DEVELOPMENT AND DEPLOYMENT OF BOTTOM MOUNTED INSTRUMENTATION (BMI) NOZZLE INSPECTION

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1 More Info at Open Access Database DEVELOPMENT AND DEPLOYMENT OF BOTTOM MOUNTED INSTRUMENTATION (BMI) NOZZLE INSPECTION 0. ABSTRACT O. Burat, M. Taglione, F. Jacquinot, AREVA, France C. Gibert, EDF - CEIDRE, France This paper deals with the recent developments, the qualification and the first deployments of the new BMI (Bottom Mounted Instrumentation) Nozzles Inspection Solutions for the French 900, 1300 MWe and N4 NPP (Nuclear Power Plants). AREVA had performed the first Inspection of the BMI Nozzles in 1993, but this was improved with a first RSEM qualification (2005) and after a new call to tender by EDF, in 2009: EDF decided to increase their inspection program through Most BMI nozzles are made of 600 alloy material which is susceptible to SCC (Stress Corrosion Cracking). Qualified UT (Ultrasonic Testing) by TOFD (Time Of Flight Diffraction) methods allow detecting these cracks, including the nozzle to weld interface examination. This RSEM qualified Inspection includes additional control by ID 0 LW (Longitudinal Waves) UT coupled with VT (Visual Inspection), for both the nozzle ID and OD surface. 1. INTRODUCTION In the context of ageing and life extension of the NPP, Inconel 600 components and alloy 182 weld material require particular attention, due to its susceptibility to SCC (Stress Corrosion Cracking). This has been known since the 1980s. As early as 1993, AREVA has performed In Service Inspection (ISI) of the Inconel 600 Bottom Mounted Instrumentation (BMI) Nozzles for EDF French 900, 1300 MWe and N4 Nuclear Power Plants (NPP). In this paper, we present firstly the component to be inspected, followed by the history of this inspection, a description of the latest requirements from EDF, the NDT methods developed and the equipment deployed to achieve the new target inspection, then lastly, we show the performance of the qualified process and tooling associated with Inspection Implementation. 2. BMI NOZZLE A Bottom Mounted Instrumentation nozzle is a tube welded into the reactor vessel bottom. Its role is to allow the introduction of nuclear instrumentation into the core of the reactor. A leak in the BMI tube or J- weld constitutes a break of the primary coolant system. Fifty BMI nozzles are typically welded at the bottom of the vessel for a 900MWe unit and there are 58 for a 1300MWe unit. Inspection of the BMI nozzle is performed from inside the vessel and it requires removing the Reactor Pressure Vessel (RPV) Internals (Figure 1). 826

2 Inconel 600 BMI 15,4m 38,1mm J-Groove Inconel Weld Stainless Steel Carbon Steel Inconel Buttering Figure 1 : BMI Forest, with internals removed Figure 2 : BMI Nozzle configuration The union between the bottom of the vessel (16MND5 carbon steel, with a 308 or 309 stainless steel cladding added) and the BMI Nozzle (Inconel 600) is formed by a buttering and a J-groove weld (alloy 182) (Figure 2). 3. HISTORY Starting in 1993, EDF began to require ISI of these components and AREVA has inspected about 2 plants per year until In United States, a first inspection (including nozzles and J-welds) and repair was performed in 2003, due to a leak of 2 BMIs in the South Texas Nuclear Station [1, 2]. After an RSEM qualification in 2005, EDF decided (in 2009) to increase inspection program of BMI Nozzles for 900, 1300 MWe and N4 NPPs through 2016, with a second RSEM qualification. In 2011, an indication was discovered in Gravelines [3]. This prompted the qualification completed in January This improved inspection has been deployed on six RPV and is planned to be implemented on three additional RPVs before the end of INSPECTION REQUIREMENTS The areas to be examined are: The entire wall thickness of the nozzle base metal. The interface base metal / weld. The examination doesn t include the volume of the weld. The axial extend of the examination volume, defined in Figure 3 is as follows: On the nozzle tube inner wall, the area extending from 15mm above the high point of the weld to 50mm below the low point of the weld. On the nozzle tube outer wall, the area following the profile of the weld, including a band between 15mm above the high profile (transition radius) and 15mm below the low profile of the weld. 827

