NTHAS9: The Ninth Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety Buyeo, Korea, November 16-19, 2014 Paper Number N9P0060

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1 NTHAS9: The Nth Korea-Japan Symposium on Nuclear Thermal Hydraulics and Saety Buyeo, Korea, Novemer 16-19, 2014 Paper Numer N9P0060 NUMERICAL STUDY ON INERT GAS BEHAVIOR IN FAST REACTOR PRIMARY COOLANT SYSTEM -INERT GAS ACCUMULATION AT HPP AND CONSIDERATION OF GAS ELIMINATION SYSTEM- Takashi Takata 1*, Yuji Konaka 1,**, Akira Yamaguchi 1 Kei Ito 2, Shuji Ohno 2 and Hiroyuki Ohshima 2 1 Osaka University, 2-1 Yamada-oka, Suita, Osaka, , Japan 2 Japan Atomic Energy Agency, 4002 Narita, Oarai, Higashi-Iaraki-gun, Iaraki, , Japan * , , takata_t@see.eng.osaka-u.ac.jp: ** Graduated rom Osaka University March o 2014 ABSTRACT In a sodium-cooled ast reactor, ert gases exist a primary coolant system either a state o dissolved gas or ree gas ules and they may cause crease reactivity the core, a nucleation site or oilg, low staility and an luence on heat transer at the heat exchanger. In this paper, a model o ule transport at entrance nozzle has een developed ased on theoretical and computational methods. For this purpose, a three-dimensional analysis o dynamic ules ehavior has een carried out y usg one-way ale trackg method which is speciied to a ule low under a low void raction. A commercial CFD tool, FLUENT Ver is used or threedimensional low ield analyses. Then the gas accumulation at a high pressure plenum (HPP) o sodium-cooled ast rector has een quantitatively-assessed y implementg the developed model SYRENA code. Furthermore, gas elimation (ule removal) systems have een considered and the quantitatively-assessment has een carried out order to vestigate the eect o each measure to the accumulation o gas at the HPP. 1. INTRODUCTION In a sodium-cooled ast reactor, ert gases exist a primary coolant system either a state o dissolved gas or ree gas ules. There are two sources o the ert gas the system; one is argon (Ar) and the other is helium (He). Ar gas, used as a reactor vessel cover gas, exists on a ree surace o the coolant at an upper plenum o the reactor vessel. The usage o the ree surace is unavoidale ecause reservoir unction is needed to asor a thermal expansion o liquid sodium. Sce the Ar cover gas exists aove the ree surace, it would dissolve to the liquid sodium and is dispersed the primary coolant system y diusion and advection. At the same time, it has a possiility that ree gas ules o Ar lows to the liquid sodium y an entrament at the surace caused y, such as, a reak-up o surace wave and a surace vortex. On the other hand, He gas is produced as a result o distegration o B 4 C control rod material and is emitted as small ules to the reactor core. Even i those gases are dissolved coolant at high temperatures, they will come out as ules as the temperature decrease at an termediate heat exchanger (IHX). Those ules are transported accordg to the coolant low the primary system and may cause crease reactivity the core, a nucleation site or oilg, low staility and an luence on heat transer at the heat exchanger. The primary coolant lows to the core region through halls o entrance nozzles stalled at a high pressure plenum (HPP) and a ree space exists aove the halls. Consequently, gases may accumulate at the HPP and accumulated gases may e carried through the entrance nozzles at one time. Thereore, it is essential to understand the dynamics ehavior o the ules exactly rom the perspective o reactor saety o the sodium-cooled ast reactor. For stance, a dipped plate will e stalled at the upper plenum o the reactor so as to elimate Ar gas entrament at the liquid surace Japan sodiumcooled ast reactor (JSFR), (Konomura M, et al., 2007). However, the gas source o He still exists side the primary coolant system and thus the quantiication o the ule ehavior is still o importance JSFR. A computational code SYRENA (Ito K, et al., 2014) or dynamics o the ules the primary coolant system had een origally developed as the VIBUL code or French ast reactor and modiied or JSFR design (Berton J.L. 1991: Yamaguchi A, et al., 2005:

