Nuclear Engineering and Technology 49 (2017) 1669E1679

Nuclear Engineering and Technology 49 (2017) 1669e1679

Contents lists available at ScienceDirect

Nuclear Engineering and Technology

journal homepage: www.elsevier.com/locate/net

Original Article Steam generator performance improvements for integral small modular reactors

* Muhammad Ilyas a, b, , Fatih Aydogan b a Pakistan Institute of Engineering and Applied Sciences (PIEAS), P. O. Nilore, 45650, Islamabad, Pakistan b Center for Advanced Energy Studies (CAES), University of Idaho, 995 University Boulevard, Idaho Falls, ID 83401, USA article info abstract

Article history: Background: Steam generator (SG) is one of the significant components in the nuclear steam supply Received 13 January 2017 system. A variety of SGs have been designed and used in nuclear reactor systems. Every SG has ad- Received in revised form vantages and disadvantages. A brief account of some of the existing SG designs is presented in this study. 9 June 2017 A high surface to volume ratio of a SG is required in small modular reactors to occupy the least space. In Accepted 20 August 2017 this paper, performance improvement for SGs of integral small modular reactor is proposed. Available online 26 August 2017 Aims/Methods: For this purpose, cross-grooved microfins have been incorporated on the inner surface of the helical tube to enhance heat transfer. The primary objective of this work is to investigate thermal Keywords: e Helical-coil Steam Generator hydraulic behavior of the proposed improvements through modeling in RELAP5-3D. Nuclear Steam Supply System Results and Conclusions: The results are compared with helical-coiled SGs being used in IRIS (Interna- Heat Transfer Coefficient tional Reactor Innovative and Secure). The results show that the tube length reduces up to 11.56% Pressure Drop keeping thermal and hydraulic conditions fixed. In the case of fixed size, the steam outlet temperature Steam Quality increases from 590.1 K to 597.0 K and the capability of power transfer from primary to secondary also Micro Fins increases. However, these advantages are associated with some extra pressure drop, which has to be Cross Grooved Fins compensated. SMRs © 2017 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the RELAP5 CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). IRIS

1. Introduction the SG, where it transfers heat to the secondary side coolant. When the feedwater absorbs sufficient heat, it starts to boil and form Advanced technologies are required to be implemented for steam. Thus, the function of SGs is heat transfer from the primary long-term deployment of nuclear power plants to meet future cooling system to the secondary side and production of high- energy challenges. Extensive research is being carried out world- quality superheated steam. wide for the development of innovative advanced reactors with Over time, a variety of SGs have been designed and used in salient features of simplified technology, inherit passive safety, nuclear reactor systems; some of the existing SG designs are sustainability and reliability, proliferation resistance, and economic described in Section 2. However, it is still required to make the SG viability. About 50 concepts and designs of advanced reactors more compact particularly for application in integral small modular including all principle reactor types (e.g., water cooled, liquid metal reactors (SMRs). In this paper, a new SG design for integral SMRs cooled, gas cooled, and molten salt cooled reactors) are under with better heat transfer characteristics is proposed. The primary development in different countries around the world; the current objective of the present work is to investigate thermalehydraulic status of these reactors can be found in references [1e6]. behavior of the proposed design under steady-state operation Steam generator (SG) is one of the major and particularly sig- through modeling in RELAP5-3D. However, comparison of the re- nificant components in the nuclear steam supply system of a nu- sults and validation of the model requires analyzing a benchmark clear reactor. It is a heat sink for the reactor core. The reactor SG design. For this purpose, the SG of the IRIS (International Reactor coolant flows in a closed loop through the reactor and the SG. It Innovative and Secure) has been selected as a benchmark. takes heat while passing through the core, and then flows through What follows is a brief account of some of the existing SGs in Section 2. A short review of the enhanced boiling surfaces is presented in Section 3. Section 4 deals with the description of the

