# Chemical and mechanical decontamination of scales from geothermal shell and tube heat exchangers in operation before anti-scaling treatment

Tomo Suzuki-Muresan, Aditya Rivonkar, Valérie Bossé, Marcel Mokili, Abdesselam Abdelouas, Gilles Montavon, Eléonore Dalmais, Guillaume Ravier, Albert Genter

► To cite this version:

Tomo Suzuki-Muresan, Aditya Rivonkar, Valérie Bossé, Marcel Mokili, Abdesselam Abdelouas, et al. Chemical and mechanical decontamination of scales from geothermal shell and tube heat exchangers in operation before anti-scaling treatment. Geothermics, 2024, 120, pp.102960. 10.1016/j.geothermics.2024.102960. hal-04610424

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# Chemical and mechanical decontamination of scales from geothermal shell and tube heat exchangers in operation before anti-scaling treatment

Tomo Suzuki-Muresan<sup>a, *</sup>, Aditya Rivonkar<sup>a</sup>, Valérie Bossé<sup>a</sup>, Marcel Mokili<sup>a</sup>, Abdesselam Abdelouas<sup>a</sup>, Gilles Montavon<sup>a</sup>, Eléonore Dalmais<sup>b</sup>, Guillaume Ravier<sup>b</sup>, Albert Genter<sup>b</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Decontamination treatments

Scales

Characterization

Waste acceptance criteria

Difficult to measure radionuclides

## ABSTRACT

Heat exchangers used in the Upper Rhine Graben (URG) area to generate heat or electricity are subject during their life cycle to scale accumulation. The main mineral precipitations related to scales in the URG are sulfates (barite, celestite), sulfides (galena, pyrite), nanocrystalline intermetallic mixed compound (Sb, As), elemental metals (Pb, As, Sb), metal oxides and carbonates. During plant operation, radionuclides from uranium and thorium decay chains, that are naturally dissolved in the geothermal fluid circulating in granitic fractured reservoirs, could be accumulated by precipitation or incorporation into the scales as observed on geothermal equipment. The direct treatment in these tubes showed an effectiveness of mechanical decontamination (>95%) and multiple cycle chemical decontamination (~90% HNO₃: HCl 1:3, ~70% DTPA, ~30% NOX-OL100+NOXOL771). However, the choice of chemical treatments is important to consider in relation to the waste acceptance criteria. In particular, the concentrations of chelating agents such as DTPA and EDTA are regulated and cannot exceed the threshold concentrations per waste package recommended by the national waste acceptance criteria. This research work also showed the importance of a good characterization of scales to model as closely as possible the geometry of the tubes, implemented in ISOCS/LabSOCS processing software for gamma spectrometry, in order to evaluate the radioactive level of radionuclides difficult to measure such as <sup>210</sup>Pb.

## 1. Introduction

The extraction of natural resources such as geothermal fluid, natural gas and oil requires facilities and equipment capable of carrying the resource to the surface with good efficiency. For example, geothermal power plants in the Upper Rhine Graben (URG) pump high-temperature water from several kilometers below the surface (5 km) and recover the natural heat produced by the earth through heat exchangers to generate heat or electricity. The natural heat is transported via natural brines circulating at depth in naturally fractured granitic rocks containing the natural chains of U and Th and their descendants including the decay chains of Ra-226 and Ra-228 (Bosia et al., 2021; Genter et al., 2010; Sanjuan et al., 2016). The geothermal fluid which is very saline (100 g L<sup>-1</sup>) contains radioactive radionuclides like K-40 (100 Bq L<sup>-1</sup>), Ra-226 (36 Bq L<sup>-1</sup>), Ra-224 (20 Bq L<sup>-1</sup>), Ra-228 (24 Bq L<sup>-1</sup>) and Pb-210 (7 Bq

L<sup>-1</sup>) as reported by (Bosia et al., 2021). Monte-Carlo simulation studies show that radium nuclei can be displaced over a distance of 10 to 50 nm (Degering et al., 2016; Eggeling et al., 2013). It implies a process of radium transport from the rock to the geothermal fluid by alpha recoil effect mainly and then co-precipitated within secondary mineral phases such as barite BaSO₄ (Eggeling et al., 2013). In addition, Ra-concentration in the fluid can be increased by the hydrothermal alteration of primary granite minerals enhanced at high temperature and pressure conditions at great depth, as well as the presence of fractured zones increasing the rock-water contact surfaces (Degering et al., 2016). In the unaltered granite, the primary minerals are K-feldspar megacrysts (bearing potassium), quartz, plagioclase, biotite and hornblende. The accessory minerals are mainly zircon, magnetite, titanite, leucoxene, and apatite. In such granitic formation, a disseminated pervasive alteration taking place at granite massif scale is characterized

<footnote><sup>a</sup> SUBATECH (IMT Atlantique, CNRS/IN2P3, Université de Nantes), BP, 20722, 44307 Nantes Cedex 3, France</footnote>

<footnote><sup>b</sup> Électricité de Strasbourg Géothermie, 26 bld Wilson, 67000, Strasbourg, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: suzuki@subatech.in2p3.fr (T. Suzuki-Muresan).</footnote>
by the occurrence of clay minerals (chlorite, illite, corrensite) and carbonates (Genter et al., 2010). Within fractured and hydrothermally altered zones, the main secondary minerals are illite, a clay mineral bearing potassium, secondary quartz, carbonates (calcite, dolomite, ankerite), barite, and sulfide (pyrite, galena). Some of those minerals constituting geothermal reservoirs contain high levels of Th, U and K (Scheiber et al., 2012). At great depths, the environment is strongly reducing, stabilizing the species (e.g. uranium) in their reduced form (U (IV)) which is almost immobile in such conditions (Eggeling et al., 2013).

