Dr Ghita Zaz

Pr Emmanuel Le Clézio

Dr Ali Dekious

Meriem Chrifi Alaoui

Dr Yoann Calzavara
email: calzavara@ill.fr

Member, IEEE Pr Gilles Despaux

Emmanuel Le Clézio
email: emmanuel.le-clezio@umontpellier.fr

Gilles Despaux

Gilles

In-situ high-resolution measurement of RHF nuclear fuel plates' spacing

Keywords: High frequency, high resolution, harsh environment, ultrasonic transducer, HPRR, RHF. I

Most of the High Performance Research Reactors (HPRR) are made of plates undergoing a limited swelling during irradiation. Measuring the fuel thickness or the inter-plate distance is then a promissing way to obtain information on the fuel element irradiation history. The experimental constraints are however quite heavy due to the aimed resolution and the element geometry. In order to perform such measurements, a high resolution ultrasonic device was designed. It was thinned down to 1 mm in order to be inserted into the reactor water-channel. The system is then excited with a 120 MHz central frequency burst and the distance measurement is carried out through the ultrasonic waves' time of flight estimation. A series of experiments was performed on a full size irradiated fuel element of the "Institut Laue-Langevin" reactor proving the feasibility of real-time in-situ measurements.

situated well below the surface of a moderating pool yielding a very difficult access. Finally, the measurement devices undergo high-levels of radiations.

Several methods can be considered to perform distance measurements. Optical [START_REF] Thiel | Interferometric measurement of absolute distances of up to 40 m[END_REF]- [START_REF] Amann | Laser ranging: a critical review of usual techniques for distance measurement[END_REF], magnetic [START_REF] Djuric | Displacement inductive sensor: Simulation tool algorithm[END_REF]- [START_REF] Yin | Thickness measurement of non-magnetic plates using multi-frequency eddy current sensors[END_REF], capacitive [START_REF] Sakai | Capacitive Sensors[END_REF], or electromagnetic [START_REF] Morris | 13 -Sensor technologies[END_REF] devices might be relevant and reach today the nanometer resolution even in high-temperature and/or high-pressure environments. However, while some of them possess a low sensitivity to some environmental conditions such as pressure and temperature, most of these systems present drawbacks conflicting with in-situ measurements inside RHF fuel elements. In particular, they may be too cumbersome to be integrated into the water-channel, may not be submerged or are deeply affected by highly radiative environments.

To take into account the experimental constraints, the development and test of a double-sided high-resolution ultrasonic device is here proposed. It includes transducers integrated on both sides of a blade with the constraint of a 1 mm total device thickness and designed with nuclear specifications. To achieve the expected resolution, the system is associated to a set of high frequency acquisition instruments and excited by bursts with a 120 MHz central frequency. At these high frequencies, the ultrasonic wavelength is sensitive to the layered structure of the devices and their design plays a central role in the shape of transmitted and received signals. To understand their behavior, the sensors are then modeled through the transfer matrix method [START_REF] William | Transmission of Elastic Waves through a Stratified Solid Medium[END_REF]- [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF]. Through a pulse-echo protocol, the device then allows distance measurements through the recording of the time of flight t (ToF) of the signals reflected by the fuel plates surfaces [START_REF] Marioli | Ultrasonic distance measurement for linear and angular position control[END_REF], [START_REF] Marioli | Digital time-of-flight measurement for ultrasonic sensors[END_REF]. To infer distance estimations from this measurement, a spectral analysis of the sensor response is proposed to identify the ultrasonic velocity in the water of the cooling channel at the measurement points.

Proving the feasibility of the in-situ distance measurements, experiments were performed on a RHF spent fuel element, demonstrating that the different components of the ultrasonic transducers showed good resistance to radiations allowing a signal to noise ratio sufficient to obtain a first in-situ estimation of the inter-plate distance.

The first section of the paper presents the sensor and device conception as well as the signal processing allowing the estimation of the ultrasonic ToF. The modeling of the sensor behavior is then proposed in the second section while the temperature dependance of the ultrasonic velocity of the waves propagating in the cooling pool water is studied in the third. The last section of the paper deals with the results and discussion on the experiments performed on one full size irradiated fuel element of a RHF.

II. SENSOR AND DEVICE CONCEPTION

A. Experimental constraints and proposed sensor conception

The experimental constraints associated with an insitu control of the fuel element are quite heavy. Regarding the RHF element presented in section VI-A, the inter-plate distance corresponding to the width of the cooling water-channel is of 1800 µm clearly limiting the possible thickness of the measuring device. Moreover, measurements have to be performed 5 m underwater in a highly radiative environment and are expected to possess a resolution of the order of the micron.

To overcome the above difficulties, the development of a high frequency ultrasonic device is here proposed. Generally speaking, ultrasonic techniques yield high levels of performance and are radiation resistant [START_REF] Laux | Determination of high burn-up nuclear fuel elastic properties with acoustic microscopy[END_REF]. Sensors can be sealed to be immersed and can be designed to reach the expected dimensions.

Distance measurements are usually based on time of flight estimations where the time delay separating two ultrasonic signals is directly linked to the distance via the velocity of the ultrasonic wave inside the medium. Several means allow this identification. The Phase Shift Method (PSM) implies the identification of the phase difference between transmitted and received continuous waves [START_REF] Folds | Transmission and reflection of ultrasonic waves in layered media[END_REF], [START_REF] Gueuning | Accurate distance measurement by an autonomous ultrasonic system combining time-of-flight and phase-shift methods[END_REF], [START_REF] Huang | Multiple-frequency ultrasonic distance measurement using direct digital frequency synthesizers[END_REF]. This method can be very accurate and overcomes issues of traditional acoustic attenuation. However, phase ambiguity will occur if the propagation path exceeds a full wavelength clearly limiting the resolution or the applicability of the expected device.

