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CRUD removal via hydrodynamic cavitation in micro-orifices
Nuclear Engineering and Design 343 (2019) 210–217
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
CRUD removal via hydrodynamic cavitation in micro-orifices a
b
a,⁎
b
Max Szolcek , Stefano Cassineri , Andrea Cioncolini , Fabio Scenini , Michele Curioni a b
b
T
School of Mechanical, Aerospace and Civil Engineering, University of Manchester, George Begg Building, Sackville Street, M1 3BB Manchester, United Kingdom Materials Performance Centre, School of Materials, University of Manchester, The Mill, M1 3AL Manchester, United Kingdom
ARTICLE INFO
ABSTRACT
Keywords: CRUD CRUD removal Cavitation Hydrodynamic cavitation Micro-orifice Micro-fluidics
Chalk river unidentified deposit (CRUD) is an acronym used to describe the corrosion deposits that form in nuclear power plants. Such deposits are often associated to reduction of heat transfer in fuel rods and to blockage of channels in regions of fast hydrodynamic flow. Given the significance of the operational consequences associated to CRUD deposition, the development of methodologies to remove it is of scientific and technical interest. In this study, a microscale flow loop setup, replicating conditions relevant to plant operations, was used to explore the potential of exploiting hydrodynamic cavitation for CRUD removal. Tests were performed on discs with micro-orifices that were previously exposed to high temperature water flow to induce CRUD deposition. CRUD removal from within the micro-orifices was performed by inducing cavitation, and monitored via periodic microstructural examinations. The results indicate that a cavitating regime can reduce the volume of CRUD deposit by 80–90% within minutes, with no detectable damage to the metal surface.
1. Introduction During the operation of light water reactors, various corrosion products typically termed CRUD (Chalk river unidentified deposit) gradually accumulate on surfaces exposed to coolant flow, particularly on fuel rods and steam generator tubing. CRUD deposition might results in multiple undesired consequences. For example, due to its porous structure, CRUD deposited on fuel rods can absorb the boron dissolved into the coolant for reactivity control, generating issues with the power management of nuclear systems. Axial offset anomalies in power density and CRUD-induced power shifts are well-known examples of this problem (Zou et al., 2013). Once present on heat-transferring surfaces, such as reactor fuel rods and steam generator tubes, CRUD provides additional thermal resistance, reducing the effectiveness of heat transfer and/or inducing surface overheating (Cinosi et al., 2011). A particularly effective cleaning technology currently available to remove CRUD deposits from nuclear fuel rods relies on cavitation, which is the formation of gas or vapor cavities in a liquid triggered by a reduction in local pressure. When the reduction in local pressure is induced via ultrasound, the phenomenon is referred to as ultrasonic cavitation. Ultrasonic cavitation cleaning has been used industrially for decades, particularly for cleaning mechanical tools and components, surgical instruments and electronic equipment (Mason, 2016). The object to be cleaned is placed in a chamber that is filled with a suitable liquid solution and equipped with ultrasound-generating transducers
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that produce high frequency pressure waves in the solution. During the rarefaction of a pressure wave, the rapid decrease in pressure triggers the formation of microscopic cavitation bubbles, which then collapse during the subsequent compression wave. Imploding cavitation bubbles generate a high-pressure shock wave that displaces materials deposited on the object surface, thereby cleaning the surface (Chahine et al., 2016; Verhaagen and Fernández Rivas, 2016). During ultrasonic cleaning of nuclear bundles (Bengtsson et al., 2014), for example, a CRUD-deposited fuel assembly is removed from the reactor and placed with the spent fuel handling tool inside the cleaner, which can be installed in the spent fuel pool or in the transfer canal, and ultrasound are applied for a few minutes. In the petrochemical industries, ultrasonic cleaning has been successfully applied to heat exchangers (Kieser et al., 2011), although it has the drawback that the heat exchanger has to be disassembled to remove the tube bundle for cleaning. Such drawback prevents the procedure to be applied to nuclear steam generators, since they are fully welded and disassembly is impossible. Currently, CRUD deposits in nuclear steam generator tubes are removed using chemical or mechanical in-situ techniques (Fujiwara et al., 2004). Chemical rinse is among the most widespread methods, but it has several drawbacks: i) a detailed understanding of the deposit chemistry is needed, ii) it might provide incomplete removal in case of complicated flow path, iii) it produces undesired waste products, and iv) it can exacerbate the corrosion of the tubes (Vepsäläinen, 2010). Mechanical cleaning via highpressure water blasting, on the other hand, is time consuming and has
Corresponding author. E-mail address: [email protected] (A. Cioncolini).
https://doi.org/10.1016/j.nucengdes.2019.01.012 Received 22 October 2018; Received in revised form 6 January 2019; Accepted 7 January 2019 Available online 12 January 2019 0029-5493/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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limited environmental sustainability because it requires large amounts of water that needs to be treated afterwards. To date, the possibility of using in situ cavitation to remove CRUD deposits from nuclear systems has received little consideration. It is well known that, when a liquid flow accelerates, the static pressure decreases. If the static pressure falls below the local liquid vapor pressure, then cavitation occurs. Being triggered by a reduction in local pressure related to the liquid flow itself, this type of cavitation is normally referred to as hydrodynamic cavitation. Since hydrodynamic cavitation could in principle be triggered, via proper tuning of the operating conditions, around regions of flow acceleration where CRUD deposition occurs, the possibility of using hydrodynamic cavitation to remove CRUD deposits from nuclear systems becomes apparent. In fact, it is worth noting that CRUD deposition can be enhanced by fluid flow and it is known to increase strongly around regions of flow acceleration, such as through flow holes in steam generator tube support plates (McGrady et al., 2017; Yang et al., 2017). A previous study (Kim and Bang, 2016) showed that orifice-induced hydrodynamic cavitation directed at a previously deposited target was capable of partially removing CRUD-like deposits over short time periods. However, the possibility of removing CRUD deposit from an orifice itself, being similar in structure to constrictions such as those found between steam generator tube support plates, has not been previously considered and is what motivated the present investigation. The aim of this work is to explore the possibility of using hydrodynamic cavitation in nuclear systems for short time periods to effectively remove CRUD. In this study, tests were performed on CRUD deposited micro-orifices to assess the possibility of removing CRUD by hydrodynamic cavitation. CRUD removal from within the micro-orifices was monitored as a function of time of exposure to cavitating flow, showing that a few minutes exposure could reduce the volume of CRUD deposit by 80–90% with no detectable damage to the metal surface. The use of microfluidics systems to recreate nuclear plant operating conditions while using a simplified experimental set-up is an effective approach recently developed by McGrady et al. (2017a), and motivated the use of micro-orifices for the present work. Though preliminary and restricted to small-scale microfluidics test systems, our results suggest that hydrodynamic cavitation may be used in nuclear systems for short time periods to effectively reduce CRUD build-up within and surrounding constricted areas, possibly in synergy with the other CRUD removal techniques previously discussed. Even though our results seem encouraging, it is worth remembering that the present work is just a proof-of-concept carried out with a simplified, though representative, experimental setup. The actual implementation of hydrodynamic cavitation cleaning in nuclear systems would require further investigation and extensive assessment. Being a novel approach, hydrodynamic cavitation cleaning cannot presently be compared in terms of cost and effectiveness with ultrasonic cavitation cleaning, which is a mature technology that has already been successfully used at multiple plants worldwide. The rest of this paper is organized as follows: the experimental setup, validation and experimental methods used in this work are presented in Section 2, while results, discussion and conclusions are presented in Sections 3 and 4, respectively.
