EN ISO 16646:2025

SLOVENSKI STANDARD
01-december-2025
Fuzijske naprave - Merila za načrtovanje in delovanje zadrževalnih in
prezračevalnih sistemov v objektih za fuzijo tritija in objektih za ravnanje s
fuzijskim gorivom (ISO 16646:2024)
Fusion installations - Criteria for the design and operation of confinement and ventilation
systems of tritium fusion facilities and fusion fuel handling facilities (ISO 16646:2024)
Fusionsanlagen - Kriterien für die Auslegung und den Betrieb von Einschluss- und
Lüftungssystemen von Tritiumfusionsanlagen und Anlagen zur Handhabung von
Fusionsbrennstoff (ISO 16646:2024)
Installations de fusion - Critères pour la conception et l'exploitation des systèmes de
confinement et de ventilation des installations de fusion avec tritium et des installations
de traitement des combustibles de fusion (ISO 16646:2024)
Ta slovenski standard je istoveten z: EN ISO 16646:2025
ICS:
13.280 Varstvo pred sevanjem Radiation protection
27.120.20 Jedrske elektrarne. Varnost Nuclear power plants. Safety
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 16646
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2025
EUROPÄISCHE NORM
ICS 13.280; 27.120.20
English Version
Fusion installations - Criteria for the design and operation
of confinement and ventilation systems of tritium fusion
facilities and fusion fuel handling facilities (ISO
16646:2024)
Installations de fusion - Critères pour la conception et Fusionsanlagen - Kriterien für die Auslegung und den
l'exploitation des systèmes de confinement et de Betrieb von Einschluss- und Lüftungssystemen von
ventilation des installations de fusion avec tritium et Tritiumfusionsanlagen und Anlagen zur Handhabung
des installations de traitement des combustibles de von Fusionsbrennstoff (ISO 16646:2024)
fusion (ISO 16646:2024)
This European Standard was approved by CEN on 22 September 2025.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 16646:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 16646:2024 has been prepared by Technical Committee ISO/TC 85 "Nuclear energy,
nuclear technologies, and radiological protection” of the International Organization for Standardization
(ISO) and has been taken over as EN ISO 16646:2025 by Technical Committee CEN/TC 430 “Nuclear
energy, nuclear technologies, and radiological protection” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by March 2026, and conflicting national standards shall
be withdrawn at the latest by March 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 16646:2024 has been approved by CEN as EN ISO 16646:2025 without any modification.

International
Standard
ISO 16646
First edition
Fusion installations — Criteria
2024-06
for the design and operation
of confinement and ventilation
systems of tritium fusion facilities
and fusion fuel handling facilities
Installations de fusion — Critères pour la conception et
l'exploitation des systèmes de confinement et de ventilation
des installations de fusion avec tritium et des installations de
traitement des combustibles de fusion
Reference number
ISO 16646:2024(en) © ISO 2024
ISO 16646:2024(en)
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 16646:2024(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General confinement specificities of tritium fusion facilities or fusion fuel handling
facilities . 7
4.1 General .7
4.2 Inventories to identify for fusion facilities confinement .9
4.3 Specific systems or loads used in fusion facilities .10
4.4 General description of confinement systems for fusion facilities .10
4.4.1 General .10
4.4.2 First confinement system . 13
4.4.3 Secondary confinement system .14
4.4.4 Fuel cycle systems . 15
4.4.5 Tritium material compatibility . 15
5 Functions and safety aspects for confinement and ventilation systems .15
5.1 General . 15
5.2 Main functions .16
5.3 General principles .17
5.4 Risk assessment procedure . .17
5.4.1 Preliminary analysis .17
5.4.2 Risk evaluation.19
5.4.3 Safety classification . 20
6 Architecture and description of the different confinement systems .21
6.1 General .21
6.2 Static confinement of radioactive material .21
6.2.1 Containment barriers and systems .21
6.2.2 Isolation function (static containment) .21
6.3 Ventilation of the volumes within the first confinement system . 23
6.3.1 General . 23
6.3.2 Vacuum vessel exhaust process . 23
6.3.3 Primary containment pressure limiter . 23
6.3.4 Local and/or centralised detritiation system .24
6.3.5 Ventilation of the volume between first and second containment barriers of the
first confinement system .24
6.3.6 Cryogenic systems .24
6.4 Ventilation of the volumes within the second confinement system . 25
6.4.1 General . 25
6.4.2 Rooms and building ventilation systems. 25
6.4.3 Detritiation systems . 25
6.4.4 Local ventilation systems . 26
6.4.5 Ventilation of the volumes located outside the secondary confinement . 26
6.4.6 Miscellaneous ventilation systems not connected with containment envelopes . 26
7 Requirements for the design of ventilation systems .27
7.1 General .27
7.2 Dynamic confinement .27
7.2.1 General .27
7.2.2 Classification of the installation into potentially contaminated areas
(confinement classification) . 28
7.2.3 Factors influencing the design of ventilation systems . 30
7.2.4 Negative pressure . 30