* Corresponding author. proposed SG design. The reference SG design is given in Section 5. E-mail address: [email protected] (M. Ilyas). Section 6 describes the RELAP5-3D model of the SG and its http://dx.doi.org/10.1016/j.net.2017.08.011 1738-5733/© 2017 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1670 M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679 validation. Results and discussion are presented in Section 7. Sec- 2.2. Recirculation SGs tion 8 covers the main conclusions. The recirculation SG is the most commonly used SG design in 2. Existing SGs pressurized water reactors. It uses vertical U-tube bundle such that hot primary coolant flows through inside of the U tubes, whereas Several types of SGs are currently in use in the nuclear industry. feedwater flows around outside of the tubes. These are classified based on flow arrangement (once through or The Westinghouse and Combustion Engineering recirculation recirculation), orientation of SG (vertical, horizontal), and tube SGs are shown in Figs. 2 and 3, respectively. In both designs, water shape (straight, helical, bayonet, or U tube). Some of the important from the steam separators mixes with the main stream secondary types are briefly described in the following subsections. coolant and rises over the U-tube bundle as it is partially converted to steam. The steamewater mixture passes through multiple levels 2.1. Once-through straight tube SGs of steam separation equipment, which returns the water to the U- tube bundle for further heating and evaporation. SGs provided by In these types of SGs, the coolant makes a single run and leaves pressurized water reactor vendors differ slightly in their designs the SG. The flow is in a countercurrent direction; however, SG and operations. The process of moisture separation and steam drier orientation could be horizontal or vertical. is so efficient that the water content in the outlet steam is less The vertical once-through straight tube SG is a straight tube 0.25% [7]. counterflow SG. In this design, the primary coolant flows through the inside of the tubes vertically downward, whereas the feedwater rises 2.3. Multilayer tube SGs upward around the tubes. By taking sensible heat, the secondary coolant changes phase and exits as superheated steam at the outlet. A specific example of multilayer tube SGs is a bayonet tube SG. It The Westinghouse SMR uses a straight tube SG with an external steam has been used in the scaled-down reactor ALFRED (Advanced Lead- separating drum as shown in Fig. 1. In this design, the primary coolant cooled Fast Reactor European Demonstrator). It is comprised of a flows vertically downward through the tubes whereas the secondary large number of bayonet tubes arranged in a prismatic array flow rises in the shell side taking sensible heat and finally steam is immersed in the lead vessel pool. The construction of a single produced and directed toward steam drum for moisture separation bayonet tube is shown in Fig. 4. It consists of four vertical coaxial [8]. tubes. The feedwater descends through the innermost tube (the The horizontal once-through SG is housed in a horizontal cy- slave tube), which is insulated from the surrounding tube (inner lindrical vessel. The steam is separated and dried by gravity at the tube) with a strongly insulating material to get the required degree top of the housing. Horizontal SGs are considered to be more robust than vertical SGs [7].

Fig. 1. Westinghouse SMR vertical once-through steam generator. SMR, integral small modular reactor. Note. From “An overview of the Westinghouse small modular reactor,” by R.J. Fetterman, A. Harkness, M. Smith, C. Taylor, 2011, Proceedings of the ASME 2011 Fig. 2. Cutaway view of Westinghouse steam generator. Note. From: “Pressurized Small Modular Reactors Symposium SMR 2011, Washington, DC, USA, American Society Water Reactor (PWR) Systems,” in Reactor Concepts Manual, USNRC Technical Training of Mechanical Engineers. Copyright © 2011 by ASME. With permission. Center. Copyright by USNRC. With permission. M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679 1671

Fig. 3. Cutaway view of Combustion Engineering steam generator. Note. From: “Pres- surized Water Reactor (PWR) Systems,” in Reactor Concepts Manual, USNRC Technical Training Center. Copyright by USNRC. With permission. of superheating of steam. The rising water in the annular region between the inner and outer tube reaches saturation and evapo- rates as it picks heat from the surrounding hot lead pool. The gap between the outermost and the outer tubes is filled with pressurized helium and high thermal conductivity particles to enhance the heat exchange capability [9]. Fig. 4. Scheme of a bayonet tube with the argon, the helium and the steam plena (not Toshiba 4s SMR also uses a double-walled heat transfer tube SG. in scale). Note. From “Conceptual design of a bayonet-tube steam generator for the ALFRED lead-cooled reactor,” by L. Damiani, M. Montecucco, A. Pini Prato, 2013, Nucl. The schematic configuration of the SG is shown in Fig. 5, whereas Eng. Des. 265,p.154e163. Copyright © 2013 Elsevier B.V. With permission. the cross-sectional view of a single heat transfer tube is shown in Fig. 6. The tubes are immersed in a pool of secondary sodium. The sodium coolant is circulated around the outside of the outer tube by increase in HTC by surface modification is associated with increase an electromagnetic pump. The feedwater pump circulates water/ in frictional pressure drop. The best configuration for the advanced steam through the inside of inner tube. The gap between the inner heat exchanger is the one with the highest ratio of HTC to pressure and outer tubes is filled with helium, and wire meshes are provided drop. Therefore, investigation of enhanced boiling surfaces is very in this gap. This system of helium and wire mesh is capable of important. detecting both inner tube failure and outer tube failure. The heat is The optimum design of a surface with enhanced two-phase flow transferred from sodium to water/sodium via the double wall tube heat transfer depends on the contact of the surface with continuous of the SG [10e12]. as well as discrete phase. Higher HTCs are observed in a flow regime in which the solid surface is in contact with both phases. 2.4. Helical-coil SGs Over the years, several enhanced boiling surfaces have been studied. One of the simplest approaches to increase the boiling HTCs is Helical-coil SG design is an advanced design. It is one of the to increase roughening of the surface. S. Choi et al. [20] studied the successful designs that offers compactness and increased heat effect of surface microgravity and surface roughness. Numerous transfer. The tubes of the SG are wound into helical coils, forming a types of geometries have been proposed to achieve significant large bundle as shown in Fig. 7. Its large surface area per unit vol- enhancement in heat transfer coefficient. ume gives it the benefit of outstanding heat transfer ability [14]. One of the earliest modified enhanced boiling tubes that became Helical-coil heat exchangers can have 16e43% higher heat transfer commercially successful was the low finned tube [15]. It has low coefficient (HTC) as compared to straight pipe heat exchangers [13]. helical fins at the outer surface of the tube as shown schematically in Fig. 8. Later on, several studies were carried out to create high 3. Enhanced boiling surfaces density of reentrant channels by mechanically notching, knurling, or bending/compressing the low finned tubes. It is highly desirable to enhance the heat transfer in the heat Nowadays, microfinned tubes are becoming popular. Several exchanger to make them economical and compact in size. However, surfaces with controlled geometries and cavities have been 1672 M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679

