Foreword

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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.

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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.

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

3 Terms and definitions

4 Symbols

5 Main tasks of ECCS strainer design and performance evaluation

6 Process steps and logical relationships

7 Process steps description and requirements

7.1 Step 1 - Initial data and requirements identification

7.2 Step 2 - Strainer design and test preparation

7.3 Step 3 - Strainer performance verification tests

7.4 Step 4 - Core integrity verification and downstream equipment qualification pertaining to debris

7.5 Step 5 - Comprehensive safety assessment

8 Guideline for minimising DST

9 Guideline for determining margins

Bibliography