iii
ISO 16646:2024(en)
7.2.5 Air velocities between areas.31
7.2.6 Basic air pattern and clean-up systems .32
7.2.7 Classification into ventilation types . 33
7.2.8 Optimization of air exchanges . 33
7.2.9 Layout and location of the ventilation ducts .37
7.2.10 Elaboration of the ventilation diagram and calculation of the pressure drops .37
7.3 Filtration .37
7.3.1 General .37
7.3.2 HEPA filters . 38
7.3.3 Air detritiation system . 38
7.3.4 Other gas-trapping devices .41
8 Management of specific risks . 41
8.1 Control of combustible gases in the buildings.41
8.2 Control of cryogenic liquid/ gases in the buildings .42
8.3 Management of ambient conditions .42
8.3.1 Tritium airborne contamination .42
8.3.2 Air conditioning of safety-classified components .43
8.3.3 Ventilation systems of the control rooms .43
8.4 Prevention of risks linked to releases of heat, gases or toxic vapours . 44
8.5 Prevention of risks linked to the deposition of matter in ventilation ducts. 44
8.6 Prevention of fire hazard .45
8.6.1 Compartmentalization .45
8.6.2 Fire compartments (fire sectors) .45
8.6.3 Fire areas .47
8.6.4 Containment compartments .47
8.7 Consideration of external hazards . 48
9 Dispositions concerning the management and the operation of the ventilation systems .49
9.1 Organization and operating procedures . 49
9.2 Technical operating instructions . 49
9.3 Operational management issues .51
9.4 Test procedures and maintenance .51
9.4.1 General .51
9.4.2 Qualification .51
9.4.3 Pre-commissioning inspection tests .51
9.4.4 Acceptance tests .52
9.4.5 Commissioning tests .52
9.4.6 Maintenance and other periodic tests. 53
9.5 Monitoring of the confinement system . 54
9.6 Control of the ventilation system to prevent fire hazards . 55
9.6.1 General . 55
9.6.2 Fire control philosophy . 55
10 Instrumentation and Control (I&C) .57
10.1 Control .57
10.2 Instrumentation .57
10.3 Alarms . 58
Annex A (informative) Guidance on airborne contamination risks .59
Annex B (informative) Examples of detritiation technologies .66
Annex C (informative) Safety classification examples .68
Annex D (informative) Typical values for leak rates .70
Annex E (informative) Requirements for ventilation system air filters used in nuclear facilities .73
Bibliography .79

iv
ISO 16646:2024(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 85, Nuclear energy, Subcommittee SC 2,
Radiological protection.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html