Tubes with external porous coatings are also important for heat transfer enhancement. These can yield augmentations of up to 10e15 times as compared to the boiling performances on plain tubes at optimum conditions. Thermal and hydraulic characteristics for selected heat transfer surface under various flow conditions are presented in this section. Table 1 compares two-phase flow heat transfer characteristics under varied flow conditions for selected surfaces [16, 18e24]. Table 2 gives a comparison of the hydraulic aspects of two-phase flow for different surface patterns and flow regimes as given in references [16, 25e29]. In most of these studies, HTC enhancement was slight as compared to the tubes area ratio. For microfin surfaces, heat transfer enhancement depends on flow ratedhigher at low velocities and low at higher velocities. The cross-grooved tubes give enhancement of 25e30% higher than microfins. It is also noticed that the pressure drop increased with increasing mass flux and heat flux.

4. Proposed improvement in the SG design

In most of the heat transfer systems, boiling of the coolant is desired to increase heat transfer. Phase change enhances heat transfer drastically as latent heat of vaporization is considerably Fig. 5. Steam generator configuration of Toshiba 4s SMR. SMR, small modular reactor. larger compared to sensible heat. HTC can be enhanced signifi- Note. From TOSHIBA, http://pbadupws.nrc.gov/docs/ML0814/ML081440765.pdf. 2008. cantly by changing smooth surface channel to one with a surface having small-scale geometric patterns. Microfinned tubes are very successful in heat transfer enhancement. Several surfaces with controlled geometries and cavities have been manufactured to find out the influence of pits and cavities on heating surfaces. The most commonly used enhanced surfaces for two-phase heat transfer inside tubes are “microfin” and “cross-grooved” as shown in Fig. 9. These are characterized with a substantial improvement in heat transfer and a minimal increase in pressure drop [16]. The proposed SG design improvement is to incorporate cross- grooved microfins on the inner surface of the helical coil tubes of the SG to improve heat transfer characteristics by sacrificing a reasonable low pressure drop increase. The modification on surface increases the density of nucleation, results in disruption of the boundary layer, and increases turbulence, thereby contributing to more heat transfer. However, the proposed modification in the Fig. 6. Double walled tube of steam generator of Toshiba 4s SMR. SMR, small modular surface of the SG tube is associated with an increase in frictional reactor Note. From TOSHIBA, http://www.uxc.com/smr/Library%5CDesign%20Specific/ pressure drop as well. Therefore, there is a trade-off between heat 4S/Presentations/2009%20-%204S%20Reactor.pdf, 2009. transfer improvement and loss due to frictional pressure drop.

