IEC 63186:2021
(Main)Nuclear power plants - Instrumentation and control systems important to safety - Criteria for seismic trip system
Nuclear power plants - Instrumentation and control systems important to safety - Criteria for seismic trip system
IEC 63186:2021 specifies the minimum requirements for the design of the seismic trip system, and the components thereof, used in a nuclear power plant to mitigate seismic effects. This system is intended to shut down the reactor in operation automatically before it is significantly impacted by the vibratory ground motion incurred by strong earthquakes. This document is applicable to both the design of new built plants and the upgrading of plants in operation. It may be used for the design of other types of nuclear facilities where normal operation shall be stopped in case of strong seismic motions.
Centrales nucléaires de puissance - Systèmes d'instrumentation et de contrôle-commande importants pour la sûreté - Critères pour système de protection sismique
IEC 63186:2021 spécifie les exigences minimales pour la conception du système de protection sismique et de ses composants, utilisés dans une centrale nucléaire de puissance pour limiter les effets d'un séisme. Le système est prévu pour arrêter automatiquement le réacteur en fonctionnement avant qu'il ne soit endommagé de manière significative par le mouvement vibratoire du sol provoqué par de forts séismes. Le présent document est applicable à la conception de nouvelles centrales comme à la mise à niveau de centrales en exploitation. Il peut être utilisé pour la conception d'autres types d'installations nucléaires dont le fonctionnement normal doit être interrompu en cas de mouvements sismiques importants.
General Information
Standards Content (Sample)
IEC 63186 ®
Edition 1.0 2021-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Nuclear power plants – Instrumentation and control systems important to safety
– Criteria for seismic trip system
Centrales nucléaires de puissance – Systèmes d'instrumentation et de contrôle-
commande importants pour la sûreté – Critères pour système de protection
sismique
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.
IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always
committee, …). It also gives information on projects, replaced have access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and
and French, with equivalent terms in 18 additional languages.
once a month by email.
Also known as the International Electrotechnical Vocabulary
(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or
need further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.
A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.
Recherche de publications IEC - IEC online collection - oc.iec.ch
webstore.iec.ch/advsearchform Découvrez notre puissant moteur de recherche et consultez
La recherche avancée permet de trouver des publications IEC gratuitement tous les aperçus des publications. Avec un
en utilisant différents critères (numéro de référence, texte, abonnement, vous aurez toujours accès à un contenu à jour
comité d’études, …). Elle donne aussi des informations sur adapté à vos besoins.
les projets et les publications remplacées ou retirées.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
Le premier dictionnaire d'électrotechnologie en ligne au
Restez informé sur les nouvelles publications IEC. Just
monde, avec plus de 22 000 articles terminologiques en
Published détaille les nouvelles publications parues.
anglais et en français, ainsi que les termes équivalents dans
Disponible en ligne et une fois par mois par email.
