Eric Rosenkrantz

Correction of diffraction effects in sound velocity and absorption measurements

The mean pressure over a receptor placed at a small distance from a circular source is calculated in order to estimate the effect of diffraction when the medium is transparent or slightly absorbing. The deviation relative to the propagation without diffraction is written in the form of a simple integral of the radial wave vector. The source and receiver are considered of the same size, but the expression for different diameters is given. Approximate analytical expressions are derived when the diffraction effect is small, i.e., when the source is much larger than the wavelength. The analytical expressions are used to show that diffraction corrections can easily be performed in a standard pulsed echo experiment.

Acoustic sensor for in-pile fuel rod fission gas release measurement

Innovative in-pile instrumentation is crucial for advanced experimental programs in research reactors. In this field, we developed a specific acoustic sensor to improve the knowledge of fission gas release in Pressurized Water Reactor (PWR) fuel rods when irradiated in Material Testing Reactors (MTR). In order to perform experimental programs related to the study of the fission gas release kinetics, the CEA (French Nuclear Energy Commission) acquired the ability to equip a pre-irradiated PWR fuel rod with three sensors, allowing the simultaneous on-line measurements of the following parameters: 1) fuel temperature with a centreline thermocouple type C 2) internal pressure with a specific counter-pressure sensor, 3) fraction of fission gas released in the fuel rod with an innovative acoustic sensor. The third detector, which has been developed and is patent pending by CEA, SCK·CEN (Belgian Nuclear Research Center) and IES (French research laboratory of Montpellier II University and French National Research Center), is the subject of this paper. This original acoustic sensor has been designed to measure the molar mass and pressure of the gas contained in the fuel rod plenum. For in-pile instrumentation, the fraction of fission gas, such as Krypton and Xenon, in Helium, can be deduced on-line from this measurement. The principle of this non destructive and on-line acoustical sensor is the following: a piezoelectric transducer generates acoustic waves in a cavity connected to the fuel rod plenum. The acoustic waves are propagated and reflected in this cavity and then detected by the transducer. The data processing of the signal gives the velocity of the acoustic waves and their amplitude, which can be related respectively to the molar mass and to the pressure of the gas. The piezoelectric material of this sensor has been qualified in nuclear conditions (gamma and neutron radiations). The complete sensor has also been specifically designed to be implemented in MTR conditions. For this purpose some technical points have been studied in details: 1) fixing of the piezoelectric sample in a reliable way with a suitable signal transmission, 2) size of the gas cavity to avoid any perturbation of the acoustic waves, 3) miniaturization of the sensor because of narrow in-pile experimental devices, 4) appropriate cables to transmit high frequency signal under nuclear conditions.

Ultrasonic method for nuclear fuel rods pressure and gas composition released measurement

The objective of this work was to develop a non-destructive acoustic method giving an easy access to two important pieces of information in the irradiated fuel rods: the pressure and the composition of the internal gas mixture in the upper plenum of a standard LWR fuel rod. A first attempt started in 1993 made possible the development of a focused sensor, able to inject acoustic power from a piezoelectric transducer, through the fuel rod cladding and excite the internal gas mixture. This step was achieved in 1999 and covered by a first patent [1]. However, the initial problem was not totally solved because of the presence of the upper spring in the LWR fuel rod. This spring mainly induced a large decrease in the acoustic amplitude response and this solution was not applicable. The method has been revised recently for several aspects: the design of the sensor to optimize the acousmtic power injection through the cladding rod, and the entire experimental protocol, including the signal processing in the time and frequency spaces. A single λ/2 layer (instead of the standard solution in λ/4) of low impedance material (compared to the transducer and tube wall impedance) as water, was found to be a better matching layer.

An Innovative Acoustic Sensor for In-Pile Fission Gas Composition Measurements

In this article we propose a new method able to determine the fission gas composition using in situ ultrasonic waves measurements. To do so an acoustic resonator was connected to a fuel rodlet, in order to perform speed of sound measurements of gas mixture (Helium and fission gases) inside the plenum. By using a dedicated signal processing the peaks due to resonant frequencies inside the gas mixture were successfully extracted from the output signal. From these data, the variations of helium and fission gas molar fraction were calculated using an adapted virial state equation. It will be proved that these data provide important information about the kinetics of gas release and about the effects of high neutron and gamma irradiation on piezoceramic sensors.