5. Description of the reference SG used in this study manufactured to find out the influence of pits and cavities on heating surfaces. The most commonly used enhanced surfaces for The SG of IRIS has been selected as a reference SG for compar- two-phase heat transfer inside tubes are “microfin” and “cross- ison of the improved SG design results. IRIS is a light water-cooled, grooved” as shown in Fig. 9. These are characterized with a sub- 335-MWe power SMR designed by an international consortium as a stantial improvement in heat transfer with a minimal increase in part of the United States Department of Energy Nuclear Energy pressure drop [16]. Research Initiative program. It incorporates an integrated reactor A typical example of a microfin tube surface is the herringbone pressure vessel (RPV) housing eight independent once-through pattern as shown in Fig. 10. A V-shaped groove pattern helps in helical coil SGs above the reactor core in an annular region be- carrying over liquid in two different circumferential flow directions tween riser and wall of the RPV as shown in Fig. 12 [32]. Detailed inside the tube. If properly designed, the herringbone pattern in- specifications of IRIS SG are given in Table 3 [31]. Each SG consists of troduces a continuous supply/removal of a non-uniform film layer, 656 tubes made of nickelechromeeiron alloy TT-690. Each tube has which results in high HTC as compared to a helical microfin tube an outside diameter of 17.6 mm and a wall thickness of 2.11 mm. [17]. The average length of tubes is 32 m. Each module consists of a Beside microfins, other patterns and enhancement elements central inner column that supports the tubes, with the lower with different shapes have also been investigated. One such feedwater header and the upper steam collector connected to the example is the Vipertex [17] enhanced heat transfer tube as shown reactor vessel. Cold feedwater at a lower pressure enters into tubes in Fig. 11. In this tube, dimples and petals are used as enhancement at the bottom of the SG. Steam is produced inside the tube as a elements. Enhanced heat transfer tubes resulted in higher HTC in result of boiling. On further moving along the length of the tube, refrigeration and air-conditioning applications. steam gets superheated and is finally collected at the top. M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679 1673

Fig. 7. Schematic view of a NuScale. Note. From “NuScale plant safety in response to extreme events,” by J.N. Reyes, 2012, Nucl. Technol., 178,p.153e163. Copyright © 2012 Journal of Nuclear Technology. With permission.

Fig. 8. Schematic representation of a low ﬁnned tube.

6. The RELAP5-3D modeling of SG side to provide boundary conditions. The inlet boundary conditions are modeled by TDV (210), whereas control of inlet temperature, 6.1. Model development pressure, and mass flow rate are provided by a time-dependent junction (215). The flow then enters into the PIPE 230, which rep- The simulation code used in this work is the RELAP5-3D, resents SG channels. The single volumes (220 and 240) prior to and developed by Idaho National Laboratory for analyses of light wa- after the PIPE 230 model the feedwater inlet and steam outlet, ter cooled reactors. The first step of simulation is division of the respectively. The steam is finally dumped to the TDV 250. entire system into nodes (called nodalization). Nodes in RELAP5-3D The section of primary loop modeled is also shown in Fig.13. The represent hydrodynamic components and heat structures. The SG boundary conditions are applied at the inlet and outlet by TDV 110 of IRIS is divided into several nodes as shown in Fig. 13. and TDV 150. At the inlet section of SG shell, a constant flow rate is The part of the secondary side that has been modeled consists of maintained through the time-dependent junction 115. This junc- SG inlet, SG coiled-tube section, and SG outlet. Time-dependent tion connects the TDV 110 to the single volume 120. The SG shell volumes (TDVs) are placed at the inlet and exit of the SG tube side is modeled by the pipe component (130). The SG shell is 1674 M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679

Fig. 9. Typical configuration of microfin and gross-grooved surfaces. Note. From: “Experimental heat transfer coefficients during refrigerant vaporisation and condensation inside herringbone-type plate heat exchangers with enhanced surfaces,” by G.A. Longo, A. Gasparella, R. Sartori, 2004, Int. J. Heat Mass Transfer, 47,p.4125e4136. Copyright © 2004 Elsevier Ltd. With permission.

results are in good agreement with the reference results. This val- idates that the model is adequate to simulate the IRIS SG.

7. Results and discussion

After validating the RELAP5-3D model in the previous section, performance analysis of the proposed SG was carried out by modeling the modiﬁed SG to improve the heat transfer characteristics with moderately increased pressure drop. In this design, the helical coil tubes with cross grooves on the inner wall surface are used. RELAP5-3D does not allow using ﬁnite element mesh to model the physical shape of the SG. It is based on conservation of mass, momentum, and energy averaged over a volume. For phase k, these equations are as follows [33]:

