IEC 61468:2021

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
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.

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and withdrawn publications.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.
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

– 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

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

– 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
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.
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.

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.
– 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.

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.
– 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.

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
σ
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.).

– 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
λ
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
-1
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]
3.8
fluence rate
φ
quotient of dΦ by dt where dΦ is the increment of particle fluence in the time interval dt:
dΦ
φ =
dt
[SOURCE: IEC 60050-881:1983, 881-04-19]
3.9
in-core neutron detector
detector, fixed or movable, designed for the measurement of neutron fluence rate at a defined
region of a reactor core
3.10
integral self-powered neutron detector
self-powered neutron detector in which the lead cable section is an extension of the detector
section, i.e. the emitter is directly attached to the core/signal wire; both sections share
common insulation, and the collector of the detector section is also the outer sheath of the
lead cable section (see Figure 1)
Note 1 to entry: An equivalent term is "cable-type self-powered neutron detector".
3.11
modular self-powered neutron detector
self-powered neutron detector made by mechanically joining, welding or brazing a detector
(emitter, insulator, collector) to a lead cable (core/signal wire, insulator, outer sheath)
(see Figure 2)
Note 1 to entry: An equivalent term is "prefabricated self-powered neutron detector".
3.12
isotope
variants of a chemical element that differ by atomic mass, having the same number of protons
and differing in the number of neutrons in the nucleus

EXAMPLE 13C refers to a carbon atom that has an atomic mass of 13.
Note 1 to entry: Radionuclides or nuclides with a non-natural isotopic ratio are shown in the structural
representation with the nuclide number displayed. Natural abundance isotopes are represented by an elemental
symbol without a nuclide number.
[SOURCE: ISO 11238: 2018, 3.37]
3.13
photoelectric effect
complete absorption of a photon by an atom with the emission of an orbital electron
[SOURCE: IEC 60050-395:2014, 395-02-08]
3.14
prompt response
signal generation from a self-powered neutron detector based on the (n, γ, e) reaction
3.15
radioactive half-life
time required for the activity of a radioisotope to decrease to half of its initial value
Note 1 to entry: The radioactive half-life is related to the decay constant λ by the expression: T½= ln2/λ ≈ 0,693/λ.
This quantity is expressed in seconds (s).
[SOURCE: IEC 60050-395:2014, 395-01-12]
3.16
radioisotope
isotope of an element with the property of spontaneously emitting particles or gamma
radiation or of emitting X-radiation
[SOURCE: ISO 5576:1997, 2.104]
3.17
self-powered neutron detector
neutron-sensitive radiation detector that requires no external power supply, consists of three
basic elements: an emitter that interacts with neutrons to emit electrons; a collector that
collects these electrons and an insulator that isolates the emitter from the collector and
converts the neutron fluence rate into electrical signal
Note 1 to entry: See Figure 1 and Figure 2.
3.18
self-shielding
self-absorption which occurs in the emitter: as emitter diameter increases, the escape
probability of an electron born in the interior of the emitter decreases, and current-producing
efficiency drops
3.19
in-core detector assembly
mechanical arrangement for positioning different detectors inside the core of a nuclear
reactor. In-core detector assembly may contain both single-type detectors as well as
detectors for various purposes and designs, for example, SPND and thermoelectric converters
3.20
sensitivity
characteristic measure of the signal of a detector to radiation. If in a given range of radiation
quantity, the response of the detector depends linearly on the applied radiation, then in this
range the sensitivity is given by the ratio:

