·Table of Contents
Qualification of Manufacturing NDT Inspection Techniques for Nuclear Power PlantsJ.M. TCHILIAN (FRAMATOME). M. DUFF (AEA TECHNOLOGY). P. LEMAITRE (JRC PETTEN), B.W.O. SHEPHERD (MITSUI BABCOCK). J. WESSELS (SIEMENS KWU).
The original objectives of this study, launched out by the European Commission DG XI, were to establish a precise snapshot of the different European practices, and :
2.1 ROLE OF MANUFACTURING INSPECTION
2.1.1 French approach
The manufacturing conditions shall guarantee an appropriate quality level of components from safety point of view. Precautions are taken in order to limit, as far as possible, the risk of defect creation.
The manufacturing inspection techniques (NDE), implemented in accordance with the RCC-M Code rules, are targeted to assess the required quality level and to detect any deviation in manufacturing process. They are not targeted for defect through wall depth sizing in comparison with critical defect dimension. Detection and characterization capabilities are requested rather than depth sizing capability.
Inspection and fabrication are to be considered as a consistent whole.
2.1.2 German approach
In Germany the significance of manufacturing inspections of nuclear power plant components is strongly linked to the so-called "Basis Safety Concept" which has been under development since about 1972 and which is now well established. The quality of manufacturing and design alone suffices to given assurance against catastrophic failures.
The ideas of the Basis Safety Concept have been transferred in the KTA safety rules.
Subject of the manufacturing inspection techniques according to the KTA safety rules, is to assess the required quality level and to detect deviation in the process of manufacture. They are not targeted for defect sizing in comparison with critical defect dimensions. Detection and characterization capability is requested rather than sizing capability.
2.1.3 UK approach
The inspections carried out during plant fabrication have the following objectives:
To provide assurance that no defects are presents which would indicate deviations from the acceptable range of quality of materials and processes used in manufacture.
To ensure that no defects are present which would impede subsequent inspection, or which have been calculated to be of safety concern.
While the manufacturing inspections in UK therefore underwrite manufacturing quality, there is a particular emphasis on demonstrating the absence of defects calculated to be of structural concern. In UK, both requirements involve measurement of defect size.
2.2 CODE PRESCRIPTIONS
2.2.1 RCC-M prescriptions
The RCC-M code defines the so-called "Reference Practices". The "Reference Practices", consistent with the components to be examined and the associated acceptance criteria, are designed to achieve the required quality level.
The RCC-M allows that "Alternative Practices" may be proposed to the contractor. A prior approval is requested, file submitted shall comprise the test results, which demonstrate that the "Alternative Practice" is equivalent to the "Reference Practice.
For UT examination of Steam Generator tubes a qualification file shall be supplied. The establishment of a defect catalogue giving assurance that any internal defect with a depth above 10% of the thickness is detected determines the acceptance level.
In order to assure and to verify the effectiveness of "reference practices" not subjected to formal qualification,.the RCC-M code defines for each NDE, its extend, the areas subjected to examination, the appropriate surface preparation and the associated acceptance criteria.
The degree of freedom left by the RCC-M is very low. For each technique a great number of influential parameters are fixed. The code prescriptions and associated acceptance criteria are mandatory and guarantee the required quality level.
Positive feedback experience on the combination of materials quality and manufacturing inspections capability, is the very low number of remaining manufacturing defects found during PSI and ISI examinations (using in some cases qualified techniques).
The performance of an NDE is never considered in isolation but in association with the manufacturing process and the complete sequence of the different NDE implemented.
2.2.2 KTA prescriptions
The inspections procedures outlined in the KTA safety rules consider manual inspections and refer to general accepted technical standards, e. g. DIN standards, AD instruction sheets, SEP rules. There is no formal demand for qualification except alternative practices or specific cases as described below.
"Alternative practices" may be proposed but a qualification is required. This is especially the case if the examination will be performed with automated or mechanized equipment, the equivalence with the specified manual examination shall be demonstrated.
Concerning the UT examination of Steam Generator tubes the KTA safety rule stipulates for the determination of the acceptance level, an evaluation of a defect catalogue (this is similar to the French practice see § 126.96.36.199).
In order to assure and to verify the effectiveness of "reference practices" not subjected to formal qualification,.subordinated inspection procedures have to be set up. These inspection procedures are part of a precheck of the manufacture documents, which in any case is performed by the authorized expert (TUeV) who plays the role of a third party.
According to the principle of "Multiple Party Testing" repetition of inspections by the client and the TUeV increase the reliability of inspection.