v 1 ða r Þþ V:ða r v AÞ¼G (1) vt k k A k k k k ! v v 1 !2 ! a r A k þ a r AV: v ¼a AV:P þ a r B A Fig. 10. Herringbone pattern in herringbone tube. Note. From: “Experimental heat k k vt 2 k k k k k k transfer coefficients during refrigerant vaporisation and condensation inside ! ! ð r Þ : þ ð herringbone-type plate heat exchangers with enhanced surfaces,” by G.A. Longo, A. ak kA FWk v k GkA v kI e © ! ! ! Gasparella, R. Sartori, 2004, Int. J. Heat Mass Transfer, 47,p.4125 4136. Copyright Þð r Þ :ð Þ 2004 Elsevier Ltd. With permission. v k ak kA FIk v k v r ! ! vð v v Þ Ca a r A k r connected to the single volume 140 (SG outlet) through the single k r m vt junction 135. The SG outlet is connected to the sink (TDV 150) ! ! ! ! þ V V through the single junction 145. v r v k v k v r The thermal interface between primary and secondary sides is (2) modeled through heat structure. The primary system is connected to outer (right) side of heat structure whereas the secondary side is v 1 va P ! connected to inner (left) side of the heat structure. ða r U Þþ V:ða r U v AÞ¼P k V:ða v AÞþQ vt k k k A k k k k vt A k k wk þ þ þ 0 þ ; 6.2. Validation of the RELAP5-3D model Qik Gighk Gwhk DISSk (3) The steady-state thermal-hydraulics of the proposed SG design and the IRIS SG has been studied using RELAP5-3D. The validation where the notations have the usual meanings and are given in the of the RELAP5-3D model is presented in this section. nomenclature. RELAP5-3D solves these equations along with the To simulate the IRIS SG using RELAP5-3D, the first step is to constitutive relationships subject to the given boundary conditions achieve steady-state and to validate the model against steady-state for a complete solution of a system. The effect of surface patterns is operating conditions. Simulation was run for a sufficient time to incorporated by the HTC, and loss coefficients are used for frictional verify that the system has achieved steady-state. Figs. 14 and 15 pressure drop to model the system. In this particular case, as show the primary/secondary flow rate and shell/tube side outlet studied by Longo et al. [16], the microgrooves enhance HTC by 50% temperature as a function of time. Steady-state is achieved when with an associated increase of 20% in pressure drop. In RELAP5-3D, the system parameters remain invariant with time. From these the HTC is enhanced by making use of the multiplying factor for figures, it can be seen that steady-state has been achieved shortly different flow regimes in the RELAP5-3D model. Two cases have after the start of the simulation. been studied, and the results are given in Table 5. In the first case, Table 4 compares the RELAP5-3D computed steady-state results thermal and hydraulic conditions are kept unchanged as compared with the reference values [31]. It is found that the model predicted to the standard design. This resulted in a more compact geometry M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679 1675

Fig. 11. EHT tube. (A) Inner and outer surface patterns (B) geometrical detail. Note. From: “Condensation and evaporation heat transfer characteristics in horizontal smooth, herringbone and enhanced surface EHT tubes,” by Si-pu Guo, Zan Wu, Wei Li, David Kukulka, Bengt Sunden, Xiao-peng Zhou, Jin-jia Wei, Terrence Simon, 2015, Int. J. Heat Mass Transfer, 85, p. 281e291. Copyright © 2015 Elsevier Ltd. With permission.

Table 1 Summary of heat transfer characteristics for selected heat transfers surfaces.

Reference Fluid Shape Surface type Flow regime Hydraulic Remarks diam. (mm)

Eckels and R-134a, R-12 Circular Smooth, microfin 75% change of quality 8,000, 8,920 Heat transfer coefficient enhancement was slight as Pate [30] from inlet to outlet compared to the tubes area ratio. Thors and R-22 Circular Smooth, microfin, d 9,500, 15,880 Enhancement ratio for the microfinned tube was the Bogart [24] and corrugated highest (four times). The corrugated tubes performance matched with the microfinned tube at higher velocities at the expense of larger pressure drop. Chamra and R-22 Circular Microfinn Annular flow, nucleate 14,660 Heat transfer coefficient enhancement (3.5 times at low Webb [19] boiling mass velocities and 1.7 times at high mass velocities). Longo et al. [16] R-22 Plate Smooth Nucleate boiling d The microfin tubes show an enhancement ranging from Herringbone microfin 80% to 180% compared to smooth tubes. The cross- and cross-grooved grooved tubes give enhancement of 25e30% higher than microfins. 1676 M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679

Table 2 Summary of hydraulic characteristics for selected surfaces.

Author Fluid Shape dh (mm) Re L/dh Remarks

Lee et al., 2002 [27] Water, air Microchannel 780e6,670 0.303e17,700 The correlation can be used in four permutations of laminar (l) and turbulent (t) regimes ll, lt, tl, and tt . Longo et al. [16] R-22 Smooth 7,594.94 500e700 38.18 The microfinned tubes resulted in heat transfer Herringbone microfin coefficient enhancement from 30% to 40% compared to and cross-grooved smooth tubes and the cross-grooved tubes give 6e10% more pressure drop than the microfin tubes. Nilpueng and Wongwises [28] Airewater Corrugated 14,030 1,000e50,000 71.276 The two-phase multiplier calculated from the measures Plate type pressure drop is in good agreement with Lockhart eMartinelli correlation. Hahne and Grigull [26] Nitrogen Smooth tube 14,000 d 37.5 The pressure drop increases with increasing mass flux and heat flux.

