# Ultrasonic detection of radiolysis bubbles generated in bitumen for dose between 6 and 100 kGy

Didier Laux, Hanaa Houjeij, Céline Monsanglant, Georges Matta, Stéphane Poirier, Francois Millet, Jean-Yves Ferrandis

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Didier Laux, Hanaa Houjeij, Céline Monsanglant, Georges Matta, Stéphane Poirier, et al. Ultrasonic detection of radiolysis bubbles generated in bitumen for dose between 6 and 100 kGy. ANIMMA 2025 – 9th International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications, ANIMMA International Steering Committee (ANIMMA), Jun 2025, Valencia, Spain. pp.07001, 10.1051/epjconf/202533807001. hal-05355284

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# Ultrasonic detection of radiolysis bubbles generated in bitumen for dose between 6 and 100 kGy

Didier Laux¹, Hanaa Houjeij², Céline Monsanglant², Georges Matta², Stéphane Poirier², François Millet³ and Jean-Yves Ferrandis¹*

¹IES, Univ Montpellier, CNRS, Montpellier, France

²Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSN-RES/SCA/LECEV, F-91400, Saclay, France

³ Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSE-ENV/SPDR/USDR, F-92260, Fontenay-aux-Roses, France

(*) jean-yves.ferrandis@umontpellier.fr

**Abstract**—The ARISE project, supported by the National French Research Center (CNRS) and the Nuclear Safety and Radiation Protection Authority (ASNR), began in 2023 to explore the feasibility of innovative ultrasonic methods to detect the appearance and growth of radiolysis bubbles in bituminized waste products (BWPs). More generally, the subject can cover the investigation of any opaque medium. In this communication, we present results on bitumen samples irradiated at the IRMA French facility, as part of the ASNR BOREE experimental platform. The samples were subjected to total absorbed doses of 6, 50 and 100 kGy with dose rates ranging from 155 to 1230 Gy/h. By using the velocity and attenuation of ultrasonic signals at around 500 kHz, combined with Foldy's model applied to polydisperse bubble clouds and X-Ray microtomography as validation technique, we demonstrate that relevant parameters can be estimated in a non-destructive manner in irradiated bitumen, namely: the void fraction, bubble radius and the polydispersity of the bubble size distribution. These results pave the way for non-destructive real time monitoring of bubble formation and growth under gamma irradiation in bitumen, as a part of the next phases of this extensive research program.

**Keywords** — Ultrasounds, Bitumen, waste products, Bubble clouds, Gamma irradiation, Radiolysis, X-Ray Microtomography.

## I. INTRODUCTION

SINCE the 1960s, bitumen has been used by the French nuclear industry as an immobilization matrix for long-lived low-level (LL) and intermediate-level (IL) radioactive waste generated during spent fuel reprocessing. The process involves concentrating radionuclides by co-precipitating them with selected salts that exhibit strong chemical affinities with them. The resulting sludge is then filtered to increase its solid content, and the final residue—now containing the radionuclides—is embedded in a bituminous matrix through an extrusion process. The resulting bituminized waste products (BWPs) are subsequently conditioned and stored in metallic drums. Within the context of nuclear safety process, it is

imperative to examine the thermal and under irradiation behavior of these BWPs. Radiolysis due to gamma self-irradiation occurs and leads to the formation of gas bubbles (essentially hydrogen) which can significantly elevate the fire and explosion risk when released [1]. Consequently, the nucleation and growth of these bubbles must be precisely determined. X-Ray microtomography, applied to pure bitumen and BWPs is commonly used to investigate such issues [2-5]. However, this technique is not suitable for direct in-line evaluation. In this framework, an extensive program (ARISE) has been initiated by ASNR (USDR and LECEV labs) and CNRS/Montpellier University (Acoustic Team of the Institute of Electronics and Systems, IES) in 2023. In our previous investigations we focused on the high frequency rheology of pure bitumen and on the influence of bubbles created by external irradiation on ultrasonic velocity [6,7]. We have demonstrated that the void fraction can be determined; however, further characteristics of the bubble cloud should be determined using ultrasonic waves attenuation in the medium, namely the median bubble radius and polydispersity of the bubble size distribution. In this communication, we focus on ultrasonic absorption by bubble clouds generated in pure bitumen through external gamma irradiation in the dose range of 6 to 100 kGy. This study represents a first step prior to investigating BWPs. X-Ray microtomography will be used as a reference method. The irradiation procedure at IRMA facility, as well as X-Ray microtomography and ultrasonic techniques, will be presented. Then, Foldy's model for polydisperse bubble cloud in a viscoelastic medium will be applied to fit ultrasonic parameters (velocity and attenuation), in order to assess to what extent, the acoustic method can provide a complete characterization of the bubble cloud. Current limitations and future optimization of the method will be discussed at the end of this paper.
## II. MEASUREMENT DESCRIPTION