In the Upper Rhine Graben where is located the Soultz-sous-Forêts power plant, there are several geothermal plants in operation which are exploiting fractured granitic reservoirs (Genter et al., 2010; Vidal and Genter, 2018). In such plants, scales are dominated by (Ba, Sr, Ca)SO₄ solid-solution containing minor amounts of galena, pyrite, or poly-metallic sulfides phases. Scales have been observed in the cold part of the geothermal infrastructures where the surface temperature is below 80 °C (Haas-Nüesch et al., 2018; Ledésert et al., 2021; Mouchot et al., 2018; Scheiber et al., 2013, 2012). Fortunately, the use of sulfate inhibitors strongly reduced barite precipitation in surface equipment (Mouchot et al., 2019). However, the geothermal fluid, a Na-Ca-K-Cl dominated brine (Bosia et al., 2021), also contains some minor elements like Ba, Sr and SO₄ and traces elements along with the presence of Ra and their descendants. During the precipitation of barite BaSO₄, radium can be substituted to barium or strontium leading to the formation of a solid solution of (Ba, Ra, Sr)SO₄ (for eg. (Grandia et al., 2008) and references therein). Concerning galena PbS, stable and natural lead as well as radioactive lead isotopes (Pb-210, Pb-212, Pb-214) co-precipitate together to form the solid phase. The thickness of these compacted and densified scales, ranging from a few millimeters to several centimeters, is sufficient to retain and trap the radionuclides generated by the radioactive decay of Ra. These scales are enriched with radioactive elements that can contain a mass activity of the order of 100 to 1000 Bq g⁻¹ (Eggeling et al., 2013). This raises the issue of the maintenance of this type of installation as well as the management of the metal equipment containing these radioactive scales. Those later might be ultimately considered to be long-lived low-level radioactive waste with toxic heavy metal content with the contribution of radionuclides from the uranium and thorium decay chain (²²⁶Ra (half-life 1600 years) in particular), and elements like As, Sn, Hg... This concern, illustrated from the example of geothermal power plants, can also be applied to gas power plants and oil platforms with the precipitation of carbonates, sulfates and/or silicates (Ali et al., 2020; Gamal et al., 2020; Hilal et al., 2014; Kamal et al., 2018). One of the challenges is to decommission long-lived intermediate-level waste into low- to very low-level waste, to reduce the costs and the volumes of waste on the one hand, and eventually the possible recycling of metallic structures after elimination of the radioactivity by decontamination. To do this, the literature is rich in processes whose methods are essentially based on the dissolution of barium/radium sulfate scales extracted from pipes and conditioned into powder form. Rare are the studies directly conducted on pipes coated with a dense, thick and compact deposit layer. The main methods include chemical, mechanical, thermal, microwave and biological methods. For this work, we were particularly interested in physical and chemical methods for decontamination scenarios involving simple to complex geometries consideration, respectively. Among the physical methods, which consist in detaching the labile contamination and tearing off the surface layer, there are techniques such as water jetting, sand blasting, dry-ice blasting, abrasion or melting (OECD/NEA, 2020), scrapping forks (Aminy, 2010)). For chemical methods, which consist in rinsing, stripping and/or degreasing the surface in order to oxidize and dissolve it, we can list the use of strong oxidants (eg. HNO₃, HCl, HF, NaNO₂ (Abdullah et al., 2017; Al Abdullah et al., 2016; Kamal et al., 2018)), strippable polymer coatings (Grav et al., 2001), surfactants (Attallah et al., 2015), chelating agents (eg. EDTA, DTPA, NTA (Dunn and Yen, 1999; Gamal et al., 2020; Kamal et al., 2018)), electrochemical

(Pujol Pozo et al., 2019) applied by soaking in a bath (batch, loop), or by pulverization (dry/liquid gel, foam). The chemical decontamination factor can be significantly increased when the method is associated with agitation, reagent recirculation, and/or high temperature.

The scales investigated in this work originate from the Soultz-sous-Forêts power plant located in Northern Alsace (France), which is operating and producing electricity with a high availability since 2016 (Seibel et al., 2021). These radioactive scales raised the question of treatment of the heat exchanger pipes for decommissioning. This work proposed to study old tubular heat exchangers used before anti-scaling treatment. Several oxidative chemical reagents have been tested along with a commercial product, NOXOL for chemical decontamination. For mechanical decontamination, scrapping method was selected for laboratory testing, compared to other methods used in industry such as sandblasting, high-pressure water jet or dry ice, which require larger samples to evaluate their efficiency. The decontamination yields were evaluated after each treatment.

## 2. Materials and method

### 2.1. Description of the samples

Sample tubes were obtained from old shell and tube heat exchangers installed on a cooling system used when the first Organic Rankine Cycle installed on site was not in operation. These heat exchangers were in operation between 2008 and 2012, before the application of anti-scaling treatment. Heat exchangers A3 and B3 were installed in series, while the third one B5 was used in backup during maintenance on the cooling system. Carbon steel TU 42 BT was the used material with the composition given in Table 1.

10 tubes were collected from each heat exchanger. Each tube was labelled according to their series and numbered from 1 to 10. Three tubes from each series were cut in half lengthwise to expose the inner oxide layer and to get directly access to the scale. Each tube had on average (19.05±0.1) mm outer diameter, (51.5 ± 1) mm length and (3.2 ± 0.1) mm thickness. These tubes were corroded on the internal surface by the geothermal fluid and by the cooling water on their external surface. The radioactivity was contained on the inner layer of the tubes. The scheme displaying the original location of the samples, as well as illustrations of the two types of samples used, one for characterization and the other for decontamination is shown in supplementary information (Figure S1).

### 2.2. Decontamination methodology

#### 2.2.1. Main methodology

The main strategy and methodology are shown in Fig. 1. As a first step, the dose rate of each tube was monitored to evaluate the dose exposure. Then the scales were characterized in order to apply a decontamination method taking into account their composition. For

<table><caption>**Table 1**
Sample characteristics used in ISOCS/LabSOCS characterization softwares.</caption><tbody><tr><td>Modelled dimensions</td><td></td><td>(19.05±0.1) × (51.5 ± 1) × (3.2 ± 0.1)</td></tr><tr><td>Outer Diameter × length × thickness (mm)</td><td></td><td>Fe: 97.38%, Cr: 0.30%, Ni: 0.30%, Mo (+Nb):0.09%, Mn: 0.90%, Si (+P + S+Al): 0.47%, C: 0.2%, Cu: v 0.30%, Ti (+V): 0.06%</td></tr><tr><td>Simplified elemental composition for the tube (in weight%) (Dinesh-Metal-Industries, 2022)</td><td></td><td></td></tr><tr><td>Tube series</td><td>A3</td><td>B3 66±1</td></tr><tr><td>Tube mass (g)</td><td>66±1</td><td>64±1</td></tr><tr><td>Apparent scale density (g.cm<sup>-3</sup>)</td><td>4.78</td><td>4.81</td></tr><tr><td>Assumed weight composition (%)</td><td></td><td></td></tr><tr><td>BaSO<sub>4</sub></td><td>52.55</td><td>48.71</td></tr><tr><td>SrSO<sub>4</sub></td><td>15.91</td><td>14.87</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>18.40</td><td>23.61</td></tr><tr><td>PbS</td><td>13.14</td><td>12.81</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Methodology followed for all the tubes of each series (A3, B3 and B5) to assess the initial dose rates, the composition of scales and the initial activities before decontamination. After mechanical decontamination, chemical decontamination may be applied, represented by a dashed arrow. For some tubes, cycles of chemical treatment were performed, represented by a double layer arrow.</figcaption></figure>

that, we collected the scales from each series of tubes cut in half lengthwise, and crushed them in a mortar. These powder samples were analyzed by X-ray fluorescence (XRF) and X-ray diffraction (XRD) to determine the chemical composition and to quantify the constituent phases. These analyses were completed, upon microwave digestion, by inductively coupled plasma mass spectrometry (ICP-MS) in order to refine the quantification of the elements contained in the scales. The solutions of mineralization were also measured by gamma spectrometry to control the activity before and after digestion, and to verify that there was no loss in the mineralizer during the reactions.

