National foreword

This British Standard is the UK implementation of ISO 18075:2026. It supersedes BS ISO 18075:2018, which is withdrawn.

The UK participation in its preparation was entrusted to Technical Committee NCE/3.

A list of organizations represented on this committee can be obtained on request to its committee manager.

Contractual and legal considerations

This publication has been prepared in good faith, however no representation, warranty, assurance or undertaking (express or implied) is or will be made, and no responsibility or liability is or will be accepted by BSI in relation to the adequacy, accuracy, completeness or reasonableness of this publication. All and any such responsibility and liability is expressly disclaimed to the full extent permitted by the law.

This publication is provided as is, and is to be used at the recipient’s own risk.

The recipient is advised to consider seeking professional guidance with respect to its use of this publication.

This publication is not intended to constitute a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 August 2026.

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, nuclear technologies, and radiological protection, Subcommittee SC 6, Reactor technology. This document is based on a standard developed by the American Nuclear Society (ANS) of which the current version is ANSI/ANS‑19.3‑2022^[2^].

This second edition cancels and replaces the first (ISO 18075:2018), which has been technically revised.

The main changes are as follows:

addition of new terms in Clause 3;
figures slightly revised;
5.8.5 was amended so that CANDU is only an example;
correction of internal references;
Table A.1 corrected;
editorial amendments in the whole document.

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.

Introduction

The design and operation of nuclear reactors require knowledge of the conditions under which a reactor will be critical, as well as the degree of subcriticality or supercriticality when these conditions change. In addition, knowledge is required of the spatial distribution of neutron reaction rates in reactor components as a prerequisite, for example, for inferring proper power and temperature distributions to ensure the satisfaction of thermal-limit and safety-limit requirements. Both reaction-rate spatial distributions and reactivity have been measured by suitable experimental techniques, either in mock-ups or in the operating reactors themselves. These quantities can also be calculated by various techniques. Available reactor experimental data have been used to validate the steady-state neutronic calculations within reasonable margins. As more accurate nuclear cross sections and improved calculation methods have become available, steady-state neutronic calculations have been utilized extensively for nuclear fuel and core designs and analyses and thus have become increasingly important.

1 Scope

This document provides guidance for performing and validating the sequence of steady-state calculations leading to prediction in all types of operating commercial nuclear reactors, of the following:

reaction-rate spatial distributions;
reactivity;
change of nuclide compositions with time.

The document provides

guidance for the selection of computational methods,
criteria for verification and validation of calculation methods used by reactor core analysts,
criteria for evaluation of accuracy and range of applicability of data and methods, and
requirements for documentation of the preceding.

2 Normative references

3 Terms, definitions and abbreviations

3.1 Terms and definitions

3.2 Abbreviations

4 Relation to other standards

5 Methods of calculation

5.1 General

5.2 Conditions to be considered

5.3 Multigroup cross sections

5.4 Collapse to few-groups and spatial homogenization

5.5 Calculation of reactivity, reaction rates and neutron flux distributions

5.6 Calculation of reaction rates in reactor components

5.7 Depletion calculations

5.8 Common practices

6 Verification and validation of the calculation system

6.1 Overview

6.2 Verification

6.3 Validation

6.4 Biases and uncertainties

7 Documentation

8 Summary of requirements

Bibliography