Project: head inspection

The first indication of cracking of a PWR reactor vessel head penetration was discovered at Bugey 3 in France in September 1991. This was originally considered to be an isolated phenomenon, until cracks were also discovered in penetrations of the vessel head at Ringhals 2 in Sweden, in May 1992. From this moment, the problem was considered to be a generic one, related to Inconel 600 penetrations. This problem has since been defined as being one of primary water stress corrosion cracking.

In July 1992, the Spanish Nuclear Safety Council (CSN) requested all the nuclear power plants in the country to provide preliminary information on the situation and on their respective action plans. A study was submitted to the CSN on the susceptibility of the Spanish plants. Following this, a requirement was issued for inspection of the vessel heads of the Asco 1 and Almaraz 1 units during their refueling outages scheduled for that same year.

In view of this situation, Tecnatom initiated a project (Petava project) in April 1993, in collaboration with some Spanish services companies, to promote the implementation of PWR reactor vessel head penetration inspections and repairs.

Tecnatom adapted the Petava equipment to develop a system to inspect sealing and fixing welds of vessel head penetrations in VVER plants.

Petava project

The scope of the project included the determination of degradation causes; the establishment of the acceptance criteria; the development of inspection techniques using ET, UT, PT and VT methods (see below) as well as thermal sleeve welding and cutting techniques; and finally the development of repair procedures.

The techniques developed were:

•Eddy currents (ET), applied for the detection and sizing of longitudinal defects.

•Ultrasonic (UT), used for sizing in depth.

•Dye penetrant (PT) and visual inspection (VT) for the characterisation of defects and determination of their morphology.

As regards inspection tools, Tecnatom has developed a motor-driven tool for penetrations without thermal sleeves, which is coupled to the robot-operated positioning system. It is also possible to adapt different probe supports to this tool for the performance of ET, UT and VT inspections. Various simpler tools were also developed. For penetrations fitted with thermal sleeves, another tool (based on Framatome Gap-scanner equipment) was adapted, allowing ET and UT inspections to be performed without the need to cut this particular element.

The study included the determination of the susceptibility of Inconel 600 to primary water stress corrosion cracking, and the acquisition of data on crack growth rate. After that, in order to establish the acceptance criteria, Tecnatom carried out various studies with three-dimensional models, developed using the finite element method.

Eddy current inspection

Eddy current data acquisition is accomplished by means of the TEDDY 4 digital multifrequency equipment. In the case of the Gap-scanner, the variable measured is the variation in impedance due to the effect of the eddy current on two coils located at the end of a blade probe inserted into the gap between the penetration and the sleeve. The availability of two coils at the end of the blade makes it possible to reduce the number of insertions, and consequently the inspection time, since two generatrices are inspected with each insertion of the probe.

The Gap-scanner imparts two movements to the blade probe: one axial and the other circumferential, which covers an arc of 360?. This allows a comb-shaped path to be generated, making it possible to inspect the entire area of interest of the penetration.

For vent pipe and non-sleeved penetrations, a rotating head is used, allowing the inspection coils to move along a helicoidal path. Three types of coils are used: pancake coils (for detection and measurement), axially oriented coils and circumferentially oriented coils (the latter allowing the orientation of the indication to be confirmed).

Mechanical equipment

The modified SM-22 equipment consists of a remotely operated polar positioner that allows the probe to be located at any penetration for the performance of eddy current vessel head inspections.

The equipment provides the following main advantages:

•Its installation and maintenance does not require human access below the vessel head, considerably reducing the dose received by the personnel during assembly, operation and disassembly.

•The inspection is performed on the vessel head stand, with no need of elevation.

•Compact modular design, robustness and accuracy.

•Easy calibration.

•Automatic positioning at the operator-selected penetration.

•Possibility of controlling from a distance of up to 150m.

The modified SM-22 fixture is made up of three clearly differentiated elements:

•Elevating column.

•Set of three articulated arms.

•Connection hose to the SIROCO control box. The SIROCO control box, which controls the remote operation equipment, communicates with the data acquisition computer LAN output interface via an IEEE 488 data bus.