v
ISO 16646:2024(en)
Introduction
Confinement and ventilation systems implemented in fusion facilities using radioactive materials and
fusion fuel handling facilities ensure a safety function aiming at protecting the workers, the public and the
environment from the dissemination of radioactive contamination, including but not limited to tritium,
likely to be released from the operation of these installations.
This document applies specifically to confinement and ventilation systems for tritium fusion facilities
and fusion fuel handling facilities and their specific buildings (such as fuel handling facilities, hot cells,
examination laboratories, emergency management centres, radioactive waste treatment and storage
station).
In such fusion installations, tritium is particularly focused, as their tritium inventory may be high and as it
is likely to have a broader impact on workers, the environment or the members of the public than the other
radionuclides.
In most countries, a tritium quantity is declared as high for tritium inventories in a facility site higher than
a range of 10 g to 100 g. In the tritium fusion facilities in the scope of this document, the tritium inventory is
deemed to be much higher than this range for the whole facility site.

vi
International Standard ISO 16646:2024(en)
Fusion installations — Criteria for the design and operation
of confinement and ventilation systems of tritium fusion
facilities and fusion fuel handling facilities
1 Scope
This document specifies the applicable requirements related to the design and the operation of confinement
and ventilation systems for fusion facilities for tritium fuels and tritium fuel handling facilities specific for
fusion applications for peaceful purposes using high tritium inventories, as well as for their specialized
buildings such as hot cells, examination laboratories, emergency management centres, radioactive waste
treatment and storage facilities.
In most countries, a tritium quantity is declared as high for tritium inventories higher than a range of 10 g
to 100 g. In the tritium fusion facilities in the scope of this document, the tritium inventory is deemed to be
higher than this range for the whole site.
This document applies especially to confinement and ventilation systems that ensure the safety function
of nuclear facilities involved in nuclear fusion with the goal to protect the workers, the public and the
environment from the dissemination of radioactive contamination originating from the operation of these
installations, and in particular from airborne tritium contamination with adequate confinement systems.
The types of confinement systems for other facilities are covered by ISO 26802 for fission nuclear reactors,
by ISO 17873 for facilities other than fission nuclear reactors and by ISO 16647 for nuclear worksite and
for nuclear installations under decommissioning. The facilities covered by these three standards, notably
ISO 17873, include tritium as a radioactive material among the ones to be confined, but tritium is not their
driver of the risks for workers and for members of the public. Nevertheless, the tritium quantities and risks
from fusion facilities create specificities for a specific standard (e.g. in fusion facilities, tritium is the driver of
routine and accident consequences). Therefore, the scope of this document does not cover the other facilities
involved in tritium releases (ISO 17873, ISO 16647 and ISO 26802), even though these other facilities create
tritium releases (e.g. non-reactor fission facilities, tritium laboratories, tritium removal facilities from
fission plants, tritium defence facilities).
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.
ISO 10648-2, Containment enclosures — Part 2: Classification according to leak tightness and associated
checking methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

