Exploring the effect of cathodic polarization to mitigate CRUD deposition

Cathodic polarization is investigated as a technique for CRUD build-up mitigation in simulated primary pressurized water reactor conditions and accelerated flow. This investigation showed that the application of 3 V could induce a 67% reduction in surface deposition whilst 30 V cell voltage completely prevented the formation of surface deposits.

Nuclear Engineering and Design 366 (2020) 110764 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Exploring the effect of cathodic polarization to mitigate CRUD deposition a a b Stefano Cassineri , Michele Curioni , Andrew Banks , Fabio Scenini a b a,⁎ T Materials Performance Centre, Department of Materials, Universtiy of Manchester, Oxford Road, M13 9PL Manchester, United Kingdom Rolls Royce plc, Kings Place, 90 York Way, N1 9FX London, United Kingdom A R T I C LE I N FO A B S T R A C T Keywords: Micro-orifices Cathodic polarization PWR CRUD mitigation Hydrogen evolution Electrokinetic deposition Cathodic polarization is investigated as a technique for CRUD build-up mitigation in simulated primary pressurized water reactor conditions and accelerated flow This investigation showed that the application of V could induce a 67% reduction in surface deposition whilst 30 V cell voltage completely prevented the formation of surface deposits This behavior can be rationalized considering the synergistic effect between the suppression of the electrokinetic component responsible for the CRUD build-up, and the hydrodynamic cleaning arising from the hydrogen evolution on the cathodically polarized surface Introduction Corrosion product build-up in the primary cycle of the pressurized water reactor is known under the acronym CRUD Such corrosion products are well known to compromise the safety and economic profitability of nuclear power plants (Uchida et al., 2011) For instance, Co-59 can transmute to Co-60 and increases the radiation levels within the primary cooling circuit when incorporated in the oxide (Holdsworth et al., 2018), whilst corrosion product build-up on the steam generator and fuel rods reduces the overall thermohydraulic performance (Uchida et al., 2011) CRUD build-up under the accelerated flow conditions observed in the steam generator is believed to be driven by electrokinetic activity across the metal solution interface (Cassineri et al., 2020b,a, 2019a,b; McGrady et al., 2017a,b; Scenini et al., 2014; Yang et al., 2017) Specifically, electrokinetic deposition of CRUD is driven by the acceleration of the flow when the zeta potential of the metal is negative, and the streaming current generated is positive (Cassineri et al., 2019c,a) In this situation the local unbalance of charges originating from the accelerated flow conditions is compensated by the generation of faradaic reactions across the metal/solution interface, resulting in the oxidation of soluble Fe2+ to insoluble Fe3+, followed by the deposition of magnetite (Cassineri et al., 2019c,a, 2020a; McGrady et al., 2017a; Scenini et al., 2014) Currently, there are no preventive or mitigating in-service methods to deal with the CRUD formation, which is removed, at a high economic cost, after shutdown, through chemical and hydrodynamic treatment (Fujiwara et al., 2004; Szolcek et al., 2019; Vepsalainen, 2010) Thus, the necessity to find an effective method to mitigate CRUD deposition ⁎ without requiring the power plant to shutdown is highly desirable In the oil and gas industry and in the civil sector, the application of cathodic protection is well established as a technique used to protect submerged pipelines and reinforced concrete structures against localised and uniform corrosion (Pedeferri, 1996) In this work, as a proof of concept, cathodic polarization is investigated as a possible technique for CRUD build up mitigation in accelerated flow regions in high temperature water, conditions that are relevant to primary PWR operation Tests were conducted in an autoclave at a temperature of 230 °C, pressure 120 bar and at Li and H2 content respectively of ppm and 2.5 ppm Boric acid is usually added to primary water of commercial PWRs to control reactivity and its concentration is reduced towards the end of the fuel cycle For consistency with previous work (Cassineri et al., 2019a), and to enable comparison of the results, boric acid was not added in the present study However, it is believed that the higher pH in the present work, compared to when boric acid is added, would not have a significant effect on the CRUD deposition mechanism The polarization tests were performed using an in-house designed flow cell that contains a micro-orifice specimen that was cathodically polarized Experimental details The tests were carried out in a 13-liter autoclave SS316 vessel schematically shown in Fig and extensively described in previous work carried out by the authors (Cassineri et al., 2019a c; Cioncolini et al., 2017; McGrady et al., 2017a,b) The driving force required to move