(shown on the right in Fig. 1) feeds the test piece via the high-pressure diaphragm pump, which was fitted with a pulsation damper to ensure pulsation-free flow conditions. The back-pressure (BP) regulator was used to control the upstream pressure to the test piece, allowing a fraction of the water flow to be circulated back to the storage tank, while the water that passed through the test piece was either collected for mass flow measurement or discharged directly to a drain. Downstream pressure to the test piece was controlled via the needle valve located at the system output. The test piece was created by inserting a disc with a micro-orifice into a bored-through 1/2″ Swagelok union joint, and face-sealed with two 1/2″ pipes using two polymeric O-rings as schematically shown in Fig. 2. The union joint was slowly tightened, compressing the O-rings against the micro-orifice, and forming a watertight seal, so that the micro-orifice was the only flow path available. This is the same test piece construction method previously used in micro-orifice flow studies by Cioncolini et al. (2015, 2016) and Szolcek et al. (2018). The straight upstream feed tube and downstream discharge tube leading to the outlet were 1/2″ tubes with 10 mm inner diameter and were 150 and 50 cm long, respectively, corresponding to a tube length to tube diameter ratio of 150 and 50. This ensured that the micro-orifices were always exposed to fully developed flow during the tests, and that the needle valve was located sufficiently far downstream so that any effects on the micro-orifice discharge could be neglected. Pressure lines were built 1 cm upstream and 15 cm downstream of the micro-orifice sample, and were connected to absolute pressure transducers (Omega model PXM 409, 0–24.5 MPa span, 0.1% full-scale accuracy). These transducers measured the static pressure upstream and downstream of the micro-orifice sample, and the pressure drop across the sample was calculated as the difference between upstream and downstream static pressures. Prior to testing, both pressure transducers were off-line calibrated at a certified metrological laboratory. Water flow temperature was measured (to within ± 1 K) by a K-type thermocouple fixed at the system output, while the water mass flow rate was measured with the weighing technique using a KERN precision balance (accuracy within 1%). 2.2. Micro-orifice samples The micro-orifice samples consisted of 1.00 mm or 1.25 mm thick 304L stainless-steel discs with a single, 300 µm or 600 µm diameter micro-hole drilled through producing a sharp-edged micro-orifice with orifice thickness to orifice diameter ratios (t/d) of 4.16 and 1.67 and orifice diameter to tube diameter ratios (d/D) of 0.024 and 0.047. The micro-orifice diameter was measured (to within ± 2 µm) at both sample faces using a Keyence VHX-5000 optical microscope. The characteristics of the four micro-orifice samples (referred to as Samples 1–4) that were produced and used in this work are summarized in Table 1. Sample 1, in particular, was machined with a 300 µm micro-orifice and was used to validate the present test system against previous work by Cioncolini et al. (2016), who studied hydrodynamic cavitation in micro-orifices with diameters of 150 µm and 300 µm and thickness to diameter ratio ranging from 3.53 to 6.93. Sample 1 was also used to assess the mechanical damage induced by cavitation on a clean microorifice, without any CRUD deposits. It is well known that, when the voids generated during cavitation implode near to a metallic surface, they cause cyclic stress that eventually results in surface fatigue and wear. For the investigated CRUD removal technique via hydrodynamic cavitation to be feasible, therefore, the time required to remove the CRUD should be significantly smaller than the time required to observe any damage to the metallic surface. Sample 2 was used as a reference, in order to determine the flow set-point at which hydrodynamic cavitation would occur in a 600 µm diameter micro-orifice and to inform the subsequent testing of CRUD deposited samples. Finally, Samples 3 and 4 (600 µm in diameter before CRUD deposition, the same as Sample 2) were deposited with CRUD prior to testing with cavitating flow, and
2. Experimental setup and methodology 2.1. Test apparatus As schematically shown in Fig. 1, the test system used in the present work comprises a storage tank filled with deionized and pre-filtered water at room temperature, and two independent flow loops. The first flow loop (shown on the left in Fig. 1) is driven by the circulation pump and was used to ensure consistent water chemistry during all tests (electrical conductivity of 0.055 μS/cm) via continuous recirculation through the deionizer (a ion exchange column). The second flow loop 211
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Fig. 1. Schematic representation of the experimental system used to test the micro-orifices that were mounted inside the test piece.
Fig. 2. Schematic view of the test piece (not to scale).
were used to assess the potential of hydrodynamic cavitation in removing the CRUD deposits. Deposition of CRUD onto Samples 3 and 4 was carried out in high purity (electrical conductivity of 0.055 μS/cm), deoxygenated (< 2 ppb O2), and hydrogenated (2.5 ppm of H2) water at a pressure of 12 MPa and temperature of 503 K, using the technique described in McGrady et al. (2017a, 2017b). As explained by the authors, these operating conditions simulate rather closely the CRUD deposition process encountered in nuclear systems. Once the CRUD deposition was completed, the upstream and downstream faces of the samples have been reversed: what was the upstream face during the CRUD buildup became the downstream face during the successive hydrodynamic cavitation cleaning. In fact, as shown by McGrady et al. (2017a, 2017b), the CRUD that builds up in microfluidics systems, and is representative in terms of morphology and composition to what observed in actual plants, preferentially forms on the upstream face of the micro-orifice. Since cavitation in micro-orifices is localized on the downstream face of the orifice, the samples orientation with respect to the flow was reversed
during the cleaning, so that the CRUD deposits could be directly exposed to hydrodynamic cavitation. Scanning electron microscope (SEM) images of Samples 3 and 4 prior to exposure to hydrodynamic cavitation are provided in Fig. 3. Notably, even though Samples 3 and 4 were CRUD-deposited following the same procedure, the amount of deposit on Sample 3 appears to be, by visual inspection, slightly larger, as quantitatively confirmed later on. The CRUD deposition process is not yet fully understood, and some variability in deposit thickness and distribution is therefore rather common, even when using a seemingly identical CRUD deposition process. This minor variability is in fact what motivated the use of two nominally identical samples (3 and 4), instead of just one. After the deposition, the morphology of the CRUD was characterized via Field Emission Gun Scanning Electron Microscopy, indicating that the deposits included a crystalline and a particulate components. The crystalline component is spread as small ridges along the micro-orifice downstream face, and is coherent with the existence of an electrokinetic deposition driven by the generation of wall currents (Scenini et al., 2014). The particulate component, on the other
Table 1 Dimensions and characteristics of the micro-orifice samples. Sample No.
