ISO 17838-1:2026

International
Standard
ISO 17838-1
First edition
Reactor technology — Design and
2026-09
performance evaluation of the
emergency core cooling system
strainer in pressurized water
reactor nuclear power plants —
Part 1:
General principles
Technologie des réacteurs - Conception et évaluation de la
performance de la filtration des systèmes de recirculation post-
accidentelle pour les réacteurs à eau pressurisée —
Partie 1: Principes généraux
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 5
5 Main tasks of ECCS strainer design and performance evaluation . 5
6 Process steps and logical relationships . 6
7 Process steps description and requirements . 9
7.1 Step 1 - Initial data and requirements identification .9
7.1.1 General .9
7.1.2 Specific considerations when setting the head loss limit .10
7.1.3 Specific requirements for upstream DST analysis .11
7.1.4 Specific requirements for DST walkdown .11
7.2 Step 2 - Strainer design and test preparation . 12
7.3 Step 3 - Strainer performance verification tests . 13
7.3.1 Basic requirement . 13
7.3.2 Head loss tests . 13
7.3.3 Bypass tests .14
7.4 Step 4 - Core integrity verification and downstream equipment qualification pertaining
to debris .14
7.4.1 In-vessel downstream effects compliance verification tests and analysis .14
7.4.2 Ex-vessel downstream equipment qualification pertaining to debris . 15
7.5 Step 5 - Comprehensive safety assessment . 15
8 Guideline for minimising DST .15
9 Guideline for determining margins .16
Bibliography . 17

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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The procedures used to develop this document and those intended for its further maintenance are described
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This document was prepared by Technical Committee ISO/TC 85, Nuclear energy, nuclear technologies, and
radiological protection, Subcommittee SC 6, Reactor technology.
A list of all parts in the ISO 17838 series can be found on the ISO website.
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.

iv
Introduction
The purpose of this document is to provide guidance for the design and performance evaluation of the
emergency core cooling system (ECCS) strainer(s) in pressurized water reactor (PWR) nuclear power
plants (NPPs). While there are many types of PWRs (including existing and under-construction units), this
document will focus on those that rely on ECCS pumps, which present enough similarities to allow a common
approach. For other technologies, this document may be used selectively, depending on the specific design
characteristics of their plants.
In the event of a relevant line break within the containment of a PWR, control or management of the accident
relies on cooling such as emergency core cooling system (ECCS), or containment cooling, via containment
heat removal system (CHRS), exemplified by the containment spray system (CSS), or reactor vessel cavity
cooling via cavity injection system (CIS), or core melt cooling via the core catcher or other features.
All the above-mentioned cooling functions need to be achieved by water supplies, pumps, and filtration
equipment to ensure the effectiveness of cooling water. The water can be obtained from locations such as
the containment sump at the lower part of the containment or the in-containment refuelling water storage
tank (IRWST). The functional requirements for the various components within these systems are derived
from their safety claims.
DBA, DBE and DEC are typically initiated by a relevant line break. Moreover, in the event of a line break
within the containment, the insulation and coatings surrounding the break can be damaged by blowdown
fluids, possibly resulting in the generation of debris. The debris, together with the blowdown fluids from the
pipe break and the water from the CSS, can flow and be directed towards the lower parts of the containment.
As this occurs, although some of the debris may be caught by barriers such as compartments, thresholds, and
steel gratings, other debris will continue to be transported by the water flow and will eventually accumulate
in the containment sump or in the IRWST or in other water storage facilities. This accumulation of debris
could render water unavailable to safety systems, thereby affecting their normal operation.
To ensure conformity with the operational requirements of the safety systems for water effectiveness, a
set of strainers can be installed at the upstream inlet of the water supply pipe to filter out debris such as
fibre and particles. In PWR NPPs, those strainers are commonly referred to as the Emergency Core Cooling
System (ECCS) strainer. Depending on specific safety case requirements, the strainer may also be used for
design extended conditions (DEC), where specific safety features are considered.
Accidents can disrupt the normal operation of various equipment, posing significant risks to plant safety.
For example, as mentioned above, the accumulation of debris on the ECCS strainer following a design basis
accident (DBA), a design basis accident (DBE) or a design basis accident (DEC) could result in the strainer
being blocked. Such accumulation can increase the head loss across the strainer, which in turn reduces
the available net positive suction head (NPSHa) to the safety systems pumps. If the NPSHa falls below the
required net positive suction head (NPSHr) of the pumps, cavitation can occur, resulting in a reduction
in pump flow rate and potentially rendering the safety functions of the system ineffective. Furthermore,
the ECCS strainer can allow the passage of fine fibre and particles, which can clog and erode downstream
equipment such as heat exchangers, valves, CSS spray nozzle(s) and orifices, and can also increase the head
loss across the reactor core, impairing the core heat transfer efficiency and potentially damaging the fuel
cladding. Additionally, fibre, particles and chemical debris can adhere to the fuel rod cladding, further
degrading heat removal conditions.

