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Boric acid and boiling time effects on critical heat flux for corrosive and non-corrosive materials
Annals of Nuclear Energy 136 (2020) 106999
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Boric acid and boiling time effects on critical heat flux for corrosive and non-corrosive materials Dong Hoon Kam, Young Jae Choi, Yong Hoon Jeong ⇑ Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 14 December 2018 Received in revised form 3 May 2019 Accepted 19 August 2019
BA (Boric Acid)-added coolant used for severe accident mitigation strategies (IVR-ERVC and core catcher) induces dissolution of heated surfaces during corrosion process; metal ion or particle concentrations inside the coolant increase with the corrosion process. In this study, various states of BA-added working fluids are considered for both corrosive and non-corrosive test sections. Before each experiments, the fluid conditions are made by the corrosion of carbon steels (not for the experiments) immersed in the BA-dissolved de-ionized (DI) water; metal elements dissolves with time. For the non-corrosive test section, CHF values increase with the metal oxide particles in the coolant where particle deposition occurs. In comparison, for the corrosive test section, CHF is strongly affected by boiling times. For relatively short boiling times, the enhancement is much more noticeable than the longer ones; degradation of CHF values are sometimes observed for the latter condition. Ó 2019 Published by Elsevier Ltd.
Keywords: Boric acid Boiling time CHF Carbon steel Corrosion
1. Introduction Continuous cooling is required to prevent damage to containment of a nuclear power plant when severe accident occurs and a reactor core has melted down. In this perspective, two representative strategies, In-Vessel Retention through the External Reactor Vessel Cooling (IVR-ERVC) and core catcher, are considered as mitigation strategies that utilize coolant supplied from the Incontainment Refueling Water Storage Tank (IRWST). The Reactor Pressure Vessel (RPV) used in a nuclear power plant is composed of carbon steel that acts as a barrier between the molten corium and coolant. Through the RPV wall, heat is transferred and removed by coolant natural circulation. Some candidate materials exist in the core catcher, but carbon steel is planned for implementation in EU-APR, one of Korean nuclear power plants. In both facilities, boiling occurs on the surface of the wall where the coolant is contacted, and it is an effective way to transfer the heat. Since boiling is an effective way of heat transfer for heat generation systems using the latent heat of the coolant, many industrial applications adopt the condition. However, there is a limit point, called the CHF (Critical Heat Flux), beyond which heat transfer performance is radically decreased; the integrity of the heating surface can be damaged under a heat flux control system. Thus, exact and correct
⇑ Corresponding author. E-mail addresses: (Y.H. Jeong).
[email protected]
https://doi.org/10.1016/j.anucene.2019.106999 0306-4549/Ó 2019 Published by Elsevier Ltd.
(D.H.
Kam),
[email protected]
CHF margins should be determined to maintain the system integrity. Numerous approaches have been considered to study CHF and determine various parameters that strongly have effects on the CHF results. Wettability, which quantifies the surface affinity with the coolant, has been widely used to explain the CHF trend. Porosity on the heating surface, if a thickness formed is not excessive, forms paths for water supply toward the bubble sites, which enhances the CHF margin. Kim et al. (2009, 2010) used various kinds of nanofluids to assess the porosity and thermal conductivity effect on CHF enhancement under flow boiling conditions. The enhancement by surface porosity was noticeable and strongly affected by boiling time during which nanoparticles were deposited. Amiri et al. (2014) used carbon nanotubes considering covalent functionalization and non-covalent functionalization to enhance the dispersion stability in the working fluid. They have measured both heat transfer coefficient and CHF values under pool boiling condition. Covalent nanofluids guaranteed highly enhanced results compared with both the non-covalent and DI water conditions. Lee et al. (2013, 2014) used a magnetite nanofluid to assess the porosity effect on the CHF results for various environments including an external magnetic field and local conditions under a flow boiling system. In the latter study, CHF enhancements with critical quality values in the nucleate boiling and annular flow regimes were investigated and analyzed. Lee et al. (2015) considered dilution and storage time effects under pool boiling conditions. Song et al. (2014) carried out nanofluid experiments using
2
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
Fig. 1. Test vessel and heater.
SiC nanoparticles under pool boiling and atmospheric pressure conditions. Kim and Kim (2007) incorporated the wicking force of porous surfaces in their analysis in addition to the wettability effect. Yang et al. (2006) used porous surface coatings on downward-facing curved hemispheres to enhance the CHF margin, and their goal was to increase the thermal limit of the IVR-ERVC strategy. In their study, CHF was enhanced for the entire orientation range on curved locations. As a similar approach, Park and Bang (2013) used a graphene nanofluid for a downward-facing curved heater to simulate the IVR-ERVC environment. Reduced graphene oxide has also been utilized by Ahn et al. (2014) to assess the enhancement, and a related analysis was carried out. A porous surface can also be produced when surface corrosion is actively engaged. For example, carbon steel is a well-known corrosive material, and a porous, iron-based oxide layer, is formed when active corrosion is occurred. Kim et al. (2012) considered coolant effects using TSP (Trisodium phosphate) and BA (Boric acid) solutions. Park et al. (2014) used SA508 carbon steel, a RPV wall
material, and coolants injected during the activation of IVR-ERVC system to describe real conditions. In addition to the surface morphology, especially porosity, thermal properties of a heater material also play an important role. Gogonin (2009) summarized material effects with thickness. Kam et al. (2015) considered SiCand Cr-coated surfaces with a thickness effect assessment. Unal et al. (1992) performed calculations using different heater materials and thickness. According to the study, size of dry patch was strongly dependent on heater materials and thickness. Raghupathi and Kandlikar (2017) assessed the overall material effects with representative parameters, and Gong and Cheng (2017) calculated heat transfer performance where material effects were considered. Kam et al. (2018) used carbon steel and stainless steel as a reference where material effect was mainly used to describe the experimental trends. In real situation, boric acid, known to expedite the corrosion process on the carbon steel, is added to the injected coolant from IRWST during operations of the severe accident mitigation
Fig. 2. Color change of BA-based coolants by dissolution (Up: relatively low concentration, Down: relatively high concentration).