Diameter (µm)
Thickness (mm)
t/d
d/D
State
Purpose
1 2 3 4
300 600 600 600
1.25 1.00 1.00 1.00
4.16 1.67 1.67 1.67
0.024 0.047 0.047 0.047
Clean Clean CRUD deposited CRUD deposited
Validation Set-point CRUD removal CRUD removal
± ± ± ±
2 2 2* 2*
± ± ± ±
0.01 0.01 0.01 0.01
(*) Measured before CRUD deposition. 212
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Fig. 3. SEM images of Samples 3 (left) and Sample 4 (right) prior to exposure to hydrodynamic cavitation (top: upstream face; bottom: downstream face).
hand, is concentrated around the micro-orifice edge and dominates (in volume) the overall build-up.
Preliminary experiments with other CRUD-deposited samples (not documented here) indicated that CRUD removal rate decreases exponentially with the time of exposure to cavitating flow. The elapsed time between successive inspections in Table 2 was therefore gradually increased as time progressed. The inspection schedule was also dictated by the availability of the SEM and CLSM equipment, and was therefore not the same for the two samples. A peculiarity of the test procedure followed here is that the set-point for inducing hydrodynamic cavitation in Samples 3 and 4 has to be known beforehand, without running any preliminary tests, so that these samples can be exposed to a cavitating flow since the very beginning of the tests. As previously noted, Sample 2 was used as a reference to determine the flow set-point to inform the subsequent testing of Samples 3 and 4. The hydrodynamic curve of Sample 2 measured for an upstream pressure of 2.5 MPa and water temperature of 295 K is presented in Fig. 4, where the pressure drop measured across the sample is plotted versus the mass flow rate. At the start of the tests, the needle valve was fully closed. After the upstream pressure was set to the desired value, the needle valve was gradually and step-wise opened, so that the downstream pressure was correspondingly gradually decreased and the mass flow rate increased. Measurements were taken after the system had reached steady-state flow conditions. The procedure was then repeated by gradually closing the needle valve, thus gradually increasing the downstream pressure and decreasing the mass flow rate. As can be noticed in Fig. 4, as long as the flow is single-phase liquid an increase in the pressure drop across the micro-orifice yields a corresponding increase in the mass flow rate, with a trend in the single-phase data that suggests a quadratic relationship between pressure drop and mass flow rate typical of single-phase channel flow. When the flow in the micro-orifice cavitates, the discharge deviates from the quadratic relationship: the flow chokes and the mass flow rate remains constant regardless of the downstream pressure value. Notably, there is no visible hysteresis in the data points in Fig. 4 generated with decreasing and increasing downstream pressure. The point of hydrodynamic cavitation inception (downstream pressure decreasing) and cessation (downstream pressure increasing) for an upstream pressure of 2.5 MPa
2.3. Methodology In order to assess the CRUD removal potential of hydrodynamic cavitation, Samples 3 and 4 were exposed to cavitating flow with constant upstream pressure for several hours. The tests were periodically stopped and the samples inspected using SEM imaging and confocal laser scanning microscopy (CLSM) analysis to assess the CRUD deposits. In particular, SEM imaging was used for a qualitative assessment of CRUD removal against exposure time to cavitating flow, while CLSM analysis was conducted to provide a quantitative evaluation of the CRUD removal process by generating 3D images of the CRUD deposits. These images were used to provide an estimate, via numerical integration, of the volume of the CRUD deposits present on the samples. Confocal analysis was conducted with a Keyence VK-X200K 3D laser scanning confocal microscope. The inspection schedule for Samples 3 and 4 is provided in Table 2. CLSM analysis was conducted at every inspection, while SEM imaging was used only at selected inspections to complement and qualitatively corroborate the CLSM analysis results. Table 2 Inspection schedule for hydrodynamic cavitation tests. Sample 3
Sample 4
Exposure time to cavitation (min)
Inspection
Exposure time to cavitation (min)
Inspection
0 5 30 90 180 360 600 1800
SEM + CLSM SEM + CLSM SEM + CLSM CLSM CLSM CLSM CLSM SEM + CLSM
0 5 180 270 345 645 1380
SEM + CLSM CLSM CLSM CLSM CLSM CLSM SEM + CLSM
213
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Fig. 4. Hydrodynamic curve of Sample 2 (red squares: decreasing downstream pressure; blue circles: increasing downstream pressure). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Square root of upstream pressure vs. choked mass flow rate during hydrodynamic cavitation (the continuous lines are linear fits through the origin).
Fig. 6. Optical images of Sample 1 downstream face before the test (left) and after 2580 min (43 h) exposure to hydrodynamic cavitation (right).
is evident at a mass flow rate of 12.5 g/s and a pressure drop of 1.5 MPa, corresponding to a downstream pressure of 1.0 MPa. Based on these results for Sample 2, Samples 3 and 4 were tested with water flowing at a temperature of 295 K, with an upstream
pressure of 2.5 MPa and with the needle valve fully open, corresponding to a downstream pressure close to atmospheric and a pressure drop across the samples of approximately 2.5 MPa. This ensured uniform test conditions for both samples. Importantly, the large pressure 214
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Sample 1 to assess the mechanical damage induced by cavitation on a clean micro-orifice, without any CRUD deposits. This test, which lasted for 43 h straight, was carried out with an upstream pressure of 9.8 ± 0.2 MPa and the needle valve partially open, corresponding to a downstream pressure of 3.4 ± 0.4 MPa, resulting in a pressure drop across the sample of about 6.4 MPa. The mass flow rate through the orifice was 6.0 ± 0.1 g/s with water flowing at a temperature of 298 ± 2 K. The minor fluctuations around the set-point were due to day-night temperature fluctuations in the laboratory. Optical images of the downstream face of Sample 1 before and after the hydrodynamic cavitation test are provided in Fig. 6. No erosion or mechanical damage to the metal surface can be noticed, indicating that an exposure to hydrodynamic cavitation on the order of tens of hours is not sufficient to induce any cavitation damage. The volumes of CRUD deposited on Samples 3 and 4, estimated via CLSM at the various stages of the hydrodynamic cavitation experiment, are reported in Table 3. At the beginning of the tests (exposure time of 0 min), the deposit on Sample 3 was about 20% larger than that present on Sample 4, confirming what previously noted when commenting the SEM images in Fig. 3. As the tests progressed, the volume of CRUD deposits on both samples gradually decreased, as evident from Table 3. The relative volume of CRUD removed, calculated as the cumulative sum of the values in Table 3 normalized by the volume of CRUD present at the beginning of the test, is presented in Fig. 7. The trend of the volume of CRUD removed is growing and saturating for both samples, with a particularly fast growth at the beginning of the tests, followed by a much slower saturation afterwards (note the logarithmic scale on the horizontal axis in Fig. 7). This indicates that after 5 min at least 85% of the CRUD deposit is removed. The presence of two different removal time scales, one small at the beginning of the tests and another much larger afterwards, is also consistent with the composition of the CRUD, which includes a crystalline and a particulate components. As previosuly noted, the particulate component is by far the most abundant and dominates the overall deposit. The results in Fig. 7 indicate that this is the component that is removed quickly in the beginnig of the tests. The crystalline component, on the other hand, is less abundant but well adherent on the samples surface, and gets removed at a much slower pace. Since corrosion products are typically more soluble at ambient temperature, rather than in high temperature condition in which they were formed, dissolution may also play a role in the slow removal of crystalline deposits. This effect, however, has not been quantified in the present setting. The difference between the trends of the volume of CRUD removed from Sample 3 and 4 in Fig. 7 is within the resolution of
