Anyone for TENIS?3 August 2002
A new system for scanning PWR pressure vessel nozzles has reduced time and costs for the operator. By Karl Quirk and Diego Molpeceres
The past three decades have seen dramatic advances in non-destructive testing (NDT) and each new generation of equipment brings further improvements.
Modern ultrasonic inspection systems offer greater reliability and repeatability than ever before. Smaller, less bulky scanners make for more convenient operations while remote systems minimise the time that personnel need to spend in hazardous areas.
New equipment and techniques have also led to significant savings for plant operators. The latest ultrasonic systems are less intrusive, helping to minimise plant downtime. Wherever possible, they are designed to scan components while still in place, avoiding the need for complex and costly dismantling operations.
RPV nozzle inspection
The TENIS scanner is a good example of how modern design can bring about such improvements. The Spanish inspection company Tecnatom was commissioned to develop a new system to test the primary circuit nozzles within PWR vessels. It worked with Phoenix Inspection Systems from the UK, a specialist in the design and manufacture of automated ultrasonic manipulators for the nuclear industry.
The reactor pressure vessel nozzles are an important focus for any PWR plant inspection, being the critical components joining the wall of the pressure vessel to the primary circuit pipework. The scope of inspection for a nozzle includes the nozzle to shell weld, the inner radius, the nozzle to safe end weld, and safe end to pipe weld. The NDT method required is ultrasonic and the inspection is performed from the inside of the nozzles.
The in-service inspection (ISI) of nuclear power plants in Spain is ruled by Section XI of the ASME Code. The whole scope of the ISI has to be completed over a period of ten years. The nozzles have to be partially inspected (50%) at the end of the first three years - this is called the partial inspection or the three-year inspection. They are then inspected again (the remaining 50%), at the end of the ten-year period, at the same time as other areas of the vessel are inspected.
The inspection is generally carried out while the plant is shut down for refuelling and, at the Vandellos 2 power plant, the partial inspection had been scheduled to coincide with the shutdown which was to take place at the end of April 2002.
Because the ten-year inspection requires the testing of large areas - circular and vertical welds, the nozzles and threaded areas - Tecnatom uses a large "universal scanner" called TIME for these inspections. However, for the partial or three-year inspection, the use of the large manipulator to scan such a small area makes no sense in terms of the cost, time and effort required to set up such a large system.
Another major problem is that, using the traditional method, the scanner is suspended from a polar (overhead) crane that lowers it into the vessel and installs it on the vessel flange surface. This restricts access to the vessel core for other activities, mainly refuelling tasks, and affects the overall schedule.
The plant operator was looking for a device which would overcome these problems and would maintain - or exceed - the quality of data collected by the universal vessel inspection scanners but reduce the use of plant resources such as the polar crane. The system was required to:
• Enter the vessel through the personnel hatch instead of the interlock door.
• Reduce the space needed on the refuelling floor during the set-up period.
• Minimise the time needed to carry out the inspection, by allowing the visual inspection to take place at the same time as the ultrasonic testing.
• Reduce decontamination time and the debris generated in the process.
• Reduce the space required for storage of contaminated containers.
Tecnatom personnel reviewed the objectives, working closely with the operators who were familiar with the plant and its infrastructure, and knew at first hand the practical problems involved.
Although use of the crane was still the best option to place the scanner inside the vessel, once inside, the scanner could be free floating and could be easily positioned by a mini-submarine or manipulation poles.
It was obvious that the scanner would have to be compact and carry on-board cameras to monitor its operation during the inspection. To achieve the objectives, it would have to be no more than 2m long and weigh no more than 80kg.
The scanner needed to be capable of being moved around by four people.
It also had to be able to carry the required number of ultrasonic probes in the correct positions to cover all the inspection zones for the inspection of the nozzles. A modular design would allow the scanner to be adapted for different configurations of nozzles.
The scanner conformed to all of Tecnatom's specifications and was named TENIS (Tecnatom Nozzle Inspection System). The scanner holds ten ultrasonic probes and can scan them axially and circumferentially over the internal surfaces of the nozzle.