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
strategies. As corrosive environments change the coolant condition with time by the dissolution, effects of the phenomenon on the CHF trends are focused in this study. The effect on non-corrosive and corrosive heaters are assessed, and the boiling time effect is considered for the corrosive one; continuous dissolution of the metal elements in the coolant is considered, which forms metal oxides, or it resides as ionic forms. 2. Experiment 2.1. Coolant and heaters For a working fluid, 2.5 wt% boric acid is prepared inside DI water considering the real condition of the coolant from the
3
IRWST, and the CHF results are compared with pure DI water conditions. To account for the effects of dissolved metal elements on the CHF trends, several concentrations of metal elements inside the working fluid are prepared before carrying out the experiments. The concentrations are adjusted with number of carbon steels (not for the CHF experiments) immersed inside the pool before the experiments. Various states BA-based coolants are used both for newly prepared non-corrosive (SS304) and corrosive (SA508) heaters; BA-added coolant without any corrosion process is notated as ‘N’, BA-added coolant with relatively low and high concentration of the dissolved metal elements are represented as ‘C-Low’ and ‘C-High’, respectively, herein after. Beyond certain time of the immersion, change of the coolant color, a measure of dissolution, does not occur anymore, and the state is notated as
Fig. 3. Concentration change of dissolved metal elements with corrosion processes (up: Fe/Cu-MS results, down: trends).
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D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
Table 1 Test matrix considering the boiling time effect for SA508. N condition
C-Low condition
C-High condition
40x100
10 min 15 min 20 min 60 min
– 15 min – –
– 15 min – 50 min
50x100
10 min 55 min
– –
10 min 55 min
Fig. 4. Experimental SA508 test section (up: DI water condition, down: BA-based coolant condition).
‘C-High’.’ Furthermore, width effect is considered on the overall trends; the widths are increased from 40 mm to 60 mm for noncorrosive heaters and from 40 mm to 50 mm for the corrosive heaters. 2.2. Test pool and procedure The amount of coolant used for the experiments is about 210 L, with 0.4 m of water depth between the water surface and center of
the heater and 0.3 m between center of the heater and the bottom part of the pool. A condenser and a pre-heater are located at the upper part and lower part of the pool, respectively, to keep the conditions. An air cylinder is located right next to the vessel to lift up the lead where copper electrodes and electrical lines are attached (Fig. 1). Power is supplied from a DC rectifier (100 kW capacity) through copper electrodes, and the voltage drop is measured on the main heaters attached to the copper electrodes. To eliminate uncertainties arisen from the heat loss through copper electrodes and joint parts, considered measuring locations are right ends of the main heater. The orientation is fixed to be vertical: 90° from the horizontally downward condition (Fig. 1). For the carbon steel test sections, brass bolts are used to connect the heaters on the copper electrodes to minimize the electrical resistance considering much lower electrical resistance of the carbon steel compared with the stainless steel plates. Under same heat flux levels for the two kinds of test sections, an ‘edge’ effect becomes too vigorous for the carbon steel when conventional stainless steel bolts are used; for the stainless steel heater, both kinds of bolts do not show noticeable difference on the CHF values. Whole tests are carried out under saturated conditions, and the bulk temperature is kept using 2 K-type thermocouples and a pre-heater located at the bottom part of the pool. The voltage has been increased by a step-wise method until reaching the CHF point. At each steps, stabilization of the boiling process has been guaranteed to prevent premature CHF occurrence. The CHF point is detected when a sudden change of electrical resistance occurs, which accompanies glowing phenomenon, induced by an abrupt increase of temperature on the heater surface. The CHF value is calculated using power supplied to the main heater when the CHF occurred and the heat transfer area.
q00 ¼ VI=Area where q00 is a heat flux from the heater surface in W/m2; V, I and Area are the voltage drop along the test section, the current and the area where the boiling occurred, respectively. The units are in Volts, Amperes, and m2, respectively. The uncertainties of the calculated CHF values are considered based on the root-sum square (RSS) method, where individual uncertainties were used from independent variables. For the calculation, square root of sum of each variables’ square values have been used. The uncertainties in power have been calculated with
Fig. 5. SEM images for surface conditions made on the SA508 test sections (left: BA coolant, right: DI water).
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
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the uncertainties of current and voltage, which are 1.0% and 0.2%, respectively. The uncertainty in heat transfer area is 0.3%, and thus, the overall CHF uncertainty is approximately 1.0%. 3. Results BA-added coolants go over color change with corrosion time as seen in Fig. 2. With corrosion processes, metal elements are dissolved in the coolant, which induces aforementioned color change (Fig. 3). To reflect the phenomenon, several conditions of BA-added coolant are considered both for the corrosive and non-corrosive test sections. Three kinds of conditions are considered to assess the overall trends of the CHF: N, ‘C-Low’ and ‘C-High’. Especially, boiling time effect is included since the boiling time is one of major contributors to the corrosion process of the carbon steel (Table 1). The corrosion of the carbon steel is confirmed by sight (Fig. 4) and from SEM images taken on the surfaces; porous structures are formed on the surface of the corrosive heaters, as noted by preceding studies (Fig. 5).
Fig. 6. BA-added coolant (N condition) & pure DI water on SS304 (40, 50 and 60 mm widths).
