A structural integrity oriented approach for defining the acceptance / rejection flaw size criteria in innovative plant components
S. Reale, University of Florence, Florence, Italy; E. Capurro, Ansaldo Nuclear Division, Genova, Italy
ABSTRACT
Codes represent an integrated system of rules and a homogeneous self-consistent body of experience. Experience is translated into rules or prescriptions having formats in accordance with the component specificity and the operating conditions (e.g. prescriptions for design, inspections, etc.). Therefore, in case of standard plants or well-characterised components, the fulfilment of a single requirement, prescribed by the Code, assure the efficiency of the single choices and the global fulfilment of the reliability requirements. In the case of an innovative plant, on one hand, there is a lack of a specific and consolidate experience, which might give a specific body of rules, on the other hand, it results a penalty to apply in an uncritical way, rules, which are related to components which partially perform the same function and which, however, are part of plants having different operating conditions. The flaw acceptance criteria, for defect detected by NDE, are usually expressed by dimensional limits with the final goal to accept flaws, the presence of which do not affect the plant safety and the required structural integrity. The safety level of the component may be considered an intrinsic requirement (i.e. the performance of the component) without any interdependence with more or less innovative quality of the plant or the component; as an intrinsic requirement it may be obtained by the analysis of the Codes relative to the considered component. Using a bottom (from prescriptions) to top (to the performance) approach, the A/R prescriptions of the code could be translated in indexes connected with the structural integrity, considering the interconnections with the design and material prescriptions. Those performance indexes are evaluated through the Reserve Factors which may be evaluated by means of Fracture mechanics (e.g. CEGB R6 / BSI PD6493). After the identification of the safety level to be enforced to the component, this level may be put as the component requirements of the innovative plant and according to this level, the requirements which the component must fulfil could be defined using a top (from the performance) to bottom (to prescriptions) approach. In this way a set of acceptance / rejection criteria, peculiar to the component will be obtained. These will consider the innovative quality of the component and reach in any case a well codified and accepted safety level equal to the safety level of analogous standard component. As example of this structural integrity oriented approach, the A/R flaw size criteria for the innovative light water nuclear reactor called ISIS (Inherently Safe Immersed System) developed by ANSALDO, are evaluated.
Publication Source: First International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components , 20 - 22 October 1998, Amsterdam, Netherlands. Held by the Joint Research Centre of the European Commission. Publisher:Woodhead Publishing Limited
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