ISO 16646:2024(en)
3.1
accidents
3.1.1
design basis accident
DBA
accident conditions against which a facility is designed according to established design criteria, and for
which the release of radioactive material is kept within authorized limits
[SOURCE: IAEA Nuclear Safety and Security Glossary (2022 interim edition)]
3.1.2
design extension conditions
DEC
postulated accident conditions not considered for design basis accidents, but considered in the design
process for the facility in accordance with best estimate methodology, and for which release of radioactive
material is kept within acceptable limits
[SOURCE: IAEA Nuclear Safety and Security Glossary (2022 interim edition)]
Note 1 to entry: This new IAEA expression has been introduced for upgrading existing facilities or designing new
facilities, following the occurrence of core melt accident situations in fission facilities. DEC cover the former situations,
that were in the past included in the Beyond Design Basis Accidents category, related to multiple failures in the facility
as well the ones that were supposed to create core melt and that are now supposed not to impact the containment of
the facility (and thus that would become a design condition for the confinement of the nuclear facility).
Note 2 to entry: For new fusion facilities using radioactive materials, this expression cover accidents scenarios that
were also considered as beyond design basis accidents for former designs, but that shall be considered in the design
process of the facility in order to limit radioactive releases within acceptable limits. For fusion facilities, examples of
DEC covered by this expression are the multiple failures scenarios (e.g. combination of loss of coolant events and loss
of vacuum accidents), explosion scenarios, generalised fire scenarios.
3.1.3
beyond-design basis accident
BDBA
postulated accident with accident conditions more severe than those of a design basis accident (3.1.1)
[SOURCE: IAEA Nuclear Safety and Security Glossary (2022 interim edition)]
Note 1 to entry: This expression was first used for fission reactors after the first core melt accident situations that
th
occurred in the 20 century, in order to identify the situations that were not considered in the design of the facility but
for which specific requirements should be considered to reduce the likelihood of fission reactors core melt situations as
well as the consequences of such situations, that are now covered by the IAEA expression “design extension conditions
(DEC)” (3.1.2). In the most recent years, for new facilities, BDBA cover only the accidents that are even beyond the DEC,
and that shall be practically eliminated.
Note 2 to entry: IAEA defines also severe accidents as “accident conditions more severe than a design basis accident
and involving significant core degradation”. In a fusion facility, there is no possibility of core degradation and therefore
this definition is not used.
3.2
aerosol
solid particles and liquid droplets of all dimensions in suspension in a gaseous fluid
3.3
air exchange rate
ratio between the ventilation air flow rate of a containment enclosure or a compartment, during normal
operating conditions, and the volume of this containment enclosure or compartment
-1 -1 -1
Note 1 to entry: The SI unit is s but the general usage is in d for leaktight volumes or in h for general ventilation.

ISO 16646:2024(en)
3.4
air conditioning
arrangements that allow sustaining a controlled atmosphere (temperature, humidity, pressure, dust levels,
gas content, etc.) in a defined volume, in order to ensure comfort of the personnel and/or the conditions for
adequate operation of safety systems used in fusion facility
3.5
balancing damper
control valve
adjustable device inserted in an aerodynamic duct allowing balancing of the fluid flow and/or the pressure
of the fluid during plant operation
3.6
barrier
physical obstruction that prevents or inhibits the movement of people, radionuclides or some other
phenomenon (e.g. fire), or provides shielding against radiation
[SOURCE: IAEA Nuclear Safety and Security Glossary (2022 interim edition)]
Note 1 to entry: In the context of this document regarding the confinement function, it concerns a structural element
that defines the physical limits of a volume with a particular radiological environment and that prevents or limits
releases of radioactive substances from this volume.
3.7
cell
shielded enclosure, shielding structure, of fairly large dimensions, possibly leak-tight
Note 1 to entry: See containment enclosure (3.10).
3.8
containment
confinement
arrangement allowing users to maintain separate environments inside and outside an enclosure, blocking
the movement between them of process materials and substances resulting from physical and chemical
reactions that are potentially harmful to workers, to the public, to the external environment, or for the
handled products
Note 1 to entry: the word containment is used for the leak-tight performances of a static physical barrier (3.6) confining
radioactive materials, whereas confinement is used for the global function of confining hazardous materials including
also the use of active systems ensuing a dynamic confinement (3.17). Therefore, confinement is used for the function
of preventing or controlling the releases of radioactive material to the environment in operation or in accidents.
Containment is used for the physical structures designed to prevent or control the release and the dispersion of
radioactive substances. In the context of facilities handling radioactive materials it covers structural elements (cans,
gloveboxes, storage cabinets, rooms, vaults, etc.), which are used to establish the physical integrity of an area.
3.9
containment compartment
CC
compartment of which the walls are able to contain radioactive substances that would be generated by any
plausible fire that breaks out in one of the fire compartments included
Note 1 to entry: It is often more practicable to limit the spread of a fire by using fire-resistant walls, and to prevent the
spread of contamination in the adjacent volumes.
3.10
containment enclosure
enclosure designed to prevent either the leakage of products contained in the pertinent internal environment
into the external environment, or the penetration of substances from the external environment into the
internal environment, or both simultaneously
Note 1 to entry: See cell (3.7).