3.1. BA-added coolant (N condition) and pure DI water on SS304 When ‘N’ state BA-added coolant, a coolant condition without any corrosion, and pure DI water are used for the SS304 CHF experiments, the former condition shows enhanced results for whole width scales. According to an analysis by Park et al. (2014), relatively high concentrations of BA in DI water, 2.5 wt% as in this study, induced further enhancement by particle deposition on the surface. Additionally, Lee et al. (2010) showed the effect of wettability change induced by BA usage. The overall trends are shown in Fig. 6. 3.2. BA-added coolants (‘C-Low’ and ‘C-High’ conditions) on SS304 The BA-based coolants, with dissolved metal ions and metal oxide particles inside, have been used for the non-corrosive test section, SS304, to exclude the corrosion effect on CHF trends. In Fig. 7, CHF results for the stainless steel are summarized with different amounts of metal elements inside the coolant (N, ‘C-Low’ and ‘C-High’). As expected, with more metal oxide particles inside the coolant, formed by interactions between oxygen and dissolved metal ions, CHF tends to further increase for the entire range of width scales considered in this study. As seen in Fig. 8, some brownish depositions can be observed after the CHF experiments on the non-corrosive test sections, which activates water supply by way of forming porous structures, as demonstrated by the numerous nanofluid experiments. When the width effect is concerned, it is most noticeable on the widest test section; the enhanced result sometimes shows even higher CHF value than the narrow one. As observed by Kam et al. (2018), width scales correspond to this range can sometimes show reversed trends with respect to CHF values depending on conditions; the range seems to be a transitional region.
Fig. 7. BA-added coolant conditions (N, C-Low and C-High) on SS304.
3.3. BA-added coolant (N condition) and pure DI water on SA508 When the SA508 test section is used for the CHF experiments with BA-added coolant (N condition) and pure DI water, BAadded one show much higher CHF results, which attributes to the accelerated corrosion phenomenon induced by an acidic condition of the boric acid (Fig. 9). As observed in Figs. 4 and 5, porous structures, formed by the corrosion process, activate water supply near the CHF points. Furthermore, some boiling time effects exist, where enhancement with BA-added coolant increases with time and almost converges for long periods of boiling time, as seen in
Fig. 8. Pictures for brownish depositions on SS304 after the experiments.
Fig. 9. For the wide test section, the CHF values converge in relatively short time. When the heater material effect is concerned, the carbon steel shows highly enhanced results compared with the stainless steel, a non-corrosive heater for whole widths. According to Kam et al.
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D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
Fig. 9. BA-added coolant (N condition) & pure DI water on SA508 (left: 40 mm-width, right: 50 mm-width).
Fig. 10. Boiling time effect on SA508 with BA-added coolants (C-High condition, left: 40 mm-width, right: 50 mm-width).
Fig. 11. Overall boiling time effects on SA508 with BA-added coolants conditions (left: 40 mm-width, right: 50 mm-width).
(2018), the carbon steel definitely shows enhanced results for the narrow geometry when DI water is used; the wide one gives small or negligible enhancement compared with the narrow one. However, with additional corrosion process in this study, the carbon steel induces highly enhanced CHF results for whole width scales by forming porous structures.
3.4. BA-added coolants (‘C-Low’ and ‘C-High’) on SA508 with boiling time According to the CHF results, the boiling time effect is strongly observable; the CHF values, especially for the ‘C-High’ condition, tend to be degraded for long periods of boiling time, as seen in
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999 Table 2 pH conditions of the BA coolants.
pH values
N condition
C-Low condition
C-High condition
4.28
4.21–4.26
4.17–4.38
Table 3 DLS results for particle sizes in the coolants. C-Low condition Mean particle size [nm] Std. error [nm]
2425 286
1886 166
C-High condition 1685 224
2343 298
7
specimen, BA-added coolant (N condition) showed enhanced results compared with the pure DI water condition. With BAadded coolant of ‘C-Low’ and ‘C-High’ conditions, the enhancement continuously increases, and it is attributed to the particle deposition. When the corrosive test section made of carbon steel is used, all BA-added coolants show boiling time effects. For short boiling times, the CHF values tend to be noticeably increased; however, sometimes, for long boiling times, the CHF value begin to decrease, especially when the metal elements are dissolved inside. Conflict of interest
2209 651
The authors declare that there is no conflict of interest. Acknowledgments
Fig. 10. In comparison, the CHF values are highly enhanced for the relatively short periods of boiling time for both width sizes. Overall trends with the boiling time effect are summarized in Fig. 11. To assess the parametric effects on the results acquired in this study, firstly, pH conditions are measured for all conditions of the BAadded coolants since pH environments strongly determines degrees of corrosion process; difference on the pH conditions are not noticeably recognizable as summarized in Table 2. Also, particle sizes are measured from the coolants of ‘C-Low’ and ‘C-High’ conditions to account for the deposition process (Table 3); size itself cannot explain the trends. Therefore, other chemicalhydraulic conditions must have played roles on the phenomena, and on this basis, metal element concentration in the coolant may have induced different degrees of corrosion, or particle concentration effect on the amount of deposition may have also contributed the trends. In addition, accelerated corrosion induced by changed environment may have formed a thick porous structure, which could role as a thermal resistance against water supply. Similarly, CRUD (Chalk River Undefined Deposit), which accompanies porous structures formed on the surfaces of fuel claddings in nuclear power plants, can generally enhance the CHF margin. However, it sometimes leads to thick layers that can form local dry-outs inside the porous structures (Jina and Short, 2014; Yeo and No, 2019). On this perspective, dry-outs could occur at lower heat flux levels even with porous structures. Cohen (1974), Pan et al. (1987), Short et al. (2013), Jina and Short (2014), Yeo and No (2017) pointed out the change of a maximum concentration factor at the interface between the cladding surface and porous structures. Especially, Pan et al. (1987) summarized the trend with wick structures, pressure, heat flux and CRUD thickness; the maximum concentration factor increases with the CRUD thickness. In this regard, solutes and particulates can form porous structures on the surface by the evaporation in this experiment, and accelerated corrosion, which induces thick porous structures, at the interface could have happened by concentrated BA concentration at the location. Depending on corrosion conditions, the CHF trend may differ according to the preceding studies, but when long periods of corrosion phenomenon is accompanied with the boiling process, there is room for reduction according to this study.