An alternative is here proposed through the implementation of a pulse-echo method [START_REF] Marioli | Ultrasonic distance measurement for linear and angular position control[END_REF], [START_REF] Marioli | Digital time-of-flight measurement for ultrasonic sensors[END_REF]. Due to the experimental constraints, a device working in a reflection mode is proposed. It includes a pair of waterproof transducers integrated into the thickness of both sides of a stainless steel blade thin enough to be inserted into the width of the water-channel (Figure 1.(a)). Each transceiver emits an ultrasonic signal in water and receives its echoes reflected from the facing plate. Knowing the speed of sound c in water and the thickness h of the blade, the total distance d between the plates can be obtained by the determination of the ToF t 1 and t 2 of the two ultrasonic signals generated and received by each transducer:

d = h + c t 1 + t 2 2 . (1)
B. Experimental set-up 1) Device geometry: To respect the experimental constraints, the ultrasonic transducers are mounted on one end of a 1 mm thick, 10 mm wide and 1500 mm long stainless steel nuclear qualified blade as presented in Figure 1.(b). This latter is connected to a 5 m long stainless steel tube with a 22 mm diameter acting as a handling device. A close view of the end of the blade including the transducers is presented in Figure 1.(c). The transducer locations are 500 µm deep circular grooves including a central hole allowing the manufacturing of the sensors connections and backings. The flat borders act like a resting zone for a LiNbO 3 piezoelectric disc corresponding to the active element of the sensor. To set up the cables, a groove is drilled on both faces of the blade. The connections are made possible thanks to notches positioned on the blade side.

2) Transduction elements: Both transducers possess the multilayered structure presented in Figure 2. They are composed of a LiNbO 3 piezoelectric element thinned down to 30 µm in a way to operate with a frequency bandwidth extending beyond 120 MHz. This disc is welded on one surface to a gold and chrome electrode and is glued to a 400 µm thick silica delay line. An aluminum layer is deposited on the second surface as an electric input and output electrode. Finally, the back surface of the transducer is covered by a filled epoxy resin layer acting like a backing material absorbing the vibration of the rear face of the piezoelectric element and ensuring forward propagation. 3) Acquisition system: To allow the emission and reception of ultrasonic signals at very high frequencies, the electronic acquisition chain includes among others:

• A dual transmitter/receiver electronic system specifically designed to improve the excitation signal of the transducers, • Two RF NI PXIe-5690 preamplifier cards with a [10 2 -3 • 10 9 ] Hz bandwidth, • A NI PXIe-5162 acquisition card sampling at 5 GS/s with a resolution of 10 bits and a 1.5 GHz bandwidth. At the high working frequencies of the device, the structure of the multilayered sensor is of the order of the ultrasonic wavelength and plays a crucial role in the shape of transmitted and received signals. Their complex nature (presented in section IV-A of the present paper) and the expected resolution then imply a close understanding of the structure of the sensor ultrasonic signature. The modeling of the acoustic behavior of the transducers is then proposed in the following section.

III. MODELING OF THE SENSOR ULTRASONIC BEHAVIOR

A. Ultrasound propagation in multilayered structures

The modeling of the ultrasonic wave propagation is based on the Stroh formalism [START_REF] Stroh | Steady state problems in anisotropic elasticity[END_REF]- [START_REF] Adler | Matrix methods applied to acoustic waves in multilayers[END_REF] associated with a recursive implementation of the hybrid matrix [START_REF] Tan | Matrix Algorithms for Modeling Acoustic Waves in Piezoelectric Multilayers[END_REF]. While a detailed presentation of the method is presented in a previous work [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF], a quick overview is proposed hereafter. N layers, piezoelectric or not, are stacked in the z direction as in Figure 3. Each homogeneous layer m is of thickness h m , of density ρ m and of elastic C m , piezoelectric e m and dielectric ε m tensors referred to some crystallographic basis. In one layer, the system of governing equations of acoustic wave propagation is:

σ ij,j = ρü i , d i,i = 0, σ ij = C ijkl u k,l + e kij ϕ ,k , d i = -ε ij ϕ ,j + e ijk u j,k (2)
where the index m is omitted for convenience. σ is the mechanical stress tensor, u is the elastic displacement, and d and ϕ are the induction and potential of the electric field induced by the piezoelectric effect. Steady timeharmonic motion inducing a e iωt time dependence is assumed along an arbitrary direction parallel to the surfaces of the plate and taken as the x-axis.

(m, n) = (x, z)
is the sagittal plane and solutions to (2) are sought in the form:

u = A(z)e ik(x-vt) , σn = F(z)e ik(x-vt) , ϕ = φ(z)e ik(x-vt) , d = D(z)e ik(x-vt) , (3)
where k(> 0) is the wavenumber along the x-axis, v is the phase velocity, and ω = kv is the angular frequency.

