants that meet the exclusion criteria of relevant guides for seismic risk assessment of nuclear power plants, so as to resist site-specific earthquakes. This guide does not involve the intensity of ground motion or the risk degree of SSCs in nuclear power plants.
1.2.2 When simplified procedures for design and verification are adopted, it is necessary to prove the suitability of these procedures for achieving safety objectives, and properly assess them from the perspective of safety.
1.2.3 This guide is applicable to the design and construction of newly built nuclear power plants, and is usually not used for re-assessment of built nuclear power plants. This guide is not applicable to the assessment of seismic design margin of built nuclear power plants.
1.2.4 This guide may also be used for the design of other types of nuclear power plants, but its applicability shall be assessed by engineering judgment according to the type of reactor and its special safety requirements.
1.2.5 The technical suggestions on modeling and qualification of SSCs in this guide may be applied to the design of vibration caused by other reasons than earthquakes, such as explosion of industrial facilities, aircraft impact and quarry explosion or accidents of high-speed rotating machinery. However, this kind of extended application shall be treated with caution, especially in terms of frequency range, duration, direction of induced vibration and influence mechanism on nuclear power plants, which shall be subjected to engineering judgment. It shall also be noted that the design for resisting such loads may be of different forms (such as anti-collision walls), or may include other different failure forms (such as scab or breakage caused by impact loads). Such special engineering measures are not considered in this guide.
2 General
2.1 Overview
2.1.1 In this clause, suggestions on seismic category are put forward according to the requirements in the Regulations, based on the safety importance of structure, system and component (SSC) in the design basis seismic events. In order to ensure proper safety margin in the design, some suggestions on the application of design standard are also given.
2.1.2 For SSC, services and processes that affect the safety of nuclear power plants covered by the applicable scope of this guide, quality assurance measures shall be formulated and effectively implemented.
2.2 Design basis earthquake
2.2.1 The seismic risk of each site shall be assessed, and two levels of design basis ground motions, namely, operational safety ground motion (SL-1) and ultimate safety ground motion (SL-2), shall be given according to relevant procedures and target probability levels or principles determined by the design of nuclear power plants.
2.2.2 In the design of nuclear power plant, SL-2 is related to the most stringent safety requirements, while SL-1 has different safety meanings, which is more likely and less serious, and can be determined by the operating unit through comprehensive assessment. Generally, SL-1 is used for load combination (for reasons related to probability, other events are combined with earthquakes with lower intensity), post-accident inspection and permit requirements. As a low-level ground motion, SL-1 is usually not related to safety requirements, but only related to operational requirements. When the actual ground motion at the site during the operation of the nuclear power plant exceeds SL-1, measures shall be taken to shut down the reactor; the safety-related SSCs of the nuclear power plant shall be assessed according to relevant requirements, and the operation of the nuclear power plant shall not be resumed until being examined and approved by the nuclear safety supervision and administration department.
2.2.3 SL-2 shall be considered for each safe-class SSC of nuclear power plant. The lowest level shall be considered as 0.15g (the value of zero period acceleration in design response spectrum), which is equivalent to the peak value of ground acceleration in free field.
2.2.4 The frequency spectrum and duration of potential ground motion are generally considered in the determination of design basis ground motion. When it is judged that there are multiple sources that have made major contributions to the hazard, special attention shall be paid to the influence of spectrum effect and duration of different sources. In this case, more caution shall be given when enveloping the ground motions (or response spectrum) originating from different seismic sources (such as far-field and near-field ones). Considering the different seismic requirements of SSCs, the bearing capacity of different ground motions should be assessed.
2.2.5 It is generally defined that the input ground motion happens in the free field at the surface of the earth or bedrock. When seismic input is needed at the foundation elevation, inversion-forward method may be used to assign values.
2.3 Seismic category for SSCs
2.3.1 Any major site expected effect caused by earthquake is related to vibration transmitted to SSCs through nuclear power plant structures. Vibration may affect the safety function of nuclear power plant through direct or indirect interaction mechanism (such as mechanical interaction between SSCs caused by earthquake, release of dangerous substances, fire or flooding, destruction of operator passage and unavailability of evacuation road or approach road).
2.3.2 All SSCs shall stand up to any possible seismic action, and the required performance during seismic events may be different from the safety functions considered in safety classification. These safety functions are based on the most demanding safety functions under all design basis conditions (postulated initiating event). Therefore, for safety-oriented design methods, besides safety classification, SSCs shall be classified according to their safety importance during and after earthquakes. Seismic resistance of SSCs can be divided into seismic category I, seismic category II and non-nuclear seismic category, or more other categories according to the design characteristics of nuclear power plant units. The purpose of classification is to help protect the public and the environment from the release of radioactive substances and ensure nuclear safety.
2.3.3 Seismic category I SSCs of nuclear power plants shall be specified, which shall be such designed that SL-2 can be borne. Seismic category I SSCs usually correspond to the highest safety category and include all SSCs important to safety. Specifically, seismic category I SSCs shall include the following ones and their supporting structures:
(1) SSCs that directly or indirectly cause an accident condition in case of failure as a result of SL-2;
(2) SSCs necessary for shutting down the reactor, keeping the reactor in a shutdown state, discharging waste heat within the required period, and those necessary for monitoring the parameters of the above functions;
(3) SSCs necessary to prevent or mitigate the radioactive release excessing the limit caused by any postulated initiating event (regardless of its occurrence probability) considered in the design;
(4) SSCs needed to prevent or mitigate unacceptable radioactive release consequences of spent fuel pool.