### A. Samples and irradiation at the IRMA facility

Duran® glass bottles were filled with 180 g of a straight-run distillation bitumen with a penetration grade of 70/100, referred to as AZALT 70/100®, and supplied by TotalEnergies. These samples were then irradiated with gamma rays at the IRMA (IRradiation of Materials) facility, a ⁶⁰Co irradiation chamber, operated by ASNR and located at the CEA (French Alternative Energies and Atomic Energy Commission) Saclay research center. Four high activity sealed ⁶⁰Co sources, each measuring 45.2 cm in length and 1.1 cm in diameter, with a total activity of 453 TBq as of March 7, 2024 (the start date of the test), were used for the irradiation. The position of the sources can be adjusted to achieve the desired dose rates. Irradiation times, dose rates and total absorbed doses are listed in Table I.

<table><caption>TABLE I
IRRADIATION PARAMETERS OF BITUMEN SAMPLES</caption><thead><tr><th>N°</th><th>Dose (kGy)</th><th>Dose rate (Gy/h)</th><th>Irradiation time (hours)</th></tr></thead><tbody><tr><td>A</td><td>6.34 ± 0.64</td><td>155±16</td><td>~41</td></tr><tr><td>B</td><td>51.9 ± 5.2</td><td>1230±120</td><td>~42</td></tr><tr><td>C</td><td>50.8 ± 5.1</td><td>304±30</td><td>~167</td></tr><tr><td>D</td><td>101 ± 10</td><td>603±61</td><td>~167</td></tr></tbody></table>

### B. X-Ray microtomography: data acquisition and processing

X-Ray microtomography tests were performed using a Bruker Skyscan 1173 micro-CT scanner with a voltage of 125 kV and an intensity of 61 µA. A full360° scan consisting of 1800 images required approximately 225 minutes. The acquired images were processed using a custom Python-based software developed in-house, to extract key bubble parameters such as volume fraction, median radius and the dispersion around the median radius. Image analysis involved six main steps: resizing, thresholding, removal of small particles, bubbles labelling and geometrical properties calculation, and data export to an Excel file. Further details on these processing steps are provided in [7].

### C. Ultrasounds: experimental method and signal processing

In our previous studies [6], non-irradiated 70/100 bitumen was investigated with ultrasonic waves. So, its ultrasonic characteristics (velocity and attenuation) are well known. We first used a Duran bottle filled with non-irradiated bitumen as a reference. Two 500 kHz (Olympus - V101 RM VISIOSCAN) longitudinal transducers were coupled to the bottle with liquid honey. These sensors were excited with a JSR Ultrasonics DPR300 pulser/receiver generator. The acoustic signal transmitted through the bottle was transferred from an oscilloscope (LeCroy WaveJet 334) to a computer using a National Instruments (USB/GPIB HS+) acquisition card. The same procedure was subsequently repeated with a Duran bottle filled with irradiated bitumen samples. Then, an intercorrelation procedure was used to estimate the difference of time of flight between the two transmitted ultrasonic signals,

$$ \Delta t = t_{\text{irradiated}} - t_{\text{non-irradiated}} \quad (1) $$

With $\Delta t$, the group velocity of the ultrasonic signal transmitted through the irradiated sample was estimated by the following relationship:

$$ c_{\text{irradiated}} = \frac{e}{\Delta t + (e/c_{\text{non-irradiated}})} \quad (2) $$

In (2), e represents the propagation distance in bitumen and is equal to 55 mm. This velocity corresponds to the non-dispersive ultrasonic velocity in low frequency regime (See Part III.). Up to now, all ultrasonic measurements were performed at 10°C, as ultrasonic signals in bitumen are less attenuated at low temperatures due to rheological reasons, as reported in [6]. For this temperature, $c_{non-irradiated} = 1993 \text{ m/s}$ [6]. The Fast Fourier Transforms of the reference signal and the signal transmitted through irradiated bitumen were also calculated, and the ultrasonic attenuation caused by the bubble cloud was estimated using the following equation:

$$ \alpha_{\text{bubbles}} = -\frac{1}{e} \ln \frac{|\text{FFT}_{\text{irradiated}}|}{|\text{FFT}_{\text{non-irradiated}}|} \quad (3) $$

