IEC 61226-2020 pdf free download

IEC 61226-2020 pdf free download.Nuclear power plants – Instrumentation, control and electrical power systems important to safety – Categorization of functions and classification of systems.
5 Categorization scheme
5.1 General
Safety functions shall be assigned to different categories according to their safety significance. The category determines the class to be assigned to the l&C systems which perform these functions and the class to be assigned to the electrical power systems which support the functions (including those which support the operation of the l&C equipment). Then the class determines the design and quality requirements for these systems.
Subclause 5.2 provides the background to the categorization scheme.
Subclause 5.3 describes the three categories that are used to categorize functions, according to their safety significance. The categories are consistent with those defined in IAEA SSG-30. Subclause 5.4 presents the assignment criteria for each category.
Clause 6 provides guidance on the categorization process and also provides guidance on the mapping of functional categorization to system classes.
Clause 7 and Annex A address high-level technical requirements for each category (requirements applying to the functions) and each class (requirements applying to the systems that perform the functions). More detailed technical requirements are presented in dedicated IEC standards.
Annex B contains typical examples of the categorization of functions and classification of systems in NPPs. It is only informative since the mentioned functions and systems may depend on the reactor type.
5.2 Background
The principle of defence in depth is firmly established in the safety design basis of nuclear power plants. The fundamental idea that there should be several layers or lines of defence in the prevention of unsafe conditions, and that the prevention of unsafe conditions, before mitigation is needed, is always to be preferred. Because of the large number of functions that are required to operate a NPP and sustain its safety, a number that increases with the principle of defence in depth, it is important that the safety significance of each function is known, All the functions performed at the different levels of defence in depth are considered.
IAEA Safety Requirements SSR-211 establishes the idea of classification of NPP systems according to their safety significance, and gives examples of the classification of the major systems of several types of NPPs. All structures, systems and components, including software for instrumentation and control (l&C), that are items important to safety, shall be first identified and then classified on the basis of their function and significance with regard to safety. They shall be designed. constructed and maintained such that their quality and reliability is commensurate with this classification.
IAEA Safety Requirements SSR-211, section 5.34 requires that the method for classifying the safety significance of items shall be based primarily on deterministic methods complemented, where appropriate, by probabilistic methods with account taken of factors such as:
a) the safety function(s) to be performed by the item:
b) the consequences of failure to perform a safety function:
c) the frequency with which the item will be called upon to perform a safety function:
d) the time following a PIE at which, or the period for which, the item will be called upon to perform a safety function.
The IAEA Safety Guide SSG-30 gives guidance on the categorization of functions and classification of SSCs according to their safety significance.
This document extends the categorization strategy presented in IAEA Safety Guide SSG-30. and establishes the criteria and methods to be used to assign the functions of NPP to one of the three categories A, B and C, depending on their safety significance, or to consider them as Thon-categorized” (NC) if they do not have any safety role.
IAEA SSG 39 chapter 5 concerning classification of l&C systems and IAEA SSG 34 chapter 3 concerning classification of electrical power systems directly refer to SSR-211 and SSG-30 and are consistent with the principles of this document.
National application of the principles and criteria of this document may assign a different nomenclature to categories A, B and C.
5.3 Description of categories
5.3.1 General
The method for determining the safety significance of a function shall primarily be based on deterministic methods, complemented where appropriate by probabilistic methods and engineering judgment.IEC 61226 pdf download.