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Charles R A Schneider and Colin R Bird
TWI, Granta Park, Cambridge CB1 6AL, UK
David A Wood
BNFL Magnox Generation
Berkeley Centre, Berkeley, Gloucestershire GL13 9PB, UK
Paper presented at BINDT Annual Conference, Southport, 17 September 2002
Magnox Generation has investigated whether it is feasible to carry out ultrasonic inspection of welds associated with boiler beam support brackets at various Magnox power stations. The welds are small, with a leg length typically of6 to 15mm, and they could not be properly inspected using conventional ultrasonic probes. A technique was therefore developed using a phased array system. This allowed the ultrasonic beam to be focussed electronically, thereby obtaining the required spatial resolution, and also allowed the beam angle to be varied in order to direct the beam into the weld volume between the cap and root. Following the development work, it was decided to apply the technique tosome boiler beam support bracket welds at Sizewell A power station.
This paper addresses the qualification of the inspection procedure, which TWI managed on behalf of Magnox Generation following the ENIQ methodology. The comprehensive technical justification document was supported by practical trials of the procedure, equipment and individual data analysts. The qualification was completed successfully and confirmed that the procedure was capable of reliably detecting crack-like defects in the weld or heat affected zone greater than 1.5mm wide by 10mm long. It also confirmed that, subject to certain provisos, the defect width would not be underestimated by more than 1.5mm.
This qualification involved a novel technique with particular challenges and this paper describes the way in which these were overcome.
The boiler tube structures inside the boilers at Sizewell A are supported by beams which are themselves attached to the boiler shells by support brackets. These brackets are welded fabrications and volumetric inspection of the welds was considered desirable to check for possible in-service degradation. The bracket design is shown in Figure 1. It consists of a landing plate and two gusset plates and is welded to the boiler shell or, in some cases, to a pad which is welded to the boiler shell. The gusset to landing plate welds are un-penetrated fillet welds and the gusset to shell/pad welds are fully penetrated, as shown in Figure 2. The landing plate to shell/pad geometry is similar to the gusset plate to shell/pad geometry, except that the plate meets the shell/pad at right angles.
Fig. 1. Geometry of support beam bracket
Fig. 2. Weld geometries and defect parameters
The plausible defects which the inspection must be capable of detecting are oxide jacking cracking, lack of fusion, hydrogen cracking and slag inclusions. The requirement was to provide highly reliable detection for crack-like defects greater than 1.5mm wide by 10mm long. The width is the dimension measured perpendicular to the weld direction on the face of the defect - dimension W for the unpenetrated welds, and W1 and W2 for the fully penetrated welds, as shown on Figure 2. Defects were assumed to be tilted at any angle between the two fusion faces - a, a1 and a2 on Figure 2, where p1 and p2 are the weld preparation angles on the fully penetrated welds. The inspection requirements were that the measured defect width should not underestimate the true defect width by more than 1.5mm.
A technique was developed for the inspection using an ultrasonic phased array system and this is described fully in another paper in this conference  . The inspection was required to be qualified according to the ENIQ methodology  and TWI was contracted to provide the independent qualification body required by ENIQ.
The use of the ENIQ methodology is well known and has been described in many papers.  The present paper will not therefore discuss the use of the ENIQ methodology in general but will instead concentrate on how this qualification dealt with the particular challenges presented by this inspection, namely:
- Use of a novel inspection technique
- Lack of evidence from previous qualifications
- Limited previous application of the technique for this component geometry
- Lack of standards that could be cited
- Lack of evidence from computer modelling
- Lack of recognised training and certification for the technique
- Limited time scale.
The qualification body consisted of three people all of whom were experienced in inspection qualification. TWI chaired the qualification body and was supported by an independent consultant. The third member of the qualification body was an employee of Magnox Generation, who provided detailed knowledge of the procedures and their application and reported to the qualification body chairman for the purposes of this qualification. The qualification body discharged the usual ENIQ responsibilities of agreeing the input information, reviewing and endorsing the inspection procedure and technical justification, carrying out practical trials and issuing qualification certificates. This work was reported in a summary of technical evidence, which provides the primary reference to the extent and limitations of the qualification. The way in which the particular challenges of this inspection were addressed is described in the following sections.
2.1 Preparatory work
Because of the novel nature of the technique and the lack of experience in this area, extensive development work was carried out. As well as laboratory work, this included trials on actual plant initially using focussed probes and later using the phased array system. At the start of qualification, members of the qualification body and a representative of the Nuclear Installations Inspectorate were given a training session, which described this preliminary work and included a demonstration of the phased array equipment. This helped to ensure that all those involved in the qualification were aware of the capabilities and limitations of the technique.
2.2 Inspection procedure
For established techniques, the inspection procedure can incorporate previous experience and can cite standards for equipment performance. These were not available in this case and the qualification body had to take account of this in assessing the inspection procedure. Particular note was taken of the following factors.
- With no established standards for equipment performance, the qualification body assessed the equipment checks in the procedure to ensure that they provided adequate control of the beam characteristics, in particular that the focal laws required by the phased array equipment provided the required beam angles and focal lengths. In addition, the data quality checks were assessed to ensure that they were capable of ensuring that the procedure was capable of collecting data of adequate quality for subsequent analysis.
- Although the technique is novel, many features of the data analysis are similar to those of other automated systems, in particular the GUIDE system which has been used for many other inspections of UK nuclear plant. The qualification body checked that experience from these systems had been taken into account.