The KTA safety rules differentiate between the supervisor for NDE and the NDE personnel. The supervisor for NDE shall have knowledge on the manufacture processes and shall be aware of the possibilities as well as the limits of the inspection technique. The NDE personnel shall master the inspection according to the requirements. It is up to the subordinated procedures, which demands certification according to EN 473.
2.2.3 UK approach
In the UK, reactors are not constructed solely in accordance with ASME code. The ASME code is a baseline set of requirements supplemented in certain instances.
Each reactor is the subject of an individual safety case of which ASME code is only one part. The information given relates exclusively to Sizewell "B" reactor.
ASME does not require the qualification of manufacturing NDE. However, the inspections of the IOF components described below in Section 2.3.3 require qualification regardless of whether they are as specified in the ASME code or modified.
In order to assure and to verify the effectiveness of "reference practices" not subjected to formal qualification, the following approach is followed : The ASME code gives guidance on important aspects of inspection techniques (influential parameters).
Although ASME specifies ASNT certification for inspection personnel, for Sizewell B the UK personnel certification scheme for NDT was specified instead.
The procedures shall define all essential parameters and were checked by Nuclear Electric experts prior to use. All ultrasonic flaw detectors and probes had to meet the requirements of relevant standard.
2.3 QUALIFICATIONS NOT REQUIRED BY THE REFERENCE CODE
2.3.1 French approach
This applies to new technique non-codified, linked to the implementation of new manufacturing process, new design and/or to the detection of particular defects.
The technique shall be qualified taking into account the following questions and targets: defects specification, sensitivity and limits of the technique, reproducibility of the results, and definition of acceptance levels.
The qualification, generally based on laboratory studies carried out on mock-ups taking into account the potential defects, permit to select the adequate technique and to optimize the examination parameters. The tests are performed as open trials.
Implementation on manufacturing permits to determine the acceptance levels.
2.3.2 German approach
In Germany there is no formalized way for manufacture inspection qualification. The qualification is obligatory where code prescriptions are not applicable.
The following necessary steps of the innovation process are carried out: research, development, prototype application, and application.
The qualification file generally contains: the demands considering inspection sensitivity and acceptance levels, technical evidence from previous practical trials, assessment of so-called essential parameters, design of tests blocks with artificial defects.
The qualification is generally performed as open trial supervised by the TUeV.
2.3.3 UK approach
In the Sizewell "B" safety case some components are designated as "Incredibility Of Failure" (IOF) components. These components are as follows: Reactor Pressure vessel (RPV), Core support structure, RPV studs, Pressuriser and Steam Generator (SG) shell, Reactor Coolant Pump (RCP) casing and flywheel, Steam Generator Divider Plate Welds, Main Steamline up to the Main Steam valve.
The ultrasonic inspections carried out on IOF components at all stages of manufacture were subject to independent qualification.
Qualification process included the following : definition of defect specification (type, orientations and roughness of potential defects), combination of blind trials on test pieces and technical justification.
Technical justification included the following: physical reasoning, mathematical modeling, the results of test piece trials, parametric studies into the influence of cladding (if any) and surface finish. The qualification is performed as blind trial.
3.1 CODIFIED MANUFACTURING INSPECTIONS
The manufacturing inspection techniques are, in most cases, implemented for the assessment of the quality level and the detection of any deviation in manufacturing process.
The codified prescriptions to assure and verify the effectiveness of the "reference practices" and associated acceptance criteria permit to guarantee the required quality level.
The confidence on such inspection techniques is also based on the use of inspectors qualified, and certified in accordance with recognized personnel certification schemes.
Moreover, there is no history of nuclear pressure part failure in Europe due to components entering in service with unacceptable manufacturing defects.
However, there are occasions, such UK Sizewell B, when manufacturing inspections are used to demonstrate that all potential defects calculated to have structural integrity significance are detected. This is on addition to their role in ensuring component quality.
In conclusion, there may be no technical need for qualification of codified manufacturing inspections if they are used to assess the quality level. However, when they are used to provide confidence in structural integrity, it may be judged that qualification is necessary (as happen in UK for Sizewell B).
3.2 CATEGORIZATION OF CASES AND TYPES OF QUALIFICATION
Through the review of national practices, it appears that the inspection qualification is generally performed only in special circumstances. The qualification approaches can be very different depending on the specific application and national practice.
3.2.1 Categorization of qualification cases
the following categorization of qualification cases could be done.
3.2.2 Categorization of qualification types
The compilation of European practices shows that for any type of qualification practical assessments are carried out. The differences are induced by the degree of representativity of the tests pieces, the defects simulation, and the extent of technical justification.
A categorization in different type could be used to define how stringent and rigorous a qualification should be.