The octet formalism of piezoelectric [START_REF] Kraut | New Mathematical Formulation for Piezoelectric Wave Propagation[END_REF]- [START_REF] Adler | Matrix methods applied to acoustic waves in multilayers[END_REF] incorporates (2), ( 3) into the first-order ordinary differential system (ODS):

Q(z) - d dz η(z) = 0, ( 4
)
where Q is the system matrix. The eight-component state vector η is expressed as follows:

η = U V , U = A φ , V = ik -1 F D . (5)
In each layer m, the eight up-going and down-going bulk waves are solutions of equation ( 4) and can be expressed through the eigen-spectrum of the system matrix Q m [START_REF] Tan | Hybrid compliance-stiffness matrix method for stable analysis of elastic wave propagation in multilayered anisotropic media[END_REF], [START_REF] Tan | Matrix Algorithms for Modeling Acoustic Waves in Piezoelectric Multilayers[END_REF], [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF]. For each layer m, the field vector elements can then be linked through the following matrix relation:

U m (z m-1 ) V m (z m ) = H m V m (z m-1 ) U m (z m ) , (6)
where z m-1 and z m are respectively the lower and upper surface of the m th layer. H m is called the layer hybrid compliance-stiffness matrix, or simply layer hybrid matrix. Its expression in terms of its eigen-solutions can be found in [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF]. It can be decomposed into 4 blocks of 4×4 matrices:

H m = H m 11 H m 12 H m 21 H m 22 . ( 7
)
The hybrid matrix H m , m = 1 . . . N , of each layer being known the global hybrid matrix is recursively constructed from the top layer (m = N ) to the bottom one (m = 1) [START_REF] Tan | Matrix Algorithms for Modeling Acoustic Waves in Piezoelectric Multilayers[END_REF], [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF]:

H (m) 11 = H m 11 + H m 12 H (m+1) 11 J -1 m H m 21 , (8)
H (m) 12 = H m 12 {I + H (m+1) 11 J -1 m H m 22 }H (m+1) 12 , (9)
H (m) 21 = H (m+1) 21 J -1 m H m 21 , (10)
H (m) 22 = H (m+1) 22 + H (m+1) 21 J -1 m H m 22 H (m+1) 12 , (11)
with

J m = [I-H m 22 H (m+1) 11
]. The superscript (m) in brackets denotes the stack matrix for layers from m to N . The global hybrid matrix is given by the recursive algorithm starting from layer N down to layer 1. The transmission coefficients of acoustic plane waves propagating through multilayered media immersed in a non-viscous fluid are then expressed through the global admittance matrix Y of the structure as presented in [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF] :

Y = i H (1)
12 (H

(1) 22 ) -1 H (1)
21 -H

(1) 11

H (1)
21 (H

(1) 22 ) -1 (H (1) 22 ) -1 H (1) 21 (H (1) 22 ) -1 .(12)
To take into account physical and dimensional parameters of each plate, the model requires as numerical input data the elastic, piezoelectric and dielectric constants of the media, their volume densities and the layer thicknesses. As presented in the next section, the above model has been implemented to understand the behavior of the ultrasonic signals propagating in the multilayered medium noted (a) in Figure 3. It includes the transducer layers (the piezoelectric element, the golden and aluminium electrodes, and the delay line of silica), the water-channel, and the fuel plate.

B. Modeling of the sensor ultrasonic response

The complex transmission coefficient T (ω n ) of each plane wave propagating through the sensor structure is computed as presented above for each frequency ω n = 2πf n , n = 1 . . . N inside the sensor bandwidth. A Fourier summation is then implemented to reconstruct the temporal signal backscattered to the sensor taking into account the initial amplitudes A(ω n ) of each frequency in the transducer spectrum:

s(t) = N n=1 A(ω n )T (ω n )e iωnt . (13)
C. Comparison between experimental and simulated signals 1) Transducer response: The temporal signal s(t) of equation ( 13) is presented as dashed lines in Figure 4. The different bursts that can be observed correspond to multi-reflections on the silica/water interface of the initial signal generated by the piezoelectric element, the progressive transmission of ultrasonic energy into the water yielding the decreasing of the amplitudes of the echoes.

This theoretical signal is compared to the experimental one, presented in straight lines. To obtain the very good agreement between these two signals, a fitting procedure has been implemented allowing the identification of the thickness of each layer constituting the sensor. This allowed both the characterization of the device behavior and the understanding of the influence of each layer on the global signal structure in view of a sensor optimization. Differences can be noticed between the two signals, in particular in the amplitude of the two first pulses, the experimental ones being much smaller than the simulated ones. This is due to instrumentation constraints imposing a high attenuation of the first echoes to avoid a saturation of the high voltage electronics.

IV. TIME OF FLIGHT ESTIMATION

A. Propagation through the total structure

To envision water-channel thickness measurements, the above simulations are extended to consider as in Figure 3 the propagation of the ultrasonic waves into the water, their reflections onto the surface of the fuel element and their back-propagations to the sensor. Results are presented in Figure 5.

The first part, noted (a), corresponds to the sensor signature of Figure 4. This composite signal is transmitted into the water and reflected back to the sensor by the surface of the nuclear fuel plate. The corresponding signal is presented in part (b) of Figure 5. Some differences in the structure of these two sets of burst can be observed, in particular the amplitude of the two first ones of part (b) is lower than the following ones. The simulation proved that this behavior is due to the presence of the glue layer, the first echo amplitude being linked to its thickness. To estimate the water-channel thickness, the determination of the ToF between parts (a) and (b) of Figure 5 is proposed in the next section.