Table 3 CRUD volume estimates from CLSM. Sample 3
Sample 4
Exposure time to cavitation (min)
CRUD Volume (µm3)
0 5 30 90 180 360 600 1800
5.1 4.6 2.8 1.6 9.1 7.1 6.1 5.9
106 105 105 105 104 104 104 104
Exposure time to cavitation (min)
CRUD Volume (µm3)
0 5 180 270 345 645 1380
4.1 6.4 5.2 4.9 4.7 4.1 3.8
106 105 105 105 105 105 105
drop across the samples maintained hydrodynamic cavitation throughout the tests, accommodating for the gradual variation in the samples flow area due to the gradual removal of the CRUD deposits. 2.4. Test rig validation The present test apparatus was validated by comparing the data generated with Sample 1 with the data provided by Cioncolini et al. (2016) for micro-orifices of the same diameter. In particular, Cioncolini et al. (2016) tested two micro-orifices with diameter of 300 µm and thickness to diameter ratios t/d of 3.33 and 6.67, whereas the thickness to diameter ratio of Sample 1 is intermediate between these two values and equal to 4.17. As described in Cioncolini et al. (2016), during cavitating choked flow the mass flow rate through the micro-orifice varies linearly with the square root of the upstream pressure, and increases with decreasing the thickness to diameter ratio t/d. Measurements taken during cavitating choked flow with Sample 1 are provided, together with the data from Cioncolini et al. (2016), in Fig. 5 where the square root of the upstream pressure is plotted versus the mass flow rate through the orifice. As can be seen in Fig. 5, the trend for Sample 1 is linear and the data points consistently fall between those from Cioncolini et al. (2016), as should be the case based on the thickness to diameter ratio values. Consequently, the test apparatus used in this work can be considered validated. 3. Results and discussion A preliminary hydrodynamic cavitation test was preformed with
Fig. 7. Volume of CRUD removed vs. exposure time to hydrodynamic cavitation (note the logarithmic scale in the horizontal axis). 215
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Fig. 8. SEM images of the downstream face of Sample 3 at various exposure times. From top to bottom: exposure time to cavitation: 0 min, 5 min, 30 min, 1800 min (30 h).
the CLSM analysis, whose accuracy gradually decreases as the volume of CRUD becomes smaller and smaller. Irrespective of this difference, the results in Fig. 7 indicate that more than 80% of the CRUD originally present on the samples is removed after a few minutes of exposure to hydrodynamic cavitation. SEM images of Samples 3 and 4 at the various stages of the
hydrodynamic cavitation experiment are presented in Figs. 8 and 9. As can be seen, the particulate deposition that dominates the overall buildup is effectively removed just after a few minutes of exposure to hydrodynamic cavitation, whilst the crystalline component is removed at a much slower pace and crystalline deposits are still evident on the samples surface at the end of the tests. This qualitatively confirms the 216
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Fig. 9. SEM images of the downstream face of Sample 4 at various exposure times to cavitation. Top: 0 min; bottom: 1380 min (23 h).
results from the CLSM analysis previously discussed. No erosion or mechanical damage to the metal surface can be noticed in Figs. 8 and 9 at the end of the tests, confirming that an exposure to hydrodynamic cavitation up to a few tens of hours is not sufficient to induce any noticeable cavitation damage.
efficiency ultrasonic fuel cleaning using a side-stream sampling system at Ringhals Unit 4. Proceedings of Nuclear Plant Chemistry Conference, Sapporo, Japan. Chahine, G.L., Kapahi, A., Choi, J., Hsiao, C., 2016. Modeling of surface cleaning by cavitation bubble dynamics and collapse. Ultrason. Sonochem. 29, 528–549. Cinosi, N., Haq, I., Bluck, M., Walker, S.P., 2011. The effective thermal conductivity of crud and heat transfer from crud-coated PWR fuel. Nucl. Eng. Des. 241, 792–798. Cioncolini, A., Scenini, F., Duff, J., 2015. Micro-orifice single-phase liquid flow: pressure drop measurements and prediction. Exp. Therm. Fluid Sci. 65, 33–40. Cioncolini, A., Scenini, F., Duff, J., Szolcek, M., Curioni, M., 2016. Choked cavitation in micro-orifices: an experimental study. Exp. Therm. Fluid Sci. 74, 49–57. Fujiwara, K., Kawamura, H., Kanbe, H., Hirano, H., Takiguchi, H., Yoshino, K., Yamamoto, S., Shibata, T., Ishigure, K., 2004. Applicability of chemical cleaning process to steam generator secondary side. J. Nucl. Sci Technol. 41, 44–54. Kieser, B., Phillion, R., Smith, S., McCartney, T., 2011. The application of industrial scale ultrasonic cleaning to heat exchangers. International Conference on Heat Exchangers Fouling and Cleaning. Crete Island, Greece. Kim, S.M., Bang, I.C., 2016. Hydrodynamic cavitation characteristics of an orifice system and its effects on CRUD-like SiC deposition. Ann. Nucl. Energy 96, 102–118. Mason, J., 2016. Ultrasonic cleaning: an historical perspective. Ultrason. Sonochem. 29, 519–523. McGrady, J., Duff, J., Stevens, N., Cioncolini, A., Curioni, M., Banks, A., Scenini, F., 2017a. Development of a microfluidic setup to study the corrosion product deposition in accelerated flow regions. npj Mater. Degrad. 1 Article 21. McGrady, J., Scenini, F., Duff, J., Stevens, N., Cassineri, S., Curioni, M., Banks, A., 2017b. Investigation into the effect of water chemistry on corrosion product formation in areas of accelerated flow. J. Nucl. Mater. 493, 271–279. Scenini, F., Palumbo, G., Stevens, N., Cook, A., Banks, A., 2014. Investigation of the role of electrokinetic effects in corrosion deposit formation. Corros. Sci. 87, 71–79. Szolcek, M., Cioncolini, A., Scenini, F., Curioni, M., 2018. Effect of thickness to diameter ratio on micro-orifice single-phase liquid flow at low Reynolds number. Exp. Therm. Fluid Sci. 97, 218–222. Vepsäläinen, M., 2010. Deposit formation in PWR steam generators. Report: VTT-R00135-10. VTT Technical Research Centre, Espoo, Finland. Verhaagen, B., Fernández Rivas, D., 2016. Measuring cavitation and its cleaning effect. Ultrason. Sonochem. 29, 619–628. Yang, G., Pointeau, V., Tevissen, E., Chagnes, A., 2017. A review on clogging of recirculating steam generators in pressurized-water reactors. Prog. Nucl. Energy 97, 182–196. Zou, L., Zhang, H., Gehin, J., Kochunas, B., 2013. Coupled thermal-hydraulic/neutronics/ crud framework in prediction of crud-induced power shift phenomenon. Nucl. Technol. 183, 535–542.