3 3 1 2 Figure 3 : Zones of the nozzle to be examined Figure 4 : Researched defects The defects to be detected (Figure 4) in the base metal are : Internal Longitudinal (IL) Stress Corrosion Cracks (SCC), deeper than (or equal to) 3 mm with a length more than (or equal to) 10 mm (1). External Longitudinal (EL) and Circumferential (EC) SCC, deeper than (or equal to) 4 mm with a length more than (or equal) 10 mm (2). A complementary inspection is made at the level of the interface base metal to the weld (3), if one of the following defects is found : Longitudinal SCC started on the ID, deeper than (or equal to) 7 mm with a length more than (or equal to) 10 mm. Longitudinal or circumferential SCC started at the OD, deeper than (or equal to) 4 mm with a length more than (or equal to) 10 mm. 5. NDT METHODS Two steps of inspection are deployed: A basic and systematic inspection of each nozzle, considered RSEM. This step is led by UT Time Of Flight Diffraction (TOFD) Longitudinal (L), whose objective is to detect SCC in the base metal of the nozzle. A complementary inspection, depending on the basic inspection results. This step is led with : o UT TOFD - Longitudinal and Circumferential (C) probe, (this complementary inspection is also considered RSEM), o Plus a 0 UT Longitudinal wave probe, o Plus a Visual Testing (VT) probe, after brushing to remove surface deposits inside the nozzle, o Plus an external VT probe. The UT TOFD technique overview is described in Figure 5 : Without a defect (or with an external defect), this technique shows a continuous echo (lateral wave 1). Faced with a crack, the UT beam is diffracted by the tip of the crack and some of this diffracted energy returns to UT receiving transducers (2). The UT wave Time Of Flight (TOF) is linked with the depth of this defect. 828

4 A second continuous echo (backwall echo - 3) is present, only interrupted by an external defect or the base metal weld interface. Figure 6 a: UT TOFD L and C Probes Figure 5 : UT TOFD Technique Overview Figure 6-b: 0 LW UT probe The two types of UT TOFD probes developed for the BMI nozzles inspection are shown in Figure 6-a. For the UT TOFD L probe, orientation of the two sensitive crystals is fit to characterize axial flaws while the orientation for the UT TOFD C probe is fit to characterize circumferential flaws. 0 Longitudinal Waves (0 LW) presented in Figure 6 b is designed for weld profiling and characterization of Lack Of weld Fusion (LOF) at the base metal to weld interface (such functionalities are also provided by the two UT TOFD probes). VT inspection is used to improve the diagnosis, when a surface indication is detected, and/or to complement the exam where disturbances like surface damage surface or acoustic noise preclude clear interpretation of the UT signals. 6. EQUIPMENT DEVELOPED AND DEPLOYED Besides the UT and the VT probes described above, the deployed equipment includes: The manipulator OBTEUS (see Figure 7), for the Inner Diameter (ID) inspection includes both UT and VT sensors. The manipulator ensures coordinated rotation and translation movement of the UT transducer and optimized positioning of the VT equipment inside the nozzle. The manipulator OTEVEX (see Figure 8) has been developed for OD visual inspection. It allows VT equipment positioning outside of the BMI Nozzle. It can be noted that these manipulators are qualified for Foreign Material Exclusion (FME). Other supporting equipment is linked to the tool via a cable bundle and is positioned on the floor reactor cavity. These equipment include : o the carrier OBTEUS remote control, o the UT acquisition hardware (Z-scan from ZETEC), associated with a PC containing the qualified UT software Ultravision, o forty meter cable bundle linking equipment operating in the vessel and these OBTEUS remote controllers and UT acquisition hardware / software. 829

5 Figure 7 : Manipulator OBTEUS Figure 8 : Manipulator OTEVEX 7. QUALIFICATION PROCESS The RSEM qualification objectives are to : Check accessibility of every BMI Nozzle, whatever its position in the bottom of the RPV. Confirm UT data quality to allow a clear diagnosis, considering the health of the inspected BMI nozzle. Check NDT performances in term of detection / characterization. Confirm repeatability and reproducibility of NDT results. Validate the inspection duration (inspection rate for basic UT inspection) Qualification Principle This qualification principle is to show and justify the performances of : The base metal inspection, on the one hand. The complementary interface base metal / weld inspection on the other hand. Base metal inspection includes area coverage, SCC detection (see 4), location, depth and length measurement, while complementary interface inspection concerns area coverage, detection, location, surface measurement and weld profile definition. These performances have been evaluated in two conditions: Best case performances considering a perfect and ideal environment. Reliability and reproducibility performances taking into account the presence of complicating parameters (CP) considering the component (including nozzle distortion and rough surface conditions), the equipment deployed (ex. : UT probes and manipulator characteristics), extending to the specific type of Stress Corrosion Cracks (ex. : morphology, ID/OD wall initiation, etc). 830