B. Signal processing

One of the simplest forms of ToF estimation is the threshold method. This technique proposes to identify the arrival of an acoustic signal to the time where its amplitude exceeds a given amplitude level. Thresholding systems do not require any complex computations, but their precision can be degraded for low signal-tonoise ratio (SNR) [START_REF] Marioli | Digital time-of-flight measurement for ultrasonic sensors[END_REF], [START_REF] Frederiksen | A single-chip monolithic sonar system[END_REF]- [START_REF] Jackson | Time-of-flight measurement techniques for airborne ultrasonic ranging[END_REF]. It is here proposed to perform the ToF estimation through a cross-correlation method [START_REF] Safak | Estimation of object location and radius of curvature using ultrasonic sonar[END_REF]- [START_REF] Queiros | Cross-Correlation and Sine-Fitting Techniques for High-Resolution Ultrasonic Ranging[END_REF] computed between the two parts (a) and (b) of the signal of Figure 5, respectively denoted x(t) and y(t). Their cross correlation R xy (t) is defined by the following equation:

R xy (t) = x(t) ⊗ y(t) = ∞ -∞
x(τ )y(t + τ )dτ. [START_REF] Haskell | The dispersion of surface waves on multilayered media[END_REF] Taking into account the structure of the ultrasonic signals of Figure 5, equation ( 14) becomes

R xy (t) = n i=1 x i (t) ⊗ n j=1 y j (t) (15)
where x i (t) = a i s(t-iτ ) and y j (t) = b j s(t-jτ -θ), τ and θ corresponding to the delays due to the ultrasonic wave propagation in silica and water, respectively. a i and b j are the echoes' amplitudes of the elementary bursts s(t). Finally,

R xy (t) = n i=1 n j=1 a i b j s(t -iτ ) ⊗ s(t -jτ -θ). ( 16
)
Compared to the correlation abs(t) ⊗ s(t -θ) of two single bursts of amplitudes a and b separated by the ultrasonic time of flight in water θ, the estimation of the maximum of similarity of the two composite signals is here optimized through the contribution of each pair of bursts increasing the signal to noise ratio.

V. THE TEMPERATURE DEPENDANCE OF THE ULTRASONIC VELOCITY c(T )

The above relation ( 16) allows a precise identification of the time delay corresponding to ultrasonic propagation to and from the fuel plates back to the sensor. To correctly infer distance estimations from this measurement, a precise knowledge of the ultrasonic velocity in the water of the cooling channel must be obtained. To do so, we propose in this section to take advantage of the thermal influence of the environment onto the sensors.

A. Temperature measurement

The spectral components of the sensors being directly dependent on the thermal expansion of transducers layers, a frequency analysis of the device response is now proposed in a way to infer the temperature at the measurement point.

1) Device structure modifications: When immersed into water, the sensor temperature variations lead to a thermal expansion or shrinkage of its multi-layered structure, yielding in particular modifications in its piezoelectric element and silica delay line thicknesses. On the expected temperature range of a cooling pool (from 25 ˚C to 35 ˚C), the thermal expansion coefficient α Si of the silica is 4.1•10 -5 /˚C [START_REF] Li | Fiber-Optic Temperature Sensor Based on Difference of Thermal Expansion Coefficient Between Fused Silica and Metallic Materials[END_REF], [START_REF] Bell | Experimental results for nulling the effective thermal expansion coefficient of fused silica fibres under a static stress[END_REF] and that of the piezoelectric element ranges between α LiN b03 = 7.5 • 10 -4 /˚C and α LiN b03 = 15 • 10 -4 /˚C [39]. These values show that it is possible to neglect the thermal expansion of the silica, the piezoelectric element being mainly responsible of a change of the transducer resonant frequency. The thermal modifications of the other layers of the sensor are here omitted due to their very small thicknesses yielding negligible modifications in the device response.

The thermal expansion of the piezoelectric element causes a change of its resonant frequency. Indeed, when the piezoelectric element is heated, its thickness d changes according to the thermal expansion law, which can be written as [START_REF] Pignatiello | Measurement of the thermal expansion coefficients of ferroelectric crystals by a moiré interferometer[END_REF]:

d(T i ) = d(T i-1 ) + δd ( 17
)
where

δd = αδT d(T 0 ). ( 18
)
The thickness of the piezoelectric element is directly related to its resonant frequency by the following equation:

f = v 2d ( 19
)
where v is the ultrasonic velocity of the longitudinal wave propagating inside the piezoelectric element. As a consequence,

δf = 0.5 δv • d -vδd v 2 . ( 20
)
On the proposed temperature range, δv ≈ 0. We can then deduce that:

δf = -0.5 δd v (21)
leading to transducer frequency changes according to the following equation:

f (T i ) = f (T i-1 ) + δf. (22)
A simulation based on the above equation was developed to anticipate the behavior of the frequency transducer according to the temperature and to evaluate the accuracy of the measurement system. Results are presented as dots in Figure 6.

2) Device calibration: A measurement system has then been implemented to confirm the above result. The ultrasonic device of Figure 1 is inserted into a thermostatic bath filled with deionized water similar to that of the cooling pool of the RHF reactor. The temperature is then cooled down from 35 ˚C to 10 ˚C by steps of 0.5 ˚C. Each step is maintained several minutes to ensure temperature stability within the sensor. The transducer is then excited by the system presented in section II-B3 and its frequency response is acquired. Acquisitions of the water temperature are also performed thanks to a thermocouple. Results are compared with good agreement in Figure 6 to the simulations confirming that the frequency variation of the sensor is mainly controlled by the piezoelectric element thermal expansion.

From 10 ˚C to 35 ˚C, the resonance frequency of the transducer is decreased by 4.3 MHz. While controlling the RHF structure, this experimental calibration law will be used to determine the in-situ temperature of the water-channel from the resonant frequency of the sensor.

