IEC 60911:2025

IEC 60911 ®
Edition 2.0 2025-06
INTERNATIONAL
STANDARD
Nuclear power plants – Instrumentation systems – Measurements for monitoring
adequate cooling within the core of pressurized light water reactors

ICS 27.120.20  ISBN 978-2-8327-0397-7

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– 2 – IEC 60911:2025 © IEC 2025
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Abbreviated terms. 13
5 Operational conditions . 13
5.1 General . 13
5.2 Cooling state with steam generator . 14
5.2.1 General. 14
5.2.2 Coolant subcooled state . 14
5.2.3 Coolant saturated state . 14
5.2.4 Coolant superheated state . 15
5.3 Cooling state with RHRS . 15
5.3.1 General situation under RHRS operation . 15
5.3.2 Cold shutdown maintenance operations . 16
5.3.3 Cold shutdown refuelling operation . 16
5.3.4 PRHRS operation. 16
5.4 Cooling state with primary loop feed and bleed . 17
5.5 Cooling state with CIS . 17
6 Measurement methods . 17
6.1 General . 17
6.2 Water level measuring devices . 17
6.2.1 General. 17
6.2.2 RPV water level measuring devices . 17
6.2.3 RPV outlet pipe water level measuring devices . 19
6.2.4 Differential pressure measurement . 19
6.2.5 Ultrasonic liquid level monitoring . 19
6.2.6 Pressurizer level . 19
6.2.7 Reactor cavity level . 19
6.2.8 Containment flood-up level . 19
6.3 Temperature measuring devices. 19
6.3.1 General. 19
6.3.2 Core exit temperature . 20
6.3.3 RPV outlet and inlet pipe temperature . 20
6.3.4 RHRS temperature . 20
6.3.5 PRHRS temperature . 20
6.3.6 RPV and cavity temperature . 20
6.4 Flow measuring device . 20
6.4.1 RPV outlet pipe flow . 20
6.4.2 RHRS flow . 20
6.4.3 RSIS flow . 20
6.4.4 CIS flow . 21
6.5 Pressure measuring device . 21
7 Instrumentation requirements . 21
7.1 General requirements. 21

7.1.1 Overview . 21
7.1.2 Safety classification . 21
7.1.3 Accuracy and response time . 21
7.1.4 Reliability . 21
7.1.5 Single failure considerations . 21
7.2 Differential pressure measurement . 22
7.2.1 Differential pressure transmitters . 22
7.2.2 Reference columns . 22
7.2.3 Differential pressure tap locations . 22
7.2.4 Hydraulic instrument line installations . 23
7.2.5 Hydraulic instrument line temperature . 23
7.2.6 Type and quality of the fluid in the instrument lines . 24
7.3 Heated sensor measurement . 24
7.4 Ultrasonic liquid level measurement. 24
7.4.1 Application . 24
7.4.2 Accuracy and response time . 24
7.4.3 Installation considerations. 24
7.4.4 Special human machine considerations . 24
7.5 Temperature sensing devices . 24
7.6 Magnetic float sensing devices . 25
8 Operator displays . 25
9 Calibration . 26
10 In-service testing and maintenance . 26
11 Qualification . 26
12 Documentation . 26
Annex A (informative) Thermodynamic analysis of the reactor coolant system . 39
A.1 General . 39
A.2 Assessment of thermodynamic conditions . 39
A.2.1 General. 39
A.2.2 Momentum and mass behaviour . 39
A.2.3 Energy behaviour . 40
A.3 Display parameters . 40
A.4 Example of displays . 41
A.4.1 General. 41
A.4.2 Pressure-temperature deviation display . 41
A.4.3 Subcooling history display . 44
A.4.4 Temperature-pressure display . 44
Bibliography . 45

Figure 1 – Different void distributions with equivalent liquid levels . 11
Figure 2 – PWR configuration 1: Cooling state with steam generator . 27
Figure 3 – PWR configuration 2: Cooling state with RHRS . 28
Figure 4 – PWR configuration 3: Cooling state with PRHRS (primary side) . 29
Figure 5 – PWR configuration 4: Cooling state with PRHRS (secondary side) . 30
Figure 6 – PWR configuration 5: Cooling state with primary loop feed and bleed (RSIS) . 31
Figure 7 – PWR configuration 6: Cooling state with primary loop feed and bleed
(recirculated in containment) . 32

– 4 – IEC 60911:2025 © IEC 2025
Figure 8 – PWR configuration 7: Cooling state after core melting . 33
Figure 9 – Water level measurement by differential pressure method . 34
Figure 10 – Heated temperature sensor measurement. 35
Figure 11 – Magnetic float actuated reed switches for water level measurements . 36
Figure 12 – Flow measurement by differential pressure method . 37
Figure 13 – Thermal-hydraulic considerations affecting water level measurements . 38
Figure A.1 – Pressure-temperature deviation display . 42
Figure A.2 – Subcooling history display . 43
Figure A.3 – Temperature-pressure display . 43

Table A.1 – Thirteen different cases of changing pressure, temperature and subcooling . 41

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
NUCLEAR POWER PLANTS – INSTRUMENTATION SYSTEMS –
MEASUREMENTS FOR MONITORING ADEQUATE COOLING WITHIN
THE CORE OF PRESSURIZED LIGHT WATER REACTORS

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 in
...