4. Conclusions BA-based coolants of various conditions have been used for CHF experiments with corrosive and non-corrosive test sections. BAadded coolant of ‘N’ condition is coolant with 2.5 wt% boric acid dissolved in DI water without any corrosion process, and BAadded coolants of ‘C-Low’ and ‘C-High’ conditions are BA-added coolant with dissolved metal ions and metal oxide particles (color changes are observed with concentrations). For the non-corrosive
This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS), granted financial resource from the Nuclear Safety and Security Commission (NSSC), Republic of Korea (No. 1305011), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. NRF-2017M2A8A4056643). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.106999. References Kim, S.J., McKrell, T., Buongiorno, J., Hu, L.-W., 2009. Experimental study of flow critical heat flux in alumina-water, zinc-oxide-water, and diamond-water nanofluids. J. Heat Transf. 131, 043204. Kim, S.J., McKrell, T., Buongiorno, J., Hu, L.-W., 2010. Subcooled flow boiling heat trasnfer of dilute alumina, zinc oxide, and diamond nanofluids at atmospheric pressure. Nucl. Eng. Des. 240 (5), 1186–1194. Amiri, A., Shanbedi, M., Amiri, H., Heris, S.Z., Kazi, S.N., Chew, B.T., Eshghi, H., 2014. Pool boiling heat transfer of CNT/water nanofluids. Appl. Therm. Eng. 71 (1), 450–459. Lee, T., Lee, J.H., Jeong, Y.H., 2013. Flow boiling critical heat flux characteristics of magnetic nanofluid at atmospheric pressure and low mass flux conditions. Int. J. Heat Mass Transf. 56, 101–106. Lee, T., Kam, D.H., Lee, J.H., Jeong, Y.H., 2014. Effects of two-phase flow conditions on flow boiling CHF enhancement of magnetite-water nanofluids. Int. J. Heat Mass Transf. 74, 278–284. Lee, J.H., Kam, D.H., Jeong, Y.H., 2015. The effect of nanofluid stability on critical heat flux using magnetite-water nanofluids. Nucl. Eng. Des. 292, 187–192. Song, S.L., Lee, J.H., Chang, S.H., 2014. CHF enhancement of SiC nanofluid in pool boiling experiment. Exp. Therm. Fluid Sci. 52, 12–18. Kim, H.D., Kim, M.H., 2007. Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids. Appl. Phys. Lett. 91, (1) 014104. Yang, J., Cheung, F.B., Rempe, J.L., Suh, K.Y., Kim, S.B., 2006. Critical heat flux for downward-facing boiling on a coated hemispherical vessel surrounded by an insulation structure. Nucl. Eng. Technol. 38, 139–146. Park, S.D., Bang, I.C., 2013. Flow boiling CHF enhancement in an external reactor vessel cooling (ERVC) channel using graphene oxide nanofluid. Nucl. Eng. Des. 265, 310–318. Ahn, H.S., Kim, J.M., Kaviany, M., Kim, M.H., 2014. Pool boiling experiments in reduced graphene oxide colloids part II – Behavior after the CHF, and boiling hysteresis. Int. J. Heat Mass Transf. 78, 224–231. Kim, T.I., Park, H.M., Chang, S.H., 2012. CHF experiments using a 2-D curved test section with additives for IVR-ERVC strategy. Nucl. Eng. Des. 243, 272–278. Park, H.M., Jeong, Y.H., Heo, S., 2014. Effect of heater material and coolant additives on CHF for a downward facing curved surface. Nucl. Eng. Des. 278, 344–351. Gogonin, I.I., 2009. Influence of the thickness of a wall and of its thermophysical characteristics on the critical heat flux in boiling. J. Eng. Phys. Thermophys. 82 (6), 1175–1183. Kam, D.H., Lee, J.H., Lee, T., Jeong, Y.H., 2015. Critical heat flux for SiC- and Cr-coated plates under atmospheric condition. Ann. Nucl. Energy 76, 335–342. Unal, C., Daw, V., Nelson, R.A., 1992. Unifying the controlling mechanisms for the critical heat flux and quenching: the ability of liquid to contact the hot surface. J. Heat Transf. 114, 972–982. Raghupathi, P.A., Kandlikar, S.G., 2017. Effect of thermophysical properties of the heater substrate on critical heat flux in pool boiling. J. Heat Transf. 139, 111502.
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Gong, S., Cheng, P., 2017. Direct numerical simulations of pool boiling curves including heater’s thermal responses and the effect of vapor phase’s thermal conductivity. Int. Comm. Heat Mass. Transf. 87, 61–71. Kam, D.H., Choi, Y.J., Jeong, Y.H., 2018a. Critical heat flux on downward-facing carbon steel flat plates under atmospheric condition. Exp. Therm. Fluid Sci. 90, 22–27. Lee, J., Jeong, Y.H., Chang, S.H., 2010. CHF enhancement in flow boiling system with TSP and boric acid solutions under atmospheric pressure. Nucl. Eng. Des. 240, 3594–3600. Kam, D.H., Choi, Y.J., Jeong, Y.H., 2018b. CHF experiment with downward-facing carbon and stainless steel plates under pressurized conditions. Int. J. Heat Mass Transf. 125, 670–680. Jina, M., Short, M., 2014. Two-Phase Fluid Flow Modeling in CRUD Using MAMBABDM, L3:MPO.CRUD.P8.02, Consortium for Advanced Simulation of LWRs.