v
International Standard ISO 17838-1:2026(en)
Reactor technology — Design and performance evaluation of
the emergency core cooling system strainer in pressurized
water reactor nuclear power plants —
Part 1:
General principles
1 Scope
This document specifies the requirements for the design and performance evaluation of the ECCS strainer,
since the performance of the strainer is important for the maintenance of nuclear safety during DBA, DBE
and DEC.
This document is applicable to the design and performance evaluation of the ECCS strainer in PWR NPPs,
particularly those with primary design philosophies that rely on ECCS pumps. Its relevance for some PWR
designs could nevertheless need some adaptations. Moreover, although tailored to PWR NPPs, it can also
serve as a valuable reference for other reactor types, providing insight into the design and effectiveness of
filtration systems in various nuclear power applications.
Additional information and insights on national practice can be found in References [1][2][3][4].
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitute
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.
ISO 12749-5, Nuclear energy, nuclear technologies, and radiological protection — Vocabulary — Part 5: Nuclear
reactors
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 12749-5 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
bypass
condition whereby debris carried by the flow to the strainer can percolate through and pass downstream
Note 1 to entry: This term is only relevant for designs that include such a system.

3.2
cavity injection system
CIS
system operating after a severe accident (SA) with core melt, designed to remove the heat of the molten core
debris by cooling the outer wall of the reactor vessel, to maintain the integrity of the reactor vessel, and to
realize the retention of molten core debris inside the reactor vessel
Note 1 to entry: This term is only relevant for designs that include such a system.
3.3
chemical effects
phenomena in which debris chemically reacts with the ECCS fluid under the post-accident operating
temperature and radiation environment, resulting in a head loss on the ECCS strainer and on the fuel
assembly
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: Chemical reactions occur not only on the strainer, but also in the liquid and vapour flows along the
containment structures, as well as in the sedimentation tanks, where the temperature may differ significantly from
the temperature indicated in either direction, but for the activity of the strainer, the operating temperature of the
IRWST (3.13) (or other water storage facilities, e.g. containment sump) is one of the main influences.
3.4
containment heat removal system
CHRS
system for reducing containment pressure and temperature following an accident and maintaining them at
an acceptably low level
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: The system may include containment spray system (CSS) (3.5).
3.5
containment spray system
CSS
system that provides a spray of cold or subcooled water into the upper containment volume to limit the
pressure and temperature of the containment atmosphere after an accident, thereby reducing the possibility
of release of airborne radioactivity to the environment
Note 1 to entry: This term is only relevant for designs that include such a system.
3.6
debris
degraded substances generated directly or indirectly as a result of an accident
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: The debris can occur through
— destruction caused by a break in the coolant, feedwater or steam pipes, leading to disintegration of insulation,
coatings, cable tray wrappings, concrete particles and sealing materials, as well as other latent materials present
in the containment, which may be transported by the water flow to the ECCS strainer (3.11) under the accident
conditions,
— post-accident chemical reactions between the coolant solution and containment materials, e.g. calcium (Ca), silicon
(Si), and
— post-accident radiation which may cause degradation of the above materials.
Note 3 to entry: It is important to consider both direct (from pipe breaks) and indirect (from radiation and chemical
reactions) source term of debris for any given accident scenario.

