Instrumentation, control and electrical power systems important to safety — Categorization of functions and classification of systems
IEC 61226-2020 pdf download.Nuclear power plants — Instrumentation, control and electrical power systems important to safety — Categorization of functions and classification of systems
Centrales nucléaires de puissance — Systèmes dinstrumentation, de contrôlecommande et d’alimentation electrique importants pour Ia sUreté — Catégorisation des fonctions et classement des systemes.
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-2/1 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-2/1, 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 “non-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.
When assessing the safety significance of a function, the following factors should be considered:
• the role of the function during normal operation (including start-up, refuelling, etc) in preventing the occurrence of PIEs;
• the role of the function in mitigating the consequences of AOO, DBA or DEC;
• the time following a PIE at which, or the period for which it is required to perform the function;
• the consequences in the event of spurious actuation in the different plant states (operational states and accident conditions);
• the role of the function following PIEs such as natural events (e.g. seismic disturbance, flood, extreme wind, lightning), human induced external events (e.g. chemical releases from other plants or industries) and internal hazards (e.g. fire, internal flood, missiles);
• the maintenance, repair and testing strategy.
Therefore, an initial safety analysis of the specific NPP design shall be completed prior to the categorization of the functions. On the basis of this safety analysis, the identification of the functions to be categorized should be performed with a sufficiently detailed granularity, in such a way that one function can be allocated to only one l&C system.
To graduate the consequences of failure to perform the function, SSG-30 defines 3 levels of severity for NPP5 keeping In mind that functions implemented to control accidents aim at making the consequences of these accidents acceptable and also as low as practicable. The assessment of the consequences is made under the assumption that the function does not respond when challenged, and without crediting any function that compensates for the failure of the function
• The severity should be considered ‘high’ if failure of the function could, at worst lead to a release of radioactive material that exceeds the limits accepted by the regulatory body for design basis accidents; or cause the values of key physical parameters (e.g pressure, temperature, etc) to exceed acceptance criteria for design basis accidents.
• The severity should be considered ‘medium’ If failure of the function could, at worst lead to a release of radioactive material that exceeds limits established for anticipated operational occurrences; or cause the values of key physical parameters to exceed the design limits for anticipated operational occurrences.
• The severity should be considered ‘low’ if failure of the function could, at worst lead to doses to workers above authorized limits.
In terms of alignment with IAEA SSG-30 categorization nomenclature, the requirements provided in this document cover the allocation of categories A, B and C to functions that can be implemented by l&C systems andlor supported by electrical power systems. These are equivalent to categories 1, 2 and 3, respectively, from IAEA SSG-30.IEC 61226-2020 pdf download.