ISO 16117:2013/Amd 1:2026

• Status: Published
• Publication Date: 10-Nov-2026
• ICS: 27.120.20 - Nuclear power plants. Safety
• Technical Committee: ISO/TC 85/SC 5 - Nuclear installations, processes and technologies
• Drafting Committee: ISO/TC 85/SC 5 - Nuclear installations, processes and technologies
• Current Stage: 6060 - International Standard published
• Start Date: 11-Nov-2026
• Completion Date: 11-Nov-2026

• Amends: ISO 16117:2013 - Nuclear criticality safety — Estimation of the number of fissions of a postulated criticality accident
• Effective Date: 17-Jun-2023

ISO 16117:2013/Amd 1 is an ISO amendment to the standard on nuclear criticality safety and the estimation of the number of fissions of a postulated criticality accident. It clarifies what information should be considered when estimating fissions, and it is aimed at people who prepare, check, or use criticality safety calculations and simplified estimation methods.

What does ISO 16117:2013/Amd 1 specify?

ISO 16117:2013/Amd 1 updates the base standard by refining the assumptions used in fission estimates and by tightening the use of simplified formulae. The amendment is focused on practical estimation work, so the main point is not a new method but clearer guidance on what must be considered before a simplified model is used.

The amendment text is organized around a replacement for Clause 3.1, one added sentence in 4.1.1, a revised paragraph in Annex D, and bibliography updates. In practice, that tells the reader where to look for the changed guidance.

What are the key requirements of ISO 16117:2013/Amd 1?

ISO 16117:2013/Amd 1 broadens the list of assumptions that should be considered when estimating the number of fissions for a critical or supercritical system. The change matters because fission estimates depend on the physical and operational conditions of the accident, not just on the fissile material itself.

Clause 3.1 - what must be considered

Clause 3.1 says the estimate should consider the system materials and their variations. The examples include composition, spatial distribution, quantities, physical form, density, chemical form, and heat capacity. In practice, this means the analyst needs to account for how the material is arranged and how it behaves, not only what it is made of.

Clause 3.1 also requires consideration of system geometry and surrounding materials. The examples include cylinder, slab, complex geometry, interactions between several units, reflector, cooling system, heat capacity, and heat transfer coefficient. This matters because geometry and surroundings can change reactivity and the resulting fission count.

The same clause includes the duration of the criticality accident, calculated or estimated with and without intervention where applicable, and the degree of confinement and environment. The examples include whether a vessel is open or closed and the pressure. That means the estimate should reflect the accident as it could actually develop, including enclosure effects.

Clause 3.1 also identifies additional information that can be estimated, such as total reactivity addition and rate of reactivity addition. It also notes that other parameters may be of interest, such as the time delay before the first persistent chain reaction for weak neutron source systems.

Clause 4.1.1 - check the model range

ISO 16117:2013/Amd 1 adds a sentence to 4.1.1 saying that it should be confirmed that the input data are within the range for which the simplified model is applicable. In practice, this is a model-validity check before using a simplified formula.

Annex D - simplified formulae

Annex D keeps the simplified formulae for estimating the number of fissions and notes that other formulae may be better for specific models and configurations, including powder, metal, and rods. This tells the user to match the formula to the actual system instead of relying on one generic approach.

What terms does ISO 16117:2013/Amd 1 define?

ISO 16117:2013/Amd 1 uses several technical terms in a practical way.

• Criticality accident - the postulated event for which the number of fissions is estimated.
• Critical or supercritical system - the system whose materials, geometry, surroundings, and confinement are assessed in the estimate.
• Reactivity addition - the change that drives the system toward or beyond criticality.
• Simplified model - a reduced calculation method that can be used only within its valid range.
• Weak neutron source systems - systems for which the delay before the first persistent chain reaction may be relevant.
• Confinement - the condition of the vessel or environment, such as open or closed and under pressure.
• Reflector - surrounding material that affects the neutron behavior of the system.

Who uses ISO 16117:2013/Amd 1?

ISO 16117:2013/Amd 1 is used by people working in nuclear criticality safety, especially in nuclear energy, nuclear technologies, radiological protection, and nuclear installations, processes and technologies. It is relevant to analysts, safety engineers, reviewers, and designers who need to estimate the number of fissions in a postulated accident and check whether a simplified calculation is appropriate.

What changed in ISO 16117:2013/Amd 1 from the previous edition?

ISO 16117:2013/Amd 1 is an amendment to ISO 16117:2013 and introduces clarifications. It replaces Clause 3.1, adds a range check to 4.1.1 for simplified models, and revises Annex D and the Bibliography to point to additional formula sources.

Which standards are used with ISO 16117:2013/Amd 1?

ISO 16117:2013/Amd 1 is prepared according to the ISO drafting and maintenance framework named in the foreword.

• ISO/IEC Directives, Part 1 - describes the procedures used to develop and maintain the document.
• ISO/IEC Directives, Part 2 - sets the editorial rules used to draft the document.

What does the ISO 16117:2013/Amd 1 document contain?

ISO 16117:2013/Amd 1 contains the revised wording of Clause 3.1, which expands the list of assumptions and examples for fission estimation. It also contains the added sentence in 4.1.1 that requires checking whether the input data fit the applicable range of the simplified model.

Annex D is revised to keep the simplified formulae and to note that other formulae may suit specific configurations such as powder, metal, and rods. The Bibliography is expanded with additional sources on simplified fission-estimation formulae and related review work.

International
Standard
ISO 16117
First edition
Nuclear criticality safety —
2013-10-01
Estimation of the number of fissions
of a postulated criticality accident
AMENDMENT 1
2026-11
AMENDMENT 1
Sécurité de criticité nucléaire — Évaluation du nombre de
fissions en cas d'un hypothétique accident de criticité
AMENDEMENT 1
Reference number
ISO 16117:2013/Amd.1:2026(en) © ISO 2026

ISO 16117:2013/Amd.1:2026(en)
© ISO 2026
All rights reserved.
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ISO 16117:2013/Amd.1:2026(en)
Foreword
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Norme
internationale
ISO 16117
Première édition
Sécurité de criticité nucléaire —
2013-10-01
Évaluation du nombre de fissions en
cas d'un hypothétique accident de
AMENDEMENT 1
criticité
2026-11
AMENDEMENT 1
Nuclear criticality safety — Estimation of the number of fissions
of a postulated criticality accident
AMENDMENT 1
Numéro de référence
ISO 16117:2013/Amd.1:2026(fr) © ISO 2026

ISO 16117:2013/Amd.1:2026(fr)
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ISO 16117:2013/Amd.1:2026(fr)
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