the water through the system was imposed by a volumetric pump able to deliver a flow rate of up to 35 l/h After being pre-heated to Corresponding author E-mail address: Fabio.Scenini@Manchester.ac.uk (F Scenini) https://doi.org/10.1016/j.nucengdes.2020.110764 Received 19 February 2020; Received in revised form July 2020; Accepted July 2020 Available online 18 July 2020 0029-5493/ © 2020 The Authors Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/) Nuclear Engineering and Design 366 (2020) 110764 S Cassineri, et al Fig Schematic representation of the flow loop used for the polarization experiments Image adapted from (McGrady et al., 2017a) Fig Flow cells used in the polarization tests: a) optical image of the flow cells used in the polarization tests b) 3D CAD of the polarization cell showing from bottom to top the female face sealing, the copper gasket, the micro-orifice, the male face sealing, the two copper washers, and the stainless steel rod screwed inside the PTFE cylinder, c) cross-section schematic of the cell back pressure regulators which set the pressure drop across the microorifice (Cassineri et al., 2019a,c; McGrady et al., 2017a,b) The accurate control and monitoring of the water chemistry was achieved through oxygen (Orbisphere 410/A/P1C00000), hydrogen (Orbisphere 510B00/T1C0P000) and conductivity sensors (ABB AX410/500001) The pressure drop across the flow cell was measured using a differential pressure cell Rosemount (model C2051CD, ± 20.7 bar, 0.075% full- 230 °C, the water was injected inside the autoclave, and after passing through the flow cells, the water was cooled, depressurized, recirculated through a mixed bed of ion exchange resin and re-injected to the 250-liter storage feed tank The tests were carried out at constant pressure drop which corresponds to constant flow velocity in the troat of the orifice even when it is partly clogged (Cassineri et al., 2019a,c; McGrady et al., 2017a,b); this was achieved thanks to the use of two Nuclear Engineering and Design 366 (2020) 110764 S Cassineri, et al Table Experimental test matrix Disc tested* Lithium/ppm Hydrogen/ppm Applied Cell potential/V Diameter/µm Time/h D400-0 V D400-1 V D400-3 V D400-30 V 2.5 30 400 24 23.5 * The nomenclature of the disc is: Diameter (µm)-Cell potential applied (V) Fig Hydrodynamic measurements showing a) the evolution of the diameter and b) the flow velocity across the test over time connection of the anode was achieved by spot welding the SS rod to a nickel–chromium wire (Ni80/Cr20) that was PTFE sleeved The disc was electrically connected to the autoclave flow loop and therefore no special electrical connection was required Three cell potentials (1, and 30 V) were applied between the SS rod and the flow cell and V cell potentials were achieved with the potentiostat using a two-electrode configuration, while the 30 V cell potential was applied with the power supply A control CRUD build-up test, using the identical setup of the other flow cells but without polarization, was carried out and used a reference It is worth noting that this investigation represents a proof of concept rather than a comprehensive study aiming at mechanicstic understanding; as such, the test domain (1–30 V) was selected to cover a broad range of applied currents that were theoretically calculated to be in the range of ≈20 to 550 µA The highest voltage was selected to counteract the low cathodic polarization efficiency associated with the high electrolyte resistance (Roychowdhury et al., 2016) During the test, the effective current was recorded by the potentiostat when the voltages applied between the micro-orifice and the SS rod were and V No electrical current was recorded when the applied voltage was the highest (30 V) All the experiments were carried out at 230 °C, 120 bar, ppm of Li and 2.5 ppm of H2 and the flow velocity was maintained constant ≈35 m/s inside the micro-orifice The operating temperature of 230 °C was selected due to experimental limitations and taking into consideration the requirements to operate under single-phase flow conditions These conditions represented the compromise between the requirements of fixing the outlet pressure at 120 bar, i.e well above the vapour pressure of water at 230 °C, and the safety requirement to limit the upstream pressure of the autoclave to a maximum value of 200 bar, i.e the sum of the outlet pressure and the pressure drop across the micro-orifice Conversely, the flow velocity value of 35 m/s was selected to induce a relatively fast deposition rate but also to avoid the mechanical removal caused by erosion (Cassineri et al., 2019a) Scanning Electron Microscopy SEM and Laser Confocal Microscopy were carried out with a FEG-SEM Zeiss Sigma and Keyence VK-X200K Table CRUD build-up estimated via laser confocal analysis D400-0 V D400-1 V D400-3 V D400-30 V Applied Cell potential/V Volume/µm3 30 662,741 654,558 216,463 scale accuracy) and the mass flow rate was measured using two variable area digital flow meters (Swagelok