16 langues additionnelles. Egalement appelé Vocabulaire
Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc
Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC 63186 ®
Edition 1.0 2021-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Nuclear power plants – Instrumentation and control systems important to safety
– Criteria for seismic trip system
Centrales nucléaires de puissance – Systèmes d'instrumentation et de contrôle-
commande importants pour la sûreté – Critères pour système de protection
sismique
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.120.20 ISBN 978-2-8322-1011-7
– 2 – IEC 63186:2021 © IEC 2021
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Abbreviated terms . 9
5 General considerations . 10
5.1 Purpose . 10
5.2 Decision issues for the seismic trip system implementation . 10
5.3 Categorization and classification . 11
5.4 Multi-unit station consideration . 11
6 System design requirements . 11
6.1 General . 11
6.2 Seismic design requirements . 11
6.3 System architecture . 11
6.4 Mounting locations . 12
6.5 Tripping variable . 12
6.6 Triggering level and trip set-point . 12
6.7 Interface requirements . 13
6.8 Human machine interface . 13
6.9 Requirements for digital technology application . 13
6.10 Environmental conditions . 13
6.11 Design provisions for maintenance and testing . 14
6.12 Power requirements . 14
7 Component design requirements . 14
7.1 General . 14
7.2 Acceleration sensor requirements . 14
7.2.1 Performance requirement . 14
7.2.2 Installation considerations . 14
7.3 Signal processing component . 15
7.4 Logic component requirements . 15
8 Qualification requirements . 15
8.1 General . 15
8.2 Environmental qualification . 15
8.3 EMC qualification . 16
8.4 Seismic qualification . 16
Annex A (informative) Implementation examples of the seismic trip system . 17
A.1 The design of the seismic trip system in CAP1400 . 17
A.2 The IAPS in a VVER plant . 19
A.3 The seismic trip system in a French research reactor . 19
Bibliography . 20
Figure A.1 – System diagram of ESS system . 18
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
NUCLEAR POWER PLANTS – INSTRUMENTATION
AND CONTROL SYSTEMS IMPORTANT TO SAFETY –
CRITERIA FOR SEISMIC TRIP SYSTEM
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 63186 has been prepared by subcommittee 45A: Instrumentation, control and electrical
power systems of nuclear facilities, of IEC technical committee 45: Nuclear instrumentation. It
is an International Standard.
The text of this International Standard is based on the following documents:
FDIS Report on voting
45A/1391/FDIS 45A/1397/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
– 4 – IEC 63186:2021 © IEC 2021
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement,
available at www.iec.ch/members_experts/refdocs. The main document types developed by
IEC are described in greater detail at www.iec.ch/standardsdev/publications.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
INTRODUCTION
a) Technical background, main issues and organization of the standard
Earthquakes pose one of the major external threats to the safe operation of nuclear power
plants. Structures, systems and components important to safety of a nuclear power plant are
generally designed to tolerate a postulated design basis earthquake. To mitigate the adverse
effects of a strong earthquake and further enhance reactor safety, the seismic trip system has
been implemented in many reactor designs.
Nevertheless, a dedicated IEC standard for the design of such I&C system was missing.
Lessons learned from the Fukushima accident emphasize the importance of installing an
automatic seismic trip system as it provides a valuable lead time and enhances the safety
margins in potential accident conditions. IEC 63186 is the first effort in this field and provides
technical guidance and requirements for the design of this system. It discusses general
considerations for a seismic trip system and explains the rationale for the system design.
Specific requirements for the system design and equipment specification are presented.
Requirements for tests used to demonstrate the functionality of the designed system are also
included.
b) Situation of the current standard in the structure of the IEC SC 45A standard series
IEC 63186 is the third level SC 45A document tackling the issue of the seismic trip system
design.
It is built upon a number of first and second level SC 45A documents. For example,
IEC 63186 refers to IEC 61513 for general requirements, IEC 61226 for classification
determination, and IEC/IEEE 60780-323 for testing requirements.
For more details on the structure of the SC 45A standard series see item d) of this
introduction.
c) Recommendations and limitations regarding the application of this standard
The design of the seismic trip system should be harmonized with overall plant designs and
the seismic design requirements in particular. Key issues to be coordinated include, but are
not limited to, classification, system architecture, acceleration sensor mounting positions,
interface to actuated device, and trip set-point.
This document focuses on the design requirements for the seismic trip system. Other parts of
the system lifecycle, particularly the operation and maintenance requirements are not within
the scope of IEC 63186.
d) Description of the structure of the IEC SC 45A standard series and relationships
with other IEC documents and other bodies documents (IAEA, ISO)
The top-level documents of the IEC SC 45A standard series are IEC 61513 and IEC 63046.
IEC 61513 provides general requirements for I&C systems and equipment that are used to
perform functions important to safety in NPPs. IEC 63046 provides general requirements for
electrical power systems of NPPs; it covers power supply systems including the supply
systems of the I&C systems. IEC 61513 and IEC 63046 are to be considered in conjunction
and at the same level. IEC 61513 and IEC 63046 structure the IEC SC 45A standard series
and shape a complete framework establishing general requirements for instrumentation,
control and electrical systems for nuclear power plants.