4. Concluding remarks The use of hydrodynamic cavitation was explored to assess the CRUD removal in micro-orifices that were previously partly clogged in simulated nuclear environment. The setup was validated against literature data and enabled predicting cavitation in micro-orifices and to monitor the CRUD removal during the experiments. Post-test materials characterization was implemented to validate the in-situ evaluations. Periodic inspections also highlighted that no erosion or mechanical damage to the metal surface was induced, confirming that an exposure to hydrodynamic cavitation up to a few tens of hours is not sufficient to induce any noticeable cavitation damage. The results presented showed that about 80–90% of the surface CRUD could be removed within the first 5 min of exposure whilst the complete removal of the CRUD required tens of hours of exposure under cavitating regime. The degree of removal was correlated with the nature of the CRUD, which consist of a particulate and poorly adherent component on the outer surface, and more crystalline, and presumably better anchored, inner oxide that was therefore more difficult to remove. Acknowledgement The financial support from EPSRC through the Centre for Doctoral Training in the Science and Technology of Fusion Energy is gratefully acknowledged (grant number EP/L01663X/1). References Bengtsson, B., Svanberg, P., Dingee, J., Pellman, A., Wells, D., 2014. Experience with high
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CRUD removal via hydrodynamic cavitation in micro-orifices a
b
a,⁎
b
Max Szolcek , Stefano Cassineri , Andrea Cioncolini , Fabio Scenini , Michele Curioni a b
b
T
School of Mechanical, Aerospace and Civil Engineering, University of Manchester, George Begg Building, Sackville Street, M1 3BB Manchester, United Kingdom Materials Performance Centre, School of Materials, University of Manchester, The Mill, M1 3AL Manchester, United Kingdom
ARTICLE INFO
ABSTRACT
Keywords: CRUD CRUD removal Cavitation Hydrodynamic cavitation Micro-orifice Micro-fluidics
Chalk river unidentified deposit (CRUD) is an acronym used to describe the corrosion deposits that form in nuclear power plants. Such deposits are often associated to reduction of heat transfer in fuel rods and to blockage of channels in regions of fast hydrodynamic flow. Given the significance of the operational consequences associated to CRUD deposition, the development of methodologies to remove it is of scientific and technical interest. In this study, a microscale flow loop setup, replicating conditions relevant to plant operations, was used to explore the potential of exploiting hydrodynamic cavitation for CRUD removal. Tests were performed on discs with micro-orifices that were previously exposed to high temperature water flow to induce CRUD deposition. CRUD removal from within the micro-orifices was performed by inducing cavitation, and monitored via periodic microstructural examinations. The results indicate that a cavitating regime can reduce the volume of CRUD deposit by 80–90% within minutes, with no detectable damage to the metal surface.
1. Introduction During the operation of light water reactors, various corrosion products typically termed CRUD (Chalk river unidentified deposit) gradually accumulate on surfaces exposed to coolant flow, particularly on fuel rods and steam generator tubing. CRUD deposition might results in multiple undesired consequences. For example, due to its porous structure, CRUD deposited on fuel rods can absorb the boron dissolved into the coolant for reactivity control, generating issues with the power management of nuclear systems. Axial offset anomalies in power density and CRUD-induced power shifts are well-known examples of this problem (Zou et al., 2013). Once present on heat-transferring surfaces, such as reactor fuel rods and steam generator tubes, CRUD provides additional thermal resistance, reducing the effectiveness of heat transfer and/or inducing surface overheating (Cinosi et al., 2011). A particularly effective cleaning technology currently available to remove CRUD deposits from nuclear fuel rods relies on cavitation, which is the formation of gas or vapor cavities in a liquid triggered by a reduction in local pressure. When the reduction in local pressure is induced via ultrasound, the phenomenon is referred to as ultrasonic cavitation. Ultrasonic cavitation cleaning has been used industrially for decades, particularly for cleaning mechanical tools and components, surgical instruments and electronic equipment (Mason, 2016). The object to be cleaned is placed in a chamber that is filled with a suitable liquid solution and equipped with ultrasound-generating transducers
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that produce high frequency pressure waves in the solution. During the rarefaction of a pressure wave, the rapid decrease in pressure triggers the formation of microscopic cavitation bubbles, which then collapse during the subsequent compression wave. Imploding cavitation bubbles generate a high-pressure shock wave that displaces materials deposited on the object surface, thereby cleaning the surface (Chahine et al., 2016; Verhaagen and Fernández Rivas, 2016). During ultrasonic cleaning of nuclear bundles (Bengtsson et al., 2014), for example, a CRUD-deposited fuel assembly is removed from the reactor and placed with the spent fuel handling tool inside the cleaner, which can be installed in the spent fuel pool or in the transfer canal, and ultrasound are applied for a few minutes. In the petrochemical industries, ultrasonic cleaning has been successfully applied to heat exchangers (Kieser et al., 2011), although it has the drawback that the heat exchanger has to be disassembled to remove the tube bundle for cleaning. Such drawback prevents the procedure to be applied to nuclear steam generators, since they are fully welded and disassembly is impossible. Currently, CRUD deposits in nuclear steam generator tubes are removed using chemical or mechanical in-situ techniques (Fujiwara et al., 2004). Chemical rinse is among the most widespread methods, but it has several drawbacks: i) a detailed understanding of the deposit chemistry is needed, ii) it might provide incomplete removal in case of complicated flow path, iii) it produces undesired waste products, and iv) it can exacerbate the corrosion of the tubes (Vepsäläinen, 2010). Mechanical cleaning via highpressure water blasting, on the other hand, is time consuming and has
Corresponding author. E-mail address: [email protected] (A. Cioncolini).