2.3.4 In 2.3.3 (3), the selection of SSCs is related to defense in depth: in an seismic event of SL-2, all levels of defense shall always be available . Physical barriers designed to defend against external events other than earthquakes shall maintain integrity and functionality during earthquakes.
2.3.5 Although the main pressure boundary of the primary circuit of light-water reactor is designed according to the borne seismic loads, as a conservative measure, it is still assumed that some design basis accidents will occur at the primary circuit pressure boundary, and SSCs to mitigate the consequences will be set up, which also belongs to the seismic category I SSCs.
2.3.6 The design, installation and maintenance of seismic category I SSCs in nuclear power plant shall meet the strictest practice, that is, higher safety margin is adopted for facilities with conventional risks. For any seismic category I SSC, appropriate acceptance criteria (such as design parameters indicating functionality, tightness or maximum deformation) shall be determined according to safety function requirements. However, in some cases, in case of assessing its impact on the safety function of nuclear power plants in detail, the acceptance criteria of physical barriers may be appropriately lowered for load combinations applicable to SL-2.
2.3.7 Seismic category II SSCs of nuclear power plant can be determined. Seismic category II SSCs shall include:
(1) all SSCs with radioactive risk but unrelated to the reactor (such as spent fuel plant building and radioactive waste plant building). These SSCs are required to have safety margins consistent with their potential radiological consequences. As these SSCs are generally related to different release mechanisms (such as waste leakage and damage of spent fuel cask), the expected consequences are different from the potential consequences of the reactor;
(2) SSCs that do not belong to seismic category I, especially those in 2.3.3 (2) and (3), but are needed to prevent or mitigate accident conditions (caused by postulated initiating events other than earthquakes) of nuclear power plants within a long enough time (there is a possibility of SL-2 or SL-1 occurring reasonably during this period);
(3) SSCs related to site accessibility and those required for implementing emergency evacuation plan.
2.3.8 The design earthquake level of seismic category II SSCs shall be determined on the following basis: the additional work done to protect the SSCs against this earthquake level must be commensurate with the risk that may be reduced for the nuclear power plant personnel or the public from the earthquake. The acceptable limits for the release of radioactive substances prescribed by the state must be followed.
2.3.9 Non-nuclear seismic category SSCs that do not belong to seismic categories I and II shall be designed according to the national non-nuclear facility specifications, i.e. facilities with conventional risks. For some of the SSCs that are important to the operation of nuclear power plants, stricter acceptance criteria may be selected according to the operation objectives. This approach can reduce the need to shut down, inspect and re-apply for a license, thus keeping the nuclear power plant operating continuously.
2.3.10 Among all SSCs of the nuclear power plant (including those that are not important to safety), for those that may have spatial interaction (such as due to collapse, fall or displacement) or other interactions (such as interaction caused by release of hazardous substances, fire, flooding or earthquake) with seismic categories I and II SSCs, the potential impact and damage caused by these SSCs shall be demonstrated that they won’t either affect the safety function of any seismic category I and II SSC, nor operator’s actions important to safety.
2.3.11 As a consequence of earthquake, according to analysis, test or experience, if it is expected that some interactions will occur and endanger the functions (including operation actions) of seismic category I or II SSCs, one of the following measures shall be taken:
(1) this kind of SSCs shall be reclassified as seismic category I or II, and redesigned;
(2) in order to avoid adverse effects on seismic category I or II SSCs, this kind of SSCs shall be subject to qualification according to SL-2;
(3) the endangered seismic category I or II SSCs shall be properly protected so as to prevent their functions from being damaged due to interaction with this kind of SSCs.
2.3.12 The SSCs mentioned in 2.3.10 shall be designed, installed and maintained according to the nuclear application practice.
However, in 2.3.11 (2), when it is considered that the frequency of interaction with seismic category I or II SSCs is very low, the safety margin may be appropriately lowered.
2.3.13 The seismic category of SSCs shall be based on a clear understanding of the functional requirements for ensuring safety during or after earthquakes. According to different safety functions, different components in the same system may belong to different seismic categories, for example, the aspects shall be considered on tightness, damage degree (such as fatigue, wear and cracking), mechanical or electrical functions, maximum displacement, permanent deformation degree and maintenance of geometric dimensions, etc.
2.3.14 Seismic loads shall be considered for all possible operation modes of nuclear power plants. In the seismic design, the seismic category of the designed SSCs shall be considered.
2.3.15 Seismic category shall be carried out according to reactor type, nuclear safety regulations and standards, and special boundary conditions of the site (such as availability of cooling water source), etc.
2.3.16 As part of the design process, a detailed list of all SSCs with relevant acceptance criteria shall be listed. See Annex I for the example list.
1 Introduction
2 General
3 Seismic design
4 Equipment seismic qualification
5 Seismic instruments
Annex I Examples of seismic category