Signal acquisition and processing were performed using an in-house Python code. Diffraction effects caused by the finite size of the ultrasonic transducers were also taken into account, although the procedure is not described here. Further information concerning this aspect can be found in [8]. A photograph of a Duran bottle containing irradiated bitumen and the acquired ultrasonic signal are shown in Fig. 1.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Ultrasonic 500 kHz sensors, Duran bottle filled with irradiated bitumen and associated ultrasonic signal.</figcaption></figure>

## III. ULTRASONIC WAVES INTERACTION WITH BUBBLE CLOUDS

A bubble cloud excited by an acoustic wave can be seen as an oscillator assembly that may resonate at the Minnaert frequency $F_M$. For a monodisperse viscoelastic bubbly medium such as bitumen [9],

$$ F_M \sim \frac{V_{To}}{\pi R_0}, \quad (4) $$

where $V_{To}$ is the shear ultrasonic velocity in the non-porous matrix and $R_0$ the bubble radius. Far below $F_M$, the porous medium is seen as a homogeneous one, the acoustic attenuation is weak and the longitudinal acoustic velocity does not depend
on frequency. For an acoustic frequency equal to $F_M$, resonance occurs and both acoustic velocity and attenuation increase a lot. At last, when frequency $>>F_M$, acoustic attenuation decreases, and acoustic velocity reaches longitudinal velocity $V_0$ in the non-porous matrix. This phenomenon has been extensively studied in the literature [9] and extended to polydisperse systems. Generally, the polydisperse bubble cloud (void fraction $\Phi$, median radius $R_0$ and polydispersity factor $\varepsilon$) is described by a log-normal distribution

$$n(R) = \frac{n_0}{\varepsilon R \sqrt{2\pi}} \exp\left(-\left(\frac{(\ln(R/R_0))^2}{2\varepsilon^2}\right)\right), \quad (5)$$

where,

$$n_0 = \frac{3\Phi}{4\pi R_0^3 \exp\left(\frac{9\varepsilon^2}{2}\right)} \quad (6)$$

As bubbles lead to an attenuation of the acoustic signal, a complex wave number $k^*$ depending on pulsation $\omega$ can be defined so that,

$$c_L = \omega/Re(k^*) \quad (7)$$

$$\alpha = -Im(k^*) \quad (8)$$

$k^*$ can be numerically calculated, with a Python code in our case, with the following relationship,

$$k^* = \frac{\omega^2}{c_0^2} + 4\pi\omega^2 \int_0^\infty \frac{Rn(R)dR}{\omega_0^2 - \omega^2 + 2ib\omega} \quad (9)$$

The factors b and $\omega_0^2$ depend on the thermal and viscoelastic properties of the bubbles and the matrix and can be calculated using the relationships given in [10,11]. These details are not recalled here.

# IV. RESULTS

## A. X-Ray microtomography

After rough data treatment, void fractions were obtained from X-Ray microtomography with an estimated accuracy of ±1.5%. Porosity histograms were then fitted using the log-normal law (5) for the median radius of bubbles estimation and polydispersity evaluation. All results are summarized in Table II.

<table><caption>TABLE II
PROPERTIES OF BUBBLE CLOUDS ESTIMATED BY X-RAY MICROTOMOGRAPHY</caption><thead><tr><th>N°</th><th>Void fraction<br>Φ (%) ± 1.5</th><th>Median radius<br>(μm) ± 10 %</th><th>Polydispersity<br>factor ε</th></tr></thead><tbody><tr><td>A</td><td>0.7</td><td>380</td><td>35</td></tr><tr><td>B</td><td>7.4</td><td>500</td><td>19</td></tr><tr><td>C</td><td>6.7</td><td>540</td><td>25</td></tr><tr><td>D</td><td>11.4</td><td>580</td><td>25</td></tr></tbody></table>

## B. Ultrasounds

In irradiated samples, longitudinal velocities ($V_{10°C}$) measured at 10°C were respectively for samples A, B, C and D: 1944 m/s, 1800 m/s, 1813 m/s and 1660 m/s, with an accuracy ±40 m/s. In [7], our results suggest that this velocity corresponds to the velocity in low frequency domain. We also demonstrated that a

first estimation of void fraction can be obtained with the following relationship:

$$\Phi = \frac{4}{3} \left[ \frac{V_{To}}{V_0} \right]^2 \frac{(V_0^2 - V_{10°C}^2)}{V_{10°C}^2}, \quad (10)$$

where $V_0 = 1993$ m/s and $V_{To} = 870$ m/s represent the longitudinal and shear velocities in bitumen with no bubbles, respectively.