Fig. 12. Internals of IRIS integral. IRIS, International Reactor Innovative and Secure. Note. From “The design and safety features of the IRIS reactor,” M.D. Carelli, L.E. Conway, L. Oriani, Nucl. Eng. Des., 230, 2004, p. 151e167. Copyright © 2004 Elsevier B.V. With permission. of the new design. The results are compared with that of the IRIS SG profile of the secondary fluid along the channel length is shown in design. The primary side temperature profile along the length of Fig. 17. It is evident from the figure that the temperature of the the SG shell side is shown in Fig. 16. The temperature of the primary secondary fluid initially increases quickly to saturation tempera- side decreases monotonically from inlet to exit. The temperature ture. The temperature does not change further during the phase M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679 1677

Table 3 Speciﬁcation of IRIS steam generator.

Parameter Value

Rated power 125 MW Tube outside diameter 17.46 mm Tube thickness 2.11 mm Tube inside diameter 13.24 mm External shell inside diameter 1,620 mm Internal shell outside diameter 610 mm Number of helical rows 21 Number of tubes 656 Tube bundle average length 32,000 mm SG height (headers centerline) 7,900 mm SG overall height 8,500 mm Primary side inlet temperature 328.4C Primary side outlet temperature 292C Feedwater temperature 223.9C Steam temperature 317C Primary side pressure 15.5 MPa Steam outlet pressure 5.8 MPa Fig. 14. Primary and secondary flow rates with respect to time. Primary flow rate 589 kg/sec Secondary flow rate 62.5 kg/sec Primary side pressure loss 72 kPa Secondary side pressure loss 296 kPa

Note. From “Steam generator of the International Reactor Innovative and Secure”, by, L. Cinotti, M. Bruzzone, N. Meda, G. Corsini, C.V. Lombardi, M. Ricotti, L.E. Conway, 2002, International Conference on Nuclear Engineering, Arlington, Virginia, USA. Copyright © 2002 by ASME. With permission. IRIS, International Reactor Innovative and Secure.

110 250

12 24

Fig. 15. Steam generator primary and secondary outlet temperature with respect to Primary side 13 23 Secondary side time. (Top to bottom 0 0 (BoƩom to top flow) ﬂow) Table 4 Comparison of the code computed steady-state operating conditions with the reference values.

Parameter Reference RELAP5-3D value [31] value

Primary pressure (MPa) 15.5 15.507 14 22 Primary coolant mass flow rate (kg/sec) 589.0 589.0 SG-shell outlet temperature (K) 565.1 565.11 150 210 SG-shell inlet temperature (K) 601.5 601.5 Secondary pressure (MPa) 5.8 5.8 Feedwater flow rate (kg/sec) 62.5 62.5 Fig. 13. Primary and secondary systems nodalization diagram of the IRIS steam Feedwater inlet temperature (K) 497.0 497.0 generator. IRIS, International Reactor Innovative and Secure. Steam outlet temperature (K) 590.1 590.61 change. Equilibrium quality for the two designs is shown in Fig. 18. Figs. 16e18 show that the proposed design achieves the required conditions at a tube length of 28.3 m. Table 5 Results of different case studies. In the second case, the size of the new design is kept unchanged. However, incorporation of the cross-grooved microfins resulted in Case Power (MWth) Secondary flow Tube length (m) Percent higher superheating of steam. The steam outlet temperature studied rate (kg/sec) difference (%) increased from 590.1 K to 597.0 K. Moreover, the capability of 1 125.0 62.5 28.3 11.56 transferring power from primary to secondary increased in this 2 126.7 62.5 32.0 1.36 case. used. Some of the existing SG designs are briefly described in this 8. Conclusions paper. For application in SMRs, a compact SG having a high surface to volume ratio is required so as to occupy the least space in the SG is one of the major components of a nuclear power plant. In power plant. Helical-coiled tube SGs with larger surface area per the nuclear industry, different types of SGs have been designed and unit volume are used especially in SMRs. HTC can be enhanced 1678 M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679

Fig. 16. Comparison of the IRIS and the proposed steam generator primary side tem- Fig. 18. Comparison of the IRIS and the proposed steam generator equilibrium quality ﬁ perature pro les under steady-state operating conditions. IRIS, International Reactor proﬁles under steady-state operating conditions. IRIS, International Reactor Innovative Innovative and Secure; SG, steam generator. and Secure; SG, steam generator.