– 6 – IEC 63186:2021 © IEC 2021
IEC 61513 and IEC 63046 refer directly to other IEC SC 45A standards for general topics
related to categorization of functions and classification of systems, qualification, separation,
defence against common cause failure, control room design, electromagnetic compatibility,
cyber security, software and hardware aspects for programmable digital systems, coordination
of safety and security requirements and management of ageing. The standards referenced
directly at this second level should be considered together with IEC 61513 and IEC 63046 as
a consistent document set.
At a third level, IEC SC 45A standards not directly referenced by IEC 61513 or by IEC 63046
are standards related to specific equipment, technical methods, or specific activities. Usually
these documents, which make reference to second-level documents for general topics, can be
used on their own.
A fourth level extending the IEC SC 45 standard series, corresponds to the Technical Reports
which are not normative.
The IEC SC 45A standards series consistently implements and details the safety and security
principles and basic aspects provided in the relevant IAEA safety standards and in the
relevant documents of the IAEA nuclear security series (NSS). In particular this includes the
IAEA requirements SSR-2/1, establishing safety requirements related to the design of nuclear
power plants (NPPs), the IAEA safety guide SSG-30 dealing with the safety classification of
structures, systems and components in NPPs, the IAEA safety guide SSG-39 dealing with the
design of instrumentation and control systems for NPPs, the IAEA safety guide SSG-34
dealing with the design of electrical power systems for NPPs and the implementing guide
NSS17 for computer security at nuclear facilities. The safety and security terminology and
definitions used by SC 45A standards are consistent with those used by the IAEA.
IEC 61513 and IEC 63046 have adopted a presentation format similar to the basic safety
publication IEC 61508 with an overall life-cycle framework and a system life-cycle framework.
Regarding nuclear safety, IEC 61513 and IEC 63046 provide the interpretation of the general
requirements of IEC 61508-1, IEC 61508-2 and IEC 61508-4, for the nuclear application
sector. In this framework IEC 60880, IEC 62138 and IEC 62566-2 correspond to IEC 61508-3
for the nuclear application sector. IEC 61513 and IEC 63046 refer to ISO as well as to IAEA
GS-R part 2 and IAEA GS-G-3.1, and IAEA GS-G-3.5 for topics related to quality assurance
(QA). At level 2, regarding nuclear security, IEC 62645 is the entry document for the
IEC/SC 45A security standards. It builds upon the valid high level principles and main
concepts of the generic security standards, in particular ISO/IEC 27001 and ISO/IEC 27002; it
adapts them and completes them to fit the nuclear context and coordinates with the
IEC 62443 series. At level 2, IEC 60964 is the entry document for the IEC/SC 45A control
rooms standards and IEC 62342 is the entry document for the ageing management standards.
NOTE It is assumed that for the design of I&C systems in NPPs that implement conventional safety functions(e.g.
to address worker safety, asset protection, chemical hazards, process energy hazards) international or national
standards would be applied.
NUCLEAR POWER PLANTS – INSTRUMENTATION
AND CONTROL SYSTEMS IMPORTANT TO SAFETY –
CRITERIA FOR SEISMIC TRIP SYSTEM
1 Scope
This document specifies the minimum requirements for the design of the seismic trip system,
and the components thereof, used in a nuclear power plant to mitigate seismic effects. This
system is intended to shut down the reactor in operation automatically before it is significantly
impacted by the vibratory ground motion incurred by strong earthquakes. This document is
applicable to both the design of new built plants and the upgrading of plants in operation. It
may be used for the design of other types of nuclear facilities where normal operation shall be
stopped in case of strong seismic motions.