For both methods of mechanical and chemical treatment, the tubes have been systematically measured before and after treatment by gamma spectrometry. For some samples, the two methods have been combined by first performing a mechanical decontamination followed by a chemical decontamination. For the chemical decontamination, several treatments could be applied (chelation, acid) under ultrasonic and temperature.

The fraction of activity removed (FR in%) is expressed as:

$$FR\% = \frac{A_{initial} - A_{treatment}}{A_{initial}} \times 100$$

where Ainitial corresponds to the initial activity before decontamination and Atreatment the activity after each step of decontamination.

The total fraction of activity removed is expressed as:

$$Total fraction removed (\%) = \frac{A_{initial} - A_{final}}{A_{initial}} \times 100$$

where Ainitial corresponds to the initial activity before decontamination and Afinal the activity after the last decontamination step.

### 2.2.2. Mechanical decontamination

A round file was used for mechanical treatment to replicate the scratching effect caused by hard scrubbers. The tool in carbon steel blade supplied by Magnusson Company was selected according to the size of the tubes and the ease of manual scrubbing. The tool was inserted into the tubes, and both the tube and the file were rotated periodically in an attempt to reach all scales. The scratching time lasted less than 10 min for each tube. This decontamination was done manually. It is to note

that this mechanical method of decontamination is adapted for laboratory scale tests and for small tube samples of a few centimeters. It can be comparable to techniques such as sandblasting or high-pressure water jetting, more adapted for tube lengths of about ten meters.

Five tubes were selected for mechanical decontamination (A3–10, B3–2, B3–4, B3–10, B5–1, B5–2, B5–9). For B5–9 tube, a different approach was used wherein the tube was scratched softly, using less force, in order to limit the amount of material base scratched off.

### 2.2.3. Chemical decontamination

Two oxidizing chemical decontamination approaches were selected: oxidizing decontaminations and chelating agents. To simulate different conditions using chemicals, either an ultrasonic vibration chamber (Fischer Scientific) or an incubator shaker, known as Thermotron (INFORS Equipment), was used which allowed for heating up the system to 80 °C along with continuous stirring at 100 rpm. Both were used for 6 h.

**Oxidizing agent.** Aqua regia HNO₃:HCl 1:3 used to oxidize lead sulfur was prepared in a glass bottle and the sample was dipped into it. Contact time between 20 min to 9 h was performed at room temperature.

**Chelating agents.** Three agents were used for the dissolution of barite containing radium: (1) solution of 0.1 M EDTA (Sigma-Aldrich, 99.4% pure powder) prepared at pH 12 adjusted with NaOH solution. The time of contact was 6 h. Ultrasonic agitation was used to increase the efficiency of decontamination; (2) solution of 0.1 M DTPA (Alfa Aesar, 98+% pure powder) at pH 12 was in contact with sample at 80 °C and under agitation; (3) solution of NOXOL-771 (EuroCorpex) prepared at 20%v/v, which is chelating product designed to dissolve barium, strontium, and radium sulfate scales, as well as some other types of inorganic deposits including aluminum and magnesium compounds. The sealed bottles containing the sample were placed in an ultrasonic agitation chamber with a total contact time of 6 h. In addition, prior to use NOXOL-771, 20 vol.% of NOXOL-100 (EuroCorpex) was used to remove the Fe-based corrosion layer. The sample was also placed in an ultrasonic agitation chamber for 6 h.

### 2.2.4. Characterization methods

**Dose rate.** The dose rate was measured using a Colibri® VLD survey meter (Mirion Technologies) from 10 nSv h⁻¹ to 1 mSv h⁻¹
(MIRION-Technologies, 2022). The background level of dose-rate was 325±20 nSv h⁻¹. Measurements were made on the side of the tube (the height of the cylinder) and at the extremities (the top and bottom of the tube). Dose rates were measured by placing the samples in clear polypropylene bottles with a wall thickness of 1 mm and placing the COLIBRI survey meter directly in contact with the bottle.

**Gamma spectrometry:** High Purity Germanium (HPGe) n-type detector and a low background lead castle (Mirion-CANBERRA) was used to ensure the high resolution and detection of the radionuclides with an energy resolution of 1.8 keV for ⁶⁰Co at 1.33MeV, and of 0.9 keV for ⁵⁷Co at 122 KeV. The relative efficiency of the detector is 45%. The gamma spectra treatments were carried out by the GENIE2K software. The calibration in energy was performed by using a reference standard solution of ¹⁵²Eu. The calibration in efficiency of the geometry was modelled using ISOCS/LabSOCS Geometry composer (V4.4), wherein a new tube geometry was created with an internal radioactive layer. The main characteristics used for the model are given in Table 1. The tubes were counted vertically therefore, avoiding self-adsorption of vertical gamma emissions (along the axis of the tube) by the tube in a steel matrix. Although, due to the smaller diameter of the tubes in comparison to the Ge detector, any non-axial gamma rays would undergo complete or partial adsorption before reaching the detector. The thickness of the scales, varying between 0.8 mm to 1 mm depending on samples, as well as the chemical composition and the density have been integrated in the geometry model in Labsocs software. The units chosen to express the level of radioactivity are for the tubes in Bq per tube and for the scales in Bq per gram of scales. Concerning more specifically the tubes, the unit "Bq tube⁻¹" indicates the total activity measured for each tube analyzed before and after decontamination, taking into account similar dimensions for each tube analyzed. Here, "total activity" is defined as the sum of all the radionuclide activities measured in a given sample. The gamma energy lines chosen during the evaluation of the radioactivity level are: ²²⁰Ra (186.21 keV), ²¹⁴Pb (351.93 keV), ²¹⁴Bi (609.31 keV), ²¹⁰Pb (46.54 keV), ²²⁸Ac (911.2 keV) for ²²⁸Ra evaluation, ²¹²Pb (238.63 keV), and ²¹²Bi (727.33 keV).

**X-ray fluorescence (XRF).** The elemental composition of the scales was measured by XRF (Shimadzu). The extracted scales were crushed and placed into the cell in sufficient quantity to form a layer on the bottom of the film. Three samples (A3, B3 and B5) were counted under ambient atmosphere without applying vacuum with a relative uncertainty of 50%. From the detected elements (Ba, Fe, Pb, Sr, S), we calculated the quantity of Ba and Sr sulfate and Pb and Fe sulfide, focusing only on the major metals.

**X-ray diffraction (XRD).** Bruker D8 Advance diffractometer equipped with a copper anode (λ = 1.5406 Å) was used to collect XRD patterns. Data were collected in the 2θ range from 18° to 90° with a step of 0.015° and a scan-speed of 5 s step⁻¹. Data were treated using Diffrac.EVA V4.3.1 software for the assessment of scale composition. Rietveld refinement using JANA2006 was used for the validation of identified phases in scale composition (Petricek et al., 2006).