Fig. 17. Comparison of the IRIS and the proposed steam generator secondary side temperature profiles under steady-state operating conditions. IRIS, International Reactor Innovative and Secure. significantly by changing the smooth surface channel to one with a that in the proposed design with incorporation of cross-grooved surface having small-scale geometric patterns. The modification on microfins and keeping thermal and hydraulic conditions fixed, surface increases the density of nucleation, results in disruption of the tube length reduced to 28.3 m with a saving of 11.56%. In the the boundary layer, and increases turbulence, thereby contributing case of fixed size, the steam outlet temperature increases from to more heat transfer. Microfinned tubes are very successful in heat 590.1 K to 597.0 K and the capability of power transfer from primary transfer enhancement. However, the modified surfaces are associ- to secondary also increases. However, pressure drop has to be ated with an increase in the frictional pressure drop as well. compensated by necessary pressure head to get these benefits. Therefore, there is a trade-off between heat transfer improvement and loss due to frictional pressure drop. Conflicts of interest An improvement in the SG design for integral SMRs has been proposed in this work. The improved SG design incorporates cross- The authors have no conflict of interest with anyone regarding grooved microfins on the inner surface of helical coil tubes of SG to this article. improve heat transfer characteristics by sacrificing a reasonable low pressure drop increase. Thermalehydraulic analysis of the Acknowledgments proposed design has been carried out using RELAP5-3D. RELAP5- 3D does not allow users to use finite element mesh to model the The authors are thankful to their respective departments for physical shape of the SG rather than HTC, and loss coefficients are providing support in completing this work. The corresponding used for frictional pressure drop to model the system. The results author acknowledges the support extended by Higher Education are compared with IRIS helical-coiled SGs. It is found from analysis Press under grant no. 1-16/PPCR/R&D/HEC/2015. M. Ilyas, F. Aydogan / Nuclear Engineering and Technology 49 (2017) 1669e1679 1679