The Tecnatom eddy current equipment, known as TEDDY, is used for the inspection. This equipment has been validated on laboratory mock-ups and on tubes at thermal and nuclear power plants.

The TEDDY software is used for data evaluation. This software runs under Windows NT or 2000 operating systems. Typical ET C-Scan and phase and amplitude measures can be displayed.

Ultrasonic inspection

The ultrasonic inspection is performed using the same fixture, with the UT-Gap equipment connected to it, for penetrations both with and without sleeves. This equipment allows UT inspections to be performed by inserting a probe into the gap between the thermal sleeve and the penetration, or between a false bushing and the penetration. The function of this inspection is to confirm by dimensioning in depth those indications that have previously been detected and located by means of the eddy current inspection.

The ultrasonic technique used is TOFD (time of flight diffraction), based on the interaction of ultrasonic waves with edges of discontinuities. As a result of this, diffracted waves are emitted, the depth of the defect being established depending on the time elapsing between emission of the pulse and its diffraction echo.

Each TOFD probe is mounted on a strip of stainless steel with a capillary tube for the supply of coupling water. The UT strap is adapted to the mechanical system of the Gap-scanner used for eddy currents (UT-Gap), allowing it to follow a comb-shaped path in order to cover the area to be inspected.

The Tecnatom SUMIAD equipment is used for the ultrasonic inspection. SUMIAD is an ultrasonic multi-channel inspection and data analysis system for the performance of non-destructive tests. It is made up of a computerised system and the multi-channel ultrasonic equipment. The SUMIAD software adds different information processing and display utilities.

PWR experience

The inspection systems can carry out the examination of a PWR reactor vessel head by ET in only three days. If any indications are detected, additional techniques such as UT, VT and PT are applied in order to fully characterise the defect.

Inspections were carried out since 1994 at Almaraz 2, Jose Cabrera, Asco 2, Vandellos 2 and Oconee (USA). 21 penetrations were inspected by means of ultrasonic techniques, and repaired.

In four penetrations repairs were carried out using the excavation tool. Once the results of the visual and PT inspections had confirmed the complete elimination of the cracks, the four thermal sleeves were re-welded.

The collective dose received at the plants in which the interventions were made were lower than the optimum values estimated by the initial forecasts.

VVER experience

The control rod penetrations in the reactor vessel head of VVER plants consist of a carbon steel tube (CRD tube) welded to the vessel head (fixing weld). In order to protect this tube against corrosion, a stainless steel tube is mounted inside it (corrosion protection tube), which is welded to the former tube at its lower end (sealing weld). Inside this there is another stainless steel tube to provide thermal protection for the control rod drive shaft.

At both the fixing and sealing welds, and in the adjacent heat affected zones, cracking could occur under normal operating conditions. In order to detect these possible cracks, eddy current, ultrasonic and visual inspections must be performed on these areas. In 1998 Tecnatom adapted Petava equipment for this kind of inspection.

The defects postulated were:

•Circumferential cracks in the CRD penetration tube.

•Circumferential cracks at the inner and outer edges of the fixing weld.

•Circumferential cracks at the edge of the sealing weld.

•Radially oriented cracks at the surface of the fixing weld.

•Circumferential cracks in the corrosion protection tube.

•Circumferential cracks at the surface of the fixing weld.

A new scanner tool (STAR module) was developed to perform the inspection cycle. It gives a radial movement to the eddy current or ultrasonic probe and a rotation movement with respect to the penetration axis. The equipment is fitted with tracking and tele-inspection cameras and is governed by a SIROCO VME control.

This equipment provides the following inspection capabilities, depending on the module used:

•Ultrasonic inspection of the fixing weld for circumferentially oriented defects.

•Ultrasonic inspection of the fixing weld for radially oriented defects.

•Eddy current inspection of the fixing and sealing welds.

•Eddy current inspection of the corrosion protection tube in the transition area (Gap-scan).

•Visual inspection of the fixing welds.

•Visual inspection of the inner surface of the RPV head.