GUIDE TUBE INSPECTION. M.Montero, M.Grataloup, P.Blachon, CEGELEC, France

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1 More Info at Open Access Database CONTEXT GUIDE TUBE INSPECTION M.Montero, M.Grataloup, P.Blachon, CEGELEC, France The SIRIUS equipment has been developed for EDF / UTO to inspect the card control guides component in nuclear reactors. The goal of the equipment is to evaluate by a NDT technique the wear of the guide before performing maintenance (possible replacement of tube guide). To point out the benefit of SIRIUS equipment the context is recalled below. The control of the activity of the nuclear reactor is critical for obvious safety reasons, but also to control the power supplied. The regulation of power is carried out by means of control rods which are inserted directly into fuel assembly (see fig1). The card guides are the components which allow this insertion. The functionality of card guide must guaranty the perfect position of control rod cluster in front of the fuel assembly. The control rod cluster (RCCA) is an assembly of 24 thin tubes (see fig 2) of zirconium of 9,7mm diameter and about 5m long. They are filled with neutron absorber hanged to a spider control part. These clad are inserted in fuel assembly at different depth to regulate the activity or shutdown the reactor. fig 1 fig 2 This set of clad is very flexible and requires a guidance system which allows aligning each ones facing the orifices of the fuel assembly to obtain a perfect insertion. 810

2 During the phase of the full production control clusters are positioned over the fuel assemblies and maintained perfectly in position in the guide tubes in front of the fuel assembly (see Figure 3). The supports are called RCCA guide tube cluster (see Figure 4) cartes RCCA Guide Tube Continuous section Card guide fig 3 fig 4 INSPECTED COMPONENT Tube Guide consist of a welded assembly card guide (24mm in thickness) on the top side and at lower side continuous guide section of length about 800mm. These guide elements (cards and continuous section) are formed by bores vertically aligned fixing in position clads of the cluster control rod. This system aims at power regulation phases but also in case of emergency shutdown of the reactor (automatic control rod drop). The way of reactor activity regulation by inserting the control rods with small movements in the assembly fuel, the vibration of clad, the water flow circulation of reactor naturally lead to the wearing of the holes by friction. The clearance due to wear could lead to a misalignment between the clad and openings of fuel assemblies, with a risk of blockage during the insertion phase. As part of maintenance programs and extend life of nuclear power stations EDF / UTO has set up an inspection program of tube guides to determine the status of these components related to the safety. INSPECTION ENVIRONMENT Constraints associated with these tests are mainly related to the environment. The inspection must be carried out, tube guides installed in the Upper Internal Assembly (see Figure 5), and immersed in the pool of the reactor building by 20m deep. Tube guides clusters are highly activated and the high dosimetry rate fig 5 811

3 WEAR DEGRADATION DEFINITION Here after the solicited area of card guide and zone to measure (in red see fig 6). EDF/UTO requirements for measurement are the followings, diameter, residual ligament and slot width. All measurements should be done at the middle of thickness, and 3 slice profiles must be done in continuous section. To take into account the dimensions to measure, it was essential to propose a technique to fulfill all requirements. fig 6 As techniques based on vision (cameras) cannot carry out measurements inside the cards or in continuous section we have decided to work with an ultrasound technique. The principle is show on fig 7. An UT beam is generated from the center of bore to the edge, to measure de distance r. If the bore hasn t degraded by a wearing this distance is r Constant in other case the variation is related to the wearing. NDT TECHNIQUE fig 7 With a beam rotation it s possible to build a complete profile. As the nominal diameter of bore is 10,67mm the design of UT measurement device introduces a mirror beam deviation (See fig 8). UT transducer Transducer 45 Mirror UT Beam Card guide fig 8 812

4 As twelve bores must be inspected at same time we design the equipment with retractable capabilities. MEASURING PERFORMANCES AND INFLUENT PARAMETERS The accuracy of measurements is theoretically affected by the influent parameters. The following type of influent parameter has been evaluated: Component and defect UT measuring technique and calibration Measuring unit position in the card and geometry UT equipment Environment To determine the theoretical accuracy we have studied 33 parameters. The uncertainties related to these influent parameters lead to the following performances which have confirmed during qualification process. Measure Uncertainty Diameter Ligament Slot Width U globale (Ø) =+/- 0,141mm U globale (Lig) =+/- 0,204mm U globale (LF) =+/- 0,492mm SIRIUS EQUIPMENT The 12 measuring unit are retracted at the introduction of equipment in the tube guide and deployed before to reach the measuring position. To achieve this mechanical function it was designed a device like "sugar tongs" based on parallelograms (see fig 9). fig 9 The mirror rotation is achieved by conical gear set with calibrated stiffness axes (see fig 10) to ensure permanent contact on 12 transmission lines. fig 10 The equipment introduction is done by the cover top plate of tube guide through a hole of 58,6mm (see fig 11) the total size of equipment is 56mm. Transmission axes 813

5 Tube Guide Centering device fig 11 Alignment between equipment and tube guide is done by means of centering device with locking ring (see fig 11) and fine adjustment with conical part (see fig 12). Tube Guide Centering device fig 12 The alignment of slot in card and arms of measuring system (see fig 13) is obtained by a frontal camera at the bottom of measuring system. (see fig 14). Clearance between arms and slot in card is about 1mm at 5m of distance from the top of tube guide. fig 13 fig 14 The measuring system is connected to a matt of 6m long which is used for translation along the tube guide. This is done by the means of control loop motor installed on centering device (see fig 15). fig

6 INSPECTION CONTROL SOFTWARE Automated movement cycles that optimize the inspection time. The software is also used for secure operation of the tool. Software continuously records images and the dynamic torque required to move. In case of over-torque especially in the approach area zones (introduction of measuring unit in cards) an automatic shutdown of the descent is enabled, then retract movements and new insertion are started to ensure the integrity of the devices and in all circumstances allow the extraction of tool from tube guide. The software allow a reel time visualization of measuring system inside de tube guide and inspection plan status (see fig 16) Plan des EIS avec les guides de grappes contrôlés ANALYSIS SOFTWARE fig 16 Analysis software has been developed to calibrate and measure the different values of guide. This software performs automated dimensions measurements required through complex processing algorithms signal (see fig 17 et 18). Zone des cartes General view of measurements in cards and continuous section fig

7 RESULTS AND PERSPECTIVES Profile characterization at center of card fig 18 SIRIUS equipment has been implemented on 8 sites. We have inspected about 500 guide tubes 1300Mw or 900Mw. Relevant results offered by the device convinced our customer to extend the inspection to more sites. SIRIUS is the first equipment to give simultaneously 12 measurements of degradations wear in guide tube allowing relevant information to utilities to proceed with maintenance action in association with other criteria. The perspective for the future is to extend the inspection to other sites and countries and to implement the equipment few cycles after guide tube replacement to evaluate the kinetic of degradation. 816