model VAF-M2, 0–1.4 cm3 s−1 and 1.1–11.1 cm3 s−1 spans) appositely calibrated prior to the tests (accuracy within 1%) Polarization tests were carried out using either a potentiostat (Gamry-reference 600) or a DC power supply (Skytronic 0–3A, 0–30 V) depending on the cell potential applied between a Stainless-steel rod (anode) and the micro-orifice specimens (cathode) The micro-orifices were mm thick and 11.7 mm wide and were manufactured from SS304L with a mechanically drilled 400 µm microorifice Before the tests, the flow entry side and the flow exit side of the discs were ground up to 600 grit using a Si-C emery paper Prior to the test, the samples were pre-oxidised for 24 h at 230 °C in lithiated and hydrogenated water Although this short oxidation time it is not sufficient to form a fully developed oxide layer, it is also known that on the stainless-steel discs, in slightly alkaline (lithiated) and hydrogenated water, the zeta potential of magnetite is negative Thus, a variation of the oxide thickness is not expected to impact significantly on the zeta potential (Hunter, 1988) The micro-orifice was accommodated in an in-house made flow cell where the sealing between the flow cell and the micro-orifice was achieved with a mm thick copper washer The anode was a threaded 304SS rod, cm long and with a diameter of mm that was screwed in the centre of a PTFE cylinder and placed at a distance of mm from the micro-orifice surface as shown in Fig Eight holes with a diameter of 1.5 mm were mechanically drilled around the anode to allow the water to flow between the surface of the anode (threaded rod) and the cathode (micro-orifice) The electrical Nuclear Engineering and Design 366 (2020) 110764 S Cassineri, et al Fig CRUD volume calculated from laser confocal analysis vs the applied potential investigated (1, and 30 V) which, for passive metals like stainless steels and Ni base alloys, corresponds to the reverse H+/H2 Nernst potential (Volpe et al., 2019) In the absence of cathodic polarization, the anodic reaction is the sum of the passive oxide film formation and the formation of CRUD deposition, where the latter is the dominant one; in fact, the steady state thickness of the oxide in PWR water is expected to be less than ~1 µm after 500 h exposure (Chang et al., 2018), whilst the CRUD deposition rate shown in this work is several microns after 24 h (Fig 5) These anodic reactions are supported by a cathodic reaction which is hydrogen evolution reaction (Birkin et al., 2015; Taqieddin et al., 2017); in fact, recalling that dissolved oxygen in PWR primary water is below ppb, the cathodic oxygen reduction reaction is negligible However, during cathodic polarization this equilibrium is shifted and the surface, which is polarized more negatively, will become a dominant cathode whilst the anodic reaction is suppressed The complete suppression of CRUD deposition during cathodic polarization can be ascribed to two concurring phenomena Firstly, the hydrogen evolved on the cathodic polarized surface can potentially promote the hydrodynamic removal of the deposited oxide In fact, atomic and molecular hydrogen are currently used for the removal of carbon contamination in semiconductor and, as a result of the cathodic charging, in the electrochemical cleaning of Al alloys (Aßmuth et al., 2007; De Graeve and Terryn, 2000) Secondly, CRUD deposition under accelerated flow conditions and alkaline pH (2 ppm of Li and 2.5 ppm of H2) was found to be driven by electrokinetic activity (Cassineri et al., 2019c,a; McGrady et al., 2017a,b; Szolcek et al., 2019) The electrokinetic activity was associated with the occurrence of Faradaic reactions responsible for the oxidation of Fe2+ to Fe3+ followed by the deposition of magnetite The hypothesis of this work is that the established reducing conditions, achieved through cathodic polarization, will counteract the electrokinetic effect by inhibiting the oxidation of ferrous to ferric iron, thus preventing the deposition of the compact and tenacious magnetite oxide (Cassineri et al., 2019a) layer In Fig the compact and crystalline layers arising from electrokinetic activity are not directly visible due to the coverage caused by the particulate build-up However, the deposition symmetry observed in Fig 5-e and f is consistent with the CRUD structure observed in previous work (Cassineri et al., 2019c) thus 3D microscope to characterize the CRUD morphology and to calculate the volume of CRUD deposited The total content of iron and nickel dispersed into the water measured by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) were respectively 1.1 ± 0.4 ppb for Fe and 0.24 ± 0.21 for a Ni The experimental test matrix is presented in Table Results and discussion The evolution of micro-orifice diameter over time and the flow velocity across the tests were calculated from hydrodynamic measurements of pressure drop and volumetric flow rate as discussed in the work carried out by the present authors (Cassineri et al., 2019c,a; Cioncolini