- There is no established personnel qualification for phased array inspections. The approach adopted in the procedure is, firstly, to invoke PCN qualification to ensure a basic understanding of ultrasonic inspection. Secondly, recognising similarities to other automated inspection systems, to invoke established qualifications for staff carrying out automated inspections on UK nuclear plant - the MIPS qualification for data collectors and the GUIDE qualification for data analysts. Finally, to provide training specific to the phased array system and to this procedure. The qualification body assessed these proposals in detail to ensure that they provided an adequate control of the inspection personnel and assessed the qualification records for all inspection personnel to ensure that they met the required criteria.
2.3 Technical justification
In the absence of prior experience, the arguments put forward in the technical justification relied very heavily on the experimental evidence produced in the development programme. Physical reasoning also broadly confirmed the plausibility of the experimental results. The qualification body assessed this evidence in detail in order to check that it provided support for the inspection capabilities claimed in the technical justification.
In many cases it is possible to invoke the results of computer modelling in the technical justification. The development team made extensive efforts to identify a suitable model but, at the time the work was done, was not able to obtain one. The qualification body was satisfied that all reasonable attempts were made and that the technical justification was acceptable without that support.
2.4 Practical trials
The qualification body considered an open trial of the equipment and data collection part of the procedure adequate but, in view of the novel nature of the inspection, considered a blind trial essential for the individual data analysts and the data analysis part of the procedure.
The inspection is carried out with one inspector manipulating the phased array probe inside the boiler shell and a second operator operating the data collection equipment outside the boiler. The inspector inside the boiler has to work in conditions of difficult access, wearing protective clothing. In designing and invigilating the data collection open trial, the qualification body took particular note of:
- The need to simulate the access conditions
- Checking for adequate communication between the inspectors inside and outside the boiler
- Checking the effectiveness of the data quality checks.
For the data analysis blind trials, the candidates were presented with data obtained from a series of test blocks containing defects induced during welding. A macrograph of a section taken from a typical example is shown in Figure 3. For the trial, the candidates were assessed against the required criteria of detecting all defects meeting the required description and not under-sizing defect width by more than 1.5mm. The blind trial was initially attempted by two of the three data analysts. Both met the detection criterion but failed to meet the sizing criterion on some defects. With the information available at that time, the qualification body could not identify if this resulted from the performance of the analysts, the capability of the procedure or poor characterisation of the defects in the test pieces (at that time, the value specified for defect implantation was taken as the 'true' value). This was resolved by (a) providing additional training for the first two analysts, (b) including the results from the blind trial on the third analyst and (c) carrying out sectioning of selected defects. With this further information, the qualification body concluded that all three analysts met the required criteria.
Fig. 3. Typical example of a sectioned test piece
2.5 Limited timescale
BNFL Magnox Generation wished to use the technique to gather data during the outage at Sizewell A in April 2001. At that stage, however, the technical justification was not complete and the inspection procedure was still in draftform. BNFL Magnox Generation therefore decided to proceed with the inspection at risk of it not being able to be qualified retrospectively. In these circumstances, the qualification body agreed to proceed with an informal datacollection open trial on a 'best endeavours' basis in order to provide information which might assist any attempt to provide retrospective qualification. A formal data collection open trial was carried out at a later stage and thisconfirmed the results of the informal trial.
The data obtained during the outage was analysed in an interim form using the draft procedure and this work provided further information to help in writing the final drafts of both the procedure and the technical justification. Thedata analysis blind trial was carried out when the procedure and technical justification were finalised and the data was then re-analysed to provide the formal inspection results.
This retrospective approach was possible in this case (and is possible in most automated inspections) because the data collection and data analysis parts of the inspection are largely independent. The qualification body did,however, check carefully that all parts of the qualification met the requirements in the final documents in all significant details. This process was documented and is available for audit.
As a result of this qualification the qualification body were able to confirm that the procedure was capable of reliably detecting crack-like defects in the weld or heat affected zone greater than 1.5mm wide by 10mm long and that,subject to certain provisos, the defect width would not be underestimated by more than 1.5mm. The three data analysts were also judged capable of applying the data analysis part of the procedure.
This qualification has also demonstrated that it is possible to apply the ENIQ methodology in a cost effective and timely manner to an inspection using a novel technique and where there is little prior experience. Some of thefactors needed to achieve this are:
- A preliminary programme of development work is needed, including experience of application of the technique under realistic conditions.
- Training should be provided for the members of the qualification body. This should include the results of any preliminary work.
- Even if the technique is novel there may be features which are similar to other established techniques. Advantage should be taken of such experience.
- Similarly, the personnel qualification may be able to take advantage of other similar inspections. However, specific training in the technique and procedure are essential.
- If data collection and data analysis can be separated, it may be possible to collect data from plant before the procedure and technical justification are complete, although this presents an economic risk of the inspection not being qualified retrospectively.
This paper is published by permission of BNFL Magnox Generation. The authors also acknowledge the assistance of the Dr K J Bowker and other members of the development team at BNFL Magnox Generation in the preparation of thepaper.
- K J Bowker, 'Ultrasonic inspection of Sizewell A boiler beam support bracket fillet welds with phased arrays', paper in this conference.
- 'European Methodology for Qualification, Second Issue', ENIQ Report No. 2, EUR 17299 EN, 1997.
- C Waites, M J Whittle and R V Booler, 'International experience in the application of the ENIQ Methodology', Proceedings of the BINDT annual conference, Buxton, 2000.