NOTE In addition to the seismic trip system, other names are possible for the system covered in this document,
e.g. automatic seismic trip system or earthquake scram/trip system.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 60671, Nuclear power plants – Instrumentation and control systems important to safety -
Surveillance testing
IEC 60709, Nuclear power plants – Instrumentation, control and electrical power systems
important to safety – Separation
IEC/IEEE 60780-323, Nuclear facilities – Electrical equipment important to safety –
Qualification
IEC 60880, Nuclear power plants – Instrumentation and control systems important to safety –
Software aspects for computer–based systems performing category A functions
IEC/IEEE 60980-344:2020, Nuclear facilities – Equipment important to safety – Seismic
qualification
IEC 60987, Nuclear power plants – Instrumentation and control important to safety –
Hardware requirements
IEC 61226, Nuclear power plants – Instrumentation, control and electrical power systems
important to safety – Categorization of functions and classification of systems
IEC 61513:2011, Nuclear power plants – Instrumentation and control important to safety –
General requirements for systems
IEC 62003, Nuclear power plants – Instrumentation, control and electrical power systems –
Requirements for electromagnetic compatibility testing
– 8 – IEC 63186:2021 © IEC 2021
IEC 62138, Nuclear power plants – Instrumentation and control systems important to safety –
Software aspects for computer-based systems performing category B or C functions
IEC 62566, Nuclear power plants – Instrumentation and control important to safety –
Development of HDL-programmed integrated circuits for systems performing category A
functions
IEC 62566-2, Nuclear power plants – Instrumentation and control systems important to safety
– Development of HDL-programmed integrated circuits – Part 2: HDL-programmed integrated
circuits for systems performing category B or C functions
IEC 62645, Nuclear power plants – Instrumentation, control and electrical power systems –
Cybersecurity requirements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
acceleration sensor
instrument capable of sensing absolute acceleration and producing an analog or digital signal
3.2
cumulative absolute velocity
time integral of absolute acceleration over the duration of the strong earthquake shaking
3.3
free field
location on the ground surface or in the site soil column that is sufficiently distant from the
site structures to be essentially unaffected by the vibration of the site structures
3.4
operating basis earthquake
OBE/S1
earthquake that could reasonably be expected to occur at the plant site during the operating
life of the plant considering the regional and local geology and seismology and specific
characteristics of local subsurface material
Note 1 to entry: It is that earthquake that produces the vibratory ground motion for which those features of the
nuclear power plant, necessary for continued operation without undue risk to the health and safety of the public,
are designed to remain functional.
[SOURCE: IEC/IEEE 60980-344, 2020, 3.25]
3.5
peak ground acceleration
PGA
the maximum absolute value of ground acceleration displayed on an accelerogram; the
greatest ground acceleration produced by an earthquake at a site
[SOURCE: IAEA Safety Glossary, 2018 Edition]
3.6
qualification
process of determining whether a system or component is suitable for operational use. The
qualification is performed in the context of a specific class of the I&C system and a specific
set of qualification requirements
[SOURCE: IEC 61513, 2011, 3.38]
3.7
safe shutdown earthquake
SSE/S2
earthquake that is based upon an evaluation of the maximum earthquake potential
considering the regional and local geology and seismology and specific characteristics of
local subsurface material. It is that earthquake that produces the maximum vibratory ground
motion for which certain structures, systems, and components are designed to remain
functional. These structures, systems, and components are those necessary to provide
reasonable assurance of the following:
a) Integrity of the reactor coolant pressure boundary
b) Capability to shut down the reactor and maintain it in a safe shutdown condition
c) Capability to prevent or mitigate the consequences of accidents that could result in
potential offsite exposures comparable to applicable regulatory requirements
[SOURCE: IEC/IEEE 60980-344, 2020, 3.39]
3.8
safety function
a specific purpose that must be accomplished for safety for a facility or activity to prevent or
to mitigate radiological consequences of normal operation, anticipated operational
occurrences and accident conditions
[SOURCE: IAEA Safety Glossary, 2018 Edition]
3.9
tri-axial
able to measure a variable in three mutually orthogonal components (directions), one of which
is usually vertical
Note 1 to entry: Applies to description of the function of an instrument or group of instruments.