Nomenclature [10] TOSHIBA [Internet], Super-safe, Small and Simple Reactor (4S, Toshiba Design), 2013 [cited 2013 Jun 3]. Available from: https://aris.iaea.org/sites/..% 5CPDF%5C4S.pdf. [11] TOSHIBA [Internet], 4S Design Description, 2008 [cited 2008 May]. Available Greek from: http://pbadupws.nrc.gov/docs/ML0814/ML081440765.pdf. a void fraction [12] TOSHIBA [Internet], 4S Reactor e Super-Safe, Small and Simple, 2009 [cited G 2009 Jun]. Available from: http://www.uxc.com/smr/Library%5CDesign% k generation rate of phase k 20Specific/4S/Presentations/2009%20-%204S%20Reactor.pdf. Gig vapor generation rate due to energy exchange in the bulk [13] V.H. Nathan, S. Piyush, A.A. Nolan, Modeling a Helical-coil Steam Generator in fluid RELAP5-3D for the Next Generation Nuclear Plant, United States, Office of the G Assistant Secretary for Nuclear Energy, Washington, D.C., 2011. w vapor generation rate due to energy exchange at the wall [14] J.N. Reyes, NuScale plant safety in response to extreme events, Nucl. Technol. rk density of phase k 178 (2012) 153e163. [15] J.R. Thome, Enhanced Boiling Heat Transfer, Hemisphere, New York, 1990. [16] G.A. Longo, A. Gasparella, R. Sartori, Experimental heat transfer coefficients English during refrigerant vaporisation and condensation inside herringbone-type A channel flow area plate heat exchangers with enhanced surfaces, Int. J. Heat Mass Transfer 47 B body force (2004) 4125e4136. C the concentration of the radionuclide in atoms per unit [17] S.-P. Guo, Z. Wu, W. Li, D. Kukulka, B. Sunden, X.-P. Zhou, J.-J. Wei, T. Simon, Condensation and evaporation heat transfer characteristics in horizontal 3 volume (atoms/m ) smooth, herringbone and enhanced surface EHT tubes, Int. J. Heat Mass e DISSk energy dissipation function of phase k Transfer 85 (2015) 281 291. FI inter-phase drag coefficient for phase k [18] T.L. Bergman, D.P. DeWitt, F.P. Incropera, A.S. Lavine, Fundamentals of Heat k and Mass Transfer, John Wiley & Sons, Hoboken, NJ, 2011. FWk wall drag coefficient for phase k [19] L. Chamra, R.L. Webb, Condensation and evaporation in micro-fin tubes at 0 e h k specific enthalpy for interface mass transfer in the equal saturation temperatures, J. Enhanced Heat Transfer 2 (1995) 219 229. fl thermal boundary layer [20] S. Choi, R.F. Barron, R. Warrington, Fluid ow and heat transfer in microtubes, * fi in: Proceedings of Winter Annual Meeting of ASME, DSC, New York, 1991. h k speci c enthalpy in phase k [21] S. Eckels, M. Pate, An experimental comparison of evaporation and conden- P average pressure sation heat transfer coefficients for HFC-134a and CFC-12, Int. J. Refrigeration 14 (1991) 70e77. Qik interface heat transfer per unit volume to phase k [22] O.B. Ergu, O.N. Sara, S. Yapici, M.E. Arzutug, Pressure drop and point mass Qwk wall heat transfer per unit volume to phase k transfer in a rectangular microchannel, Int. Commun. Heat Mass Transfer 36 Uk specific energy for phase k (2009) 618e623. [23] A. Kawahara, P.M.Y. Chung, M. Kawaji, Investigation of two-phase flow vk velocity of phase k pattern, void fraction and pressure drop in a microchannel, Int. J. Multiphase Flow 28 (2002) 1411e1435. References [24] P. Thors, J. Bogart, In-tube evaporation of HCFC-22 with enhanced tubes, J. Enhanced Heat Transfer 1 (1994) 365e377. [1] M.M. Abu-Khader, Recent advances in nuclear power: a review, Prog. Nucl. [25] P. Gao, S. Le Person, M. Favre-Marinet, Scale effects on hydrodynamics and Energy 51 (2009) 225e235. heat transfer in two-dimensional mini and microchannels, Int. J. Thermal Sci. [2] H. Hidayatullah, S. Susyadi, M.H. Subki, Design and technology development 41 (2002) 1017e1027. for small modular reactors d safety expectations, prospects and impediments [26] E. Hahne, U. Grigull, Heat Transfer in Boiling, Hemisphere, Washington, DC, of their deployment, Prog. Nucl. Energy 79 (2015) 127e135. 1977. [3] IAEA-TECDOC-1451, Small and Medium Sized Reactors: Design Features, [27] P.S. Lee, J.C. Ho, H. Xue, Experimental study on laminar heat transfer in Safety Approaches and R&D Trends, International Atomic Energy Agency, microchannel heat sink, in: Eighth Intersociety Conference on Thermal and Vienna, 2005. Thermomechanical Phenomena in Electronic Systems, ITHERM 2002, IEEE, [4] IAEA-TECDOC-1474, Natural Circulation in Water Cooled Nuclear Power 2002. Plants Phenomena, Models, and Methodology for System Reliability Assess- [28] K. Nilpueng, S. Wongwises, Flow pattern and pressure drop of vertical upward ments, International Atomic Energy Agency, Vienna, 2005. gaseliquid flow in sinusoidal wavy channels, Exp. Thermal Fluid Sci. 30 (2006) [5] IAEA-TECDOC-1536, Status of Small Reactor Designs without On-site Refuel- 523e534. ing, International Atomic Energy Agency, Vienna, 2007. [29] D.B. Tuckerman, R. Pease, High-performance heat sinking for VLSI, IEEE [6] M.K. Rowinski, T.J. White, J. Zhao, Small and medium sized reactors (SMR): a Electron, Device Lett. 2 (1981) 126e129. review of technology, Renew. Sust. Energy Rev. 44 (2015) 643e656. [30] S. Eckels, M. Pate, Evaporation and condensation of HFC-134 a and CFC-12 in a [7] Pressurized water reactor (PWR) systems, in Reactor Concepts Manual, USNRC smooth tube and a micro-fin tube, ASHRAE Trans. 97 (1991). Technical Training Center. [31] L. Cinotti, M. Bruzzone, N. Meda, G. Corsini, C.V. Lombardi, M. Ricotti, [8] R.J. Fetterman, A. Harkness, M. Smith, C. Taylor, An overview of the West- L.E. Conway, Steam generator of the international reactor innovative and inghouse small modular reactor, in: Proceedings of the ASME 2011 Small secure, in: International Conference on Nuclear Engineering, Arlington, Vir- Modular Reactors Symposium SMR 2011, American Society of Mechanical ginia, USA, 2002. Engineers, Washington, DC, USA, 2011. [32] M.D. Carelli, L.E. Conway, L. Oriani, The design and safety features of the IRIS [9] L. Damiani, M. Montecucco, A. Pini Prato, Conceptual design of a bayonet-tube reactor, Nucl. Eng. Des 230 (2004) 151e167. steam generator for the ALFRED lead-cooled reactor, Nucl. Eng. Des. 265 [33] G.A. Roth, F. Aydogan, Development of governing equations based on six fields (2013) 154e163. for the RELAP code, Nucl. Sci. Eng. 182 (2016) 71e82.