ULTRASONIC TRANSDUCERS FOR SODIUM COOLED REACTORS..F.Saillant, O.Martin, S.Charrier, J.Sibilo, AREVA, France F.Baqué, CEA - DEN, France

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1 More Info at Open Access Database 1. INTRODUCTION ULTRASONIC TRANSDUCERS FOR SODIUM COOLED REACTORS.F.Saillant, O.Martin, S.Charrier, J.Sibilo, AREVA, France F.Baqué, CEA - DEN, France The framework of the French Act, dated June 28th 2006, which requested an assessment of the industrial perspectives of transmutation by 2012, concluded that Generation IV systems, and especially the sodium-cooled fast reactors, seemed to be the most suitable technologies to be developed. Thus, since 2007, French partners CEA, EDF and AREVA launched a coordinated research program on Sodium cooled Fast Reactors (SFR) which is now being used for the next ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) prototype. Fig. 1. Pre-conceptual Design for ASTRID reactor block Several axes of improvement are targeted in this program [1] [3]. Amongst them, the improvement of the reactor s In-Service Inspection and Repair (ISI&R) is a major transverse issue. This includes: - Safety analysis o checking the state of structures during the reactor s life span o detection of premature failures o in-operation detection of significant failures - Economic reliability (implementation of delays), - Investment protection (repair). Deployment of ISI&R in sodium cooled reactors is not an easy task. Sodium has several characteristics which tend to make ISI&R complex, especially by comparing with that of Light Water Reactors: - Sodium is very reactive with air and water. This leads to monitor carefully the confinement and steam generators leakage (if any), and to control the tightness during the interventions. - Due to the risk of under stress caustic corrosion formation, sodium draining of the reactor is only planned for exceptional interventions. Routine inspections and operations have to be realized in immersion under liquid sodium. - ISI&R devices have to work at high temperature levels. Because of sodium s melting point (97.8 C), the temperature corresponding to plant cold shutdown condition is maintained to a minimum of 180 C. Operation temperatures are comprised between 400 C and 550 C. 920

2 Liquid sodium being opaque, each intervention (inspection / repair) beneath the free surface of sodium cannot be performed using optical devices. Ultrasonic techniques are largely studied to provide means of under sodium investigations [2]. This paper reports on the development and tests of ultrasonic transducers specifically designed for performing Non Destructive Testing (NDT) on structures immersed under liquid sodium. First, the technical objectives will be specified. Then, the experimental setup and the design of transducers will be presented. Finally, experimental results will be shown and discussed. 2. NDT ULTRASONIC TRANSDUCERS FOR IMMERSION UNDER LIQUID SODIUM The harsh environment of Sodium Fast Reactors makes the design of ultrasonic transducers difficult. Next paragraphs will detail the specifications for the different transducer design parameters. 2.1 Temperature NDT of the reactor s structures will be performed during cold shutdown phases. Sodium temperature is then kept at 180 C. This temperature is not conventional for ultrasonic transducers. Especially, the transducer will have to be immersed continuously for periods of time of several days. Cooling down the transducer will also be difficult as it can be immersed under 20m of hot sodium, meaning that the coolant can difficultly be cold when reaching the transducer. Moreover, sodium in the reactor has to be very pure and the risk of having coolant leaking in the sodium primary circuit is not acceptable. Therefore we aim at making a transducer that can work continuously at 200 C (180 C + 20 C margin) without external cooling. Several technologies can be envisaged for generating ultrasound. The choice here was made to use piezoelectric transduction for converting electrical energy into ultrasonic pressure waves, and vice-versa. If the choice is made to use a ferroelectric ceramic, then one has to pay attention that its Curie temperature is not be exceeded so that it remains piezoelectric. High temperatures means that passive materials conventionally used in NDT transducers fabrication cannot be used, their physical integrity could be damaged. Moreover, thermal expansion coefficients of the materials of the different components (backing, matching layers, piezoelectric material ) leads to different strains at the interfaces, generating stresses which may cause adhesion failure and therefore malfunction of the transducer. Electrical connections are also affected by the temperature level. Cabling and soldering have to be chosen carefully. 2.2 Chemical compatibility It was mentioned in Section 1 that sodium was reactive to air and water. Sodium can also be aggressive towards different materials and much care has to be taken for the conditioning of the probe. For instance, epoxies are not all compatible to sodium. All materials used for the external packaging of the probe must be compatible to sodium. Chemical compatibility tests are mandatory to determine the potential use of particular nuances of materials. Materials inside the probe do not necessarily need to be compatible; however, particular attention has to be paid to the sealing of the packaging. 2.3 Acoustic parameters Pure sodium is a metal and one would expect acoustic properties similar to that of steel or alloys. However, the acoustic properties of sodium at 200 C are surprisingly relatively similar to that of water: - Density = kg/m 3 - Velocity = m/s - Acoustic impedance = 2.23 MRayl Consequently, the same design rules as those for underwater immersion transducer can be applied. The problem mainly consists in finding suitable backing and matching layer materials for operating at a continuous temperature of 200 C. The objective in terms of acoustics is to produce a reasonably short pulse shape: a maximum of 4 cycles at -20dB, or a relative bandwith > 40%. Short pulses are necessary to have sufficient resolution during the inspection. 921