Yeo, D.Y., No, H.C., 2019. Modeling heat transfer through corrosion product deposits on fuel rods in pressurized water reactors. Nucl. Eng. Des. 342, 308–319. Cohen, 1974. Heat and mass transfer for boiling in porous deposits with chimneys. In: AICHE Symposium Series, pp. 71–80. Pan, C., Jones, B.G., Machiels, A.J., 1987. Concentration levels of solutes in porous deposits with chimneys under wick boiling conditions. Nucl. Eng. Des. 99, 317– 327. Short, M.P., Hussey, D., Kendrick, B.K., Besmann, T.M., Stanek, C.R., Yip, S., 2013. Multiphysics modeling of porous CRUD deposits in nuclear reactors. J. Nucl. Mater. 443, 579–587. Yeo, D.Y., No, H.C., 2017. Modeling heat transfer through chimney-structured porous deposit formed in pressurized water reactors. Int. J. Heat Mass Transf. 108, 868–879.
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Boric acid and boiling time effects on critical heat flux for corrosive and non-corrosive materials Dong Hoon Kam, Young Jae Choi, Yong Hoon Jeong ⇑ Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 14 December 2018 Received in revised form 3 May 2019 Accepted 19 August 2019
BA (Boric Acid)-added coolant used for severe accident mitigation strategies (IVR-ERVC and core catcher) induces dissolution of heated surfaces during corrosion process; metal ion or particle concentrations inside the coolant increase with the corrosion process. In this study, various states of BA-added working fluids are considered for both corrosive and non-corrosive test sections. Before each experiments, the fluid conditions are made by the corrosion of carbon steels (not for the experiments) immersed in the BA-dissolved de-ionized (DI) water; metal elements dissolves with time. For the non-corrosive test section, CHF values increase with the metal oxide particles in the coolant where particle deposition occurs. In comparison, for the corrosive test section, CHF is strongly affected by boiling times. For relatively short boiling times, the enhancement is much more noticeable than the longer ones; degradation of CHF values are sometimes observed for the latter condition. Ó 2019 Published by Elsevier Ltd.
Keywords: Boric acid Boiling time CHF Carbon steel Corrosion
1. Introduction Continuous cooling is required to prevent damage to containment of a nuclear power plant when severe accident occurs and a reactor core has melted down. In this perspective, two representative strategies, In-Vessel Retention through the External Reactor Vessel Cooling (IVR-ERVC) and core catcher, are considered as mitigation strategies that utilize coolant supplied from the Incontainment Refueling Water Storage Tank (IRWST). The Reactor Pressure Vessel (RPV) used in a nuclear power plant is composed of carbon steel that acts as a barrier between the molten corium and coolant. Through the RPV wall, heat is transferred and removed by coolant natural circulation. Some candidate materials exist in the core catcher, but carbon steel is planned for implementation in EU-APR, one of Korean nuclear power plants. In both facilities, boiling occurs on the surface of the wall where the coolant is contacted, and it is an effective way to transfer the heat. Since boiling is an effective way of heat transfer for heat generation systems using the latent heat of the coolant, many industrial applications adopt the condition. However, there is a limit point, called the CHF (Critical Heat Flux), beyond which heat transfer performance is radically decreased; the integrity of the heating surface can be damaged under a heat flux control system. Thus, exact and correct
⇑ Corresponding author. E-mail addresses: (Y.H. Jeong).
[email protected]
https://doi.org/10.1016/j.anucene.2019.106999 0306-4549/Ó 2019 Published by Elsevier Ltd.
(D.H.
Kam),
[email protected]
CHF margins should be determined to maintain the system integrity. Numerous approaches have been considered to study CHF and determine various parameters that strongly have effects on the CHF results. Wettability, which quantifies the surface affinity with the coolant, has been widely used to explain the CHF trend. Porosity on the heating surface, if a thickness formed is not excessive, forms paths for water supply toward the bubble sites, which enhances the CHF margin. Kim et al. (2009, 2010) used various kinds of nanofluids to assess the porosity and thermal conductivity effect on CHF enhancement under flow boiling conditions. The enhancement by surface porosity was noticeable and strongly affected by boiling time during which nanoparticles were deposited. Amiri et al. (2014) used carbon nanotubes considering covalent functionalization and non-covalent functionalization to enhance the dispersion stability in the working fluid. They have measured both heat transfer coefficient and CHF values under pool boiling condition. Covalent nanofluids guaranteed highly enhanced results compared with both the non-covalent and DI water conditions. Lee et al. (2013, 2014) used a magnetite nanofluid to assess the porosity effect on the CHF results for various environments including an external magnetic field and local conditions under a flow boiling system. In the latter study, CHF enhancements with critical quality values in the nucleate boiling and annular flow regimes were investigated and analyzed. Lee et al. (2015) considered dilution and storage time effects under pool boiling conditions. Song et al. (2014) carried out nanofluid experiments using
2
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
Fig. 1. Test vessel and heater.
