Nuclear power plants - Instrumentation systems important to safety - In-core instrumentation: Characteristics and test methods of self-powered neutron detectors

IEC 61468:2021 applies to in-core neutron detectors, viz. self-powered neutron detectors (SPNDs), which are intended for application in systems important for nuclear reactor safety: protection, instrumentation and control. This document contains SPND characteristics and test methods. In this document, the main sources of errors, and the possibilities for their minimization are also considered. This document contains requirements, recommendations and instructions concerning selection of SPND type and characteristics for various possible applications.
This document about SPNDs uses the basic requirements of IEC 61513 and IEC 60568 and complements them with more specific provisions in compliance with IAEA Safety Guides.
This second edition cancels and replaces the first edition, published in 2000, and its Amendment 1, published in 2003.
This edition includes the following significant technical changes with respect to the previous edition:
a. Title modified.
b. Justify the requirements for SPND characteristics in terms of influencing factors.
c. Align the terminology with the current state of the regulatory framework.

General Information

Status
Published
Publication Date
25-Apr-2021
Current Stage
PPUB - Publication issued
Completion Date
26-Apr-2021
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IEC 61468
Edition 2.0 2021-04
INTERNATIONAL
STANDARD
Nuclear power plants – Instrumentation systems important to safety –
In-core instrumentation: Characteristics and test methods of self-powered
neutron detectors
IEC 61468:2021-04(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 61468
Edition 2.0 2021-04
INTERNATIONAL
STANDARD
Nuclear power plants – Instrumentation systems important to safety –
In-core instrumentation: Characteristics and test methods of self-powered
neutron detectors
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.120.20 ISBN 978-2-8322-9706-3

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 61468:2021 © IEC 2021
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms and definitions ...................................................................................................... 8

4 Abbreviated terms ......................................................................................................... 12

5 Self-powered neutron detectors general advantages and disadvantages ....................... 13

6 Composition and construction ........................................................................................ 13

7 Application recommendations ........................................................................................ 16

7.1 General ................................................................................................................. 16

7.2 Fluence rate mapping – Core monitoring and surveillance .................................... 16

7.3 Power regulation – Feedback control .................................................................... 16

7.4 Core protection ..................................................................................................... 16

7.5 Reactor noise analysis .......................................................................................... 16

7.6 Classification ........................................................................................................ 16

8 Design recommendations .............................................................................................. 17

8.1 General ................................................................................................................. 17

8.2 Reproducibility of SPND characteristics ................................................................ 17

8.3 Background signal ................................................................................................ 17

8.4 Electrical interference noise .................................................................................. 17

8.5 Lifetime ................................................................................................................. 17

9 Test methods ................................................................................................................. 17

9.1 General ................................................................................................................. 17

9.2 Prototype testing ................................................................................................... 18

9.3 Production tests .................................................................................................... 18

10 Detector calibration ....................................................................................................... 18

10.1 Place of calibration ............................................................................................... 18

10.2 Absolute calibration .............................................................................................. 19

10.3 Comparison calibration ......................................................................................... 19

10.4 In-core calibration ................................................................................................. 19

10.5 Calibration procedure ............................................................................................ 19

10.6 Recommended calibration periods ........................................................................ 20

Annex A (informative) Self-powered detector principles and characteristics ........................ 21

A.1 SPND response mechanisms ................................................................................ 21

A.2 Beta decay (delayed response) ............................................................................. 21

A.3 Neutron capture (prompt response) ....................................................................... 21

A.4 Photoelectric effect (prompt response) .................................................................. 21

A.5 Compton effect (prompt response) ........................................................................ 21

A.6 Nature of SPND response ..................................................................................... 22

A.7 Thermal neutron interactions................................................................................. 22

A.8 Gamma interactions .............................................................................................. 22

A.9 Dynamic characteristics of SPND .......................................................................... 22

A.10 Detector burn-up life ............................................................................................. 23

A.11 Measurement errors .............................................................................................. 23

A.11.1 General ......................................................................................................... 23

A.11.2 Error for determination of SPND actual response ........................................... 23

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IEC 61468:2021 © IEC 2021 – 3 –

A.11.3 Error determined by gamma-component of SPND current .............................. 24

A.11.4 Error determined by leakage currents ............................................................ 24

A.11.5 Error determined by signal wire current ......................................................... 25

A.12 Self-powered detector operating characteristics .................................................... 25

A.12.1 General ......................................................................................................... 25

A.12.2 Vanadium emitter characteristics ................................................................... 26