3 Controlling the pulse shape is necessary; however, the most critical acoustic issue to be addressed is the acoustic transmission at the interface between sodium and the transducer s emissive face. Indeed, liquid sodium does not wet all material surfaces. By comparison, it has similar behaviour to a drop of mercury on a table which would always remain more or less spherical, rather than spreading flat like water. When a liquid does not wet a surface, it means that there is a thin film of gas in between. Transmission of acoustic energy between solid/gas or between liquid/gas is very inefficient; the acoustic transmission coefficient is in the order of 0.001% compared to optimal state. This means the transducers emissive face should be made from a material that can be wetted at 200 C. Temperature again is an influent parameter in the wetting of materials. For example, sodium does not wet stainless steel at 200 C; however, it wets well at a temperature above 400 C (note that when wetting is achieved, it remains wetted even if temperature decreases). Surface state is also an important parameter. A non oxidized polished surface helps wetting at lower temperature. 2.4 Irradiation Transducers will be exposed to high levels of gamma irradiation. Indeed, unlike PWRs, fuel rods won t be removed when performing NDT inspections. Preliminary calculations stated that when the transducer is located near the assembly rods, it could be exposed to approximately Gy/hr which is considerable. 3. TUCSS TRANSDUCER DESIGN Transducers developed in this project are called TUCSS, french acronym for Ultrasonic transducer for NDT under sodium (Transducteur à Ultrasons pour CND Sous Sodium). We previously reported in [2] on the TUCSS G1, the first generation of TUCSS which showed that we were able to make a transducer emitting ultrasound at a temperature < 180 C and to last for more than 2 weeks in sodium (expected duration of NDE campaign) without degradation. However, its performances were quite low in terms of sensitivity and pulse length. Ø20 polished Nickel emissive surface Seal Metallic housing Elementary transducer with Nickel emissive surface Seal Fig 2. Sketch and photograph of a TUCSS G2 transducer. The transducers presented in this paper, TUCSS G2, are designed to have improved performance, especially in terms of sensitivity. A photograph of a TUCSS is shown in Fig.2. The piezoelectric element is based on PZT material. Its resonant frequency is 2 MHz and the emissive surface is a Ø20 mm diameter disc. Acoustic transmission was obtained by using a polished Nickel front face. An elementary transducer was fabricated and then encapsulated in a metallic housing by means of a silicone seal. Two transducers were fabricated and tested. 922