https://doi.org/10.1016/j.nucengdes.2019.01.012 Received 22 October 2018; Received in revised form 6 January 2019; Accepted 7 January 2019 Available online 12 January 2019 0029-5493/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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limited environmental sustainability because it requires large amounts of water that needs to be treated afterwards. To date, the possibility of using in situ cavitation to remove CRUD deposits from nuclear systems has received little consideration. It is well known that, when a liquid flow accelerates, the static pressure decreases. If the static pressure falls below the local liquid vapor pressure, then cavitation occurs. Being triggered by a reduction in local pressure related to the liquid flow itself, this type of cavitation is normally referred to as hydrodynamic cavitation. Since hydrodynamic cavitation could in principle be triggered, via proper tuning of the operating conditions, around regions of flow acceleration where CRUD deposition occurs, the possibility of using hydrodynamic cavitation to remove CRUD deposits from nuclear systems becomes apparent. In fact, it is worth noting that CRUD deposition can be enhanced by fluid flow and it is known to increase strongly around regions of flow acceleration, such as through flow holes in steam generator tube support plates (McGrady et al., 2017; Yang et al., 2017). A previous study (Kim and Bang, 2016) showed that orifice-induced hydrodynamic cavitation directed at a previously deposited target was capable of partially removing CRUD-like deposits over short time periods. However, the possibility of removing CRUD deposit from an orifice itself, being similar in structure to constrictions such as those found between steam generator tube support plates, has not been previously considered and is what motivated the present investigation. The aim of this work is to explore the possibility of using hydrodynamic cavitation in nuclear systems for short time periods to effectively remove CRUD. In this study, tests were performed on CRUD deposited micro-orifices to assess the possibility of removing CRUD by hydrodynamic cavitation. CRUD removal from within the micro-orifices was monitored as a function of time of exposure to cavitating flow, showing that a few minutes exposure could reduce the volume of CRUD deposit by 80–90% with no detectable damage to the metal surface. The use of microfluidics systems to recreate nuclear plant operating conditions while using a simplified experimental set-up is an effective approach recently developed by McGrady et al. (2017a), and motivated the use of micro-orifices for the present work. Though preliminary and restricted to small-scale microfluidics test systems, our results suggest that hydrodynamic cavitation may be used in nuclear systems for short time periods to effectively reduce CRUD build-up within and surrounding constricted areas, possibly in synergy with the other CRUD removal techniques previously discussed. Even though our results seem encouraging, it is worth remembering that the present work is just a proof-of-concept carried out with a simplified, though representative, experimental setup. The actual implementation of hydrodynamic cavitation cleaning in nuclear systems would require further investigation and extensive assessment. Being a novel approach, hydrodynamic cavitation cleaning cannot presently be compared in terms of cost and effectiveness with ultrasonic cavitation cleaning, which is a mature technology that has already been successfully used at multiple plants worldwide. The rest of this paper is organized as follows: the experimental setup, validation and experimental methods used in this work are presented in Section 2, while results, discussion and conclusions are presented in Sections 3 and 4, respectively.
(shown on the right in Fig. 1) feeds the test piece via the high-pressure diaphragm pump, which was fitted with a pulsation damper to ensure pulsation-free flow conditions. The back-pressure (BP) regulator was used to control the upstream pressure to the test piece, allowing a fraction of the water flow to be circulated back to the storage tank, while the water that passed through the test piece was either collected for mass flow measurement or discharged directly to a drain. Downstream pressure to the test piece was controlled via the needle valve located at the system output. The test piece was created by inserting a disc with a micro-orifice into a bored-through 1/2″ Swagelok union joint, and face-sealed with two 1/2″ pipes using two polymeric O-rings as schematically shown in Fig. 2. The union joint was slowly tightened, compressing the O-rings against the micro-orifice, and forming a watertight seal, so that the micro-orifice was the only flow path available. This is the same test piece construction method previously used in micro-orifice flow studies by Cioncolini et al. (2015, 2016) and Szolcek et al. (2018). The straight upstream feed tube and downstream discharge tube leading to the outlet were 1/2″ tubes with 10 mm inner diameter and were 150 and 50 cm long, respectively, corresponding to a tube length to tube diameter ratio of 150 and 50. This ensured that the micro-orifices were always exposed to fully developed flow during the tests, and that the needle valve was located sufficiently far downstream so that any effects on the micro-orifice discharge could be neglected. Pressure lines were built 1 cm upstream and 15 cm downstream of the micro-orifice sample, and were connected to absolute pressure transducers (Omega model PXM 409, 0–24.5 MPa span, 0.1% full-scale accuracy). These transducers measured the static pressure upstream and downstream of the micro-orifice sample, and the pressure drop across the sample was calculated as the difference between upstream and downstream static pressures. Prior to testing, both pressure transducers were off-line calibrated at a certified metrological laboratory. Water flow temperature was measured (to within ± 1 K) by a K-type thermocouple fixed at the system output, while the water mass flow rate was measured with the weighing technique using a KERN precision balance (accuracy within 1%). 2.2. Micro-orifice samples The micro-orifice samples consisted of 1.00 mm or 1.25 mm thick 304L stainless-steel discs with a single, 300 µm or 600 µm diameter micro-hole drilled through producing a sharp-edged micro-orifice with orifice thickness to orifice diameter ratios (t/d) of 4.16 and 1.67 and orifice diameter to tube diameter ratios (d/D) of 0.024 and 0.047. The micro-orifice diameter was measured (to within ± 2 µm) at both sample faces using a Keyence VHX-5000 optical microscope. The characteristics of the four micro-orifice samples (referred to as Samples 1–4) that were produced and used in this work are summarized in Table 1. Sample 1, in particular, was machined with a 300 µm micro-orifice and was used to validate the present test system against previous work by Cioncolini et al. (2016), who studied hydrodynamic cavitation in micro-orifices with diameters of 150 µm and 300 µm and thickness to diameter ratio ranging from 3.53 to 6.93. Sample 1 was also used to assess the mechanical damage induced by cavitation on a clean microorifice, without any CRUD deposits. It is well known that, when the voids generated during cavitation implode near to a metallic surface, they cause cyclic stress that eventually results in surface fatigue and wear. For the investigated CRUD removal technique via hydrodynamic cavitation to be feasible, therefore, the time required to remove the CRUD should be significantly smaller than the time required to observe any damage to the metallic surface. Sample 2 was used as a reference, in order to determine the flow set-point at which hydrodynamic cavitation would occur in a 600 µm diameter micro-orifice and to inform the subsequent testing of CRUD deposited samples. Finally, Samples 3 and 4 (600 µm in diameter before CRUD deposition, the same as Sample 2) were deposited with CRUD prior to testing with cavitating flow, and
2. Experimental setup and methodology 2.1. Test apparatus As schematically shown in Fig. 1, the test system used in the present work comprises a storage tank filled with deionized and pre-filtered water at room temperature, and two independent flow loops. The first flow loop (shown on the left in Fig. 1) is driven by the circulation pump and was used to ensure consistent water chemistry during all tests (electrical conductivity of 0.055 μS/cm) via continuous recirculation through the deionizer (a ion exchange column). The second flow loop 211
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Fig. 1. Schematic representation of the experimental system used to test the micro-orifices that were mounted inside the test piece.
