ABSTRACT

The inspection of the inner radius areas of the nozzles in the BWR reactor vessels, in addition the problems of access to these areas and their radiological conditions, present other difficulties resulting from the complexity of their geometry. The supposed orientation of the possible defects adds a new difficulty to the automatic inspection of these areas.
The new ultrasonic inspection techniques, developed by new equipment and reinforced by ARRAY technology, have made it possible to advance in the field of defect characterization, localization and sizing of these components.
This report represents the recent development realized by TECNATOM for inner radius inspection of the Boiling Water Reactor Vessel.

RÉSUMÉ
On rencontre quelques difficultés, de l´ordre géométrique, radiologique. et d´accéssibilité, pour línspection de certaines paroies internes des tuyauteries primaires des centrales BWR. L´orientation des défauls augmentes d´autant plus les difficultés de contrôle dans ses zônes pour des ensembles mécanisés. Les nouvelles techniques dínspection par ultrason, et le développement de nouveaux ensembles renforcent la technique ARRAY. Cela a permis de faire avancer le champs de caractérisation et de localisation en dimensionnement, au niveau des défauts dans ces ensembles.
Ce rapport décrie la nouvelle technologie que TECNATOM à développer pour l´inspection des paroies internes des cuves des centrales BWR.

INTRODUCTION

The nozzles of boiling water reactor vessels are difficult to access and present high levels of radioactivity. Added to these difficulties, for the ultrasonic inspection of the areas of the inner radius of these nozzles, are those arising from their specific geometry. This geometry due to its being the result of connection of two cylindrical bodies - the reactor vessel and the piping - gives rise to particularly complex volumes of interest and surfaces from which to perform the inspection.
The difficulty involved in accessing these areas and the adverse conditions of environmental radiation encountered make it necessary for the inspections to be performed by remote control. Consequently,

BACKGROUND - PREVIOUS EXPERIENCE

FIGURE 1

Tecnatom has been performing ultrasonic y nspections of nozzles similar to the one shown in FIGURE 1 since 1984. The inspection is carried out from the outside of the nozzle using the direct contact pulse-echo technique.
The objective of these inspections is to orient the ultrasonic beams from the probes suitably, such that the entire volume of interest is covered, with the beams impinging perpendicularly to the possible defects. With a view to achieving this objective, different items of mechanical equipment, probes and shoes are combined.
Tecnatom has used two different types of mechanical equipment: one which scans from the surface of the vessel wall (surface A-B) and another which performs explorations from the surface of the nozzle body (surface B-C).

When the inspection is performed from surface A-B, a probe is used to carry out two examinations, each with a different probe-holding module orientation (examinations to the left and right). When the inspection is performed from the B-C surface, two similar probes are used coupled to different sets of shoes. Depending on the set of shoes selected, the angle of entry of the ultrasonic beam into the steel will differ.

Given that the probes used are ceramic and with a fixed angle, ensuring that the ultrasonic beams are introduced into the nozzle along optimum paths requires that the movements of the mechanical equipment (three degrees of freedom) be combined and that advantage be taken of the specific geometry of the nozzle.

FIGURE 2 shows schematically the inspection performed using both items of equipment.

These inspections guarantee the detection of defects measuring more than 2.5 mm. - this having been demonstrated on 1:1 scale mock-ups - but on the basis of using more than one inspection system and various probes, shoes and scans. Unless diffractions appear, sizing requires the performance of laborious additional explorations and calculations, in order to determine the position of the probe and the defects, and even then the accuracy of the results obtained is not entirely satisfactory.

NEW TECHNIQUES

The design of new techniques was initiated with the aim of meeting two objectives:
• To simplify and improve the ultrasonic inspection of the volume of interest in order to reduce data acquisition times and facilitate sizing of the defects.
• To automate positioning calculations.

With a view to meeting the first objective, ARRAY probes manufactured from piezo-composite materials were used.

The angle of entry and focal depth of these probes may be controlled by means of electronic equipment. This allows better ultrasonic inspection of the volume of interest. By selecting suitable probes and correctly combining the angles and focuses it is possible to use a smaller number of items of equipment and probes and reduce scan times. In addition, availability of a tool allowing the ultrasonic beam to be controlled means that the sizing of defects can be accomplished more accurately.

For automation of positioning calculations, the second objective mentioned above, use was made of computer-based simulation models representing the probes and the ultrasonic beams emitted by them onto a nozzle. The path followed by these beams inside the nozzle, determined by the position of the probes on the nozzle and the characteristics (angle and direction) of the beams emitted, makes it possible to position the defects and calculate the optimum position of the probes for performance of the acquisitions. Characterization of the ultrasonic beams and calculation of the focal laws are accomplished using simulation programs representing the shape of the beam depending on the characteristics of the probe, the medium and the focal laws with which the crystals are excited.

Thanks to the computer-based simulation model, scanning and the focal laws are optimized, this actually implying optimization of the angles, focuses and electronic scans. Application of these computer models in actual inspections is effective and practical only if a relationship is established between the model and reality. In this type of inspection, the position of the probe on the nozzle has been seen to be highly critical for establishment of the scan plans, correct focal laws and the position of the indications detected, as a result of which efforts should be made to gain accurate insight into its position. Finally, the technique should be validated on a 1:1 scale mock-up, the entire radiofrequency signal being acquired.

RESULTS

The first inspections performed on mock-ups with application of these techniques allow the following results to be predicted:
• Detection: Detection of 100% of the defects in the volume of interest will be maintained with a single examination, this reducing the time required by 50% with respect to previous inspections. Ultrasonic inspection of complex nozzles. Application of new technologies.
• Sizing: Defects measuring more than 1.5 mm in depth will be correctly sized, an accurate determination of defect position and orientation and of whether or not they are open onto the outer surface being achieved.

CONCLUSIONS

In recent years important developments have been made in the manufacturing of new probes and electronic components and also in the design of programs simulating the impingement of ultrasonic beams on specific geometric models. This progress has made it possible to address ultrasonic inspections from a new perspective. Thus, inspections previously requiring more than one item of mechanical equipment, various probes and complicated scans may now be accomplished in a simpler manner and with a smaller number of items of equipment. Thanks to these new technologies, improvements have been made to the quality and reliability of inspections. In addition inspection times have decreased, this having a favourable impact on the average doses received, and the economic costs involved have been reduced.

Antonio Tanarro
Head of Ultrasonic Research Group. R&D Division.
Tecnatom, s.a.
Avda. Montes de Oca, 1.
28709 San Sebastián de los Reyes. Madrid. Spain.
Telephone: (341) 651 67 00
Telex: 45.831 TCOM
Telefax: (341) 654 15 31
E-mail: tanarro@tecnatom.es
Homepage: http://www.tecnatom.es