B. Ultrasonic velocity measurement

The temperature being evaluated at the measurement point, the next step deals with the identification of the thermal dependance of velocity of the ultrasonic wave propagating in the water pool. To do so, a home-made quartz tank with a controlled thicknesses of 1800±10 µm is used as reference reflector and the ToF is measured by the above device in the [10 -35] ˚C range by steps of 0.1 ˚C. The ultrasonic velocity variation with the temperature is presented in Figure 6 and compared to values proposed in [START_REF] Bilaniuk | Speed of sound in pure water as a function of temperature[END_REF]. Velocity (m/s) Fig. 6. Variations with the temperature of the sensor response frequency (straight line: experimental data; dots: modeling) and of the ultrasonic velocity (dashed line from [START_REF] Bilaniuk | Speed of sound in pure water as a function of temperature[END_REF]; circles: experimental data).

C. Distance estimation

The two experimental laws of Figure 6 allow a realtime in-situ estimation of the local ultrasonic wave velocity at the measurement point through the measurement of the resonant frequency of the sensor. Then, with known local time of flight and ultrasonic velocity, the water-channel width d can be calculated from equation [START_REF] Sannen | From High to Low Enriched Uranium Fuel in Research Reactors[END_REF]. The uncertainty δd on its estimation is given by:

δd = d × δt t + δv v (23)
where δt and δv are respectively the absolute uncertainty of the time of flight and of the ultrasonic velocity measurements. The maximum difference between experimental and theoretical velocities presented in Figure 6 being of about 4 m/s, the total error on the inter-plate distance estimation is estimated in our experiments to less than 20 µm.

VI. IN-SITU MEASUREMENTS

A. The fuel element structure

The High Flux Reactor of the Laue-Langevin Institute is a research reactor that produces neutrons for fundamental science experiments. Heavy water is used as a moderator in order to slow down the neutrons produced by the fission reaction allowing the generation of the most intense continuous flux of thermal neutrons, about 1.5 • 10 15 neutrons per second per cm 2 , with a thermal power of 58 MW. The RHF fuel element is made of 280

B. Environment

To validate the distance measurement approach, an insitu experiment was carried out in the above RHF positionned 5 m under water in a cooling pool. To evaluate the radiation resistance of the device and estimate the measurement resolution achievable in a highly radiative environment, measurements have been performed inside three different water-channels in which the sensor was first moved down to a depth of about 10 cm. During the experiment, the mean water-channel temperature was measured equal to 28.3 ± 0.1 ˚C.

C. Results

In each channel, acoustic signals were recorded every 50 ms while slowly manually ascending the device. As previously mentioned, the experimental constraints associated with an in-situ control are quite heavy. In particular, the device has to be handled by an operator situated over the cooling pool. One of the consequences linked to this experimental implementation deals with the control of the ultrasonic fields' alignment with the normal of the reflecting surfaces. To illustrate this point, Figure 8 presents the energy of the reflected signals received in one of the water-channel by one of the sensors. It clearly shows that, when the sensor is not correctly oriented, the amplitude of the acquired signal is greatly degraded. The influence of this misalignment on the thickness estimation has been quantified through the model presented in section III-A and [START_REF] William | Transmission of Elastic Waves through a Stratified Solid Medium[END_REF]- [START_REF] Lam | Acoustic wave transmission through piezoelectric structured materials[END_REF] and it was found that due to the fuel plates' curvature, the maximum inclination of the transducer is less than 3ẙ ielding a maximum error of 4 µm on the global distance estimation. However, while the sensors alignment is clearly an issue due to the experimental constraints, results from Figure 8 give an indication of the quality of the measurements. As a consequence, the energy of each signal is being computed and used to select relevant signals yielding reliable identifications of the water-channel thickness. The corresponding estimations

D. Discussion

First of all, these results clearly prove the feasibility of a in-situ non destructive control of the RHF thanks to an ultrasonic method. Indeed, the experiments demonstrated that the different components of the ultrasonic transducers showed good resistance to radiations during at least 5 hours. Moreover, in spite of the highly irradiative nature of the environment, the quality of the ultrasonic signals was sufficient to ensure a stable identification of the inter-plate distances.

Some remarks can however be made on the quality of the above results. It is here important to emphasize that due to the manual positioning of the device and the removing of the low energy signal no information on the sensor depth can be here obtained and the reader should avoid to draw a mental line between the measurements. These results however allow the estimation of a mean water-channel thickness, here equal to 1829 ± 11 µm. Moreover, the resolution of the device can be estimated thanks to the dispersion of neighboring points corresponding to measurements performed at very close locations. It is here estimated to 2 µm.

VII. CONCLUSION

This paper presented a prototype of a ultrasonic device integrating two high frequency transducers designed to measure water-channel thickness variations inside a High Flux Reactor positioned 5 meters below the water surface of a cooling pool. With a total thickness of 1 mm, the device is attached to a holder to allow its insertion into the nominally 1800 µm thick water-channel of the fuel element. To achieve the expected resolution, the system is excited with frequencies up to 120 MHz and integrated into a set of high frequency acquisition instruments. Knowing the speed of sound in water, the device allows the distance measurement through the evaluation of the ultrasonic wave's time of flight. To do so, the influence of the thermal expansion of the transducer layers on its resonant frequency is taken into account to infer the local temperature, and the corresponding ultrasonic wave velocity, at the measurement point. A first experiment was performed in the High Flux Reactor of the Laue-Langevin Institute, allowing the identification of the in-situ experimental constraints. Moreover, the corresponding results clearly proved the resistance of the sensor components to the radiations and demonstrated the feasibility of the distance measurement in a highly radiative environment. Future works will deal with the improvement of the measurement reliability. In particular, to study the sensor behavior under gamma radiations and the potential evolution of its characteristics, three transducers will be tested in an irradiation chamber. An automated system supporting the transducers and a guiding system are moreover being designed to ease the insertion of the blade into the fuel element and stabilize its position between the fuel plates. Associated to a control of the position of the sensor, it should allow quantitative measurements of the water-channel variations along a vertical generating line of the fuel-element-water-channel.

