Ultrasonic Inspection of Reactor Pressure Vessel from Outside Surface, Qualification and Assessment of Inspection at LOVIISA NPP

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Ultrasonic Inspection of Reactor Pressure Vessel from Outside Surface, Qualification and Assessment of Inspection at LOVIISA NPP

Authors:
Raimo Paussu
Fortum Engineering Ltd. - ISI expert of technical support group for Loviisa NPP
POB 10, FIN - 00048 FORTUM, Finland
Jorma Pitkänen
VTT Manufacturing Technology - experts of the vendor
POB 1704, FIN - 02044 VTT, Finland
Co-Authors:
Bernhard Elsing
Fortum Power and Heat Ltd., Loviisa Power Plant - ISI expert of the Utility
Pauli Särkiniemi, Harri Jeskanen
VTT Manufacturing Technology - experts of the vendor
POB 1704, FIN - 02044 VTT, Finland
Contact

ABSTRACT

Welds are one of the main objects in inservice inspections of reactor pressure vessel (RPV). Circumferential core region weld is most critical for RPV integrity and therefore complementary ultrasonic inspection from the outside surface is found necessary.

In reactor pressure vessel inspection, there are some factors that have an effect on the ultrasonic inspection. It is important to know the temperature of outside vessel wall during inspection. Temperature changes the angles of incidence and affects on the detection capability of defects. The state of cladding is important also for detection of the defects at the interface between cladding and base material.

The basic inspection techniques and the measurement of transducer contact will be discussed in this overview. It is very important to have similar noise level in the validation block as in the cladding of the component. By this way, the best verification of the technique for real situation will be achieved.

The crack detection is depending on orientation, skew and tilt angle of the crack. The optimisation for detection of smaller defects requires certain angle of incidence. Noise from the cladding can enlarge the size of detectable defect. These factors are handled in the qualification of ultrasonic inspection of pressure vessel at Loviisa NPP performed from the outside surface.

INTRODUCTION

Fortum Power and Heat operates two Russian designed PWR units of type VVER 440/213 at Loviisa.

Inservice inspections (ISI) of Loviisa NPP are based on the requirements of ASME Code Section XI (inspection scope, objects, methods and interval) following the guides of Finnish regulator. The applied inspection interval is ten years for ISI inspections at Loviisa NPP. Practically, the reactor pressure vessel (RPV) will be totally inspected every eight years from the inside surface through the cladding.

An additional ultrasonic inspection of the critical core section weld from the outside surface will be carried out between the inside inspections of RPV. The specified volumes of inner and outer near surface areas of this core section weld are to be covered by this outside inspection.

The objective of this complementary inspection of core weld is to detect surface breaking defects in the outer surface area, and also, underclad defects and defects penetrating the cladding and growing into the ferritic steel in the inner surface area.

QUALIFICATION ACTIVITIES

Qualification approach in Finland
Finnish Regulator (STUK) has in 1996 set requirements for the qualification of inservice inspections following the "Common position" document of European Nuclear Regulators (NRWG).

The Finnish Utilities together with inspection companies and VTT Manufacturing Technology, as a national qualification Steering Committee, have started to outline and organise the Finnish approach for qualification of inspection systems following the principles of the "European methodology" document and published ENIQ recommendations.

The steering committee will nominate a task-based qualification body formed by independent NDT level 3 experts, and with complementary experts of other fields when necessary, for qualifications of separate inspection cases.

Additional education and training for detection and sizing of cracks in austenitic stainless steel welds has started in 1982 for Finnish level 2 and 3 inspectors participating in the pre- and inservice inspections. Blind tests of personnel for crack detection after training courses have been implemented since 1997.

Qualification Body
Key tasks of the qualification body are to assess inspection procedure, technical justification and documentation of laboratory trials, and to supervise practical trials with test blocks and their defects, and evaluate the results versus defined qualification requirements. The statement of qualification body summarises the implementation of inspection qualification.

Level of Qualification
The utility together with the vendor will prepare the document specifying qualification level for approval of steering committee. The basic level of qualification (normal, medium or high) depends on the safety importance of the inspection object and on the safety importance of the inspection to be performed for the integrity of the inspection object. In Finnish approach, the level & rigour of different activities in the process of inspection qualification may deviate from the basic level.

In the case of RPV inspection, normal qualification level & rigour is given for personnel and equipment qualification activities, medium for procedure qualification and technical justification and high for grouping of items.

Qualification plan
The utility will prepare the qualification plan giving the input data for component, defects and inspection conditions, and the objectives for inspection qualification (capability requirements for inspection system and inspection personnel).

In this inspection task the volumes to be covered are:

Outer surface area up to 30 mm thickness and 100 mm on both sides of core weld
Inner surface area up to 40 mm thickness and 100 mm on both sides of core weld.

The defect types and orientations (skew ±10° and tilt ±15°) to be detected in outer and inner surface areas of core weld are defined as follows:

Axial and circumferential surface breaking cracks in outer surface area
Circumferential subsurface defects of core weld in outer surface area
Circumferential subsurface defects of core weld in inner surface area
Circumferential, but also axial, surface breaking cracks penetrating cladding and grown into ferrite base or weld metal in inner surface area.
Underclad cracks in inner surface area.

Detectable defect size is defined based on the crack size used in the deterministic safety analysis of RPV divided with the applied safety factor.

Detection capability of inspection system is qualified if the defect size 5 mm * 10 mm (depth * length) will be detected with over 6dB signal-to-noise ratio in the open trials.

INSPECTION PROCEDURE

ENIQ Recommendations are considered in the content and structure of inspection procedure. The mechanised ultrasonic equipment Sumiad III with Masera analysing software (Tecnatom, Spain) is used for defect detection and characterisation. SAFT equipment (IzfP, Germany) is available for accurate analysing of indications and sizing of defects.

