Development of new and novel low cost robot inspection methods for in-service inspection of nuclear installation

Final Report Summary - RIMINI (Development of new and novel low cost robot inspection methods for in-service inspection of nuclear Installation)

The strategic objectives of the RIMINI project were:
1. to significantly reduce the cost of inspection of nuclear reactor pressure vessels by reducing the time for inspection;
2. improved safety through higher reliability and repeatability of the inspections.

The following Non-destructive testing (NDT) methods and equipment were developed.
- MicroPulse 5PA - 128 channel phased array instrument. Later proofed and enabled for full matrix capture. The instrument is complete with its own driving and display software. It has been produced by Peak NDT.
- Simulus - conventional and phased array ultrasonic beam modelling software. The software has been validated by TWI against two other models and physical trials.
- ACFM system - calibrated on implanted defects and verified on real and artificial stress corrosion cracking. Produced by TSC.
- Multichannel, eddy current array, validated on SCC. Designed and tested by TWI Ltd.
- Phased array TRL probe with 2 x 64 element immersion array. Concept designed by TWI. Detail design and manufacture by Vermon.
- Wall climbing robot designed and manufactured by South Bank University. Designed and manufactured to work aside a PWR with nuclear compatible materials and control system.
- Robot arm for scanning inside surface of nozzles after deployment from climbing robot.

A 2.5 MHz Transmit-receive longitudinal phased array (TRLPA) probe was selected with two 64 channel arrays the transmitter and receiver system. The design was optimised using the Peak-NDT Simulus modelling package and the specification was as follows: roof angle 3.1 degrees, scan axis 4 elements with 3 mm pitch, index axis 16 elements with 2mm pitch.

The theoretical ultrasonic beam model Simulus developed by Peak NDT was validated during the course of the RIMINI project. Simulus was then used to model and optimise the Transmit-receive longitudinal (TRL) phased array probe.

Simulus was validated using evidence from experimental data collected at TWI and from past validation programmes. Simulus is primarily a continuous wave model designed to run fast and allow on site personnel to verify their inspection strategy.

The validation programme for Simulus involved previously well characterised single crystals and linear phased array probes. In conclusion, Simulus was validated and showed good agreement to experiment and other models such as CIVA. Simulus will provide a commercially powerful tool.

Overall it was concluded the TRLPA probe demonstrates better signal-to-noise performance when compared to single crystal probes. It is able to detect defects which lie along the weld-nozzle and weld-RPV fusion faces. Two configurations were studied and both proved viable for use in inspection.

Although an array probe is good for inspecting flat plates, it is much harder to inspect the doubly curved geometry around a reactor pressure nozzle. It is not possible to make a rigid array to fit the geometry without having excessive lift-off between sensors and metal surface. On the other hand, a flexible probe would be quite vulnerable to damage when deployed by robot, and difficult to seal against water ingress.

It was therefore decided that a non-array probe be used for the ACFM inspection. This is possible because the robot deployment system was designed to be able to scan such a probe in a well-controlled manner over the nozzle surface once it has docked.

To evaluate ACFM and EC probes a sample of stainless steel cut from the curved outer edge of a nozzle was supplied by Babcock. Surface flaws were introduced by True Flaw.

In order to scan a large area and thereby increase the coverage on each scan and reduce the inspection time an array probe is required. However, to cope with the doubly curved geometry of the reactor nozzle, a conformable (semi flexible) array is required. Several types of probe were evaluated.

The robot uses a common principle to climb; which is to create a negative force to stick the robot to the wall. This is achieved using three sliding suction cups, with the suction created by centrifugal pumps driven by high speed air motors. The key advantage of this technique is that expelling water creates a thrust force when the system is not touching the wall. The force pushes the robot towards the wall till the suction cup becomes attached to the wall.

The wall climbing robot was initially made neutrally buoyant so that very little traction at the wheels is required to climb the wall. However, the scanning robot that the climber piggy backs is not neutrally buoyant. Its mass in air is 55 kg and in water it is 20 kg. This represents a 20 kg negative buoyancy force. The climber compensates for this by adding 20 kg of positive buoyancy. The resulting combined system is then neutrally buoyant and the system climbs on the wall in exactly the same way.

However, when the scanner robot leaves the climber to enter a nozzle, the climber is left with a positive buoyancy force of 20 kg which lifts the robot to the surface. The solution to this problem was to add two underwater high friction suction cups that do not slide to the wall climbing robot. The cups are lowered to the surface by two pneumatic cylinders. Tests showed that the robot can hold itself in a parked state with up to 100 kg of positive buoyancy.

The underwater robot developed by London South Bank University is designed to convey the scanning robot which carries and deploys the NDT sensors to the RPV nozzles.

The objective was to design develop and manufacture a fully functioning submarine robot, capable of carrying and deploying the NDT sensors and systems inside the RPV orifice. It is evident that the robot must be compatible with the LSBU carrier robot described above which travels to RPV nozzle and delivers the 'orifice crawler' onto the orifice opening. Consequently, the system should be able to tackle and operate over all possible curvatures encountered during the travelling of the orifice crawler operation inside.

A principal component in the RIMINI crawler project is the software controller that will guide the mechanical system and will enable it to perform the various tasks for which it was developed. The software provides functionality for tele-operating the robot using position and speed control of all nodes, as well as displaying readings from the various sensory modules and health information such as current draw. Two-way communications implemented using standard RS232 connection using the serial data protocol provided by Maxon. Control is achieved using a client GUI application running on the operator's laptop.

The robot software executes on two interacting computer systems: the robot controllers; and the off-robot PC. The controllers communicate with the off-robot PC via a serial interface (RS232->RS485->RS232->CANOpen). The RIMINI control software was written entirely in the C++ programming language, using a number of in-house, purpose-built, and third-party API libraries.

Overall the inspection-arm MK2 design consists of the sensor holder / positioning subassemblies (EC and ACFM holders, P/A Holders, encoders) and a mechanism that controls the angle / opening of the two arms. The two arms seen in that system are able to rotate around their edge. The rotational Degrees of freedom (DOF) in each arm is driven by a lead screw mechanism actuated by a marinised motor. The principle is similar to the one of an umbrella. By rotating the lead screw a stage is moved back and forth and an articulated mechanism is actuated.

The implementation of the NDT technologies embodied in the RIMINI system will provide higher reliability and repeatability for the inspection of RPV's. Initially this will only be of 'Safe end' welds; however, by modifying the design of the robot it will be possible to use the technologies on many other applications.

Final Report Summary - RIMINI (Development of new and novel low cost robot inspection methods for in-service inspection of nuclear Installation)

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