This led to void fractions equal to 1.3% (A), 5.7% (B), 5.3% (C) and 11.2% (D). Ultrasonic attenuation caused by the bubble clouds in the 100-200 kHz frequency range is shown in Fig. 2. For sample (A), the bandwidth has been reduced because at low frequencies attenuation is too small. One can notice that even if 500 kHz sensors were used, the effective bandwidth after propagation in bitumen was modified because of sample attenuation equivalent to a filtering.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Ultrasonic attenuation induced by bubble clouds versus frequency measured in irradiated samples</figcaption></figure>

The model presented in the previous section was used to fit ultrasonic attenuation measured in order to deduce both the median radius and the polydispersity of the bubble cloud. Classical physical parameters required for the simulation (thermal conductivity, matrix density, surface tension, etc.) were extracted from literature. Matrix viscosity evolution versus frequency was described with a Carreau-Yasuda law, established in our previous works [6]. Regarding Fig. 2, for samples B, C and D at low frequencies, one can notice that the attenuation does not tend to 0 as predicted by the models. This is certainly due to an offset caused by the coupling between sensors and bottles, imperfect correction of sensors diffraction effects, edge effects in the Fast Fourier transform, low sensors efficiency at the considered frequency, etc. In order to adjust our data, the existing offset was corrected. The void fraction first estimated with (10), was slightly adjusted when necessary. The adjustment of experimental data is presented in Fig.3 for samples A, B and D. The adjustment for sample C is not presented for more clarity, as it led to results equivalent to those of sample B. The physical parameters of the bubble clouds, as estimated from the simulation, are listed in Table III.
<figure><img src="image_4.png" /><figcaption>Fig.3. Ultrasonic attenuation adjusted with model presented in Part III for samples A, B and D</figcaption></figure>

<table><caption>TABLE III
PROPERTIES OF BUBBLE CLOUDS ESTIMATED BY ULTRASOUND</caption><thead><tr><th>N°</th><th>Void fraction<br>Φ (‰) ± 1.5</th><th>Median radius<br>(µm) ± 10 %</th><th>Polydispersity<br>factor<br>ε ± 10 %</th></tr></thead><tbody><tr><td>A</td><td>1.5</td><td>530</td><td>40</td></tr><tr><td>B</td><td>5.7</td><td>500</td><td>17</td></tr><tr><td>C</td><td>5.3</td><td>500</td><td>25</td></tr><tr><td>D</td><td>11.4</td><td>540</td><td>18</td></tr></tbody></table>

Ultrasonic evaluation yields results that are consistent with X-ray microtomography, although the current level of uncertainty remains significant, especially for estimates of median radius and polydispersity.

## V. CONCLUSIONS

Until now, only results at 10°C have been presented. Indeed, as previously explained in [6] ultrasonic attenuation increases up to 40-50°C, making measurements at room temperature more challenging due to the high thickness of the Duran flasks used in these experiments. We are currently developing smaller sample cells (thickness = 2 cm) made of aluminum holders. These new cells are expected to improve the contact between the sensors and the bitumen, allowing for a more accurate evaluation of ultrasonic attenuation versus frequency, reduce error bars, and measurements to be performed at room temperature. In addition, signal processing will also be adapted to assess the phase velocity of the ultrasonic signal over the sensor's bandwidth. Although the results presented in this study still require refinement, they are very encouraging and, in our opinion, clearly demonstrate that the ultrasonic approach constitutes a promising method for the non-destructive characterization of bubble clouds in bitumen used for nuclear waste immobilization.

## ACKNOWLEDGMENT

The authors sincerely acknowledge Mejdi Neji (ASNR/PSE-ENV/SPDR/LETIS), Pascal Lemaitre and Soleiman Bourrous (ASNR/PSN-RES/SCA) for their precious help with the Python code dedicated to the 3D analysis of X-Ray microtomographic

data. We also thank Gaetan Touzé from the LECBA laboratory at CEA for the 3D reconstructions performed using VgStudio, TotalEnergies for providing the AZALT 70/100 bitumen samples and Oluwaseyi Miracle Adebola for his help during his Master 2 internship. We are also very grateful to Valentin Leroy from Paris Cité University for his valuable insights into ultrasonic waves interaction with bubble clouds, which were particularly helpful for the simulation aspects. Finally, we sincerely acknowledge Lucie Millot for launching the ARISE project.

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