The device is based upon an axial chassis assembly consisting of an inboard and an outboard end plate, with three beams between them. This assembly holds together all the other components. The rotation mechanism is driven along the beams and can rotate the probes by up to 380° using a slide ring. The ten probes are arranged in five groups of two, equally spaced around the slide ring. They are operated pneumatically and are retractable by design to protect them from damage while they are being inserted into the nozzle.
There are also two flotation chambers, one on the inboard and one on the outboard part of the scanner. The 5kg negative buoyancy controls the rate of descent while the shape prevents it from spinning around and so ensures that the orientation is maintained.
There were several challenges in designing the TENIS manipulator.
In order to achieve a 5kg weight in water, precise calculations of component weights and displacements were required, and aircraft technology had to be applied to the design of the flotation chambers.
Cable management was difficult since the space available for cabling and other services on the manipulator meant that special cable chains had to be developed allowing rotation through 380°.
The probe holders were possibly the most complex components on the system. It was important that the probes should follow the contours of the nozzle precisely and yet combine this fine movement with robustness during scanning and protection during insertion.
The length of the scanner was restricted to 2m to allow it to be introduced into the containment building via the personnel airlock. This produced conflicts with the coverage required to meet the inspection criteria. The final design met both criteria.
TENIS is designed to work with an existing ROV, a mini-submarine consisting of a framework of extruded aluminium beams, with a moveable propeller. The ROV clamps onto an interface bracket on the back of the scanner and, once lowered into the water, will guide it into position inside the nozzle. Another possibility would have been to use manipulation poles instead of the ROV.
The two units are also linked by the umbilical cable that carries the power supply, telemetry and ultrasonic signal from outside of the vessel. Finally, there is an emergency retrieval cord that is attached to the scanner for use only in the event of an equipment failure.
Testing and training
The go-ahead for the project was given in July 2001, which left just nine months to complete it - six months to design and build a system and the remainder for commissioning and trialling the equipment and training the operators.
When the TENIS scanner was completed, it was shipped to Tecnatom's premises in Madrid. This was the first time the partners were able to fully integrate the system. While Phoenix had been working on the equipment, Tecnatom had designed a programme of trials which would reproduce in the most realistic way possible the conditions within the vessel. A life-size model of an outlet nozzle was created from stainless steel and was placed in a pit 5m deep.
Two types of trials were carried out. The first took place in dry conditions, primarily to test the alignment, repeatability and functioning of the equipment. The second trial was carried out underwater and simulated the whole operation, testing the way in which the device would enter the vessel and be positioned inside the nozzle with the help of the ROV, right through to the point where it is removed from the vessel once the inspection has been carried out.
A portable crane was used in place of the polar crane that would be used in the plant, and distances between the key points were accurately reproduced. Finally, two full months were given over to the training of personnel who would be carrying out the inspection.
TENIS in operation
With trials successfully out of the way, and the training programme completed, the moment of truth finally arrived. TENIS went into operation in April on the inspection of vessel nozzles at Vandellos 2. The inspection showed:
• Introducing the TENIS scanner through the personnel hatch interlock, instead of the containers interlock door, reduced the overall time for the operation.
• The scanner was prepared for inspection on the refuel floor in a space of just 2x2m.
• The use of the ROV or manipulation poles to guide the scanner into the nozzle meant the overhead crane was in use for one hour only.
• Within 48 hours of the scanner being delivered to the plant packed in a container, it was ready to start the inspection.
• The use of poles to deploy the scanner into the nozzles turned out to be easier in a real-life situation than it had been in the testing due to the conditions - distance, illumination and so on - being more favourable.
• Both inspections - the ultrasonic and the visual inspection - were performed in parallel and took an overall time of 26 hours.
• A few hours after the inspection was completed, the scanner was removed from the vessel successfully with the use of the polar crane.
• The decontamination of the scanner was carried out without any need for the crane or any other equipment.
The whole process took less than half the time typically required for the decontamination of the universal inspection scanner. Considering this and the reduction in the volume to be cleaned, there was an estimated saving of over 50% in the amount of waste generated during the decontamination.
The application of the TENIS scanner produced huge benefits in terms of the reduction in inspection time and in the requirement for plant resources. TENIS proves the benefits of using purpose-built manipulators for the in-service inspection of reactor components, instead of complex, multi-purpose inspection machines. Innovative design can overcome many of the problems associated with particular types of NDT inspection and maintain or improve safety levels while reducing costs for operators.