Fig. 1 .

 1 Fig. 1. (a) General principle of inter-plate distance measurement and geometry of the sensor holder: (b) device holder and blade; (c) image of the end of the blade with one inserted transducer.

Fig. 2 .

 2 Fig. 2. Ultrasonic transducer structure.

Fig. 3 .

 3 Fig. 3. The modeled multilayer medium: (a) sensor and (b) fuel element structures.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Reflection of the signals within the multilayer structure of the transducer. Dashed line: Theoretical. Solid line: measured

  aluminum curved plates containing high-enriched uranium aluminide-aluminum (UAlx/Al) dispersion fuel and forming a cylindrical tube (Figure 7.(a)). The plates possess the following dimensions: 903 mm long, 69.54 mm wide and a nominal thickness of 1.27 mm. They are regularly spaced by a 1.8 mm wide channel where a water stream flows as coolant and moderator to the nuclear reaction (see Figure 7.(b)).

Fig. 7 .

 7 Fig. 7. (a) Top view of a mockup of the ILL RHF fuel element; (b) Fuel element plates' arrangement and ultrasonic sensor position.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Evolution during the acquisition process of the energy of the reflected signals measured by one of the sensors. are presented in 9 where spurious points corresponding to signals with low energy have been removed.

ACKNOWLEDGMENT

The authors would like to thank Laurent Podevin, Yannick Launay and Alain Portal, respectively assistant engineer, technician and study engineer at the IES -UMR CNRS 5214, for their strong contributions to manufacture the sensors and the associated electronics.


Lisa Jean Moore

Claire Beaudevin

Missing bodies. The politics of visibility by Casper

The increasing visibility of the body in the American society is paradoxically concomitant with the invisibility of manifold 'bodies conspicuously missing in action ' (p.3).

This observation constitutes the root of the book's problematic, a different approach to "corporeity": 'exploring how certain places, spaces, policies, and practices in contemporary society […] exhibit and celebrate some bodies while erasing and denying others' (p.3): States, not content with making live and letting die, grant social visibility as well.

Chapter 2, the first of six American case studies, depicts the social invisibility of children's bodies in contemporary society. The strongly socially constructed "innocence" of childhood is the reason put forward by diverse institutions justifying their protection behaviour. It leads to the absence of empowerment of children and lack of interest in their voices: this 'cultural veil of innocence' (p.36) silences and obliterates them. Their bodies and sexual experiences are set up as sanctuaries. Academics wanting to focus their research on them thus face manifold difficulties.

Chapter 3 concerns the social invisibility of infant mortality: even if the number of infant deaths is surprising considering the country's GDP, it has decreased tremendously during the 20 th century and has thus been socially "forgotten". This chapter relevantly shows interactions between demography and biopolitics: demographical statistics construct criteria and targeted by preconception care policies, since her body appears as the only "place" where the issue of high infant mortality rate could be resolved.

Drawing on the concept of exposure ('the degree of coverage and the amount of attention a body receives' p.79), chapter 4 addresses the recent shift of HIV/AIDS politics from the field of public health to that of national and international security. Even if HIV/AIDS is considered as pandemic, in America, it is supposed to be under control.

Therefore, attention and fears target foreign (developing) countries, many of them in Africa, depicted as actual "biological weapons", potentially endangering social systems. This results in a contrasted picture of nearly invisible American people living with HIV/AIDS along with intensely mediatised African sufferers' bodies, presented as the main reason of their States' failures.

Chapter 5 deals with biomonitoring: attempts to assess environmental risks through surveillance of human fluids' pollution. After an account of the American legislative process that led, in some States, to the large scale implementation of biomonitoring, the authors emphasize the lack of applicability of the obtained data. Breast milk, the archetypally mentioned fluid, represents a social commodity, monitored for the sake of the entire society (in contrast with semen, which is not monitored for collective benefits). This status of 'sentinel fluid' (p.122), in passing, obliterates women's bodies and individual interests (women are not provided with an individual exposure assessment), while it assumes an unquestioned link between women and nature and an obvious superiority of breastfeeding over formula. Thus, one can observe a synecdochical process, where women's very bodies fade away, giving way to breast milk as an isolated pollution-monitoring commodity.

The section 'Heroes' deals with social overexposition of certain bodies. Chapter 6 examines the story of Jessica Lynch, an American soldier, wounded, captured and then "rescued" in Iraq in 2003. The authors draw upon her case to address the interactions between gender and the military. The hiatus between military and media accounts of Lynch's story and her own version is analyzed through the prism of defined gender roles. She is presented as helpless and mistreated, thus emphasizing the key intervention of her fellow male soldiers. Her personal memories and her body have been erased from the social scene. Her defenceless mediatic portrait personifies all her fellow female soldiers, thus hiding their bravery (and deaths).

In contrast, chapter 7 presents Lance Armstrong's cycling career and survival of testicular cancer as the archetype of the American masculine hero. Media portray Armstrong as superhuman, with a famous multicommentated physique. This overexposure embodies the very notion of American masculinity and success (winning several times the Tour de France after cancer treatment) and constitutes a sort of 'screen' dissimulating the actual experience of other cancer patients.

In conclusion, the authors stress again the different invisibilities they tackled: 'innocents' themselves or parts of their bodily experience (dead babies, children's sexuality) are invisible, "conjured away". 'Exposed' bodies are not necessarily visible: 'hidden in plain sight' (p.183) as Americans living with HIV/AIDS dissimulated by their scary African counterparts, or widely biomonitored breast milk, whose assumed statistical relevancy erases women. Bodies of 'heroes' are the trees preventing us from seeing the woods of actual social heroes.