4 Abbreviated terms
ALARA As Low As Reasonably Achievable
CAV Cumulative Absolute Velocity
CRDM Control Rod Drive Mechanism
EMC Electromagnetic Compatibility
ESS Earthquake Scram System
FRS Floor Response Spectrum
FPGA Field Programmable Gate Array
HMI Human Machine Interface
IAPS Industrial Anti-Seismic Protecting System
I&C Instrumentation and Control
MEMS Micro-Electromechanical Systems
NPP Nuclear Power Plant
– 10 – IEC 63186:2021 © IEC 2021
PGA Peak Ground Acceleration
SOE Sequence of Event
SSCs Structures, Systems and Components
VVER Water-Water Hull Nuclear Power Reactor with Pressurized Water
5 General considerations
5.1 Purpose
Earthquakes pose one of the major external threats to the safe operation of nuclear power
plants. To withstand the potential effects of earthquakes, structures, systems and components
important to safety of a nuclear power plant are generally designed to tolerate a postulated
design basis earthquake. To mitigate the adverse effects of a strong earthquake and further
enhance reactor safety, the seismic trip system has been implemented in some reactor
designs.
The seismic trip system may be used for different purposes, e.g. as a precautionary measure
against inadequate seismic designs identified during operation. In cases where geological
studies or observed earthquakes near the site indicate that the risk of seismic hazard is
higher than the design basis, then the seismic trip system may be applied as an additional
measure to enhance the seismic robustness for an operating plant.
In other cases, where the expected seismic level is bounded by design basis earthquakes,
safety systems are adequately engineered to implement the desired safety functions during
and after seismic motions. In such cases the seismic trip system may be deployed to further
improve reactor safety against earthquakes. Both experience and technical studies have
indicated that an immediate seismic trip during a strong earthquake can provide a favourable
lead time, e.g. between 5 s to 20 s, which helps to reduce the loads during the seismic event
and there will be less burden incurred on the plant systems. Such lead-time also helps to
reduce the likelihood of a loss-of-coolant accident or severe transient after a seismic event.
For example, earlier trip can reduce transient pressure and loads, as well as the heat
generation rate in the core. In the event of a loss-of-coolant accident, an earlier trip will
reduce the fuel rod temperature transient and the containment vessel pressure.
A seismic trip system can contribute to the safety of an operating nuclear power plant by
mitigating the risk from seismic activity. The timing of the seismic trip system tripping
actuation may be of relevance to facilitate post-trip analysis, therefore the actuation status of
the seismic trip system may be transmitted to an SOE system.
Some examples of implementation of the seismic trip system are given in Annex A.
5.2 Decision issues for the seismic trip system implementation
The decision to install a seismic trip system will depend upon national regulatory
requirements, the conclusions from the plant safety analysis and the utility owner’s
considerations. In general, a well-informed decision should take into account the following
factors:
a) The level, frequency and duration of earthquake activity at the plant site.
b) The robustness of nuclear power plant systems against seismic shocks.
c) Considerations relating to spurious trips.
d) Impact on national or local electricity grid stability.
e) Mandatory requirements manifested in national regulations.
f) Other factors as applicable and appropriate.
A decision may be made based on one or a combination of these factors, and the relative
importance of each of the issues may vary from one country to another.
5.3 Categorization and classification
The safety class of a seismic trip system shall correspond to its assigned category of
functions in accordance with IEC 61226 and IEC 61513 and in particular mandatory national
regulation requirements. In international practices, seismic trip systems classified as Category
B or C are both found.
The seismic trip function may either be implemented in a standalone system or integrated as
a part into other I&C systems. In the case of integration into other systems, the seismic trip
system shall be designed in such a way as to prevent the propagation of failures from or to
the integrated system.
5.4 Multi-unit station consideration
If the seismic trip system is shared by multiple units located at the same site, then the
transient affect on the grid due to simultaneous tripping by spurious actuation should be
considered, especially for countries with small national grids, as this might lead to failure of
the national grid.
Where it is determined appropriate to share the seismic trip system trip signals, annunciations
shall be made in each plant. Caution should be taken when transmitting the trip actuation
signals across the units, to ensure their functionality during and after S2 earthquakes.