A.12.3 Cobalt emitter characteristics ........................................................................ 26

A.12.4 Rhodium emitter characteristics ..................................................................... 26

A.12.5 Silver emitter characteristics .......................................................................... 27

A.12.6 Platinum emitter characteristics ..................................................................... 27

A.12.7 Hafnia emitter characteristics ........................................................................ 27

A.13 Self-powered detector assemblies......................................................................... 28

A.13.1 General ......................................................................................................... 28

A.13.2 Typical bottom-mounted rhodium self-powered detector assembly for

pressurized light water reactors ..................................................................... 28

A.13.3 Typical top-mounted rhodium self-powered detector assembly for

VVER–type light water reactors ..................................................................... 28

A.13.4 Typical top-mounted cobalt self-powered detector assembly for

pressurized light water reactors ..................................................................... 28

A.13.5 Typical heavy water reactor self-powered detector assembly ......................... 29

Bibliography .......................................................................................................................... 34

Figure 1 – Typical integral self-powered neutron detector ..................................................... 13

Figure 2 – Typical modular self-powered neutron detector .................................................... 13

Figure 3 – Typical background detector ................................................................................ 15

Figure 4 – Typical SPND with built-in background detector ................................................... 16

Figure A.1 – Simplified equivalent circuit of the SPND .......................................................... 25

Figure A.2 – Bottom-mounted rhodium self-powered detector assembly for pressurized

water reactors ....................................................................................................................... 29

Figure A.3 – Top-mounted rhodium self-powered detector assembly for VVER reactors

with four thermocouples ........................................................................................................ 30

Figure A.4 – Top-mounted rhodium self-powered detector assembly for VVER reactors

with level sensor ................................................................................................................... 31

Figure A.5 – Top-mounted cobalt self-powered detector assembly for pressurized

water .................................................................................................................................... 32

Figure A.6 – CANDU pressurized heavy water reactor self-powered detector assembly ........ 33

Table 1 – Characteristics of SPND emitters .......................................................................... 14

Table A.1 – Examples of specifications for typical SPNDs used in power reactors ................ 26

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– 4 – IEC 61468:2021 © IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
NUCLEAR POWER PLANTS – INSTRUMENTATION SYSTEMS IMPORTANT
TO SAFETY – IN-CORE INSTRUMENTATION: CHARACTERISTICS AND
TEST METHODS OF SELF-POWERED NEUTRON DETECTORS
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

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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.

International Standard IEC 61468 has been prepared by subcommittee 45A: Instrumentation,

control and electrical power systems of nuclear facilities, of IEC technical committee 45:

Nuclear instrumentation.

This second edition cancels and replaces the first edition, published in 2000, and its

Amendment 1, published in 2003. This edition constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous

edition:
a) Title modified.

b) Justify the requirements for SPND characteristics in terms of influencing factors.

c) Align the terminology with the current state of the regulatory framework.
---------------------- Page: 6 ----------------------
IEC 61468:2021 © IEC 2021 – 5 –
The text of this International Standard is based on the following documents:
FDIS Report on voting
45A/1381/FDIS 45A/1383/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.

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.
---------------------- Page: 7 ----------------------
– 6 – IEC 61468:2021 © IEC 2021
INTRODUCTION
a) Technical background, main issues and organisation of the Standard
This International Standard focuses on self-powered neutron detectors (SPNDs).

It is intended that this document be used by operators of NPPs (utilities), systems evaluators

and by licensors.

b) Situation of the current Standard in the structure of the IEC SC 45A standard series

IEC 61468 is a third level IEC/SC 45A document.
IEC 61468 is to be read in conjunction with IEC 61513 which establishes general

requirements for I&C systems and with IEC 60568 which establishes general requirements for

in-core instrumentation for neutron fluence rate (flux) measurements in power reactors.

For more details on the structure of the IEC SC 45A standard series, see item d) of this

introduction.
c) Recommendations and limitations regarding the application of the Standard

To ensure that the Standard will continue to be relevant in future years, the emphasis has

been placed on issues of principle, rather than specific technologies.

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.

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,

cybersecurity, 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 45A standard series, corresponds to the Technical

Reports which are not normative.
---------------------- Page: 8 ----------------------
IEC 61468:2021 © IEC 2021 – 7 –

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 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.
---------------------- Page: 9 ----------------------
– 8 – IEC 61468:2021 © IEC 2021
NUCLEAR POWER PLANTS – INSTRUMENTATION SYSTEMS IMPORTANT
TO SAFETY – IN-CORE INSTRUMENTATION: CHARACTERISTICS AND
TEST METHODS OF SELF-POWERED NEUTRON DETECTORS
1 Scope

This document applies to in-core neutron detectors, viz. self-powered neutron detectors

(SPNDs), which are intended for application in systems important for nuclear reactor safety:

protection, instrumentation and control. This document contains SPND characteristics and

test methods. In this document, the main sources of errors, and the possibilities for their

minimization are also considered.