One can notice some facile phrasing -'demographers perform a type of numerical magic, erasing actual bodies [of dead infants] […] and replacing them with statistics' (p.66) -and caricatural oppositions -between the dead infants' invisibility and the mediatic visibility of the Emperor penguins through the documentary March of the Penguin.

Nevertheless, this book is written in a vivid style, endowed with a straightforward and pleasant way of expressing the authors' engaged analysis.

Despite its focus on United States, the book's content -including its theoretical framework -is relevant in other contexts too. It can usefully be part of undergraduate teaching, as example of ethnographies (especially chapters 2 and 6) showing interactions between citizens' bodies and State's power.

Claire BEAUDEVIN

Cermes3 / Paris Descartes University, France

  Casper, Monica J. and Moore, Lisa Jean. 2009. Missing Bodies. The Politics of Visibility. New York and London: New York University Press. 223 pp. Pb: 17.59 €. ISBN: 978-0814716786

  of normality and contribute to the definition of infant mortality rate as a social clue, an indicator of social troubles. The authors stress the way risk factors for infant death are constructed by epidemiology and demography: every woman is treated as a potential womb Published in Social Anthropology, 19(2) -May 2011 (journal of the European Association of Social Anthropologists -EASA)


Everardo Reyes

Andrés Burbano
email: aburbano@uniandes.edu.co

-2020

Keywords: ACM, SIGGRAPH, computer graphics, interactive techniques, art, science, humanities, conference

This institutional presentation introduces the ACM SIGGRAPH Art Papers programs 2019 and 2020. The 2019 edition marks the 11th version of the program and our main goal is to highlight its principal novelties and projections onto 2020. At this occasion, we will discuss the main scientific and artistic contributions of art papers, but also different ways of accessing, experiencing, and enhancing the published content.

Introduction

ACM SIGGRAPH is the Special Interest Group in Computer Graphics and Interactive Techniques (SIGGRAPH) of the Association for Computing Machinery (ACM). While its history can be traced back to 1967, when a small group of specialists organized a seminar in interactive computer graphics, it was in 1974 when the first annual conference was organized. Nowadays, SIGGRAPH is an international community of researchers, scientists, technologists, artists, filmmakers, developers, and business professionals. Together with SIGGRAPH Asia (SA), which began in 2008, the conferences are attended by tens of thousands of professionals every year [START_REF]ACM SIGGRAPH[END_REF].

In 2019, the 46th International Conference and Exhibition on Computer Graphics & Interactive Techniques includes twenty-four programs and two special events distributed in five main areas: Production & Animation, Research & Education, Arts & Design, Gaming & Interactive, and New Technologies [START_REF]Thrive[END_REF]. This year marks the 11th edition of the Art Papers Program, which brings together communities of researchers and practitioners working at the intersections of art, design, humanities, science, and technology. This institutional presentation introduces the ACM SIGGRAPH Art Papers programs 2019 and 2020.

ACM SIGGRAPH Art Papers Program

The ACM SIGGRAPH Art Papers Program has been organized since 2009. It consists of a series of cutting-edge artscience-technology research presented in the form of scien-tific papers. To be included in the program, authors first submitted their papers as proposals that go through a rigorous blind peer-review process. Accepted papers are presented in person by at least one of its authors at the SIGGRAPH venue and published in a special issue of Leonardo, the journal of the International Society for the Arts, Science, and Technology (ISAST) (MIT Press) [START_REF] Burbano | Official website[END_REF], also available from the ACM Digital Library [4] and, since 2015, as open access conference content accessible from the SIGGRAPH website [START_REF]ACM SIGGRAPH Conference Content, Official website[END_REF].

Over the time, the program has grown substantially, from an average of 40 submissions during the early years to more than 130 in 2018. One of the reasons of such increase in the volume was the introduction, in 2018, of Short Papers to the already existing Long Papers format. The main difference between both formats is that Long Papers describe concepts, processes, and applicability of advances in science, computer graphics, and interactive techniques, while Short Papers discuss on-going works and visions.

2019

Although we did not define an explicit topic for 2019, we did call for submissions that explored innovative and provocative projects in a self-reflective manner, i.e. multiple views (or entry points), multicultural translations, and multigenerational communication [START_REF]Art Papers Submissions -SIGGRAPH 20019[END_REF].

From the beginning of the planning process we had clear objectives in mind: • To attract high-quality scientific papers that explore the most innovative intersections of arts, technology, humanities, and computer graphics. • To provide rigorous and thoughtful reviews to all submissions by at least 3 members of the jury. • To enhance the visibility of art-based research within the SIGGRAPH communities. To cope with these goals, it was paramount to increase the number of jurors, to consolidate relationships with other artistic communities associated to SIGGRAPH, and to maintain the publishing recognition of an academic journal such as Leonardo while amplifying the accessibility through the exploitation of web-based platforms.

To foster dialogue between approaches we proposed four broad categories: 1) Project descriptions; 2) Theory/criticism of contemporary digital art; 3) Meth-ods/techniques of creative practices; and 4) History/Media Archaeology of materials and roles of artists. This organization was also helpful to the review process. We gathered an international jury composed of 10 renowned artists, scientists, and scholars in such areas. Moreover, the jurors could extend an invitation to other experts and specialists to serve as external reviewers and we had 28 by the end of the process.