SiC nanoparticles under pool boiling and atmospheric pressure conditions. Kim and Kim (2007) incorporated the wicking force of porous surfaces in their analysis in addition to the wettability effect. Yang et al. (2006) used porous surface coatings on downward-facing curved hemispheres to enhance the CHF margin, and their goal was to increase the thermal limit of the IVR-ERVC strategy. In their study, CHF was enhanced for the entire orientation range on curved locations. As a similar approach, Park and Bang (2013) used a graphene nanofluid for a downward-facing curved heater to simulate the IVR-ERVC environment. Reduced graphene oxide has also been utilized by Ahn et al. (2014) to assess the enhancement, and a related analysis was carried out. A porous surface can also be produced when surface corrosion is actively engaged. For example, carbon steel is a well-known corrosive material, and a porous, iron-based oxide layer, is formed when active corrosion is occurred. Kim et al. (2012) considered coolant effects using TSP (Trisodium phosphate) and BA (Boric acid) solutions. Park et al. (2014) used SA508 carbon steel, a RPV wall
material, and coolants injected during the activation of IVR-ERVC system to describe real conditions. In addition to the surface morphology, especially porosity, thermal properties of a heater material also play an important role. Gogonin (2009) summarized material effects with thickness. Kam et al. (2015) considered SiCand Cr-coated surfaces with a thickness effect assessment. Unal et al. (1992) performed calculations using different heater materials and thickness. According to the study, size of dry patch was strongly dependent on heater materials and thickness. Raghupathi and Kandlikar (2017) assessed the overall material effects with representative parameters, and Gong and Cheng (2017) calculated heat transfer performance where material effects were considered. Kam et al. (2018) used carbon steel and stainless steel as a reference where material effect was mainly used to describe the experimental trends. In real situation, boric acid, known to expedite the corrosion process on the carbon steel, is added to the injected coolant from IRWST during operations of the severe accident mitigation
Fig. 2. Color change of BA-based coolants by dissolution (Up: relatively low concentration, Down: relatively high concentration).
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
strategies. As corrosive environments change the coolant condition with time by the dissolution, effects of the phenomenon on the CHF trends are focused in this study. The effect on non-corrosive and corrosive heaters are assessed, and the boiling time effect is considered for the corrosive one; continuous dissolution of the metal elements in the coolant is considered, which forms metal oxides, or it resides as ionic forms. 2. Experiment 2.1. Coolant and heaters For a working fluid, 2.5 wt% boric acid is prepared inside DI water considering the real condition of the coolant from the
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IRWST, and the CHF results are compared with pure DI water conditions. To account for the effects of dissolved metal elements on the CHF trends, several concentrations of metal elements inside the working fluid are prepared before carrying out the experiments. The concentrations are adjusted with number of carbon steels (not for the CHF experiments) immersed inside the pool before the experiments. Various states BA-based coolants are used both for newly prepared non-corrosive (SS304) and corrosive (SA508) heaters; BA-added coolant without any corrosion process is notated as ‘N’, BA-added coolant with relatively low and high concentration of the dissolved metal elements are represented as ‘C-Low’ and ‘C-High’, respectively, herein after. Beyond certain time of the immersion, change of the coolant color, a measure of dissolution, does not occur anymore, and the state is notated as
Fig. 3. Concentration change of dissolved metal elements with corrosion processes (up: Fe/Cu-MS results, down: trends).
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Table 1 Test matrix considering the boiling time effect for SA508. N condition
C-Low condition
C-High condition
40x100
10 min 15 min 20 min 60 min
– 15 min – –
– 15 min – 50 min
50x100
10 min 55 min
– –
10 min 55 min
Fig. 4. Experimental SA508 test section (up: DI water condition, down: BA-based coolant condition).
‘C-High’.’ Furthermore, width effect is considered on the overall trends; the widths are increased from 40 mm to 60 mm for noncorrosive heaters and from 40 mm to 50 mm for the corrosive heaters. 2.2. Test pool and procedure The amount of coolant used for the experiments is about 210 L, with 0.4 m of water depth between the water surface and center of
the heater and 0.3 m between center of the heater and the bottom part of the pool. A condenser and a pre-heater are located at the upper part and lower part of the pool, respectively, to keep the conditions. An air cylinder is located right next to the vessel to lift up the lead where copper electrodes and electrical lines are attached (Fig. 1). Power is supplied from a DC rectifier (100 kW capacity) through copper electrodes, and the voltage drop is measured on the main heaters attached to the copper electrodes. To eliminate uncertainties arisen from the heat loss through copper electrodes and joint parts, considered measuring locations are right ends of the main heater. The orientation is fixed to be vertical: 90° from the horizontally downward condition (Fig. 1). For the carbon steel test sections, brass bolts are used to connect the heaters on the copper electrodes to minimize the electrical resistance considering much lower electrical resistance of the carbon steel compared with the stainless steel plates. Under same heat flux levels for the two kinds of test sections, an ‘edge’ effect becomes too vigorous for the carbon steel when conventional stainless steel bolts are used; for the stainless steel heater, both kinds of bolts do not show noticeable difference on the CHF values. Whole tests are carried out under saturated conditions, and the bulk temperature is kept using 2 K-type thermocouples and a pre-heater located at the bottom part of the pool. The voltage has been increased by a step-wise method until reaching the CHF point. At each steps, stabilization of the boiling process has been guaranteed to prevent premature CHF occurrence. The CHF point is detected when a sudden change of electrical resistance occurs, which accompanies glowing phenomenon, induced by an abrupt increase of temperature on the heater surface. The CHF value is calculated using power supplied to the main heater when the CHF occurred and the heat transfer area.
q00 ¼ VI=Area where q00 is a heat flux from the heater surface in W/m2; V, I and Area are the voltage drop along the test section, the current and the area where the boiling occurred, respectively. The units are in Volts, Amperes, and m2, respectively. The uncertainties of the calculated CHF values are considered based on the root-sum square (RSS) method, where individual uncertainties were used from independent variables. For the calculation, square root of sum of each variables’ square values have been used. The uncertainties in power have been calculated with
Fig. 5. SEM images for surface conditions made on the SA508 test sections (left: BA coolant, right: DI water).