et al., 2017; McGrady et al., 2017a,b) and are shown in Fig The calculated flow velocity across the test ranged between 34 and 38 m/s as shown in Fig As expected, no reduction in micro-orifice diameter was observed during either of the tests because in an alkaline environment containing ppm of Li there is no significant radial buildup (Cassineri et al., 2019c) However CRUD build-up was found on the front surface of the discs and the volume of CRUD deposited was measured with a laser confocal microscope after post-mortem examination of the discs following the procedure described in reference (Szolcek et al., 2019) The CRUD deposition volumes calculated in this work are shown in Table and Fig 4, whilst the corresponding SEM images of the front face of the micro-orifice specimens are shown in Fig The application of V cell potential had a negligible effect on the deposition value Conversely, after the application of V and 30 V a partial reduction (67%) and complete suppression of deposition were observed The SEM images revealed a significant reduction in build-up when the cell potential applied was increased from to V and complete suppression of deposition at the highest potential 30 V as shown in Fig 5-d and h Under pressurized water reactor conditions, the electrochemical corrosion potential is dictated by the dissolved hydrogen content Nuclear Engineering and Design 366 (2020) 110764 S Cassineri, et al Fig SEM images of the flow entry side of the micro-orifice specimens a–d) low magnification e–h) high magnification Nuclear Engineering and Design 366 (2020) 110764 S Cassineri, et al suggesting that a crystalline layer is formed underneath the amorphous layer In light of these observations, the results can be rationalized considering that when the applied potential was V (24 µA), the unchanged CRUD build-up observed in Fig 5-b suggests that neither the hydrogen generated on the cathodic site nor the reducing conditions established on the micro-orifice surface were sufficient to avoid the electrokinetic deposition of CRUD, or to hydrodynamically clean the specimen Conversely, at V cell potential 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https://doi.org/10.1016/j.nucengdes.2011.04.009 Vepsalainen, M., 2010 Deposit formation in PWR steam generators VTT Tech Res Cent Finl VTT-R-0013, 1–33 Volpe, L., Bertali, G., Curioni, M., Burke, M.G., Scenini, F., 2019 Replicating PWR primary water conditions in low pressure H2-steam environment to study alloy 600 oxidation processes J Electrochem Soc 166 (2), C1–C8 https://doi.org/10.1149/2 0081902jes Yang, G., Pointeau, E., Tevissen, E., Chagnes, A., 2017 A review on clogging of recirculating steam generators in Pressurized- Water Reactors Progress Nucl Energy https://doi.org/10.1016/j.pnucene.2017.01.010 Summary Cathodic polarization was investigated as a technique for CRUD mitigation in regions of accelerated flow and simulated primary PWR conditions The results show that cathodic polarization results in a synergistic effect between the establishment of reducing conditions, that stop the electrokinetic deposition of CRUD, and the evolution hydrogen, which is responsible for the hydrodynamic cleaning of the specimens and it is able to suppress deposition It is worth recalling that this work was intended as a proof of concept and a more comprehensive work and validation using representatively scaled facility will be considered in future work In addition, the potential issues concerning hydrogen embrittlement also needs to be explored Data availability statement All the data discussed are directly presented in the paper and therefore they are automatically accessible CRediT authorship contribution statement Stefano Cassineri: Conceptualization, Methodology, Data curation, Formal analysis, Writing - original draft, Visualization Michele Curioni: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition Andrew Banks: Conceptualization, Funding acquisition Fabio Scenini: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition 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 Acknowledgments The authors wish to acknowledge support from the Engineering and Physical Sciences Research Council (grant EP/L01680X/1), Rolls Royce plc and the Centre for Doctoral Training in Materials for Demanding Environment (CDT M4DE) References Aßmuth, A., Stimpel-Lindner, T., Senftleben, O., Bayerstadler, A., Sulima, T., ... corresponds to the reverse H+/H2 Nernst potential (Volpe et al., 2019) In the absence of cathodic polarization, the anodic reaction is the sum of the passive oxide film formation and the formation of CRUD. .. between the requirements of fixing the outlet pressure at 120 bar, i.e well above the vapour pressure of water at 230 °C, and the safety requirement to limit the upstream pressure of the autoclave to. .. tests b) 3D CAD of the polarization cell showing from bottom to top the female face sealing, the copper gasket, the micro-orifice, the male face sealing, the two copper washers, and the stainless