**Mineralization of scales.** An ETHOS EASY Milestone mineralizer equipped with SK-15eT Rotor was used to solubilize the scales collected by mechanical decontamination in high-purity concentrated acids (Merck and Sigma-Aldrich). Among the protocols proposed by the supplier (internal database of Milestone), protocols for the solubilisation of CaSO₄, SrSO₄, BaS, PbSO₄, PbS, FeSO₄, Fe₂O₃, iron ore, Fe-Cr High carbon, Si-Cr alloy were tested according to the presence of sulfide and sulfate phases. After an intensive optimization of digestion parameters by adjusting the mass of sample, the concentration and volume of chemical reagents as well as the chemical composition, two protocols were therefore selected named BaS and Si-Cr alloy. The detailed conditions are reported in supplementary information (Table S1).

**Inductively coupled plasma mass spectrometry (ICP-MS).** A Thermoelectron Xseries 2 ICP-MS was used to determine the elemental composition of the microwave mineralized deposits. NIST certified calibration standards from SCP Science were used to perform the

external calibration of the ICP-MS and to correct the signal drift by internal calibration. Lead, barium and strontium were quantified by hot plasma, while iron was determined by collision reaction cell (CCR) in order to minimize interference on mass 56. Calibration standards and samples were prepared by dilution in 2% ultrapure HNO₃ matrix. The limits of quantification are 0.29 µg L⁻¹ for Ba, 2.31 µg L⁻¹ for Fe, 0.31 µg L⁻¹ for Pb and 0.24 µg L⁻¹ for Sr, with relative uncertainties at 4–8%.

# 3. Results

## 3.1. Radiological characterization before decontamination

The Table 2 reports the dose rate measurements made at the extremities of the tube and along the tube. The results show dose rates higher for the A3 series than the other two series, indicating differences between the series studied. One can also notice the variation of the dose rates between the different A3 samples. As the A3 series has been operated for a longer period of time, the deposition of radioactive material may have been accumulated indicating a higher activity in the scales. For the B3 series, the dose rate results are heterogeneous indicating a heterogeneous accumulation of the scales along the tube. This heterogeneity is also observable in the series A3 where hot spots can be measured at 2 to 3 times the background. As for the B5 series, having been used only during maintenance of primary exchanger with few hours of service, its dose rate is therefore low and confirms the results.

The Fig. 2 shows the total initial activity measured by gamma spectrometry before decontamination for the A3, B3 and B5 series tubes. Series A3 and B3 have activities of 4276 ± 408 Bq tube⁻¹ and 4064 ± 1619 Bq tube⁻¹, respectively. The heterogeneity in dose rate measurements found on the B3 series are also found in the activity measurements along the tubes of the series (from B3–2 to B2–10). The B5 series counts 922 ± 273 Bq tube⁻¹, confirming the low activity of this series. Details of the activities per radionuclide are provided in supplementary information (Tables S2-S4).

## 3.2. Characterization of scales and their composition

XRF characterization (Fig. 3) of the scales indicates the main presence of Ba, Fe, Pb, Sr and S. For A3 and B3 scales, between 56% and 59% are composed of Ba and Sr sulfate and the rest being attributed to Pb and Fe sulfide. The phase distribution remains homogeneous for these two samples. Concerning the B5 scale, the XRF analysis indicates a contribution dominated by an Fe-rich phase, nearly 89% of FeS phase. The origin of this important contribution in iron can be explained by the frequent stop of this exchanger with a continuous preservation of the system in the geothermal fluid. Very little maintenance has been done, leading to corrosion and the formation of an iron-enriched scales. This first step of characterization confirms the existence of sulfate and sulfide scales, however this technique cannot detect light element such as oxygen and does not allow to access to oxide compositions. Therefore, the characterization has been completed by XRD (Fig. 3) revealing the presence of several Fe-phases such as Fe-oxide of hematite type Fe₂O₃ and Fe-sulfide FeS. The complex mineralogical nature of the scales did not allow to give a more precise quantification by Rietveld refinement between the different Fe phases. However, the results obtained from the XRF and XRD techniques allow to select the adapted mineralization protocols for the scales allowing a more accurate elemental quantification.

To validate the protocol of mineralization, triplicates for each series of scale powders were performed and the solutions of mineralization were measured by gamma spectrometry. Each repetition corresponds to the mean value obtained on the base of the two protocols selected as described in supplementary information (Table S1). Then the activity of each radionuclide in the scale was compared before and after mineralization. The results show that for the three series A3, B3 and B5, the activities measured in the solution of mineralization are close to the
<caption>**Table. 2**</caption>

External net dose rates (nSv h⁻¹, after subtracting the background) measured by a Colibri® VLD survey meter of series A3, B3 and B5. The background level was 325 ±20 nSv h⁻¹.

<table><thead><tr><th>Tube</th><th>On the side</th><th>At the extremities</th><th>Mean</th><th>Tube</th><th>On the side</th><th>At the extremities</th><th>Mean</th><th>Tube</th><th>On the side</th><th>At the extremities</th><th>Mean</th></tr></thead><tbody><tr><td>A3-1</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td><td>B3-1</td><td>background level</td><td></td><td></td><td>B5-1</td><td>background level</td><td></td><td></td></tr><tr><td>A3-2</td><td>469</td><td>411</td><td>440</td><td>B3-2</td><td>657</td><td>518</td><td>588</td><td>B5-2</td><td>background level</td><td></td><td></td></tr><tr><td>A3-3</td><td>529</td><td>725</td><td>627</td><td>B3-3</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td><td>B5-3</td><td>background level</td><td></td><td></td></tr><tr><td>A3-4</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td><td>B3-4</td><td>363</td><td>407</td><td>385</td><td>B5-4</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td></tr><tr><td>A3-5</td><td>538</td><td>695</td><td>617</td><td>B3-5</td><td>background level</td><td></td><td></td><td>B5-5</td><td>background level</td><td></td><td></td></tr><tr><td>A3-6</td><td>1085</td><td>925</td><td>1005</td><td>B3-6</td><td>background level</td><td></td><td></td><td>B5-6</td><td>background level</td><td></td><td></td></tr><tr><td>A3-7</td><td>735</td><td>675</td><td>705</td><td>B3-7</td><td>background level</td><td></td><td></td><td>B5-7</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td></tr><tr><td>A3-8</td><td>411</td><td>444</td><td>428</td><td>B3-8</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td><td>B5-8</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td></tr><tr><td>A3-9</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td><td>B3-9</td><td>Tube cut in half, dose rate not measured</td><td></td><td></td><td>B5-9</td><td>background level</td><td></td><td></td></tr><tr><td>A3-10</td><td>353</td><td>440</td><td>397</td><td>B3-10</td><td>553</td><td>596</td><td>575</td><td>B5-10</td><td>background level</td><td></td><td></td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Total gamma activity measured from ²²⁶Ra, ²¹⁴Pb, ²¹⁴Bi, ²¹²Pb and ²¹²Bi on series A3, B3 and B5. The activity of ²¹⁰Pb is not integrated.</figcaption></figure>