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999
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the uncertainties of current and voltage, which are 1.0% and 0.2%, respectively. The uncertainty in heat transfer area is 0.3%, and thus, the overall CHF uncertainty is approximately 1.0%. 3. Results BA-added coolants go over color change with corrosion time as seen in Fig. 2. With corrosion processes, metal elements are dissolved in the coolant, which induces aforementioned color change (Fig. 3). To reflect the phenomenon, several conditions of BA-added coolant are considered both for the corrosive and non-corrosive test sections. Three kinds of conditions are considered to assess the overall trends of the CHF: N, ‘C-Low’ and ‘C-High’. Especially, boiling time effect is included since the boiling time is one of major contributors to the corrosion process of the carbon steel (Table 1). The corrosion of the carbon steel is confirmed by sight (Fig. 4) and from SEM images taken on the surfaces; porous structures are formed on the surface of the corrosive heaters, as noted by preceding studies (Fig. 5).
Fig. 6. BA-added coolant (N condition) & pure DI water on SS304 (40, 50 and 60 mm widths).
3.1. BA-added coolant (N condition) and pure DI water on SS304 When ‘N’ state BA-added coolant, a coolant condition without any corrosion, and pure DI water are used for the SS304 CHF experiments, the former condition shows enhanced results for whole width scales. According to an analysis by Park et al. (2014), relatively high concentrations of BA in DI water, 2.5 wt% as in this study, induced further enhancement by particle deposition on the surface. Additionally, Lee et al. (2010) showed the effect of wettability change induced by BA usage. The overall trends are shown in Fig. 6. 3.2. BA-added coolants (‘C-Low’ and ‘C-High’ conditions) on SS304 The BA-based coolants, with dissolved metal ions and metal oxide particles inside, have been used for the non-corrosive test section, SS304, to exclude the corrosion effect on CHF trends. In Fig. 7, CHF results for the stainless steel are summarized with different amounts of metal elements inside the coolant (N, ‘C-Low’ and ‘C-High’). As expected, with more metal oxide particles inside the coolant, formed by interactions between oxygen and dissolved metal ions, CHF tends to further increase for the entire range of width scales considered in this study. As seen in Fig. 8, some brownish depositions can be observed after the CHF experiments on the non-corrosive test sections, which activates water supply by way of forming porous structures, as demonstrated by the numerous nanofluid experiments. When the width effect is concerned, it is most noticeable on the widest test section; the enhanced result sometimes shows even higher CHF value than the narrow one. As observed by Kam et al. (2018), width scales correspond to this range can sometimes show reversed trends with respect to CHF values depending on conditions; the range seems to be a transitional region.
Fig. 7. BA-added coolant conditions (N, C-Low and C-High) on SS304.
3.3. BA-added coolant (N condition) and pure DI water on SA508 When the SA508 test section is used for the CHF experiments with BA-added coolant (N condition) and pure DI water, BAadded one show much higher CHF results, which attributes to the accelerated corrosion phenomenon induced by an acidic condition of the boric acid (Fig. 9). As observed in Figs. 4 and 5, porous structures, formed by the corrosion process, activate water supply near the CHF points. Furthermore, some boiling time effects exist, where enhancement with BA-added coolant increases with time and almost converges for long periods of boiling time, as seen in
Fig. 8. Pictures for brownish depositions on SS304 after the experiments.
Fig. 9. For the wide test section, the CHF values converge in relatively short time. When the heater material effect is concerned, the carbon steel shows highly enhanced results compared with the stainless steel, a non-corrosive heater for whole widths. According to Kam et al.
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Fig. 9. BA-added coolant (N condition) & pure DI water on SA508 (left: 40 mm-width, right: 50 mm-width).
Fig. 10. Boiling time effect on SA508 with BA-added coolants (C-High condition, left: 40 mm-width, right: 50 mm-width).
Fig. 11. Overall boiling time effects on SA508 with BA-added coolants conditions (left: 40 mm-width, right: 50 mm-width).
(2018), the carbon steel definitely shows enhanced results for the narrow geometry when DI water is used; the wide one gives small or negligible enhancement compared with the narrow one. However, with additional corrosion process in this study, the carbon steel induces highly enhanced CHF results for whole width scales by forming porous structures.
3.4. BA-added coolants (‘C-Low’ and ‘C-High’) on SA508 with boiling time According to the CHF results, the boiling time effect is strongly observable; the CHF values, especially for the ‘C-High’ condition, tend to be degraded for long periods of boiling time, as seen in
D.H. Kam et al. / Annals of Nuclear Energy 136 (2020) 106999 Table 2 pH conditions of the BA coolants.
pH values
N condition
C-Low condition
C-High condition
4.28
4.21–4.26
4.17–4.38
Table 3 DLS results for particle sizes in the coolants. C-Low condition Mean particle size [nm] Std. error [nm]
2425 286
1886 166
C-High condition 1685 224
2343 298
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specimen, BA-added coolant (N condition) showed enhanced results compared with the pure DI water condition. With BAadded coolant of ‘C-Low’ and ‘C-High’ conditions, the enhancement continuously increases, and it is attributed to the particle deposition. When the corrosive test section made of carbon steel is used, all BA-added coolants show boiling time effects. For short boiling times, the CHF values tend to be noticeably increased; however, sometimes, for long boiling times, the CHF value begin to decrease, especially when the metal elements are dissolved inside. Conflict of interest
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The authors declare that there is no conflict of interest. Acknowledgments
Fig. 10. In comparison, the CHF values are highly enhanced for the relatively short periods of boiling time for both width sizes. Overall trends with the boiling time effect are summarized in Fig. 11. To assess the parametric effects on the results acquired in this study, firstly, pH conditions are measured for all conditions of the BAadded coolants since pH environments strongly determines degrees of corrosion process; difference on the pH conditions are not noticeably recognizable as summarized in Table 2. Also, particle sizes are measured from the coolants of ‘C-Low’ and ‘C-High’ conditions to account for the deposition process (Table 3); size itself cannot explain the trends. Therefore, other chemicalhydraulic conditions must have played roles on the phenomena, and on this basis, metal element concentration in the coolant may have induced different degrees of corrosion, or particle concentration effect on the amount of deposition may have also contributed the trends. In addition, accelerated corrosion induced by changed environment may have formed a thick porous structure, which could role as a thermal resistance against water supply. Similarly, CRUD (Chalk River Undefined Deposit), which accompanies porous structures formed on the surfaces of fuel claddings in nuclear power plants, can generally enhance the CHF margin. However, it sometimes leads to thick layers that can form local dry-outs inside the porous structures (Jina and Short, 2014; Yeo and No, 2019). On this perspective, dry-outs could occur at lower heat flux levels even with porous structures. Cohen (1974), Pan et al. (1987), Short et al. (2013), Jina and Short (2014), Yeo and No (2017) pointed out the change of a maximum concentration factor at the interface between the cladding surface and porous structures. Especially, Pan et al. (1987) summarized the trend with wick structures, pressure, heat flux and CRUD thickness; the maximum concentration factor increases with the CRUD thickness. In this regard, solutes and particulates can form porous structures on the surface by the evaporation in this experiment, and accelerated corrosion, which induces thick porous structures, at the interface could have happened by concentrated BA concentration at the location. Depending on corrosion conditions, the CHF trend may differ according to the preceding studies, but when long periods of corrosion phenomenon is accompanied with the boiling process, there is room for reduction according to this study.
