Other Major Component Inspection II

Underwater Remotely Operated Vehicle System for Core Internals Inspection of Nuclear Power Plant

M. Tooma, H. Mori, M. Senoo, Energy and Environmental Systems Laboratory, Japan Y. Takatori, M. Sonobe, K. Kimura, M. Koike, Hitachi- GE Nuclear Energy, Japan

ABSTRACT

An underwater remotely operated vehicle system for core internals has been developed. This system is composed of remotely operated vehicle, automatic inspection data acquisition system and position detecting system. The underwater vehicle and its systems have been confirmed to offer sufficient moving performance and measuring accuracy of moving distance for core internals inspection. Subsequently, the vehicle and its systems were applied to actual nuclear power plants, where they are making a contribution to reduction of the number of inspection tasks and the time needed to carry them out.

INTRODUCTION

Electrical power demand as well as an increasing number of aged nuclear power plants requires in-serve inspection and core internals inspection to cover various structural components for plant reliability [1,2]. Stress corrosion cracking (SCC) has been found on the shroud and shroud support of nuclear power plants. In core internals visual inspection has generally been performed with a suspended camera or swimming type remotely operated vehicle with CCD camera [3,4]. If SCC was found, the SCC length and depth sizing were done by eddy current inspection and ultrasonic inspection using a mast type scanning mechanism. However, using the mast type scanning mechanism requires many preliminary and supplemental tasks and many operators. In order to reduce inspection tasks and the time needed to do them, we developed an underwater vehicle system with its position detecting system and its accompanying automatic inspection data acquisition system that can perform eddy current and ultrasonic inspections.

PRINCIPLE AND APPARATUS

Figure 1 shows underwater remotely operated vehicle (ROV) system. Figure 2 shows system configuration of ROV system. This system is composed of remotely operated vehicle, automatic inspection data acquisition system and position detecting system. Our underwater vehicle can attach itself to a wall, such as a shroud or a shroud support surface, and move on the wall. The attachment force, i.e. a negative pressure, is generated by water ejected from underneath the vehicle by two thrusters. The underwater vehicle has two wheels which allow it to move and rotate on the wall by speed control. Moving distance and attitude of the vehicle is controlled by a feedback control algorithm. The vehicle has an automatic inspection data acquisition system which can control a flexible multi-coil eddy current inspection system and a phased array ultrasonic inspection system. The vehicle system is also equipped with a position detecting system which provides a backup confirmation of the inspection position and moving distance of the vehicle. The vehicle position is calculated by a triangulation method that detects four LED lights on the back side of the vehicle using an external camera. The underwater vehicle and its installed systems were verified to be able to acquire inspection data in actual plant environments and they could deal with such issues as radiation dose, water depth and configuration of core internals.

Core Plate

ROV Top Guide

Shroud Camera

【 Position Detecting System 】【Remotely Operated Vehicle】

ECTUT

Shroud

Thruster

Water Flow

driving

Travel Distance

Travel Distance

Camera Image

LED Light

Trajectory

Initial FinalPosition

Evaluation Result

Center Evaluation

Image Processing Binarization

Distortion Correction

LED Light

Figure 1 - Underwater Remotely Operated Vehicle System

ROV (with Feedback Control )

Travel Distance, Pitch; Odometry and Attitude

ROV System

Controller

Automatic Inspection

Data Acquisition System

Multi Coil ECT

Phased Array UT

Position Detecting

System

Camera

(Radiation-proof)

Figure 2 - System Configuration of ROV System

RESULTS AND CONCLUSION

Figure 3 shows experimental results of ROV moving performance, i.e. travel distance accuracy, meandering and changing of attitude angle. Accuracy of travel distance and meandering were measured and evaluated by scale and eddy current testing method. Changing of attitude angle was measured and evaluated by scale and tiltmeter in underwater vehicle. Figure 4 shows image acquisition and processing methodology, and evaluation principle of travel distance. Travel distance can be evaluated by displacement for gravity center of four LED lights. Figure 5 shows image processing of distortion correction. Barrel distortion, unbalance distortion and optical center can be corrected by image processing. Figure 6 shows position detecting results of ROV system. Initial position, final position and trajectory of underwater vehicle can be evaluated by this position detecting system.

The underwater vehicle and its systems have been confirmed to offer sufficient moving performance and measuring accuracy of moving distance for core internals inspection. Subsequently, the vehicle and its systems were applied to actual nuclear power plants, where they are making a contribution to reduction of the number of inspection tasks and the time needed to carry them out.

Initial Point

Angle

dyw

Final Point

Travel Distance

dθ

X：Travel Distance [mm]

+1.8mm

90deg90deg90deg

-1.8mmTravel Distance 200mm

< ±1[deg]

Y[m

m]

dθ

Meandering

Attitude

Figure 3 - Experimental Results of ROV Performance

x

y

Coordinate

(xcg0,ycg0)ΔＸ = xcg1 – xcg0

ΔY = ycg1 – ycg0

LED

Camera

Camera Image

LED Light

Center Evaluation

Image Processing

BinarizationDistortion Correction

Sub-pixel Analysis

LED Center

Initial Position Final Position

Center of Gravity (xcg1,ycg1)Evaluation of Travel

Distance

ROV

Figure 4 - Position Detecting System for ROV

Before Distortion Correction

After Distortion Correction

Correction; Barrel Distortion,

Unbalance, Optical center

Figure 5 - Image Processing of Distortion Correction

Initial Position

Final Position

Trajectory

Closeup

Circumferential Direction

Initial Position

Final Position

Trajectory CU

Axial Direction

Figure 6 - Position Detecting Results of ROV System

CONCLUSIONS

We have developed an underwater remotely operated vehicle system for core internals. This system is composed of remotely operated vehicle, automatic inspection data acquisition system and position detecting system. The underwater vehicle and its systems have been confirmed to offer sufficient moving performance and measuring accuracy of moving distance for core internals inspection. Subsequently, the vehicle and its systems were applied to actual nuclear power plants, where they are making a contribution to reduction of the number of inspection tasks and the time needed to carry them out.

REFERENCE

[1] Masahiro Tooma, Yutaka Kometani and Kojiro Kodaira, “Inspection Technology for Nuclear Power Plant”, Systems, Control and Information Vol.53, No.3(2009).

[2] Mitsuru Odakura, Yutaka Kometani, Masahiro Koike, Masahiro Tooma amd Yoshiaki Nagashima, “Advanced Inspection Technologies for Nuclear Power Plants”, HITACHI HYORON, Vol.91 No.02 (2009).

[3] Yosuke Takatori1, Kenichi Otani1, Kazuhiro Kimura1, Yoshio Nonaka1, Masahiro Tooma and Satoshi Okada, “Development of Underwater Vehicle System for Reactor Internals Inspection”, 7th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, P828 (2009).

[4] Satoshi Okada, Ryosuke Kobayashi, Hiroshi Yamamoto and Yukihiko Ono, “Development of Tandem Underwater Vehicle System for Narrow Section Inspection”, Journal of Robotics Society of Japan Vol.26 No.6 (2008).