activities in the scales themselves (details are given in supplementary information – Figure S2). A3 and B3 scale samples give similar results confirming their similar behavior during operation at the geothermal plant. Concerning B5 scale sample, the lower activities confirm the weak deposition of radioactive matter within the scale. Activities of ²²⁸Ac and ²¹²Bi were below the detection limit. The validation of the mineralization method allows to continue the characterization of scales by performing ICP-MS measurement on the same digestion solutions. The results give an average value of: 13 wt.% SrSO₄, 43 wt.% BaSO₄, 11 wt.% PbS, 16–20 wt.% Fe₂O₃ for A3 and B3 series, and 2 wt.% SrSO₄, 8 wt.% BaSO₄, 1 wt.% PbS, 86 wt.% Fe₂O₃ for B5 series (details are given in supplementary information – Table S5). Firstly, the total composition is about 83 wt.% to 97 wt.%, lower than 100wt% due to the contribution of other chemical compounds such as Sb, As, Ca, Cr, Cu and Zn. These minor phases can contribute up to 15–17% for the series A3 and B3, and 4% for the series B5. However, these elements were not integrated in the final design of the model geometry which bring more complexity to the scale composition. Then, taking into account the heterogeneous distribution of scale within one series and getting a representative scale composition, the main elements retained are Sr, Ba, Pb and Fe. Concerning Fe, the difficulty was to determine the distribution between FeS and Fe₂O₃ mainly identified by XRD refinement, but we think that the composition is more complex including other phases of Fe such as FeSO₄ and FeCO₃. Considering the limitation of XRF technique to quantify oxide phases and the characterization of Fe₂O₃ phase by XRD, we assume that the entire inventory of Fe corresponds to hematite Fe₂O₃. This is an approximation and a simplification of the scale composition integrated in ISOCS/LabSOCS geometry model. Finally, the total composition mentioned in Table 1 is re-calculated to 100% composed by SrSO₄, BaSO₄, PbS and Fe₂O₃ phases.

### 3.3. Validation of the geometry for gamma spectrometry

The next validation concerned the geometry tube by using the ISOCS/LabSOCS geometry composer. For this purpose, the A3–10 tube was taken as a reference. A refined analysis was performed to determine the mass balance between the activity measured in the tube and the extracted scale. The Fig. 4 shows the detailed activities of the scale in the tube deduced from the difference of the activities before and after decontamination. This activity is directly compared with that of the extracted scale. The results show that for the two measurements, the activities of scale deduced from the tube are close to the activity measured directly in the scale with a deviation of about (15±5)%. Moreover, these results reveal an overestimation of the ²¹⁰Pb activity when it is deduced from the difference of the activities before and after decontamination. This important deviation of Pb-210 estimated in the tube before any treatment is discussed in the Section 4.3 of this paper. Therefore, the composition of scales and tube, as well as the geometry and the characteristics of the tubes give an acceptable validation of the method. This validated method is applied and adapted for the series A3, B3 and B5.

### 3.4. Mechanical decontamination combined with chemical decontamination

Mechanical decontamination was applied to the samples for all three series with a decontamination factor ranging between 30% and ~100%. Fig. 5-a shows that for the tubes A3–10 and B3–4, the decontamination factor is about (99.5 ± 0.3)%, revealing a very good efficiency of this method when high mechanical pressure is applied. The minimum of decontamination factor ((31±3)%) is attributed to low mechanical pressure as it is the case for the tubes B3–2, B5–9-cycle1 and B5–9-cycle2. For tubes B3–10, B5–1 and B5–2, the decontamination factor is about 75%. The differences in the values of the removed fractions can be linked to the pressure applied with the decontamination tool (a round file). But the nature of the scales may also explain the differences in hardness between the sulfides and the sulfates. For a same high mechanical pressure, in A3 and B3 series the scales are dominated by the presence of Ba and Sr sulfate with a hardness of the order of 2.5–3.5, lower than hematite considered as the dominant phase in the B5 series with a hardness of 5–6 (Anthony et al., 1990). This method of decontamination allows to obtain the greatest factors making it effective in particular for simple geometries as in the present application with straight tubes.

The A3 and B3 series tubes were subsequently decontaminated by chemical treatment. Two types of media have been used: EDTA 0.1 M pH12 and NOXOL771 20 vol.%. The tubes were put in contact with the solutions for 6 h under agitation at 80 °C. The results in Fig. 5-a show that completing the mechanical treatment with the EDTA chemical treatment brings a slight improvement in the performance. A soft mechanical decontamination completed with a chemical decontamination (NOXOL 771) produced equivalent efficiency to that of hard mechanical
<figure><img src="image_6.png" /><figcaption>Fig. 3. (left) distribution of the main phases identified by XRF and (right) X-ray diffraction patterns obtained on powders A3, B3 and B5.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. activities of $^{226}$Ra, $^{214}$Pb, $^{214}$Bi, $^{212}$Pb, $^{212}$Bi and $^{210}$Pb of scale from A3–10 tube: activity of scales contained in the tube before mechanical treatment compared to the direct measurement of scale after extraction.</figcaption></figure>

decontamination. Nevertheless, mechanical treatment remains the decontamination method of choice.

### 3.5. Chemical decontamination: single and double stages

Fig. 5-c shows the result of treatment performed by contacting complexing agents such as EDTA and DTPA under agitation at 80 °C during 6 h on tubes A3–8, B3–5, B5–6 and B5–10. These molecules are known to be strong complexing agents, particularly towards metals. The studies carried out by (Abdullah et al., 2017) also show an important efficiency on the dissolution of barium sulfate close to 90% at 60 °C. The results indicate that for a decontamination based on EDTA, the decontaminated fraction represents at most 23%, whereas in the presence of DTPA this fraction presents in average 60%. For tube B5–10, two cycles of decontamination with DTPA were applied. It can be seen that after one cycle about (42±1)% of the radioactivity is removed. After measuring the activity on the final tube, a total of nearly (61±4)% of radioactivity was removed after 2 cycles. These results also show the efficiency of DTPA compared to EDTA, notably by the presence of a higher number of ligands (octadentate) and complexation sites.
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Fraction of activity removed (%) after mechanical and/or chemical decontaminations of A3, B3 and B5 series. The total activity is obtained from $^{226}$Ra, $^{214}$Pb, $^{214}$Bi, $^{212}$Pb and $^{212}$Bi. (a) Overview of the mechanical treatment; (b) Detailed view of the dotted box in (a); (c) Contact with a solution of EDTA or DPTA 0.1 M at pH12 under agitation at 80 °C; (d) Contact with HNO$_3$:HCl 1:3 solution; (e) Contact with NOXOL771 20 vol.% under agitation at 80 °C and NOXOL100 20 vol.%.</figcaption></figure>

Treatment with DTPA with multicycle steps in this study appears to be encouraging.