Fig. 2. Schematic view of the test piece (not to scale).
were used to assess the potential of hydrodynamic cavitation in removing the CRUD deposits. Deposition of CRUD onto Samples 3 and 4 was carried out in high purity (electrical conductivity of 0.055 μS/cm), deoxygenated (< 2 ppb O2), and hydrogenated (2.5 ppm of H2) water at a pressure of 12 MPa and temperature of 503 K, using the technique described in McGrady et al. (2017a, 2017b). As explained by the authors, these operating conditions simulate rather closely the CRUD deposition process encountered in nuclear systems. Once the CRUD deposition was completed, the upstream and downstream faces of the samples have been reversed: what was the upstream face during the CRUD buildup became the downstream face during the successive hydrodynamic cavitation cleaning. In fact, as shown by McGrady et al. (2017a, 2017b), the CRUD that builds up in microfluidics systems, and is representative in terms of morphology and composition to what observed in actual plants, preferentially forms on the upstream face of the micro-orifice. Since cavitation in micro-orifices is localized on the downstream face of the orifice, the samples orientation with respect to the flow was reversed
during the cleaning, so that the CRUD deposits could be directly exposed to hydrodynamic cavitation. Scanning electron microscope (SEM) images of Samples 3 and 4 prior to exposure to hydrodynamic cavitation are provided in Fig. 3. Notably, even though Samples 3 and 4 were CRUD-deposited following the same procedure, the amount of deposit on Sample 3 appears to be, by visual inspection, slightly larger, as quantitatively confirmed later on. The CRUD deposition process is not yet fully understood, and some variability in deposit thickness and distribution is therefore rather common, even when using a seemingly identical CRUD deposition process. This minor variability is in fact what motivated the use of two nominally identical samples (3 and 4), instead of just one. After the deposition, the morphology of the CRUD was characterized via Field Emission Gun Scanning Electron Microscopy, indicating that the deposits included a crystalline and a particulate components. The crystalline component is spread as small ridges along the micro-orifice downstream face, and is coherent with the existence of an electrokinetic deposition driven by the generation of wall currents (Scenini et al., 2014). The particulate component, on the other
Table 1 Dimensions and characteristics of the micro-orifice samples. Sample No.
Diameter (µm)
Thickness (mm)
t/d
d/D
State
Purpose
1 2 3 4
300 600 600 600
1.25 1.00 1.00 1.00
4.16 1.67 1.67 1.67
0.024 0.047 0.047 0.047
Clean Clean CRUD deposited CRUD deposited
Validation Set-point CRUD removal CRUD removal
± ± ± ±
2 2 2* 2*
± ± ± ±
0.01 0.01 0.01 0.01
(*) Measured before CRUD deposition. 212
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Fig. 3. SEM images of Samples 3 (left) and Sample 4 (right) prior to exposure to hydrodynamic cavitation (top: upstream face; bottom: downstream face).
hand, is concentrated around the micro-orifice edge and dominates (in volume) the overall build-up.
Preliminary experiments with other CRUD-deposited samples (not documented here) indicated that CRUD removal rate decreases exponentially with the time of exposure to cavitating flow. The elapsed time between successive inspections in Table 2 was therefore gradually increased as time progressed. The inspection schedule was also dictated by the availability of the SEM and CLSM equipment, and was therefore not the same for the two samples. A peculiarity of the test procedure followed here is that the set-point for inducing hydrodynamic cavitation in Samples 3 and 4 has to be known beforehand, without running any preliminary tests, so that these samples can be exposed to a cavitating flow since the very beginning of the tests. As previously noted, Sample 2 was used as a reference to determine the flow set-point to inform the subsequent testing of Samples 3 and 4. The hydrodynamic curve of Sample 2 measured for an upstream pressure of 2.5 MPa and water temperature of 295 K is presented in Fig. 4, where the pressure drop measured across the sample is plotted versus the mass flow rate. At the start of the tests, the needle valve was fully closed. After the upstream pressure was set to the desired value, the needle valve was gradually and step-wise opened, so that the downstream pressure was correspondingly gradually decreased and the mass flow rate increased. Measurements were taken after the system had reached steady-state flow conditions. The procedure was then repeated by gradually closing the needle valve, thus gradually increasing the downstream pressure and decreasing the mass flow rate. As can be noticed in Fig. 4, as long as the flow is single-phase liquid an increase in the pressure drop across the micro-orifice yields a corresponding increase in the mass flow rate, with a trend in the single-phase data that suggests a quadratic relationship between pressure drop and mass flow rate typical of single-phase channel flow. When the flow in the micro-orifice cavitates, the discharge deviates from the quadratic relationship: the flow chokes and the mass flow rate remains constant regardless of the downstream pressure value. Notably, there is no visible hysteresis in the data points in Fig. 4 generated with decreasing and increasing downstream pressure. The point of hydrodynamic cavitation inception (downstream pressure decreasing) and cessation (downstream pressure increasing) for an upstream pressure of 2.5 MPa
2.3. Methodology In order to assess the CRUD removal potential of hydrodynamic cavitation, Samples 3 and 4 were exposed to cavitating flow with constant upstream pressure for several hours. The tests were periodically stopped and the samples inspected using SEM imaging and confocal laser scanning microscopy (CLSM) analysis to assess the CRUD deposits. In particular, SEM imaging was used for a qualitative assessment of CRUD removal against exposure time to cavitating flow, while CLSM analysis was conducted to provide a quantitative evaluation of the CRUD removal process by generating 3D images of the CRUD deposits. These images were used to provide an estimate, via numerical integration, of the volume of the CRUD deposits present on the samples. Confocal analysis was conducted with a Keyence VK-X200K 3D laser scanning confocal microscope. The inspection schedule for Samples 3 and 4 is provided in Table 2. CLSM analysis was conducted at every inspection, while SEM imaging was used only at selected inspections to complement and qualitatively corroborate the CLSM analysis results. Table 2 Inspection schedule for hydrodynamic cavitation tests. Sample 3
Sample 4
Exposure time to cavitation (min)
Inspection
Exposure time to cavitation (min)
Inspection
0 5 30 90 180 360 600 1800
SEM + CLSM SEM + CLSM SEM + CLSM CLSM CLSM CLSM CLSM SEM + CLSM
0 5 180 270 345 645 1380
SEM + CLSM CLSM CLSM CLSM CLSM CLSM SEM + CLSM
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Fig. 4. Hydrodynamic curve of Sample 2 (red squares: decreasing downstream pressure; blue circles: increasing downstream pressure). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Square root of upstream pressure vs. choked mass flow rate during hydrodynamic cavitation (the continuous lines are linear fits through the origin).