In this institutional presentation we present highlights of the 15 accepted papers. Authors will present their research in three sessions. In the first session, the main themes comprised spaces and territories including our relationship to fictional and psychological constructions such as borders, frontiers and our inherent human vantage point. In the second session, some of the steering topics dealt with digital tools and archives, preservation of artworks and memorializing social events with digital media. In the third session, the contributions shared a consideration of volume and space as a key component for interaction and interpretation from 3D printing techniques to multimedia installations.

2020

At ISEA 2019 we will evoke the overall plan for SIGGRAPH 2020, which will take place in Washington DC from July 19 to 23. We will announce on-site the overall procedure, the main dates, and the potential topics under discussion. In the end, we also share some novelties about the program in cooperation with the recently created Arts Advisory Group at SIGGRAPH.

Acknowledgements

We want to acknowledge the guidance and support from the SIGGRAPH 2019 conference chair Mikki Rose and the SIGGRAPH 2020 conference chair Kristy Pron. We are also thankful for the collaboration and assistance from the SIGGRAPH DAC chair Victoria Szabo, the Leonardo Executive Editor Roger Malina, the Leonardo Managing Editor Erica Hruby, the Conference Staff at ACM Stephen Spencer and Marc Barr, and SmithBucklin Leona Caffey and Elizia Artis.

Authors Biographies

Everardo Reyes is Associate Professor in the Information Sciences Dept. at Université Paris 8. He is member of the Paragraphe Lab, collaborator of the Cultural Analytics Lab, and board member of the International Association for Visual Semiotics. His research investigates relationships between humanities, the arts, and computer sciences, particularly the graphical and interactive forms.

Andres Burbano is Associate Professor in the Department of Design at Universidad de los Andes. Burbano holds a PhD in Media Arts and Technology from the University of California Santa Barbara. Burbano, originally from Colombia, explores the interactions of science, art and technology in various capacities: as a researcher, as an individual artist and in collaborations with other artists and designers. The broad spectrum of his work illustrates the importance, indeed the prevalence, of interdisciplinary collaborative work in the field of digital art.


est destinée au dépôt

Study of colonization mechanisms of construction products by micromycetes applied to the development of a preventive treatment

CONCLUSION AND PERSPECTIVES

 Fungal colonization of building materials involves same biotic relationships than those observed in natural environments → a succession of species with functional relationships like antagonism.  Interaction mechanisms between microorganisms are mediated by diffusible substances in substrates (Bacillus sp). These antagonist active substances, whose safety must be verified, could be exploited to develop a dedicated eco-compatible treatment to limit fungal growth.  Study of the involvement of MVOC in biotic interactions will be prosecuted.

Study of colonization mechanisms of construction products by micromycetes applied to the development of a preventive treatment

Isabelle

Maastricht, Netherlands

In nature, saprotrophic fungi have an essential ecological role in decomposition of organic matter. In soils, they break down material like leaves, wood, dead bodies, etc. Fungal colonization in natural environments requires advanced functional relationships, including various antagonisms and synergisms, between the different components of the biotope. Colonization of natural substrates has been described at different scales (macroscopic and molecular) and modeled.

In indoor environments, fungi find niches favorable to their development. They are able to induce pathologies to the occupants (allergies, poisoning and infections) and can also cause irreversible damages to the materials they infest.

Colonization of man-made substrates (construction and decoration products) by mold is insufficiently described to develop colonization models. A better knowledge of the dynamic of fungal colonization would be helpful to imagine preventive strategy to limit fungal growth.

Chemical ecology of species inoculated individually or in combination (PCA)

→ Specific VOCs were identified and appear to be specific for each biotic interaction.

OBJECTIVES RESULTS AND DISCUSSION

Towards a better understanding of building material colonization dynamic

Before their implementation, building materials are variably contaminated by micromycetes in quantity and quality.

T0

Initial contamination of a new bio-based insulation material by Trichoderma. → Raw material is probably contaminated in fields.

21-28 days

Without water restriction, fungal development mainly consists of species belonging to genera Penicillium, Aspergillus and Cladosporium, ubiquist airborne mycoflora.

New invasive species such as Stachybotrys and Alternaria, known for their antagonist interactions, appear.

After 28 days

Biotic interactions between micro-organisms

  Isabelle Lacaze, Stéphane Moularat, Marjorie Draghi, Enric Robine, Philippe Silar To cite this version: Isabelle Lacaze, Stéphane Moularat, Marjorie Draghi, Enric Robine, Philippe Silar. Study of colonization mechanisms of construction products by micromycetes applied to the development of a preventive treatment. FEMS 2015, 6th Congressof European Microbiologists, Jun 2015, Maastricht, Netherlands. hal-02167654

  to raw material +++

1 Centre Scientifique et Technique du Bâtiment (CSTB), 84, Avenue Jean Jaurès Champs-sur-Marne 77447 Marne-la-Vallée Cedex 2 2 Laboratoire Interdisciplinaire des Energies de Demain (LIED), UMR 8236, Bâtiment Lamarck B, 35 rue Hélène Brion 75013 Paris *corresponding author: isabelle.lacaze@cstb.fr MATERIALS AND METHODS

	Stachybotrys proliferation
	on wallpaper after
	12 weeks incubation
	(25°C, RH 100%)

 Building materials Wallpaper, glass fiber wall covering, plasterboard and biobased insulation material  Fungal species most frequently identified Aspergillus versicolor, Penicillium chrysogenum and Cladosporium sphaerospermum

Fungal growth in presence (top) and in absence (bottom) of Bacillus

	→ Bacillus are able to produce cyclic
	lipopeptides belonging to three families
	(Iturins, Fengycins & Surfactins) with
	antifungal properties.