2 Tatsumi E, et al., 2007). This code quantiies the amount o ree ules and dissolved gas sodium coolant. However, simple module models o ule transport (pot approximation) each component are implemented and one-dimensional low o models is assumed the code. Furthermore, it is diicult to validate the code well ecause o less actual plant data. However, it is still considerale that the simpliication may not e suiciently accurate to descrie the ule ehavior especially the complicated components such as an upper plenum o the reactor, IHX and HPP. In this paper, the authors have proposed the model o ule transport at entrance nozzle ased on theoretical and computational methods. For this purpose, a three-dimensional analysis o dynamic ules ehavior has een carried out y usg one-way ale trackg method which is speciied to a ule low under a low void raction. A commercial CFD tool, FLUENT Ver (Ansys: is used or three-dimensional low ield analyses. Then the gas accumulation at HPP has een quantitativelyassessed y implementg the developed model SYRENA code. Additionally, gas elimation (ule removal) systems have een considered and the quantitatively-assessment has een carried out order to vestigate the eect o each measure to the accumulation o gas at the HPP. Computational region at entrance nozzle 2. BUBBLE BEHAVIOR AT ENTRANCE NOZZLE 2.1 Analytical Conditions In the modelg o ule ehavior at the entrance nozzle, a multi-dimensional CFD is carried out y FLUENT at irst. Then one-way ule trackg analysis is done ased on the computational geometry and result (Konaka Y, et al., 2012). In the analysis, one-sixth symmetrical geometry is assumed as shown Fig. 1. Let us mention the rie computational condition (details Konaka Y, et al., 2012). The total numer o nodes is approximately 100,000 and a steady state computation is carried out with the standard k-ε turulent model and the standard wall unction. Ater the steady state o the low and pressure ield is otaed, one-way ule trackg simulation is done usg a Lagrange method. In the terpolation o the low and pressure ields, the same manner with that used the MPS method is applied. In the ule trackg simulation, the ule size is segmented to 50 s rom 1µm to 300µm radius and 200 ules are placed randomly at the ottom o the computational geometry and are tracked each. In order to develop the commonly-used model, the ollowg are selected as a parameter the simulation; the radius o nozzle (R1), the radius o let hole (R2), the height o nozzle (H), the numer o holes (Nhole) and the let velocity (V ). Tale 1 summarizes the analytical parameter and the location o let holes are shown Fig. 2. As shown Fig.2, the holes are assumed to e troduced the same terval rom the topside. Computational geometry Fig. 1 Computation geometry at entrance nozzle In the actual entrance nozzle, coolant lows not only rom vertical direction (ottom side) ut also rom horizontal direction. In the present computations, coolant let is only applied to the ottom side as shown Fig. 1. It is attriuted to the act that the vertical upward low will e domant rom the viewpot o pressure loss and that there is less luence o the location o the coolant let on the result the prelimary simulation. It is also noted that the ase case o V corresponds to an average velocity at a rated operation. Geometric parameter Tale 1. Analytical parameters BASE case R1 [mm] R2 [mm] H [mm] Nhole [-] V [m/s]

3 Fig. 2 Location o let holes 2.2 Results and Modelg o Bules Figure 3 shows an example o the analytical result (snap shot ule trackg (50µm radius) at ase case). As seen Fig.3, the coolant lows to the entrance nozzle through upper-side holes and thus the ules also ollows the low le domantly at the ule size o 50µm radius. In the ule ehavior model, three categories are considered. One is the total amount o lowg to the nozzle (Inlow). The second is that o dissolvg the coolant durg the trackg (Dissolution). The last is that o accumulatg at the top o the analytical geometry (Residual). Furthermore, a non-dimension numer o F /F d, deed the ollowg, is troduced to summarize the parametric study. Fig. 3 Flow le and snap shot p ule trackg (ule radius: 50µm, ase case) ρ ρ g F ρ 8rg = = F C V ( ρ ρ ) 2 3 ρ 2 d 3 Dρ CD ( V V ) 8r ρ. (1) Fig. 4 Bule ehavior at ase case geometry (Fractions o Inlow, Dissolution and Residual) Here, F and F d mean the uoyance and drag orces o the ule respectively. Figure 4 shows the raction o each category at the ase case geometry with various let velocities. It is demonstrated that the present non-dimension numer (F /F d ) 1/2 is preerale as an dependent variale the model. As Fig. 4, the raction o dissolution is quite small comparg with that o low and residual. It is attriuted to the act that the existg duration at the low region near the entrance nozzle is an order o second and thus the quite small raction (smaller than at 1µm radius) the analysis. Accordgly, the raction o dissolution at the entrance nozzle is neglect the model. The result o the low raction at various parameters is dicated Fig. 5. Then the ollowg model is proposed ased on the itted curve. Fig. 5 Non-dimensional correlation or low raction