A second type of chemical decontamination was applied by treating two tubes A3–7 and B3–5 with aqua regia (Fig. 5-d). Tube A3–7 was contacted with a concentrated (~10 mol L⁻¹) HNO₃:HCl solution. The results show that after one treatment cycle of 20 min of contact at room temperature, almost (26.0 ± 0.7)% of the activity was removed. This result is promising considering the short contact time and the low temperature (room temperature). To validate, a second test was conducted with the B3–5 tube. Three treatment cycles have been applied to the tube at (~4 mol L⁻¹) HNO₃:HCl solution with each time the renewal of the solution. The contact time was 1 h, then twice 4 h. This treatment cycle was also intended to simulate dynamic conditions such as a loop. It can be seen that nearly (89±1)% of the activity was removed. This result is extremely promising because it allows to act on both the sulfide and sulfate phases, although aqua regia is known to oxidize sulfides such as Pb and Fe sulfides.

A third treatment of scales by NOXOL products has been also tested

for two grades: NOXOL771 and NOXOL100. This product is used and applied for the dissolution of sulfate scales and iron oxides, respectively (Feltcorn, 2006). A direct treatment with a 20 vol.% solution of NOXOL771 gives results with a maximum of 14% of removed activity (Fig. 5-e). Decontamination with NOXOL771 gives similar heterogeneous results to that with EDTA on the samples studied. One of the reasons may be due to the heterogeneity of the scales themselves, where it was seen during their characterization that they are not homogeneously distributed. On the other hand, these complexing agents are selective, and in particular they act in an effective way with the sulfate phases. However, the presence of oxides can inhibit the dissolution reaction of the sulfate phases. Therefore, a pre-treatment with NOXOL100 was carried out to dissolve these oxides. The results show that after treatment with NOXOL100 which has removed about (4 ± 1)% the following treatment by NOXOL771 does not provide expected results without enhancing significantly the decontamination factor. Similar result has been obtained with tube A3–7 for which an aqua regia solution treatment followed by a NOXOL771 treatment was tested. It was
found that NOXOL771 did not increase the fraction removed although the tube underwent two successive chemical treatments in presence of oxidative and complexing agents. Compared to DTPA with satisfying decontamination factor with 2 cycles of treatment, supplementary experiments on cycle treatment with NOXOL110+NOXOL771 need to be performed. In this work the concentration of NOXOL771 solution at 20 vol.% might have been too low and a higher concentration could be tested in a cycle mode.

This chemical decontamination study demonstrates that the concept of cycles is important because solutions can be quickly saturated and therefore limit the effectiveness of their action. The renewal of the solution is an important asset to consider, and in the case of industrial application the presence of a decontamination loop is a necessity to increase the decontamination factors.

# 4. Discussion

## 4.1. Efficiency of decontamination methods

In this study, two types of decontamination methods were used: mechanical and chemical. The results showed and confirmed the effectiveness of mechanical decontamination. This method is also used at an industrial level and is suitable for decontaminating simple geometries, i. e. straight pipes (Aminy, 2010). Scrapping is an environment-friendly decontamination without secondary waste, which prevents an accumulation of contaminated dust and wastes, saves valuable resources and is an automated decontamination process. Other methods such as high-pressure water jetting can also be effective but may generate wet secondary waste in the form of sludge to be processed for subsequent conditioning. Sandblasting can also be an interesting mechanical method but has the disadvantage of generating large volumes of waste. However, these methods are not very suitable for complex geometries such as pumps, valves, bent pipes, grids.... Thus, chemical decontamination allows to overcome these difficulties.

Following the different chemical reactions tested to either oxidize or chelate the scales and the dissolved elements, multicycle aqua regia HNO₃:HCl 1:3 treatment has shown promising results with an efficiency close to the mechanical decontamination. Aqua regia solution allows the oxidation and mobilization of the sulfides and sulfates upon dissolution. Moreover, the implementation of cycles with fresh solutions avoids the system from reaching saturation, allowing to maintain the continuous oxidation reaction. After each cycle, the activity collected in the decontamination solutions and the filtered scales residues detached from tube B3-5 were measured. The cumulated activity (solution, filter) is 846 Bq after the 1st cycle (1 h), 457 Bq after the second cycle (4 h) and 70 Bq after the third cycle (4 h). These values correspond to the total activities excluding the activity of Pb-210. It can be seen that the largest fraction of the radioactivity is removed after the two first cycles while the variation between cycle 2 and 3 is very small. A trend emerges and a multi-cycle decontamination with aqua regia effectively and significantly reduces the level of activity in the tubes. To optimize this method, the set-up could be heated to 60 °C and stirred in an extracting fume hood to accelerate the reaction, especially after the 2nd cycle, and to create turbulence which could better represent the environment within the loop system in the industry thereby allowing the scale to fall off the tubes.

An important issue in chemical decontamination is the chemical composition of the secondary effluent wastes generated. Indeed, the optimization of the treatment by aqua regia has in practice produced large quantities of non-radioactive waste volume characterized by the dissolution of the metallic tubes. In the present work with the B3-5 tubes, 5% of its total weight has been lost after treatment with a loss equivalent of ~3.8 g-Fe-Cr-Ni per liter of decontamination solution. The contribution of radioactive solid is estimated to 1.4 ng, which corresponds to a ratio of 10⁹ between the quantity of Fe dissolved from the metallic pipe and the total quantity of radioactive contaminants. A

preliminary treatment of these secondary effluents by selective extraction of metals (Fe, Ni, Cr) by means of resin or co-precipitation will then allow the reduction of radioactive waste volume. Furthermore, the addition of corrosion inhibitor during these chemical treatments could reduce and prevent an excessive loss of the base metal. Thus in the case of straight pipes, the implementation of a loop injection system flowing inside the pipe could reduce the volume of secondary effluents, as well as the quantity of dissolved iron. For metallic wastes with complex geometries, segmentation remains an efficient way to reach the different parts and ensure a good decontamination.