The RSE-M 1997 defines, for In Service Inspections, different type of qualification depending of the probability of existence of defects induced in service, and safety consequences.
Even if the goal of manufacturing inspections is different from the objective of In Service inspections, such type of categorization applies for manufacturing inspections qualifications listed in our compilation of practices.
3.2.3 Choice of a qualification type
A flowchart establishing correlations between qualification cases and qualification types is given in figure 1.
|Fig 1: EXAMPLE CHOICE OF QUALIFICATION TYPE|
3.3 EXAMPLE APPROACHES FOR QUALIFICATION
3.3.1 Lessons emerging from the review of national practices.
Even if, like in France and Germany, there is no formalized way for manufacture inspections qualifications, the qualification is gained by a combination of trials on tests pieces and technical justification. As general rule, the inspection techniques are qualified independently.
The following items shall be considered : involved parties, technical justification (technical evidence), NDT procedure, definition of blocks test and defects simulation, type of trials (blind or non-blind), assessment of the essential parameters, evaluation and acceptance levels, and qualification of personnel (extra complementary to European schemes, if necessary or required).
The current national practices are different, in terms of the detailed implementation.
3.3.2 Documented European methodologies for qualification of NDT
In Europe the utilities operating nuclear plants have joined together to form the European Network for Inspection Qualification (ENIQ). The ENIQ has published a document (REPORT N°2) which contains general guidelines for the qualification of NDT.
These guidelines were developed specifically for In-Service Inspection (ISI). Only the general principles for qualification are also applicable to manufacturing inspections. The decision on whether an inspection is or not qualified is out of the scope of this document. Even with its limitations, this document is very useful, namely because, written in general terms, it is relevant to any non-destructive method.
The Nuclear Regulators Working Group (NRWG) has established the "Common position of European regulators on qualification of NDT systems for pre- and in-service inspection of light water reactor components". The final report has been issued in 1996. This document is developed specifically for In-Service Inspection.(ISI).
Nevertheless the principles for the derivation of basic qualification requirements are given and could be useful, after the necessary adaptation, for the manufacturing inspections (introduction of qualification levels and types). Introduction of the notions of "essential variables" and "technical justification", like in ENIQ report N°2).
The RSE-M (In Service Inspection Rules for the Mechanical components of PWR nuclear power islands) 1997 gives the conditions for qualification of an application. An application is defined as the implementation of a NDT method on a given zone.
RSE-M 1997 defines different types of qualification (see § 3.2.2). Moreover, an appendix provides "acceptable" methodology of qualification for each type of qualification.
Despite it is applicable only for In-Service Inspection, the RSE-M 1997 is very interesting, because it is, as far as we know, the first "translation" in terms of application of the philosophy and principles of ENIQ and NRWG documents.
3.3.3 Recommended qualification schemes for manufacturing NDT
The goal of the present chapter is only to list the different steps to be followed, and for each step to give some guidelines on the basis of the compilation of practices carried out, and on the basis of documented European methodologies.
4.1 CODIFIED MANUFACTURING INSPECTIONS
This study has confirmed that, in general, when a non-destructive code "reference technique" is implemented for manufacturing, there may be no technical need to qualify it. This is the current situation in France and Germany.
This conclusion is based on the mains following facts : the manufacturing inspections are generally implemented for the assessment of the quality level and the detection of deviation in manufacturing process, the codes generally fix a great number of influential parameters, the inspectors are qualified in accordance with recognized personnel certification schemes.
If manufacturing inspections are used to detect and sentence defects in order to demonstrate structural integrity (as Sizewell B), qualification may be judged necessary. This applies regardless of whether the inspection is a code "reference technique" or not.
The final decision on the need for qualification of manufacturing inspections can be very different depending on the role of the NDT inspections in the whole manufacturing process, the specific application and the national practice.
4.2 CORRELATION BETWEEN QUALIFICATION CASES AND TYPES
After a review of the industrial practices, it appears that inspection qualification of manufacturing inspections in the nuclear field is in general performed only in special circumstances, as given below (qualification cases) : explicit requirements by the code, the technique deviates from a codified inspection by one or more essential parameters, application of new non-codified techniques, structural integrity concern.
From the compilation of European practices and the existing prescriptions for ISI, a categorization of qualification types has been defined. An example flowchart illustrates possible correlation between the qualification cases and the qualification types.
4.3 QUALIFICATION SCHEMES FOR MANUFACTURING INSPECTIONS
If it is decided to perform qualification, the detailed approach and definition of levels of qualification need to be agreed upon between the different involved parties.
In this document an example is given on how such an approach could be implemented in practice step by step.
Independently of the detailed approach selected the qualification is performed using the appropriate mix of technical justification and test piece trials.
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