3.7
head loss test
test designed to measure the head loss caused by the build-up of debris (3.6) transported by the water flow
on the ECCS strainer (3.11) and by potential chemical effects
Note 1 to entry: This term is only relevant for designs that include such a system.
3.8
debris source term
DST
quantity and characteristics of debris that are expected to be generated and possibly transported by water
flow during a postulated accident scenario
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: This notion is used to analyse the debris transport to the containment sump or IRWST (3.13) or other
water storage facilities, the subsequent debris accumulation, and resulting head loss across the strainer, which affects
the overall effectiveness of the ECCS.
3.9
walkdown
activities performed to check in-situ the real potential DST (3.8) and flow paths within the containment
against their design documentation
Note 1 to entry: Walkdown related activities include the development of a procedure, the performance of on-site
inspections and the examination of flow paths leading to the containment sump or IRWST (3.13) or other water storage
facilities.
Note 2 to entry: This term is only relevant for designs that include such a system.
3.10
emergency core cooling system
ECCS
safety system having as central function to ensure the automatic initiation of a number of actions ensuring
adequate core cooling and primary coolant inventory losses compensation, following an accident
Note 1 to entry: This term is only relevant for designs that include such a system.
[SOURCE: ISO 12749-5:2026, 3.10.7.2, modified — Notes to entry have been deleted.]
3.11
emergency core cooling system (ECCS) strainer
filtration equipment of the ECCS (3.10) and other safety systems used to filter water from the containment
sump or IRWST (3.13) or other water storage facilities
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: The strainer surface need not necessarily be meshed.
Note 3 to entry: The word "strainer," as used throughout this document, may be considered as singular, i.e. strainer or
collective, i.e. strainers, as content dictates.
3.12
ex-vessel downstream effects
phenomenon linked to bypass debris leading to potential blockage, abrasion or both in downstream
equipment outside the reactor pressurized vessel such as pumps, heat exchangers, valves, orifices, CSS spray
nozzle(s) and pipes due to debris (3.6) passing through the ECCS strainer (3.11)
Note 1 to entry: This term is only relevant for designs that include such a system.

3.13
in-containment refuelling water storage tank
IRWST
tank or pool is installed inside the containment, and is designed to provide sufficient water for ECCS (3.10),
CSS (3.5) etc., following a relevant line break (3.18) accident, and for CIS (3.2) following a DBA, DBE or DEC
Note 1 to entry: This term is only relevant for designs that include such a system.
3.14
in-vessel downstream effect
phenomenon linked to bypass debris having an impact on heat transfer of the fuel assemblies to the coolant
inside the reactor pressurized vessel
Note 1 to entry: This term is only relevant for designs that include such a system.
3.15
in-vessel downstream effects compliance verification
test designed to measure the coolant flow head loss throughout the reactor core and to detect the deposition
of debris (3.6) in narrow gaps or adhesion to fuel rod surfaces in fuel assemblies
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: Such deposits of debris may lead to degradation of the heat transfer in the fuel assemblies.
3.16
net positive suction head
NPSH
for a pump, difference between the pressure at the pump inlet and the vapour pressure of the fluid
Note 1 to entry: This term is only relevant for designs that include such a system.
3.17
reference plant
existing power plant whose main characteristics are referenced in the design of new plants
Note 1 to entry: This term is only relevant for designs that include such a system.
3.18
relevant line break
sudden failure of the pressure boundary in an energised line system, manifested as a circumferential
fracture or an abrupt, unconfined axial crack
Note 1 to entry: This term is only relevant for designs that include such a system.
3.19
safety case
collection of arguments and evidence in support of the safety of a facility or activity
Note 1 to entry: This term is only relevant for designs that include such a system.
Note 2 to entry: This normally includes the findings of a safety assessment and a statement of confidence in these
findings.
Note 3 to entry: For a disposal facility, the safety case may relate to a given stage of development. In such cases, the
safety case should acknowledge the existence of any unresolved issues and should provide guidance for work to
resolve these issues in future development stages.
[SOURCE: IAEA Nuclear Safety and Security Glossary, 2022 (Interim)]

3.20
upstream DST analysis
process of evaluating, by analysis and calculation, the types, characteristics, mass/volume, and size
distribution of debris (3.6) that could result from relevant line breaks (3.18) within the containment
Note 1 to entry: This term is only relevant for designs that include such a system.
3.21
zone of influence
ZOI
zone affected by a relevant line break (3.18) where the fluid ejected from the break has sufficient energy to
cause the generation of debris (3.6) from insulation, coatings, and other materials within it
Note 1 to entry: The method for determining the shape or dimension of the ZOI is not in the scope of this document and
should be described separately.
Note 2 to entry: This term is only relevant for designs that include such a system.
4 Symbols
For the purposes of this document, the following symbols shall apply.
Symbol Description
CHRS containment heat removal system
CIS cavity injection system
CSS containment spray system
DBA design basis accident
DBE design basis event
DEC design extended conditions
DST debris source term
ECCS emergency core cooling system
I&C instrumentation and control
IRWST in-containment refuelling water storage tank
NPP nuclear power plant
NPSH net positive suction head
NPSHa available net positive suction head
NPSHr required net positive suction head
PWR pressurized water reactor
SA severe accidents
ZOI zone of influence
5 Main tasks of ECCS strainer design and performance evaluation
ECCS strainer design and performance evaluation should take into account the following:
— Systems safety assessment, which forms the basis for addressing the ECCS strainer issues. The
requirements for the design and performance evaluation of the ECCS strainer are derived from the plant
global safety assessment. However, in order to focus on the ECCS issue, this document does not include
the requirements on the safety assessment.
— Assessment of upstream DST, including quantification of debris generation and transport analysis, as
well as debris characteristics.
— Performing walkdown, as a complementary activity to the upstream DST analysis for accuracy and
reliability, including at least verification of DST, flow paths and water retention.