Piezoelectric thick film sensors: Fabrication and characterization

The screen printing of piezoelectric materials such as Pb(Zrx,Ti1-x)O3 (PZT) offers a wide range of possible applications for the development of acoustic sensors and the measurement of small size deformations. Material properties characterizations of these PZT thick-films are essential for devices design and applications. To optimize the fabrication process, we measured the electrical properties for different ranges of frequencies and temperature -from 10μHz up to 13MHz between 30 and 400°C-of these thick-films. The piezoelectric coefficients d33 and pyroelectric coefficient are also investigated. Various thicknesses of (PZT) thick films were deposed, up to 60 μm by screen printing process. The deposits were obtained by superposition of several layers thick of 20 microns. We implemented different screen printing processes firing and co-firing duration, controlled temperature rate to bring the better piezoelectric material as possible. We will highlight the use of these thick films to determine liquid properties.

REMORA 3: The first instrumented fuel experiment with on-line gas composition measurement by acoustic sensor

With the aim to improve the knowledge of nuclear fuel behaviour, the development of advanced instrumentation used during in-pile experiments in Material Testing Reactor (MTR) is necessary. To obtain data on high Burn-Up MOX fuel performance under transient operating conditions, especially in order to differentiate between the kinetics of fission gas and helium releases and to acquire data on the degradation of the fuel conductivity, a highly instrumented in-pile experiment called REMORA 3 has been conducted by CEA and IES (Southern Electronic Institute — CNRS — Montpellier 2 University). A rodlet extracted from a fuel rod base irradiated for five cycles in a French EDF commercial PWR has been re-instrumented with a fuel centerline thermocouple, a pressure transducer and an advanced acoustic sensor. This latter, patented by CEA and IES, is1 used in addition to pressure measurement to determine the composition of the gases located in the free volume and the molar fractions of fission gas and helium. This instrumented fuel rodlet has been re-irradiated in a specific rig, GRIFFONOS, located in the periphery of the OSIRIS experimental reactor core at CEA Saclay. First of all, an important design stage and test phases have been performed before the irradiation in order to optimize the response and the accuracy of the sensors: — To control the influence of the temperature on the acoustic sensor behaviour, a thermal mock-up has been built. — To determine the temperature of the gas located in the acoustic cavity as a function of the coolant temperature, and the average temperature of the gases located in the rodlet free volume as a function of the linear heat rate, thermal calculations have been achieved. The former temperature is necessary to calculate the molar fractions of the gases and the latter is used to calculate the total amount of released gas from the internal rod pressure measurements. — At the end of the instrumented rod manufacturing, specific internal free volume and pressure measurements have been carried out. Preliminary calculations of the REMORA 3 experiments have been performed from these measurements, with the aim to determine free volume evolution as a function of linear heat rate history. — A tracer gas has been added to the filling gas in order to optimize the accuracy of the helium balance at the time of the post irradiation examination. The two phases of the REMORA 3 irradiation have been achieved at the end of 2010 in the OSIRIS reactor. Slight acoustic signal degradation, observed during the test under high neutron and gamma flux, has led to an efficiency optimization of the signal processing. The instrumentation ran smoothly and allowed to reach all the experimental objectives. After non destructive examination performed in the Osiris reactor pool, typically gamma spectrometry and neutron radiography, the instrumented rod and the device have been disassembled. Then the instrumented rod has been transported to the LECA facility in Cadarache Centre for post irradiation examination. The internal pressure and volume of the rodlet as well as precise gas composition measurements will be known after puncturing step performed in a hot cell of this facility. That will allow us to qualify the in-pile measurements and to finalize the data which will be used for the validation of the fuel behaviour computer codes.

Full-scale hot cell test of an acoustic sensor dedicated to measurement of the internal gas pressure and composition of a LWR nuclear fuel rod

A full-scale hot cell test of the internal gas pressure and composition measurement by an acoustic sensor was carried out successfully between 2008 and 2010 on irradiated fuel rods in the LECA-STAR facility at CADARACHE Centre. The acoustic sensor has been specially designed in order to provide a non-destructive technique to easily carry out the measurement of the internal gas pressure and gas composition (mainly Helium-Xenon mixture, with a small amount of Krypton) of a LWR nuclear fuel rod. This sensor has been achieved in 2007 and is now covered by an international patent. We performed the gas characterisation contained in irradiated fuel rods. The acoustic method accuracy is now ±5 bars on the pressure measurement result and {±}0.3% on the evaluated gas composition. The results of the acoustic method were compared to puncture results (destructive sampling). Another significant conclusion is that the efficiency of the acoustic method is not altered by the irradiation time, and possible modification of the cladding properties. The sensor-operating characteristics have not been altered by a two-year exposure in the hot cell conditions. These results make it possible to demonstrate the feasibility of the technique on irradiated fuel rods. The transducer and the associated methodology are now operational.