Adequate electrical isolation shall be provided for the trip actuation signal.
6 System design requirements
6.1 General
The seismic trip system continuously monitors the site seismic activities, and automatically
generates a trigger signal when the monitored variable exceeds the set-point or prescribed
criterion. The trip signal is sent to trip the reactor by selected means, e.g. opening circuit
breakers controlling power feed to control rod drive mechanisms (CRDMs).
Design efforts shall be commensurate with the safety significance and classification assigned
to the seismic trip system, which is the result of overall plant design considerations. Special
considerations shall be taken into account to avoid spurious reactor trip as much as
technologically possible.
6.2 Seismic design requirements
The seismic trip system is intended to implement automatic reactor trip on occurrence of a
strong earthquake; subsequently it is an inherent requirement for the system and its
components to remain functional during and after an earthquake. In order to guarantee such
capabilities, the seismic trip system shall be designed, fabricated, and tested against the
design basis earthquake known as S1/S2. The level shall be chosen according to the system
design.This shall be ensured by qualification in conformance with IEC/IEEE 60980-344.
6.3 System architecture
The engineered system architecture for the seismic trip system shall comply with applicable
requirements from IEC 61513. In particular, the seismic trip system architecture shall be
designed to prevent spurious reactor trips.
– 12 – IEC 63186:2021 © IEC 2021
To achieve the goal of high system reliability and low spurious actuation risk, the seismic trip
system shall employ a redundant structure to implement coincidence between redundant
channels to initiate a correct triggering output. Adequate provisions, especially physical and
electrical separation, shall be used to ensure independence, as required by IEC 60709.
The categorization and classification of the system shall be determined taking into account
the issues addressed in 5.3.
6.4 Mounting locations
To enable reliable set-point exceedance determination, tri-axial acceleration sensors used by
the seismic trip system shall be placed where floor or ground vibration has been theoretically
modelled and verified from dynamic calculation of seismic responses in the civil structures
following an earthquake. The acceleration sensors should be placed at locations on the civil
structures with the same seismic response so that the coincidence can be accurately detected.
The sensors should be located where the seismic response has been shown to be close to the
incoming earthquake, or elsewhere justified. This may be on the nuclear building basement.
In contrast, it is not recommended to place acceleration sensors in the free field for the
purpose of reactor tripping, which makes it vulnerable to inadvertent impact as well as cabling
failures.
To avoid spurious actuation incurred by localized vibration, acceleration sensors shall be
dispersed and located at an adequate distance from all equipment which generates vibration
or impacts during operation, either normally or accidentally. Some implementation may install
redundant groups of sensors at different elevations to offer more diversity.
The principle of ALARA (as-low-as-reasonably-achievable) shall be followed to reduce the
occupational risk of radiation for maintenance staff.
6.5 Tripping variable
Ground vibration signals may be processed using different algorithms to generate the trip
signal. This includes peak ground acceleration (PGA), cumulative absolute velocity (CAV),
and response spectrum, etc. In order to achieve the desired lead time, PGA should be used
as the preferred tripping variable.
6.6 Triggering level and trip set-point
Consistent with the seismic design practices, the trip set-point may be determined with
reference to S1 or S2. Internationally, the set-points close to S1 or S2, known as the lower
and higher triggering levels respectively, are both found in practical applications. There are
-3
even examples of very low level trips in the order of 20 milli g (equivalent to 20 × 10 ×
-2
9,81 ms ) to increase a site’s readiness for beyond-design earthquakes.
The lower triggering level is associated with S1 level earthquake, which is usually concerned
with normal operation limits. At this level, SSCs important to safety are expected to remain
functional, though the NPP may be required to shutdown in order to comply with licensing
conditions. A lower automatic triggering level is expected to lead to earlier reactor trip before
maximum shaking of the earthquake. This enables the heat generation rate in the core to be
reduced which should reduce the transient pressure and other loads on the NPP.
