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·Nuclear Industry | Detection of Thermal Fatigue Cracks in Stainless Steel Pipe: Application to the Inspection of RHRS line of Type N4/PWR
Yves BOUVERET,
Framatome Technical Center, ZIP, BP13, 71380 Saint Marcel Cedex, France, ybouveret@framatome.fr.
Jean Michel TCHILIAN,
Framatome QR, 92084 Paris la Défense Cedex, France, jmtchilian@framatome.fr.
Gilbert THERON,
EDF / SQR Immeuble France-Evry, Boulevard de France BP 128, 91004 EVRY Cedex, France gilbert.theron@edf.fr.
Olivier de VAREILLES,
Intercontrôle, 13 rue du capricorne, SILIC 433, 94583 Rungis cedex.
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ABSTRACT:
Further to a leakage in the Residual Heat Removal System (RHRS) of the PWR at Civaux Unit 1, EDF, Framatome and Intercontrôle initiated a program of thorough expert evaluations. For which purpose ? Improve the design of the elements of 10" and 4' line pipes to reduce the thermal fatigue cracking mechanisms. The development, and the implementation of a dedicated ultrasonic examinations to the detection of the thermal fatigue cracks have had an essential role in this evaluation program.
Introduction
The program of evaluations undertaken by the CNEN and Framatome originated in an incident which occurred in May 1998 in the line A of the Residual Heat Removal System (RHRS) of the N4 model PWR at Civaux Unit 1. The primary water leaked through a 100% through wall crack, located in the weld of a 10" pipe elbow near the main mixing region of the cooling system. The RHRS lines are the cooling systems used to remove the residual heat during cold shutdown. Located in the reactor building, the RHRS is only used when the reactor is shut down. It serves to cool down the reactor, which still generates heat when it is shut down. Similarly to all systems ensuring a major safety function, the RHRS is doubled : line A and line B. When a leakage is detected in one RHRS line in operation, the leaking line may be isolated by closing the insulation valves. The residual heat is then extracted through the other way.
The program of evaluations have been carried out namely by ultrasonic examinations of the components. A specific ultrasonic examination technique has been developed for the expertise of the damaged components. This technique has been improved through numerous examinations and validated by comparison with penetrant test examinations and metallographic examination, after cutting, of damaged zones. The second step has been to define the parameters, as type of transducers and reporting levels, applicable to the PSI and ISI of the new components in order to guarantee the performance of detection requested. The last step was the formal qualification of the PSI and ISI ultrasonic examination technique according to RSE-M 97 prescriptions.
Extended evaluations
Then the national center of nuclear equipment (CNEN) of EDF, in collaboration with Framatome, initiated a program of thorough expert evaluations aiming at a new design of the RHRS. In this region of the system, consisting in a T-junction, a cold source is mixed up with a hot source. The studies carried out by the CNEN and Framatome showed that the thermal fatigue cracking phenomena may be due to complex thermohydraulic phenomena depending on the line geometry, the respective arrangements of the various components (T, elbows, pipes, diaphragms), the hot and cold flow distribution and mixture, and generally the operating conditions. The evaluation of this mixing regions revealed some indications of thermal stratification or crackling leading to cracking along the weld beads.
The program of thorough inspections implemented by Framatome and Intercontrôle, was not limited to the only region comprising a 10" pipe elbow. The investigations were extended to the regions likely to be affected by the temperature variations. In addition to the 10" line, the inspections then covered the "no flow" RHRS line of 4" dia. and the 4" PCTS/RHRS junction.
Ultrasonic examinations
Thanks to such evaluations, the CNEN and Framatome decided to improve the RHRS design. To limit the number of welds in the mixing region, the new lines now include machined and long T-junctions. The elbows made up of welded half-shells are replaced by one-piece elbows. As far as the inside surface condition of the lines is concerned, it was improved to prevent any crack initiation region.
Since 1998, initial inspection and 100 hours inspections have been carried out on the new designed items of the 10" and 4" line pipes. These checks were performed by ultrasonic testing according to the procedures prepared by the Technical Center of Framatome for the RHRS and PCTS lines. So, searching of the thermal stratification or crackling is mainly carried out using 45° refracted shear waves in half skip. Scanning is made in both directions parallel and normal to the line axes.
Searching of the cracks on the edge of the weld beads is made using 60°-angled shear waves in half skip.
For stainless steel RHRS and PCTS systems, the 2.25 and 3.5 MHz frequencies are used depending on the thickness and attenuation. Validation of these examination techniques was carried out by the Technical Center of Framatome on several mockups representative of the components and welds to be examined.
RSE-M 97 qualification of ISI techniques of the RHRS
Electricité de France wanted to obtain the RSE-M 97 qualification of the in-service inspection of the RHRS for the new design of 10" and 4" line pipes of the N4 model PWR. In order to achieve this goal, the Equipment Department of EDF defined the inspection policy in a specification. Whereas the nature of the searched defects remain of course the same as before, their dimensions and the extent of the regions to be examined and the performance to be guaranteed were adapted to the "in-service" follow-up.
The qualification of "in-service" inspections is a "specific" qualification according to the RSE-M 97. Firstly, it consists in compiling a file containing the technical justification, an in-service inspection procedure and a qualification program describing the tests to be carried out on mockups representative of the components to be examined and containing artificial defects.
This file is supplemented by the inspection performance validation, after comparison of the results obtained by UT testing on coupons containing actual crackling defects and cracks on the bead edge (coupons taken from the defective components of the Chooz and Civaux N4 units) with the results of the destructive evaluations of these coupons.
When the qualification file was compiled, the qualification tests were carried out by Intercontrôle under the supervision of the Manufacturing Quality Division of EDF (SQR). The qualification file, qualification test results and a summary as well were presented by the Group of Laboratories of EDF (GDL) to the qualification commission in May 2000.
The qualification commission expressed a favourable opinion. The qualified procedures were implemented by Intercontrôle on the 4" RHRS of the Chooz B2 Unit during the first refueling shutdown.