6 To supply these two conditions, a series of mock-ups (without and with CP) was defined and manufactured. Reflectors representative of targeted flaws were implanted in these samples. Performance evaluations of the techniques described above have been achieved based on experimental testing (see Figure 9, examples of experimental test results that are exploited for detection and characterization of external notches detected with the UT TOFD L probe). Reference nozzle Acoustical Figure 9 : B scan cartography of external notches To improve the experimental validation, inspection models using CIVA software were developed to predict the UT performance allowing detectability to be inferred based on a limited number of implanted sample flaws. An example of the UT TOFD signal taking the expected morphology of a reflector into account is shown in Figure Qualified UT Performances Figure 10 : CIVA modeling configuration and D scan result Considering the inspection requirements (described in 4), the qualified UT performances (including the contribution of every complicating parameters) are the following : Area coverage is in accordance with the requirement, excepted for some nozzles of the N4 PWR, for which the area extension is limited to 10mm below the low point of the weld, Base Metal Stress Corrosion Cracks (SCC) to be detected, that s to say : o Internal Longitudinal (IL) SCC (ID SCC) deeper than (or equal to) 3 mm with a length more than (or equal to) 10 mm, o External Longitudinal (IL) and Circumferential (IC) SCC (OD SCC) deeper than (or equal to) 4 mm with a length more than (or equal to) 10 mm, can be detected, Base Metal SCC detected can be sized, if their depth are between : o 3 and 9 mm, for ID SCC o 4 and 8 mm for OD SCC. Deeper SCC are said to be potentially crossed SCC Depth and Length and locating SCC accuracies are given in the table below. These are systematically better than accuracies that are required. 831

7 Type of defect Inspection Requirements ID Long SCC OD Long SCC OD Circ SCC Height Accuracy ±1,5 mm ±0,50 mm ±0,50 mm ±0,50 mm Length Accuracy (mm) ±5 mm -2.5 mm / +3 mm -0.5 mm / +5 mm -1.5 / +3 mm Longitudinal Location ±5 mm ±2 mm ±2 mm ±2 mm Azimut Location +/-30 +/-15 +/-15 +/-15 Table 1 : Height, Length and Location accuracies of SCC detected (requirements and performances) Interface flaws to be detected and sized (diameter more than (or equal to) 3 mm) can be detected and sized, Interface flaws surface sizing accuracy is -0/+1,5 mm; Required surface sizing was -0,5/+1,0 mm. 8. INSPECTION IMPLEMENTATION The equipment implementation conditions during an inspection includes the following : The pool is full during the inspection. The water is necessary to shield bridge workers from high levels of radiation inside the vessel. The refueling machine is used during the operation of installation (see Figure 11). Once the appropriate probe is attached to the tool and placed in-line with the calibration nozzle, the manipulator is lowered into the reactor vessel cavity and positioned on the nozzle to be inspected. With the help of a funnel, it is centered (to ensure the proper alignment for the probe to be inserted without damaging the nozzle) and fixed to this nozzle. For each UT probe, the initial tool setup requires calibration of the tool motors and encoders. The UT probe is then driven down to the designated start position. Once in position, the examination begins by a continuous helical rotation movement of the UT probe. The UT calibration standard is ultrasonically scanned as the probe exits the top of each BMI nozzle. Figure 11 : Use of the refueling machine during the inspection operation The duration of a vessel bottom inspection is approximately 112 hours as broken down below: Installation of the equipment in the reactor building (24h). Inspection duration (56h), including : o initialization in reactor vessel, 832

8 o UT examination, o withdrawal of the equipment from the reactor vessel (including a vessel bottom VT inspection). Removal (32h), including decontamination and packing. 9. INSPECTION EVOLUTION AREVA NDE Solutions has already performed six inspections following the RSEM Qualification, which was obtained in January The main milestone events for these inspections are : The implementation with the first set of equipment by INTERCONTROLE - AREVA NDE - Solutions at Tricastin 2 in March The use of second equipment at BUGEY 3, deployed in June After six inspections already achieved, three more are planned before the end of this year. Third equipment is expected to be qualified by EDF for It will allow AREVA to make three simultaneous operations. REFERENCES [1] NRC Bulletin NUREG 1863, Leakage from Reactor Pressure Vessel Lower Head - Penetrations and Reactor Coolant Pressure Boundary Integrity, August 23, 2003, OMB control No: [2] Inspection Technology for BMI Penetrations (M.S. Lashley, South Texas Project, S. W. Glass and R.F. Cole, Framatome ANP Inc.) Conference on Vessel Penetration Inspection, Crack Growth and Repair - Washington Center Gaithersburg, MD (September 29 October 2, 2003) [3] Note d information de l ASN (20/12/2011), concernant la Réparation de défauts détectés dans une PFC du réacteur de Gravelines 1 833