– 12 – IEC 61468:2021 © IEC 2021
∆I
S =
∆ϕ
where
∆I is the variation of the output signal (detector response);
∆ϕ is the variation of the quantity of applied radiation (neutron fluence rate or gamma dose
rate)
3.21
sensitivity to conditional neutron fluence rate
sensitivity to the neutron fluence rate with the Maxwell energy density distribution of neutrons
having the most probable velocity equal to 2 200 m/s corresponding to the energy of
0,0253 eV
Note 1 to entry: It is convenient to use the Westkott formalism to determine the sensitivity of the detector to the
neutron flux density with a soft spectrum other than the one indicated above.
3.22
thermal neutron
neutron with a kinetic energy close to that of neutrons in thermal equilibrium with the atomic
nuclei of the surrounding medium
Note 1 to entry: In the case of a maxwellian nuclei distribution, the most probable energy is equal to 0,025 eV at
293 K, which corresponds to the neutron velocity equal to 2 200 m/s.
Note 2 to entry: In practice the upper limit is about 1 eV.
[SOURCE: IEC 60050-395:2014, 395-02-17]
3.23
useful life
operational life, under irradiation and environmental conditions restricted within specified
limits, after which the detector characteristics exceed the specified tolerances
Note 1 to entry: The limitation of the useful life is associated, as a rule, with burn-up process and the capabilities
of the measuring equipment.
Note 2 to entry: Useful life can be expressed in incident particle flux, time, etc.
4 Abbreviated terms
BWR  boiling water reactor
CANDU Canada, Deuterium, Uranium (Canadian reactor design featuring natural uranium
fuel and heavy water moderator and coolant)
HWR heavy water reactor (heavy water (D O) cooled and moderated reactor)
ICDA in-core detector assembly
LWR  light water reactor (light water cooled and moderated reactor. Commercial types
include the pressurized water reactor (PWR) and the boiling water reactor (BWR))
PWR  pressurized water reactor
RBMK graphite moderated light water-cooled reactor
____________
See Westcott C.H., Walker W.H., Alexander I.K., "Cross-Sections and Cadmium Rations for the Neutron
Spectra of Thermal Reactors", Proceedings of the Second United Nations International Conference of the
Peaceful Uses of Atomic Energy, Geneva, 1958, V.16, United Nations, 1959.

SPND self-powered neutron detector
VVER abbreviation from the Russian name "water-water power reactor" – PWR reactor
type
5 Self-powered neutron detectors general advantages and disadvantages
In self-powered neutron detectors (SPNDs), the interactions of neutrons and atomic nuclei are
used to produce a current which is proportional to the neutron fluence rate. The principles and
characteristics of SPND design and operation are given in Annex A.
When compared to other types of detectors, they have the following advantages:
– no need of power supply;
– simple and robust construction;
– relatively small mechanical "size" well-suited for in-core installation;
– good stability under temperature, pressure, radiation and other conditions.
The main disadvantages of SPNDs are the low signal level and the inertia of the neutron
component of the signal.
6 Composition and construction
A typical SPND consists of an emitter, made of a material with a large neutron interaction
cross-section, its surrounding insulator, collector and lead cable.
SPNDs are divided into two types according to their design and manufacturing technology:
– integral (or cable-type) SPND (see Figure 1), in which the signal wire of the lead cable
mates directly to the emitter; isolation around the emitter is identical to the isolation of the
signal line, and the collector is an extension of the signal line sheath;
– modular (or prefabricated) SPND (see Figure 2), which is made from separate detector
and lead cable sections.
Figure 1 – Typical integral self-powered neutron detector

Figure 2 – Typical modular self-powered neutron detector

– 14 – IEC 61468:2021 © IEC 2021
SPNDs are available in a variety of designs with sensitive lengths ranging from a few
centimetres to full core height. The design of an SPND shall incorporate proper selection of
emitter type and thickness, as well as sheath and insulation material types and dimensions, to
optimize the mechanical design for a specific task.
SPND emitters shall be fabricated with materials resistant to operating conditions of
detectors, with cross-sections of interaction with neutrons suitable for measured neutron
energy range.
For power reactor applications, typical emitter materials used in SPNDs include vanadium,
cobalt, rhodium, silver, platinum and hafnia. These materials should be used because they
possess relatively high melting temperatures, relatively high cross-sections to thermal
neutrons and are compatible with the SPND manufacturing process. Other emitters such as
cadmium, gadolinium and erbium may be used in SPNDs for low temperature experimental
reactors, but are not practical for power reactor applications.
Table 1 gives an overview of some of the important characteristics of SPND emitters used in
power reactor applications.
Table 1 – Characteristics of SPND emitters
Emitter Stable Composition Cross- Resulting Half-life Applications
material isotope % section σ isotope
50 51
Vanadium V 0,24 100 V Stable HWR fluence rate mapping
23 23
51 52
LWR fluence rate mapping
V 99,76 4,9 V 3,76 min
23 23
59 60
Cobalt Co 100 37 Co 5,27 years LWR fluence rate mapping
27 27
LWR control
LWR local core protection
103 104
Rhodium Rh 100 11 (8 %) Rh 4,4 min LWR fluence rate mapping
45 45
135 (92 %) Rh 42 s
107 108
Silver Ag 51,82 35 Ag 2,42 min Upgraded RBMK fluence rate
47 47
mapping
109 110
Ag 48,18 93 Ag 24,4 s
47 47
174 175
Hafnia Hf 0,18 390 Hf 70 d LWR control
72 72
176 177m
Hf 5,20 15 Hf 51,4 min HWR control
72 72
177 178m
Hf 18,50 380 Hf 31 years
72 72
178 179m
Hf 27,14 75 Hf 25,1 d
72 72
179 180m
Hf 13,75 65 Hf 5,5 h
72 72
180 181m
Hf 35,23 14 Hf 42,4 d
72 72
192 193m
Platinum Pt 0,78 14 Pt 4,3 d RBMK fluence rate mapping
78 78
194 195m
Pt 32,90 2 Pt 4,1 d RBMK local control
78 78
195 196
RBMK local protection
Pt 33,80 24 Pt Stable
78 78
196 197m
Pt 25,30 1 Pt 1,3 h
78 78
198 199
Pt 7,22 4 Pt 30,8 min
78 78
The material of the collector and lead cable sheath shall be resistant to radiation and
corrosion effects, the latter is especially important for detectors placed directly in the primary
coolant. To eliminate the effects of corrosion and extend the lifetime, SPNDs may be installed
in assemblies in which the SPND elements are not in contact with the primary coolant.