Fig. 6. Optical images of Sample 1 downstream face before the test (left) and after 2580 min (43 h) exposure to hydrodynamic cavitation (right).
is evident at a mass flow rate of 12.5 g/s and a pressure drop of 1.5 MPa, corresponding to a downstream pressure of 1.0 MPa. Based on these results for Sample 2, Samples 3 and 4 were tested with water flowing at a temperature of 295 K, with an upstream
pressure of 2.5 MPa and with the needle valve fully open, corresponding to a downstream pressure close to atmospheric and a pressure drop across the samples of approximately 2.5 MPa. This ensured uniform test conditions for both samples. Importantly, the large pressure 214
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Sample 1 to assess the mechanical damage induced by cavitation on a clean micro-orifice, without any CRUD deposits. This test, which lasted for 43 h straight, was carried out with an upstream pressure of 9.8 ± 0.2 MPa and the needle valve partially open, corresponding to a downstream pressure of 3.4 ± 0.4 MPa, resulting in a pressure drop across the sample of about 6.4 MPa. The mass flow rate through the orifice was 6.0 ± 0.1 g/s with water flowing at a temperature of 298 ± 2 K. The minor fluctuations around the set-point were due to day-night temperature fluctuations in the laboratory. Optical images of the downstream face of Sample 1 before and after the hydrodynamic cavitation test are provided in Fig. 6. No erosion or mechanical damage to the metal surface can be noticed, indicating that an exposure to hydrodynamic cavitation on the order of tens of hours is not sufficient to induce any cavitation damage. The volumes of CRUD deposited on Samples 3 and 4, estimated via CLSM at the various stages of the hydrodynamic cavitation experiment, are reported in Table 3. At the beginning of the tests (exposure time of 0 min), the deposit on Sample 3 was about 20% larger than that present on Sample 4, confirming what previously noted when commenting the SEM images in Fig. 3. As the tests progressed, the volume of CRUD deposits on both samples gradually decreased, as evident from Table 3. The relative volume of CRUD removed, calculated as the cumulative sum of the values in Table 3 normalized by the volume of CRUD present at the beginning of the test, is presented in Fig. 7. The trend of the volume of CRUD removed is growing and saturating for both samples, with a particularly fast growth at the beginning of the tests, followed by a much slower saturation afterwards (note the logarithmic scale on the horizontal axis in Fig. 7). This indicates that after 5 min at least 85% of the CRUD deposit is removed. The presence of two different removal time scales, one small at the beginning of the tests and another much larger afterwards, is also consistent with the composition of the CRUD, which includes a crystalline and a particulate components. As previosuly noted, the particulate component is by far the most abundant and dominates the overall deposit. The results in Fig. 7 indicate that this is the component that is removed quickly in the beginnig of the tests. The crystalline component, on the other hand, is less abundant but well adherent on the samples surface, and gets removed at a much slower pace. Since corrosion products are typically more soluble at ambient temperature, rather than in high temperature condition in which they were formed, dissolution may also play a role in the slow removal of crystalline deposits. This effect, however, has not been quantified in the present setting. The difference between the trends of the volume of CRUD removed from Sample 3 and 4 in Fig. 7 is within the resolution of
Table 3 CRUD volume estimates from CLSM. Sample 3
Sample 4
Exposure time to cavitation (min)
CRUD Volume (µm3)
0 5 30 90 180 360 600 1800
5.1 4.6 2.8 1.6 9.1 7.1 6.1 5.9
106 105 105 105 104 104 104 104
Exposure time to cavitation (min)
CRUD Volume (µm3)
0 5 180 270 345 645 1380
4.1 6.4 5.2 4.9 4.7 4.1 3.8
106 105 105 105 105 105 105
drop across the samples maintained hydrodynamic cavitation throughout the tests, accommodating for the gradual variation in the samples flow area due to the gradual removal of the CRUD deposits. 2.4. Test rig validation The present test apparatus was validated by comparing the data generated with Sample 1 with the data provided by Cioncolini et al. (2016) for micro-orifices of the same diameter. In particular, Cioncolini et al. (2016) tested two micro-orifices with diameter of 300 µm and thickness to diameter ratios t/d of 3.33 and 6.67, whereas the thickness to diameter ratio of Sample 1 is intermediate between these two values and equal to 4.17. As described in Cioncolini et al. (2016), during cavitating choked flow the mass flow rate through the micro-orifice varies linearly with the square root of the upstream pressure, and increases with decreasing the thickness to diameter ratio t/d. Measurements taken during cavitating choked flow with Sample 1 are provided, together with the data from Cioncolini et al. (2016), in Fig. 5 where the square root of the upstream pressure is plotted versus the mass flow rate through the orifice. As can be seen in Fig. 5, the trend for Sample 1 is linear and the data points consistently fall between those from Cioncolini et al. (2016), as should be the case based on the thickness to diameter ratio values. Consequently, the test apparatus used in this work can be considered validated. 3. Results and discussion A preliminary hydrodynamic cavitation test was preformed with
Fig. 7. Volume of CRUD removed vs. exposure time to hydrodynamic cavitation (note the logarithmic scale in the horizontal axis). 215
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Fig. 8. SEM images of the downstream face of Sample 3 at various exposure times. From top to bottom: exposure time to cavitation: 0 min, 5 min, 30 min, 1800 min (30 h).
the CLSM analysis, whose accuracy gradually decreases as the volume of CRUD becomes smaller and smaller. Irrespective of this difference, the results in Fig. 7 indicate that more than 80% of the CRUD originally present on the samples is removed after a few minutes of exposure to hydrodynamic cavitation. SEM images of Samples 3 and 4 at the various stages of the
hydrodynamic cavitation experiment are presented in Figs. 8 and 9. As can be seen, the particulate deposition that dominates the overall buildup is effectively removed just after a few minutes of exposure to hydrodynamic cavitation, whilst the crystalline component is removed at a much slower pace and crystalline deposits are still evident on the samples surface at the end of the tests. This qualitatively confirms the 216
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Fig. 9. SEM images of the downstream face of Sample 4 at various exposure times to cavitation. Top: 0 min; bottom: 1380 min (23 h).
results from the CLSM analysis previously discussed. No erosion or mechanical damage to the metal surface can be noticed in Figs. 8 and 9 at the end of the tests, confirming that an exposure to hydrodynamic cavitation up to a few tens of hours is not sufficient to induce any noticeable cavitation damage.
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4. Concluding remarks The use of hydrodynamic cavitation was explored to assess the CRUD removal in micro-orifices that were previously partly clogged in simulated nuclear environment. The setup was validated against literature data and enabled predicting cavitation in micro-orifices and to monitor the CRUD removal during the experiments. Post-test materials characterization was implemented to validate the in-situ evaluations. Periodic inspections also highlighted that no erosion or mechanical damage to the metal surface was induced, confirming that an exposure to hydrodynamic cavitation up to a few tens of hours is not sufficient to induce any noticeable cavitation damage. The results presented showed that about 80–90% of the surface CRUD could be removed within the first 5 min of exposure whilst the complete removal of the CRUD required tens of hours of exposure under cavitating regime. The degree of removal was correlated with the nature of the CRUD, which consist of a particulate and poorly adherent component on the outer surface, and more crystalline, and presumably better anchored, inner oxide that was therefore more difficult to remove. Acknowledgement The financial support from EPSRC through the Centre for Doctoral Training in the Science and Technology of Fusion Energy is gratefully acknowledged (grant number EP/L01663X/1). References Bengtsson, B., Svanberg, P., Dingee, J., Pellman, A., Wells, D., 2014. Experience with high
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