To overcome this effect, adapted chemical decontamination methods to the nature of the scales were intended for this purpose and selected. EDTA and DTPA as well as NOXOL-grades are complexing agents which have largely demonstrated their effectiveness, in particular for dissolving barium sulfate and radium scales. Pure barium and radium sulfate compounds are insoluble in water and acidic media such as HNO₃ and HCl (Matyskin et al., 2017, 2016). Their very low solubility (KsBaSO₄=10⁻⁹.⁹⁷, KsRaSO₄=10⁻¹⁰.⁴⁴ (Dean and Lange, 1999)) makes chemical decontamination difficult (Morris and Paul, 1990). However, they become highly soluble in the presence of organic ligands at high pH such as chelates composed of ethyleneamine molecules with several carboxylic acid functions capable of complexing barium from the solid to the liquid form (Jones et al., 2007). The most common are molecules belonging to the polyamino-carboxylate family including ethylene diamine tetraacetic acid (EDTA) and diethylene triamine pentaacetic acid (DTPA) (Abergel, 2014; Dunn and Yen, 1999). EDTA and DTPA are hexadentate and octadentate ligands, respectively, i.e. they have 6 to 8 metal-complexing sites (Bucheli-Witschel and Egli, 2001; Kelland, 2014). The ligand binds to Ba²⁺ ions through the oxygen atoms of the carboxylate groups as well as through lone pairs of the nitrogen atoms to form soluble [Ba(EDTA)]²⁻ and [Ba(DTPA)]³⁻ species. The stability of the complex is attributed to the ability of the chelating agent to form more than one bond with the metal ion, resulting in the formation of a cyclic structure incorporating the metal ion (Putnis et al., 2008). The formation constants of the metal-chelate complex increase according to Ra<Ba<Sr<Ca and the reactions are favored in the presence of DTPA (Abergel, 2014; Smith and Martell, 1987). Thus, the presence of Ca may limit the decontamination efficiency of Ra by EDTA and DTPA (Jones et al., 2007). XRF analyses on scales (results not shown) revealed the contribution of Ca up to 2wt% in A3 and B3 only, but with an uncertainty of 20%. Therefore Ca might be present in the scales but as minor to trace contributor. Furthermore, EDTA is resistant to ionizing radiation as well as high temperatures up to 200-250 °C (Keith-Roach, 2008). The tests carried out on tubes B5-6 and B5-10 in the presence of DTPA seem to be promising compared to EDTA and could be considered as an alternative solution for the decontamination of tubes from geothermal exploitation. Metal sulfides such as PbS, CuS, FeS, on the other hand, are insoluble in reducing and anoxic media characterized by low solubility constants (KsPbS=10⁻²⁷.¹⁰ (Dean and Lange, 1999). The dissolution of PbS is conditioned by the presence of oxygen and an acidic environment (Hsieh and Huang, 1989). Dissolution becomes unfavorable (almost zero) for pH values above 8 (Hsieh and Huang, 1989). Hsieh and Huang propose a two-step mechanism of PbS dissolution with (1) adsorption of oxygen molecules on the protonated surface of PbS forming a surface complex, followed by (2) an oxidation of sulfides to sulfates resulting in the release of lead into solution (Hsieh and Huang, 1989). HNO₃:HCl 1:3 (aqua regia), a medium commonly used to dissolve metal sulfides (e.g. (Davidson, 2013; Helser and Cappuyns, 2021; Jung, 2008)), was employed to dissolve Pb sulfides, typically found on the surface of scales (Scheiber et al., 2013). Considering the simultaneous presence of sulfate and sulfide scales, a double chemical combined treatment (oxidizing and chelating agents) reveals the necessity to include an oxidizing agent (HNO₃:HCl or NOXOL100) to dissolve the sulfide scales as well as the Fe-oxide phases. However, the major difference between the literature data and the present work is the form of the scale utilized. Indeed, in almost all the studies listed previously, the tests are generally carried out
on scales extracted and put in fined and sieved powder form, thus favoring the contact surface (eg. (Abdullah et al., 2017)). A direct application and transfer of methodology to the deposits fixed inside the tube require combined methods, either mechanical, electrochemical or vibration, to initiate the detachment or the dissolution of sulfate and sulfide scales.

## 4.2. Waste acceptance criteria

The strategies implemented to optimize decontamination methods are based on several criteria: reduction of waste volumes, solidification of liquid effluents, reduction of secondary effluents, concentrations of chemical toxics and heavy metals, regulatory limits imposed by the national nuclear waste management agencies, pH adjustment, conditioning of wastes in a long-term performance matrix, etc.

To take the example of the tests conducted on the B3–5 tube, we have noted that from all the radioactivity concentrated in ~430 mm³ of scale, the 3 cycles of aqua regia acidic treatment generated 1.35 L of liquid waste with pH adjustment to 7 and one 30 L bag of solid wastes (filtered solids, papers, gloves, plastic bottles...) during the whole test. In this example, the liquid waste was composed of nitric and hydrochloric acids which have limited concentrations accepted in the final waste. In France, for waste producers such as universities, research laboratories, industries outside of the electronuclear sector, the general accepted waste conditioning for a drum containing short half-life (<31 years) without alpha emitters, Cl concentration needs to be <50 g kg⁻¹ and NO₃ concentration <11 g kg⁻¹ in solid and liquid wastes (Andra, 2018). Moreover, the concentration of heavy metals including As, Cr, Cu, Ni, Pb, Sb, and Zn needs to be <70 mg kg⁻¹ in solid wastes and <280 mg kg⁻¹ in liquid wastes (Andra, 2018). After aqua regia treatment, the concentration of Cr and Ni in liquid waste reached a maximal value of ~1000 ppmCr and ~290 ppmNi, conducting to a high amount of these metal elements by a factor 3.5 compared to the regulatory limit. Since these elements are stable, a physico-chemical separation by (co-)precipitation and filtration would allow to decrease their contribution and concentration in the final waste package. These limits are given as an order of magnitude, and it could not be directly applicable due to the presence of long lived radionuclides such as ²²⁶Ra (1600 years). Specific agreement with the waste management agency needs to be therefore fulfilled.

In addition in this study we have shown that DTPA could be an efficient decontamination agent which could be applied for complex geometry components. These complexing agents are known for their advantage of keeping dissolved elements in solution without precipitation and can also solubilize radionuclides by complexation by competing with the mechanisms of sorption and retention on solid phases. Nevertheless, they have the disadvantage of promoting the migration and transport of radionuclides in the environment of the disposal site. The recommendation states that the concentration of these complexing agents (EDTA, DTPA) should be as low as possible per waste package. For example, in Canada or at Los Alamos in United States, the concentration must be less than 0.1 wt.% (Besner, 2012; Harvey et al., 2020; SKB, 2015). Another alternative route might be the use of NOXOL-grades because of its biodegradable and low toxicity properties and the absence of chlorine (EuroCorpex-Ltd, 2011).

## 4.3. Difficulty to characterize and measure low energy radionuclide: case of ²¹⁰Pb

All the results presented and discussed in this study are based on the measurement of ²²⁶Ra, ²¹⁰Pb, ²¹⁴Bi, ²¹²Pb and ²¹²Bi which have gamma emission lines high enough to be detected by gamma spectrometry. One of the main difficulties was that the tubes were too thick (3 mm) to totally attenuate the only gamma ray emitted by ²¹⁰Pb at very low energy (46 keV). This difficulty is due to the high density of the metal tube

(~7.8 g cm⁻³) and its high mass attenuation coefficients (⟨μ⟩<sub>Fe</sub> ~2.7 cm² g⁻¹, ⟨μ⟩<sub>Ni</sub> ~3.3 cm² g⁻¹, ⟨μ⟩<sub>Cr</sub> ~2.2 cm² g⁻¹ compared to ⟨μ⟩<sub>Al</sub> ~0.4 cm² g⁻¹ or ⟨μ⟩<sub>Ti</sub> ~1.5 cm² g⁻¹ (Berger et al., 1998)) at the photoemission energy of ²¹⁰Pb. Therefore, two positions horizontal and vertical were tested. In the horizontal position, no ²¹⁰Pb-photoemission peak was detected, confirming the high attenuation within the thickness of the metal tube, whereas in the vertical position ²¹⁰Pb becomes the main contributor. Fig. 6 compares the results for all the radionuclides activities of the U-, Th-chains measured with A3–2 tubes. Not only the energy peak of ²¹⁰Pb is missing, but a strong reduction of the activity with horizontal position is obtained with 100% for ²¹⁰Pb and between 30% to 50% for the rest.