Thick films acoustic sensors devoted to Material Testing Reactor environment measurements

The development of advanced instrumentation for in-pile experiments in Material Testing Reactor (MTR) constitutes a main goal for the improvement of the nuclear fuel behavior knowledge. An acoustic method for fission gas release detection was tested with success during a first experiment called REMORA 3 in 2010 and 2011, and the results were used to differentiate helium and fission gas release kinetics under transient operating conditions. The maximal temperature on the sensor during the irradiation was about 150 °C. In this paper we present a thick film transducer produce by screen printing process. The screen printing of piezoelectric offers a wide range of possible applications for the development of acoustic sensors and piezoelectric structure for measurements in high temperature environment.

Irradiation behavior and post-irradiation examinations of an acoustic sensor using a piezoelectric transducer

The development of advanced instrumentation for in-pile experiments in Material Testing Reactor constitutes a main goal for the improvement of the nuclear fuel behavior knowledge. In the framework of high burn-up fuel experiments under transient operating conditions, an innovative sensor based on acoustic method was developed by CEA and IES (Southern Electronic Institute).This sensor is used to determine the on-line composition of the gases located in fuel rodlet free volume and thus, allows calculating the molar fractions of fission gases and helium. The main principle of the composition determination by acoustic method consists in measuring the time of flight of an acoustic signal emitted and reflected in a specific cavity. A piezoelectric transducer, driven by a pulse generator, generates the acoustic wave in the cavity. The piezoelectric transducer is a PZT ceramic disk, mainly consisting of lead, zirconium and titanium. This acoustic method was tested with success during a first experiment called REMORA 3, and the results were used to differentiate helium and fission gas release kinetics under transient operating conditions. However, during the irradiation test, acoustic signal degradation was observed, mainly due to irradiation effect but also due to the increasing of the gas temperature. Despite this acoustic signal degradation, the time of flight measurements were carried out with good accuracy throughout the test, thanks to the development of a more efficient signal processing. After experiment, neutronic calculations were performed in order to determine neutron fluence at the level of the piezoelectric transducer. In order to have a better understanding of the acoustic sensor behavior under irradiation, Post Irradiation Examination program was done on piezoelectric transducer and on acoustic coupling material too. These examinations were also realized on a non-irradiated acoustic sensor built in the same conditions and with the same materials and the same design as the one which has been irradiated. The comparison between both results highlights microstructural evolutions due to irradiation effect. Firstly, metallographic examinations of polished low-angle cross-section of the sensor were performed. The use of low-angle cross-section sample gives geometrical characteristics (macroscopic swelling measurements for instance) with a very good accuracy. This preparation induces a significant magnification of observed region of interest, especially for the acoustic coupling material whose thickness is very small. Optical microscopy on irradiated and non-irradiated samples provides microstructural information about a possible evolution during irradiation test. Secondly, X-ray diffraction measurements on both samples give information about crystalline phase behavior under irradiation. In addition with the important feedback acquired during the in-pile test, these examinations allow us to improve the acoustic sensor design for the next experiments.

Validation of the first prototype high temperature ultrasonic sensor for gas composition measurement

The measurement of substances mixtures properties by ultrasonic approach represents a high interest for industrial sector. In particular, the evaluation of fission gas mixtures in nuclear combustion rod under harsh temperature and radiative environment is of a great interest for fuel characterization and notably lifetime optimization. Previously, an acoustic sensor for in situ fission gases (mainly He and He-Xe mixtures) measurements was developed in our laboratory and tested under experimental condition in OSIRIS reactor (Alternative Energies and Atomic Energy Commission facility). Its operational temperature was limited by 200°C [1]. In this paper, a first prototype for measurement in the temperature range from 300 to 400°C is presented. Piezoelectric material was deposit on alumina substrate by screen printing technique. Sodium Bismuth Titanate (NBT) was used as active material. Studies on dielectric properties and impedances behaviors as function of temperature confirm the sensor use in the range of u...