4 res dis ( ρ ρ ) F 8rg = exp = exp , 2 F d 3CDρ V 2 (2) ( R = ) = 1, = 0. Here, R 2 is the correlation coeicient. As shown Eq. (2) and Fig. 5, only dependent variale o (F /F d ) 1/2 is suicient the correlation at the entrance nozzle although many geometrical parameters are examed. Firstly, the range o ule radius is comparatively small (one to several hundred µm). Thereore the ules are mostly aected y the termal velocity o the coolant (V ). Consequently, (F /F d ) 1/2 will e a most important parameter. At the same time, it is vestigated that the coolant lows to the entrance nozzle mostly through several holes at the top side the CFD simulation as shown Fig. 3. Sce the holes le up order rom the top side (Fig. 2), the numer o holes has less luence on the present correlation. 3. ACCUMULATION AT HPP AND GAS ELIMINATION SYSTEM In the vestigation o an accumulation at the HPP and the consideration o gas elimation systems, ule dynamics analyses with the SYRENA code are carried out the loop-type sodium cooled ast reactor, JSFR design (Yamaguchi A. et al., 2005). 3.1 Gas Accumulation at HPP Primary Coolant System The low network (node coniguration) o the primary coolant system the SYRENA code is shown Fig. 6. Encircled numers mean the volume and numers at red pots dicate the low path. The HPP is located at the low path 12 as Fig. 6. In the present analysis, the gas accumulation model (Eq. (2)) is implemented at that pot. In the primary coolant system, there are two ree suraces at the upper plenum and primary pump. In this low network, the gas ule will e removed only at the ree suraces due to uoyancy. In the analysis, a simpliied model is applied to the removal rate ased on a termal velocity o ule, a geometry dimension and a coolant low rate. It is also noted that the upper plenum is divided to two sections (1 and 2) through the low path 2 and 3. In JSFR, a dipped plate (D/P) will e stalled at that location so as to reduce an Ar gas entrament rom the ree surace. The exchange rate is set to approximately 0.7% o the total low rate ased on the experimental oservation (Kimura N. et al., 2005) With regard to the gas sources, 0.4cc/s o He ules release with 50µm radius is assumed rom the control rods to the core region (9 Fig. 6) (Yamaguchi A, et al., 2005). On the other hands, the Ar gas entrament rate rom the ree surace at the upper plenum is selected as a parameter (0-0.4cc/s with 50µm radius) takg to account the acceptant level (5% o ackground level) (Eto K, et al., 2012). Fig. 6 Flow network SYRENA code In each analysis, ule size is divided to 50 s rom 1 to 300µm considerg a critical Weer numer caused y a share orce at the pump (Mignot G, 1997). And a steady state calculation has een done. The accumulation rate at the HPP at each ule size case o 0.4cc/s Ar gas entrament is shown Fig. 7. As Fig. 7, it is ound that the accumulation o He gas is domant at the HPP although the same source (0.4cc/s) is assumed. This is attriuted to the act that He gas is emitted at the core and less He gas move to the ree surace at the upper plenum (1) ecause o the small exchange rate via the D/P (0.7%). Furthermore, the soluility o He is much higher than Ar. Hence, the He gas emitted at the core dissolves easily due to high temperature resultg small numer density at the upper plenum. Then a nucleation will appear at the IHX (4). Accordgly, the peak radius o accumulated ule shits rom 50µm (itial source) to 80µm as Fig. 7. Fig. 7 Bule accumulation rate at HPP (Ar: 0.4cc/s) On the other hand, the Ar gas entrament occurs at the ree surace o the upper plenum and less Ar ules move to the primary system. As a result, the luence o the Ar gas entrament on the accumulation rate is comparatively low as shown Fig. 8.