4 4. EXPERIMENTAL SETUP Under sodium tests were performed at CEA facilities in Cadarache, France. They took place in the glove box named PENELOPE, which had a Nitrogen atmosphere. The objective of the study was to show that it is possible to detect a flaw inside a stainless steel structure immersed under liquid sodium. Fig. 3 illustrates the setup that was used to carry out this demonstration. TUCSS placed against the block Sodium TUCSS Lever Ultrasonic wave 316L stainless steel block Ø2mm BDH Block with BDH Fig 3. Sketch and CAD view of the under sodium NDT demonstration device Indexation finger This device is made of: - A 316L stainless steel block including a Ø2 mm Bottom Drilled Hole (BDH), - A system for locating the transducer so that the BDR would be on the trajectory of the ultrasonic beam, - A system for pressing on the transducer against the block with a controlled force. The block was 140 mm long and the BDR was located 30 mm away from the backwall. The hole was made from the bottom and not drilled through so that there would be no sodium inside (therefore filled with Nitrogen gas). The positioning of the transducer is done by a V-block. Once it is placed on the V-block, an indexation finger pushes it against the block to enable a firm contact. The momentum generated by the mass at the end of the lever allows producing a controlled 10N contacting force. All the parts are mounted on a support plate which is put on the bottom of a 300mm diameter pot filled with sodium. For reasons of clarity, several parts have not been represented on the CAD view of Fig.3. For instance: legs to pull the device in and out of the sodium, a lid to prevent sodium aerosols to diffuse in the glove box, or the slide way to guide the transducer to the V-block. Note that although the temperature of the test is 200 C, the stainless steel block needs to be wetted before trying to sending ultrasound in it. As mentioned earlier, stainless steel wets at 400 C and our TUCSS G2 transducer can only withstand a maximum temperature of approximately 220 C. Wetting of the structure and wetting of the transducer have to be separate and testing has to be carried out according to the following sequence: - Immersion of the test device without the transducer in low temperature sodium (approximately 110 C), - Sodium temperature risen to 400 C to achieve wetting, - Sodium cooled down back to approximately 110 C - Immersion of the transducer at 110 C to minimize thermal shock, - Sodium temperature risen to 200 C. The transducer was excited using a SEPEMA pulser/receive circuit was made of an amplifier (Data precision, Analogic D1000) and a bandpass filter (KH3940). Acquisitions were done using a Lecroy Wave surfer 24MXs oscilloscope. 923

5 5. RESULTS This TUCSS G2 transducer was immersed in the pot containing liquid sodium at 105 C and the test device. Fig. 4 shows the oscillogram obtained from the pulse/echo measurement when the transducer Dead zone at excitation SDH echo Backwall echo 105 C Repetitions of Backwall echo Fig 4. Oscilloscope snapshot showing echoes from the Ø2mm Side Drilled Hole and multiple reflections from the backwall at 110 C was placed in contact against the block s front face. The BDH could clearly be seen as well as multiple backwall echoes. The amplitude of the backwall echo was 20 mv and the echo from the BDH was 2.5mV (square excitation of 50V, gain = 0). The temperature was then increased. The level of noise became much more important, making the identification of the echoes impossible. Fig. 5 shows an oscillogram taken at 130 C. The length of the dead zone is considerably increased. This increase in noise was due to the thermal expansion of the seal around the Ni emissive face. It expended more than foreseen, preventing a close contact between the transducer and the block to be made. This gap consequently generated multiple reflections between the transducer and the block, hence an important amount of noise. Integrity tests of the TUCSS were carried out in liquid sodium up to 200 C. These tests showed that this type of transducer can survive in conditions representative of the In Service Inspection conditions for ASTRID. However, some additional design work is required to make an effective contact between the transducer s front face and the test block. Dead zone at excitation BDH echo 130 C Backwall echo Repetitions of Backwall echo Fig 5. Oscilloscope snapshot showing echoes from the Ø2mm Side Drilled Hole and multiple reflections from the backwall at 130 C 924

6 6. CONCLUSIONS AND PERSPECTIVES The present work is an important step towards the improvement of In Service Inspection of sodium fast reactors. This significant milestone indicates that NDT can be performed in a structure immersed under liquid sodium. The results shown in this paper are work in progress. Optimization of the design of the probe has to be done for operation at 200 C. Once this optimization done, future work will consist in performing NDT in a mock-up that includes representative welds and flaws. This will require special designs for L0, angled or phased array probes. Irradiation has yet not been taken into account: tests are planned for end REFERENCES [1] F. Jadot, F. Baqué, J.Ph. Jeannot, J.M. Augem, G. De Dinechin, J. Sibilo, "Astrid sodium cooled fast reactor program for improving in service inspection and repair," Animma Int. Conf., Ghent, 6-9 June [2] C. Lhuillier, O. Descombin, F. Baqué, B. Marchand, J.F. Saillant, J.M. Augem, "In Sodium Tests of Ultrasonic Transducers," Animma Int. Conf., Ghent, 6-9 June [3] F. Baqué, F. Jadot, F. le Bourdais, J. Sibilo, JM. Augem, O. Gastaldi, ASTRID In Service Inspection and Repair : review of R&D program and associated results FR13, Paris mars