Self-powered neutron detectors can be used for measurement of neutron fluence rate and

associated parameters in nuclear reactors. Most popular for the indicated applications are

detectors with rhodium emitters.

In this document dynamic characteristics, emitter burn-up, identity and other factors

influencing operational characteristics of detectors are considered.

Besides SPNDs with rhodium emitters, SPNDs with emitters from other materials and their

main characteristics are also considered in this document.
This document contains requirements, recommendations and instructions concerning

selection of SPND type and characteristics for various possible applications. This document

about SPNDs uses the basic requirements of IEC 61513 and IEC 60568 and complements

them with more specific provisions in compliance with IAEA Safety Guides.
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 60515:2007, Nuclear power plants – Instrumentation important to safety – Radiation

detectors – Characteristics and test methods

IEC 60568:2006, Nuclear power plants – Instrumentation important to safety – In-core

instrumentation for neutron fluence rate (flux) measurements in power reactors

IEC/IEEE 60780-323:2016, Nuclear facilities – Electrical equipment important to safety –

Qualification

IEC 61226, Nuclear power plants – Instrumentation, control and electrical power systems

important to safety – Categorisation of functions and classification of systems

IEC 61513, Nuclear power plants – Instrumentation and control important to safety – General

requirements for systems
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
---------------------- Page: 10 ----------------------
IEC 61468:2021 © IEC 2021 – 9 –

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
background-compensation

method employed for compensation of background contribution to the self-powered neutron

detector current

Note 1 to entry: This is usually accomplished by placing an "emitterless" background detector in the in-core

assembly, or by using detectors with an internal compensating lead wire.
Note 2 to entry: An equivalent term is “lead-compensation”.
3.2
beta decay

radioactive decay process in which mass number A remains unchanged, but the atomic

number Z changes

Note 1 to entry: Processes include electron emission (b– decay), electron capture, and positron emission (b+

decay).
3.3
burn-up

depletion or reduction of target atoms when exposed to a thermal neutron flux density over

time, due to conversion to other radioisotopes
3.4
burn-up life

time after which, at a given value of the neutron fluence rate of given energy distribution, the

amount of emitter sensitive material will decrease to such an extent that the characteristics of

the detector go beyond the tolerance established for their given application
3.5
Compton effect

effect which occurs when an incident high-energy photon is deflected from its original path by

an interaction with an electron

Note 1 to entry: The electron is ejected from its orbital position and the x-ray photon loses energy because of the

interaction but continues to travel through the material along an altered path. Energy and momentum are

conserved in this process. The energy shift depends on the angle of scattering and not on the nature of the

scattering medium. Since the scattered photon has less energy, it has a longer wavelength than the incident

photon.
Note 2 to entry: An equivalent term is “Compton scattering”.
[SOURCE: IEC 60050-395:2014, 395-02-07]
3.6
cross-section

measure of the probability of a nuclear reaction of a specific type, stated as the effective area

which targets particles present to incident particles for that process

Note 1 to entry: The standard unit for measuring a nuclear cross-section is the barn, which is equal to 10−28 m

or 10−24 cm .

Note 2 to entry: A microscopic cross-section can be measured for each process of nuclear reaction (capture,

fission, n-n′, n-2n, n-g,etc.).
---------------------- Page: 11 ----------------------
– 10 – IEC 61468:2021 © IEC 2021

Note 3 to entry: The macroscopic cross-section allows the calculation of the number of interactions for a given

nuclear reaction in a given material; this value is the produce between the corresponding cross-section and the

-1 -1
number of particles in volume of this material; it is expressed in m or cm .
[SOURCE: IEC 60050-395:2014, 395-01-23]
3.7
decay constant

number of disintegrations per unit time dN/dt for an atomic nucleus divided by the number of

nuclei N existing at the same time t
1 dN
λ = − ×
N dt
Note 1 to entry: The decay constant is expressed in reciprocal seconds (s ).

Note 2 to entry: The decay constant may be considered the total probability of radioactive decay (disintegration

and/or nuclear transition).
[SOURCE: IEC 60050-395:2014, 395-01-11]
...

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