— Identification of safety requirements related to the performance of the strainer and associated
downstream safety equipment such as pumps, heat exchangers, valves, orifices, CSS spray nozzle(s) and
pipes which are critical to the safety case.
— Sizing of the ECCS strainer equipment, including determining the screen type, screen area, mesh size,
layout and 3D structure of the strainer.
— Estimation of the bounding case(s) to be tested in subsequent steps, including at least the determination
of test objectives, requirements, parameters, items, inputs, and acceptance criteria.
— Performing head loss tests, designed to measure the head loss resulting from debris and chemical effects
built up on the ECCS strainer.
— Performing bypass tests, in which either the mass of debris retained by the strainer is measured, which
allows deriving the quantity of debris passing through the strainer, or the mass of debris passing through
the strainer is measured directly.
— Performing in-vessel downstream effects compliance verifications, targeted to measure or estimate the
head loss of the coolant flow in the core or the heat removal behaviour due to fine debris not filtered by
the ECCS strainer, providing a basis for analytical comparison with the heat removal requirements of the
safety case (the method used for this verification is outside of the scope of this document).
— Performing ex-vessel downstream equipment qualification (possibly reusing existing test results if
they can be shown to be applicable) and if needed analysis designed to verify the effective operation of
downstream equipment, including pumps, heat exchangers, valves, orifices, CSS spray nozzle(s), pipes,
etc. Equipment qualification to debris for pumps and valves should be performed as appropriate (if
required, the qualification of pumps and valves is outside of the scope of this document).
6 Process steps and logical relationships
The design and performance evaluation of the ECCS strainer should follow the principles of these
recommended five steps. Their description as specified in Clauses 6 and 7 should be used as a basis for their
application (for details, see Figure 1).
NOTE 1 The numbers listed in this clause all refer to their corresponding numbers in Figure 1.
NOTE 2 The dotted line is the dividing line between two adjacent steps.
NOTE 3 The dashed line around the comprehensive safety performance assessment means it is outside the scope of
this document, step 5 exists here only to better understand the integrity of the workflow.
NOTE 4 The double dotted line around the words of walkdown indicates that the upstream DST analysis is feasible
and the DST results are acceptable even if the on-site walkdown could not be carried out during the construction
phase of NPP.
Figure 1 — Process steps and logical relationships
a) Step 1, Initial data and requirements identification.

Step 1 provides guidelines to identify the relevant requirements and input data to perform the design
process of the strainer. In this step, the input data are derived from the basic technical requirements, which
should take into account at least the following:
— Requirements for the quantity and characteristics of the debris reaching the strainer, which compose
the upstream DST (Item 1). Also, if necessary and feasible (if the plant to be evaluated has already been
constructed), an on-site walkdown (Item 2) can be performed to confirm the actual status of the initial
DST conditions, the flow path and water retention, and to check the quantity and characteristics of
potential debris present within the containment.
— Flow rate of safety systems (ECCS, etc.) pumps, head loss limit of strainer and particles size limit (if
applicable) (Item a, b and c) derive from systems functional requirements and safety claims (availability,
duration, etc.).
— Fibre quantity limit and other applicability conditions associated to the literature or previous study used
for fuel assemblies (Item d) derive from core safety requirements (if applicable).
— Strainer dimensions, fixation methods, and transfer matrix derive from plant layout characteristics
(such as constraints of space).
— Test bench specification, which specify type of testing and characteristics (size, instrumentation, etc.).
— List of parameters considered, such as temperature, pH, concentration, influence of chemicals, etc.
b) Step 2, Strainer design and test preparation.
Step 2 provides the basic requirements for ECCS strainer design and test preparation. The work in step 2
should be based on the input either data or requirements, or both from step 1.
When designing the ECCS strainer (Item 3), the following should be done as a minimum:
— Determine the screen type of the strainer - grid, perforated plate, 3D mesh and slotted plate are generally
accepted.
— Determine t
...