The collector may be fabricated from nickel-based alloys, characterized by excellent corrosion
resistance. When choosing the material of the collector (as well as insulator), it shall be borne
in mind that the presence of even a small amount of impurities with a large cross-section of
interaction with neutrons can contribute to the detector reading.
The insulation should have an extremely low probability of interaction with neutrons or gamma
rays. Under operating conditions, the insulation resistance of the detector should be
substantially greater than the resistance of the signal line conductor and the input resistance
of the measuring device in order to avoid leakage current through the insulation. Ceramic
oxides are the preferred materials because they have a high resistivity and can withstand the
hostile environment inside a nuclear reactor.
Typically, three materials are used for the insulation in SPNDs employed in power reactor
O , MgO and SiO . Al O has been chosen for most applications as
applications, namely, Al
2 3 2 2 3
it is readily available and, in powder form, is less sensitive than MgO to the effects of
humidity. However, the currents of the signal line using Al O insulation may be larger than
2 3
the currents of the signal line with MgO insulation due to the formation of the beta-active
isotope Al . MgO is hygroscopic and is therefore sensitive to cable swelling if moisture
penetrates the cable sheath. SiO may be used in some applications because its low density
enhances emitter to collector electron transmission and thus maximizes sensitivity
characteristics. However, SiO has lower insulation resistance at high temperature than MgO
or Al O .
2 3
The SPND connection cable should be a single-wire or two-wire cable with mineral insulation
and a metal outer sheath. In order to minimise the contribution to the SPND signal from the
interaction of neutrons and gamma radiation with the signal cable, appropriate materials
should be selected in the cable design and construction, taking into account the relative
geometric dimensions of the wires and cable sheath.
For SPNDs using a single core cable, the contribution to the SPND signal from the interaction
of the signal cable with neutrons and gamma radiation may be minimised by using a
background detector to provide a compensation signal. The background detector should be
constructed without an emitter and using the same cable (see Figure 3).

Figure 3 – Typical background detector
For SPNDs using a two-wire cable, one wire should be connected to the emitter of the SPND
with the second wire being used as a background detector. In this case, the identity of the
material and the geometry of the wires plays an important role. Special attention should be
given to the selection of the materials and the geometry of the wires to minimise effects on
the SPND signal. In order to compensate for possible differences in the irradiation conditions
of the two wires, they should be arranged in a twisted pair arrangement (see Figure 4).

– 16 – IEC 61468:2021 © IEC 2021

Figure 4 – Typical SPND with built-in background detector
7 Application recommendations
7.1 General
For each specific task, an SPND shall be selected with optimal characteristics for the solution
of this problem. At the same time, for a solution of practically all problems, a small burn-up
rate of the emitter is essential. High burn-up may be acceptable if a calibration system is
available or burn-up is accounted for by the calculation method.
7.2 Fluence rate mapping – Core monitoring and surveillance
SPNDs may be used for fluence rate mapping and core monitoring and surveillance with or
without an associated calibration system.
7.3 Power regulation – Feedback control
SPNDs may be used for feedback control purposes, however, acceptability of a signal delay
will depend on the following limitations:
– axial power shape control may tolerate moderate delay;
– integral power control shall not permit signal delay.
7.4 Core protection
The characteristics required for SPNDs that are used for local core protection purposes sh
...