Inspection is carried out in assembly of 4 probes in two separate scans, see figure 1. Scanning steps in rotation direction are 5 mm for outer surface inspection and 8 mm for inner surface inspection. The measurement step in scanning direction is 1 mm.

Fig 1: RPV inspection assembly for scanning from outside surface.

Figure 2 shows the different defect size levels (based on standard EN 12062) and how the indications and defects are handled during site inspection.

Fig 2: The principles of recording and reporting of indications

Inspection technique of outer surface area
The inspection technique for outer surface area is validated first time in 1994 using old probes and old inspection system of VTT, and validation was renewed in 1996 with today's inspection system and optimised probes of VTT.

Twin crystal 70°TRL-St probes are selected for inspection of outer surface area. Surface breaking defects and near surface defects up to 30 mm depth can be clearly detected and analysed with this probe (Särkiniemi P, 1994). Two gates are used during scanning - gate 1 for detection and gate 2 for contact control of probes, see figure 3. Multiple peaks with sound path information are stored in both gates from the measured data.

Fig 3: Scanning of outer surface area.

Inspection technique of inner surface area

Fig 4: Indication from an underclad fatigue crack in test block PS13.
The best S/N ratio was about 30 dB.
The inspection technique to be used for inner surface area is developed in 1994-1996 for outside inspection of core weld and fixing welds of radial supports welded to RPV cladding.

Transversal probes 41° T1-St are selected for inspection of inner surface area. The noise from cladding is applied for contact control of probes. This incidence angle gives an excellent S/N ratio. This 41° T1-St probe is sensitive for detection of both small and large defects. According to Wüstenberg (1972) with angles just below 40° the sensitivity is better than 45° for smaller notches. Similar results were gained also from practical experience, NESC and open trial in 1994, see figure 4. The same probe type were used for scanning of both axial and circumferential defects.

The validation of inspection system and personnel for core weld, for defect detection in inner surface area, is implemented just before site inspection in 1999 under the supervision of qualification body. This open trial is performed using one test block of Loviisa NPP with machined underclad reflectors of qualification size and orientations.

The whole A-Scan is stored from measured signals coming from inspection area.

TECHNICAL JUSTIFICATION

Technical Justification (TJ) is based on physical reasoning, results of laboratory trials, round robin and field experience and on the results of open trial. The variables of component, defects, procedure, equipment, scanner and analysis of results are defined, assessed and justified in this TJ according to ENIQ recommendations.

Effects of some inspection conditions are studied in more details in this TJ as new aspects. The temperature of RPV outer wall can be about 60°C when starting UT inspection. The significance of that temperature for capability of specified UT inspections is studied and measured with laboratory trials.

Increased angle of incidence of 70° TRL probe due to higher temperature causes better sensitivity for detection of surface breaking defects, and decreased sensitivity for detection of defects located deeper in outer surface area. That temperature has no relevant effect on UT inspection of inner surface area.

The inspection systems of inner and surface areas are optimised and validated using Finnish and Czech fabricated test blocks simulating the real base material cladding structure. Inner surface cladding has a clear noise effect on UT inspection decreasing detection capability of defects. Noise levels of all test blocks applied and the real cladding in RPV are measured and evaluated. The noise levels are confirmed as similar in all cases and the test blocks can be considered representative. So the implemented open trial gives a reliable verificationg of detection capability of inspection system.

EXPERIENCES OF INSPECTION and QUALIFICATION ACTICITIES

The smoothness of outer surface guarantees proper scanning contact for inspections. In local grinding areas, the contact surface can produce extra indications because of swinging of the probe on the surface. The slag inclusions and slag lines of cladding are slightly disturbing the analysis of inspection results of inner surface area.

UT inspection of outer surface area should be performed first during the outage for getting the best sensitivity for defect detection of surface breaking defects.

The qualification activities of the utility and vendor started too late and time enough was not reserved for preparing necessary documentation and open trials. Nomination of the members of qualification body happened so late that their real contribution for selection of test blocks and reflectors to be used in open trial was limited.

Inspection procedure, technical justification and results of laboratory and open trials could be assessed after site inspections some months later. On the other hand, qualification body was able, time to time to supervise the site inspections and analysis of inspection results, and see how the inspection procedure was really followed.

Also the vendor performed more additional laboratory trials before delivering technical justification with the results of trials. The positive statement of qualification body summarised the succeeded implementation of inspection qualification.

SUMMARY

When a cladding is present in the inspection area of a component, it is really important for validation of detection capabilities of inspection system, that the properties and noise levels of test block cladding fully simulate the real component cladding.

Like in this specific case, the number of defects in the test blocks is quite limited and not sufficient as an evidence for statistical assessment of detection capability.

The inspection of RPV core weld is one of the first cases for breaking-in Finnish approach of qualification in practice. One of the challenges was to create proper and practical ways for each organisation to work together. The whole qualification documentation together with the statement of qualification body is now delivered for approval process of Finnish regulator.

REFERENCES

Wüstenberg, H. & Mundry, E., 1972 Nuten und Kanten als Bezugsreflektoren in der Materialprüfung mit Ultraschall, Materialprüfung 14 (1972) 2 s. 58 - 61.
Särkiniemi, P. & Pitkänen, J. 1999 Technical Justification; Mechanized ultrasonic inspection of the RPV weld of the cylinder part.
Jeskanen, H., 1999 Inspection Procedure; Mechanized ultrasonic inspection of the RPV weld of the cylinder part.
Särkiniemi, P & Jeskanen, H., 1994 Document LO1-K311-961-21. Lo1 Mechanized UT-inspection of RPV, Outage 1994 Technical assesment and Open trial, 12.7.1994.