4. Conclusions BA-based coolants of various conditions have been used for CHF experiments with corrosive and non-corrosive test sections. BAadded coolant of ‘N’ condition is coolant with 2.5 wt% boric acid dissolved in DI water without any corrosion process, and BAadded coolants of ‘C-Low’ and ‘C-High’ conditions are BA-added coolant with dissolved metal ions and metal oxide particles (color changes are observed with concentrations). For the non-corrosive
This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS), granted financial resource from the Nuclear Safety and Security Commission (NSSC), Republic of Korea (No. 1305011), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. NRF-2017M2A8A4056643). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.106999. References Kim, S.J., McKrell, T., Buongiorno, J., Hu, L.-W., 2009. Experimental study of flow critical heat flux in alumina-water, zinc-oxide-water, and diamond-water nanofluids. J. Heat Transf. 131, 043204. Kim, S.J., McKrell, T., Buongiorno, J., Hu, L.-W., 2010. Subcooled flow boiling heat trasnfer of dilute alumina, zinc oxide, and diamond nanofluids at atmospheric pressure. Nucl. Eng. Des. 240 (5), 1186–1194. Amiri, A., Shanbedi, M., Amiri, H., Heris, S.Z., Kazi, S.N., Chew, B.T., Eshghi, H., 2014. Pool boiling heat transfer of CNT/water nanofluids. Appl. Therm. Eng. 71 (1), 450–459. Lee, T., Lee, J.H., Jeong, Y.H., 2013. Flow boiling critical heat flux characteristics of magnetic nanofluid at atmospheric pressure and low mass flux conditions. Int. J. Heat Mass Transf. 56, 101–106. Lee, T., Kam, D.H., Lee, J.H., Jeong, Y.H., 2014. Effects of two-phase flow conditions on flow boiling CHF enhancement of magnetite-water nanofluids. Int. J. Heat Mass Transf. 74, 278–284. Lee, J.H., Kam, D.H., Jeong, Y.H., 2015. The effect of nanofluid stability on critical heat flux using magnetite-water nanofluids. Nucl. Eng. Des. 292, 187–192. Song, S.L., Lee, J.H., Chang, S.H., 2014. CHF enhancement of SiC nanofluid in pool boiling experiment. Exp. Therm. Fluid Sci. 52, 12–18. Kim, H.D., Kim, M.H., 2007. Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids. Appl. Phys. Lett. 91, (1) 014104. Yang, J., Cheung, F.B., Rempe, J.L., Suh, K.Y., Kim, S.B., 2006. Critical heat flux for downward-facing boiling on a coated hemispherical vessel surrounded by an insulation structure. Nucl. Eng. Technol. 38, 139–146. Park, S.D., Bang, I.C., 2013. Flow boiling CHF enhancement in an external reactor vessel cooling (ERVC) channel using graphene oxide nanofluid. Nucl. Eng. Des. 265, 310–318. Ahn, H.S., Kim, J.M., Kaviany, M., Kim, M.H., 2014. Pool boiling experiments in reduced graphene oxide colloids part II – Behavior after the CHF, and boiling hysteresis. Int. J. Heat Mass Transf. 78, 224–231. Kim, T.I., Park, H.M., Chang, S.H., 2012. CHF experiments using a 2-D curved test section with additives for IVR-ERVC strategy. Nucl. Eng. Des. 243, 272–278. Park, H.M., Jeong, Y.H., Heo, S., 2014. Effect of heater material and coolant additives on CHF for a downward facing curved surface. Nucl. Eng. Des. 278, 344–351. Gogonin, I.I., 2009. Influence of the thickness of a wall and of its thermophysical characteristics on the critical heat flux in boiling. J. Eng. Phys. Thermophys. 82 (6), 1175–1183. Kam, D.H., Lee, J.H., Lee, T., Jeong, Y.H., 2015. Critical heat flux for SiC- and Cr-coated plates under atmospheric condition. Ann. Nucl. Energy 76, 335–342. Unal, C., Daw, V., Nelson, R.A., 1992. Unifying the controlling mechanisms for the critical heat flux and quenching: the ability of liquid to contact the hot surface. J. Heat Transf. 114, 972–982. Raghupathi, P.A., Kandlikar, S.G., 2017. Effect of thermophysical properties of the heater substrate on critical heat flux in pool boiling. J. Heat Transf. 139, 111502.
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