ISO 16646:2024(en)
Note 2 to entry: This is a generic term used to designate all kinds of enclosures, including glove boxes, leak-tight
enclosures and shielded cells equipped with remotely operated devices.
3.11
containment envelope
volume allowing the enclosure, and thus the isolation from the environment, of those structures, systems
and components whose failure can lead to an unacceptable release of radionuclides
3.12
containment system
confinement system
system constituted of a coherent set of physical barriers (3.6) and/or dynamic systems intended to confine
radioactive substances in order to ensure the safety of the workers and the public and the protection of the
environment and to avoid releases of radioactive materials in the environment
Note 1 to entry: According to IAEA definitions, a containment system concerns the containment structure and the
associated systems with the functions of isolation, energy management, and control of radionuclides and combustible
gases. This containment system also protects the facility against external events and provides radiation shielding
during operational states and accident conditions. These two last functions are not described in this document, due to
the absence of link with the ventilation systems. In a fusion facility, the dynamic confinement (3.17) is more important
than in other facilities because of the tritium dispersion and permeation properties. Therefore, in fusion facilities, the
term confinement system is more generally used.
3.13
contamination
presence of radioactive substances on or in a material or a human body or any place where they are
undesirable or can be harmful
3.14
cubicle
generic term used to describe enclosures containing electrical equipment (power or instrumentation and
control) or cables
EXAMPLE Cabinets, junction boxes, switchboards.
3.15
decontamination factor
ratio of the contaminant concentration or particle number upstream of a filtration system to the contaminant
concentration or particle number downstream of the filtration system
Note 1 to entry: definition derived from ISO 29464:2017.
3.16
discharge stack
duct (usually vertical) at the termination of a system, from which the air is discharged to the atmosphere
after control and monitoring of contaminants
3.17
dynamic confinement
action allowing, by maintaining a preferential air flow circulation, the limitation of back-flow between two
areas or between the inside and outside of an enclosure, in order to prevent radioactive substances being
released from a given physical volume
3.18
event
any occurrence unintended by the operator, including operator error, equipment failure or other mishap,
and deliberate action on the part of others, the consequences or potential consequences of which are not
negligible from the point of view of protection and safety
[SOURCE: IAEA Nuclear Safety and Security Glossary (2022 interim edition)]
Note 1 to entry: In the context of this standard regarding the confinement function, the events are those challenging
the confinement function, whether the event is internal or external to the plant.

ISO 16646:2024(en)
Note 2 to entry: EXAMPLES of internal events for fusion type facilities are plasma events, human errors, loss of coolant
(LOCA), loss of vacuum (LOVA), loss of cryogenic inventories, electromagnetic loads, failures in steam piping systems,
leakage or failure of a system carrying radioactive fluid; fuel handling accident, loss of electrical power, drop loads,
internal missile, explosion, fire, and internal flooding.
Note 3 to entry: Examples of external events are aircraft crash, external explosion, earthquake, flood or drought,
winds and tornados, extreme temperature (high and low), human induced accidents, neighbouring facilities accidents,
external fires.
3.19
filter
device intended to trap particles suspended in gases or to trap gases themselves
Note 1 to entry: A particle filter consists of a filtering medium, generally made of a porous or fibrous material (glass
fibre or paper) fixed within a frame or casing. During the manufacturing process, the filter is mounted in a leak-tight
manner in this frame, using a lute. Gas or vapour filters are generally found in physical or chemical process units
where the primary aim is to trap certain gases. They cover in particular iodine traps (activated charcoal).
3.20
fire area
fire zone
volume comprising one or more rooms or spaces, surrounded by boundaries (geographical separation)
constructed to prevent the spreading of fire to or from the remainder building for a period of time allowing
the extinction of the fire
Note 1 to entry: It is often more practicable to limit the spread of a fire, and to prevent the spread of contamination in
the adjacent volumes by using fire-resistant walls (
...