The vertical position has been nevertheless selected to avoid any missing radionuclide counting. For this simulated geometry as described in Table 1 using ISOCS/LabSOCS characterization softwares and the results shown in Fig. 4 comparing the activities of Pb-210 in the tube and in the extracted scale, the activity values on ²¹⁰Pb reveal an over-estimation by nearly a factor of 165. To determine the expected activity in the scales, the scales were directly measured by gamma spectrometry during mechanical stripping. The results (supplementary data, Figure S2) show on one hand that the ²¹⁰Pb inventory remains high compared to the other radionuclides of the decay chain (²²⁶Ra, ²¹⁴Pb, ²¹⁴Bi) but this time by a factor of 5. On the other hand, thanks to these measurements, we confirmed a higher activity in ²¹⁰Pb compared to the other radionuclides of its same decay chain whose mechanism remains to be confirmed.

The geothermal fluid is characterized by its very high salinity in Cl, Na, Ca and K, with minor species (<0.5 g/L) like Ba, Sr and SO₄ and trace elements (< 0.05 g/L) like Pb, As and Sb. The brine temperature decrease leads to scale formation. Major mineral precipitations are Ba, Sr,Ca-sulfates (e.g., barite, celestite) with a presence of sulfides (galena, pyrite) (Haas-Nüesch et al., 2018). The solubility constants for barium sulfate and lead sulfide of 10⁻²⁷ and 10⁻⁹.⁹⁷ (Dean and Lange, 1999), respectively, indicate the favored precipitation of Pb sulfide. However, characterization of the scales untreated by scale inhibitor reveals a greater amount of Ba/Sr sulfate than Pb sulfide and the presence of a thin layer of Pb sulfide (Scheiber et al., 2012). Sulfide concentrations in the surface fluid are below the limit of quantification. Microbial activity was not detected in the geothermal fluid (Bosia et al., 2021) and this process cannot explain the formation of galena. In order to understand the origin of PbS formation, geochemical modeling performed using the open-source software Phreeqc with PHREEQC-ThermmodemV1.10_15Dec2020 database suggests that thermodynamic equilibrium variation explains the formation of sulfide. The temperature variation leads to equilibrium variation between SO₄²⁻ and HS⁻. The low solubility leads to the fast consumption of HS⁻ by stable lead present in the geothermal brine thus co-precipitating radioactive lead isotopes (²¹⁰Pb, ²¹²Pb, ²¹⁴Pb) with galena (Degering et al., 2015).

## 5. Conclusion and perspectives

This study conducted on the mechanical and chemical decontamination of scales in metal tubes from geothermal activities confirmed the effectiveness of mechanical decontamination. As the stripping method is adapted to the laboratory scale, it is recommended to test methods suitable for industrial scale such as high pressure water jets at 1000–3000 bars for linear tubes and dry ice for complex geometries. The chemical decontamination, particularly adapted for more complex geometries, shows high efficiency by the use of strong oxidizing agents like aqua regia to dissolve sulfides or complexing agents like DTPA to dissolve sulfates. Decontamination thus makes it possible to reduce the volumes of low-level long-lived wastes by changing their category to very low-level (VLW) waste. Another issue will be the management of chemical toxic substances (Pb, As, Sn...) that are also naturally present in the geothermal fluid but for which restrictions exist in the disposal sites and quotas are imposed by the national waste management
<figure><img src="image_9.png" /><figcaption>Fig. 6. Comparison of the horizontal and vertical positions of A3–2 tube for the optimization of the geometry position during gamma spectrometry acquisition.</figcaption></figure>

agencies. An economic gain can be noted since the cost of the geological disposal of a VLW waste is estimated at 500€·m⁻³ compared to intermediate low level waste at 5000 €·m⁻³ (Bataille and Namy, 2017). Injection of inhibitors is also an alternative solution applied by geothermal industry to minimize significantly the quantity of scales deposited (Scheiber et al., 2012). With those scaling inhibitors, operators succeeded to strongly reduce the formation of sulfates scaling and the main residual scales are poly-metallic sulfides (Mouchot et al., 2019).

The results raised many questions, in particular on the adequate use of chemical decontamination solution which must meet the criteria of waste acceptance if these secondary effluents were to be conditioned and solidified in a sustainable matrix ready for disposal as a waste form.

Finally, an important point that could not be investigated in depth during this study is the measurement and characterization of radionuclides that are difficult to measure with the case of ²¹⁰Pb encountered here. Indeed having only one gamma emission at 46 keV does not allow an easy measurement of Pb-210. To go a step further, metallic wastes present a strong issue on the characterization of radionuclides difficult to measure (DTM) for decontamination and sanitation of the wastes arising from nuclear power plants, oil and gas plants, or geothermal plants. These DTM are characterized by their very low emission energies such as ⁵⁹Ni, ⁹³Zr, ²¹⁰Pb. Finally, the refinement of radiochemical characterization is important and the development of geometry models (e.g., Monte Carlo simulations) for complex matrices is a necessary need for the future, especially in connection with decontamination, dismantling, sanitization and cleanup activities.

### CRediT authorship contribution statement

**Tomo Suzuki-Muresan:** Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. **Aditya Rivonkar:** Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing – original draft, Writing – review & editing. **Valérie Bossé:** Investigation, Methodology. **Marcel Mokili:** Conceptualization, Formal analysis, Methodology, Validation, Writing – review & editing. **Abdesselam Abdelouas:** Methodology, Writing – review & editing, Supervision, Validation. **Gilles Montavon:** Writing – review & editing. **Eléonore Dalmais:** Validation, Writing – review & editing. **Guillaume Ravier:** Validation, Writing – review & editing. **Albert Genter:** Writing – review & editing.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

The authors would like to thank the EEIG Heat Mining, the site owner from the Soultz-sous-Forêts geothermal power plant for giving access to scaling samples, as well as ES (Electricité de Strasbourg) and Ademe (the French ecological transition agency) for partially funding the research work. We would like to express our gratitude to Mr. H. Lemelletier from Mirion for his help in the design and development of the tube geometry for the ISCOS/LabSOCS software, and to Ms. C. Bosia, former employee of ESG during the project, for all the fruitful discussions and constructive questions and remarks. We also wish to thank Dr. K. Chaillou from DSEE department (IMT Atlantique) for the XRF measurements as well as Mr. Matias Alvarez-Gomez for his contribution in tube decontamination, Ms. C. Bailly and Mr. M. Bailly (Subatech, IMT Atlantique) for the measurement of some tubes by gamma spectrometry and the analysis of the corresponding results. We would also like to thank M. Hadjistephanou from EuroCorpex company for providing free NOXOL solutions to conduct the tests. Finally, we acknowledge the Radioprotection team SPRI of Subatech for their help in the reception of the samples and in the implementation of workspace.

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.geothermics.2024.102960.

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