5 ule remover. With regard to the exchange low rate at the puriication system, a comparative small rate (0.1t/h) is selected as a rated (ase) condition. Fig. 8 Total accumulation rate at HPP In the present analysis, only the ree surace at the pump is set to the removal (sk) pot at the primary system side the D/P. Thereore, a comparative large accumulation (more than a dozen liter per day) is vestigated. However, it is careully noted that the ypass low should exist at the HPP the actual system and thus the actual accumulation rate would e smaller than the present result. 3.2 Consideration o Gas Elimation systems As a design study to reduce the gas accumulation rate at the HPP, the ollowg three systems are taken to account. I. Increase o exchange low rate at the D/P II. Increase o exchange low rate at the pump III. Gas removal at a puriication system As mentioned Sec. 3.1, the ree suraces are only the removal pot the analysis. Hence, the crease o exchange rate will reduce the accumulation rate. It is sure that the crease o the exchange rate at the pump (II) is eective or the reduction. On the contrary, the crease o exchange rate at the D/P (I) will also cause the crease o an additional comg o Ar ules rom the ree surace. Hence, the sensitivity o the exchange rate is vestigated terms o case I. In the primary coolant system, a puriication system must e required to keep the purity o the coolant. For this purpose, the coolant temperature is lowered and impurities, such as sodium oxide and hydride, are precipitated as a solid phase and are trapped at the ilter. The soluility o He also decrease exponentially as the coolant temperature decreases. Accordgly, the puriication will have a high potential as a gas elimator. Figure 9 shows the low network o the present design study. In JSFR, the puriication system will e stalled at the upper plenum. However, a detail design is not determed yet. In the present study, as an engeerg judgment, simple pipe shape geometry o 3cm and 15cm diameter and the temperature decrease o coolant rom 823K to 393K are assumed to estimate the coolant average velocity and the soluility. The same nucleation model with the IHX is applied to the economizer. Furthermore, the ree surace o the same cross-section area with the pipe is implemented as a Fig. 9 Flow network SYRENA code or design study As a parameter study, various exchange rates are examed each system. Figure 10 dicates the eectiveness o the systems (conventional ratio o accumulation rate). Solid symols Fig. 10 mean the rated (ase) conditions. In case o the D/P, the accumulation rate decrease slightly accordance with the crease o the exchange low rate. As mentioned eore, the D/P has eect oth o comg and outgog o ules. Sce the Ar gas entrament has less eect on the accumulation rate as Fig. 8. Accordgly, it would e concluded that the exchange rate o the D/P has less luence on the accumulation rate. Fig. 10 Inluence on gas accumulation rate When the cross-section area is small case o the puriication system (3cm diameter), a local optimization low rate is vestigated as seen Fig. 10. When the exchange low rate creases, the eectiveness also crease rom the viewpot o the mass amount o the throughput. However, the crease o the average velocity side the puriication system results a small nucleation size and high entraed ules at the outlet. When the large size (slow coolant velocity) puriication is applied, quite high reduction o

6 the accumulation can e achieved. This is attriuted to the act that the puriication system only can remove the dissolved gas. However, it is also noted that a comparative large exchange rate would e required ecause o the throughput. For stance, almost the same order o the exchange rate with Japanese prototype ast reeder reactor, MONJU (approximately 0.05% o the rated condition, JNC, 2005) The most important issue or the gas removal is the low coolant velocity durg the decrease o the temperature (large ule size o nucleation) and the ree surace section (relatively high uoyancy eect). Thereore, an optimization o the