The higher triggering level is associated with S2 level earthquake, beyond which significant
damage may be incurred by strong seismic motion. A higher triggering level may reduce the
probability of spurious trip induced by lower level earthquake, equipment vibration or
inadvertent impact.
The set-point chosen for a specific application shall be carefully evaluated with set-point
analysis based on Floor Response Spectrum (FRS) at seismic sensor mounting locations. In
cases where the seismic trip system is intended to cope with concerns that actual
earthquakes will probably exceed the design basis, a trip set-point close to the S2 level
should be implemented. If earlier reactor trip before maximum shaking of the earthquake is
the rationale for the seismic trip system implementation, then trip set-point near S1 is
preferred. For the latter situation, if spurious tripping is a major concern, a higher tripping
level may be adopted.
6.7 Interface requirements
The interfaces between the seismic trip system and other systems shall be designed to
minimize the effect of equipment failures, prevent the propagation of failures between
systems and minimize the probability of spurious tripping of the seismic trip system. Particular
attention shall be given to the type of trip logic to be used (e.g. de-energize to trip versus
energize to trip), which dramatically impacts the likelihood of spurious actuation.
The seismic trip system actuation shall be indicated in the main control room by visual and
audio alarms, alerting the operators that a triggering event has happened. To facilitate event
synchronization and post-trip analysis, the seismic trip system actuation signals should be
transmitted to and recorded by the sequence-of-event (SOE) computer or controller.
In some cases, an unplanned trip of the reactor without tripping the turbine may result in
undesirable results, e.g. unnecessary actuation of an engineered safety feature. Accordingly,
appropriate interlocks, with sufficient isolation and suitable time delay, should be implemented
between the seismic trip system and turbine trip, to prevent such undesirable consequences.
Those signals from the seismic trip system sent to systems of lower safety class shall be
isolated, to prevent interference due to failure of the system to which the signal is sent. The
requirements of IEC 60709 shall be satisfied.
6.8 Human machine interface
The seismic trip system should provide user-friendly human machine interface (HMI). The
actuation status and system operating conditions should be indicated via the HMI at the local
cabinet or control panel. There shall be sufficient HMI resources to support various operations
such as testing, parameter setting and bypassing. Online testing capabilities should be
provided as far as possible, in order to alleviate maintenance staff workload and enhance
system reliability. The HMI shall fulfil cyber security requirements to avoid spurious trips.
6.9 Requirements for digital technology application
If the seismic trip system is implemented using programmable digital devices, such as micro-
processors, FPGAs, or MEMS device using embedded digital technologies, requirements in
IEC 60987, IEC 60880, IEC 62138, IEC 62566 and IEC 62566-2 shall be satisfied, as
appropriate.
To mitigate cyber security threats posed by application of digital technologies, the
requirements provided in IEC 62645 shall be implemented.
6.10 Environmental conditions
The seismic trip system shall be designed to withstand environmental conditions of the
location where it is installed, including temperature, humidity, pressure, radiation and
electromagnetic interference, during all applicable operation conditions.
The endurance and service life under the prescribed conditions, including operations and
potential hazard conditions, shall be verified through testing or analysis.
– 14 – IEC 63186:2021 © IEC 2021
6.11 Design provisions for maintenance and testing
The seismic trip system shall be designed with service functions to facilitate maintenance and
testing activities. The testing, calibrating and maintenance program should be specified during
the design phase and performed periodically during service. Acceleration sensors should be
testable in power operation without interference with the performance of the intended function
of the seismic trip system.
The applicable requirements of IEC 60671 shall be satisfied in the seismic trip system design.
Self-diagnosis should be provided to reveal component failures. Particular attention should be
given to the calibration of tri-axial acceleration sensors. A shaker table may need to be
available to facilitate routine calibration.
6.12 Power requirements
The seismic trip system shall be provided with uninterrupted electrical power, which may be
achieved in different ways, to ensure reliable system operation. The power supply shall
ensure system functioning during and after the expected earthquake,
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.
Loading comments...