puriication system would e worthy to discuss or saer sodium ast reactor plant. The exchange rate o the pump also has a high potential or gas removal as shown Fig. 10. This is attriuted to the act that the exchange low rate is generally much larger than the puriication system. However, the ree surace at the pump is implemented or a seal o hydraulic earg. Thereore, it might e challengg to crease the exchange low rate. In the design study, it is demonstrated that the gas accumulation ehavior can e vestigated with the present method. It can e said that the present method is also useul or an optimization o the systems rom the view pot o a low accumulation rate at the HPP. However, it is also mentioned that the ule removal model the present study is quite simple as mentioned eore. Consequently, a modiication o the removal model considerg the complicated geometrical eect is necessary or a quantitative assessment. 4. CONCLUSION A ule ehavior model at an entrance nozzle o sodium cooled ast reactor has een developed and implemented to SYRENA code or dynamics o the ules the primary coolant system so as to enhance a predictive accuracy and an accumulation rate o gas at a high pressure plenum (HPP). In the model development, a quantitative multidimensional CFD has een carried out and one-way ule trackg analyses have een done ased on the CFD results. Then commonly-used correlation has een estalished with non-dimensional numer ased on the uoyance and drag orces o ule. As a quantitative-assessment o the accumulation rate, the numerical vestigation o the primary coolant system o JSFR has een carried out. As a result, it is ound that the accumulation o He gas, which is emitted rom B 4 C control rods, is domant at the HPP and that Ar gas entrament rom the ree surace at the upper plenum has less luential on the accumulation. Sce the ypass low at the HPP is not taken to account the present study, a comparative large rate (more than a dozen liter per day) is vestigated. A design study or reducg the gas accumulation at the HPP has also een carried out. As a gas elimation system, the crease o exchange low rate at the dipped plate (D/P) and the ree surace section o the pump are considered as well as an implementation o gas removal unction the puriication system. As a result, it is demonstrated that the exchange low rate at the D/P has less luence on the gas accumulation. On the other hands, the puriication system with a comparative low coolant velocity (pipg shape o 15cm diameter) and the ree surace at the pump have a high potential to reduce the gas accumulation. It is also concluded that the present method is useul or an optimization o the systems rom the view pot o a low accumulation rate at the HPP, although a detailed validation o the present model should e required the actual plant vestigation. NOMENCLATURE C D drag coeicient [-] F uoyancy orce [-] F d drag orce [-] dis dissolution raction [-] low raction [-] res residual raction [-] g gravity [m/s 2 ] H length o the entrance nozzle [m] Nhole numer o let hole [-] r ule radius [m] R1 entrance nozzle radius [m] R2 let hole radius o entrance nozzle [m] V velocity magnitude [m/s] Greek Letters ρ density [kg/m 3 ] Suscripts gas phase liquid phase let REFERENCES Berton J.L. (1991), Um modèle de calcul de la voe des ullesen réacteur, NTCEA, SSAE, LSMI91, 023. Eto K. et al. (2012), NUMERICAL QUANTI- FICATION OF DISSOLVED GAS BEHAVIOR IN PRIMARY COOLANT SYATEM OF FAST REACTOR -SUPPRESSION OF GAS ENTRAI- NMENT USING DIPPED PLATE-, 8 th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Saety, N8P1113, Beppu, Japan, Dec Ito K. et al. (2014), Private Communication. JNC (Japan Nuclear Cycle Development Institute, Sodium Technology Education Committee), (2005) Sodium Technology Handook, JNC TN , ( Japanese). Kimura N. et al. (2006), Experimental Study on low optimization upper plenum o reactor vessel or a compact sodium cooled ast reactor, Nuclear Tech., 152, 2, Konaka Y. et al. (2012), STUDY ON BUBBLE BEHAVIOR OF INERT GASES AT ENTRANCE

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