Investigation of fuel assembly by using subchannel analysis for supercritical-water cooled power reactor

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1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1100 Investigation of fuel assemly y using suhannel analysis for superritial-ater ooled poer reator Kazuaki Kitou 1*, Kouji Nishida 1, Masayoshi Matsuura 1 and Shigenori Shiga 2 1 Hitahi, Ltd., Omika-ho, Hitahi-shi, Iaraki, Japan Toshia Corporation, 8 Shinsugita-ho, Isogo-ku, Yokohama-shi, Japan One of the prinipal advantages of the superritial-ater ooled poer reator (SCPR) is high thermal effiieny, hih is due to the use of superritial pressure ater as a oolant. On the other hand, fuel ladding surfae temperature inreases loally due to a synergy effet from the inrease of oolant temperature and derease of heat transfer oeffiient, if the oolant flo distriution is non-uniform in the fuel assemly. Therefore the SCPR fuel assemly as designed using suhannel analysis to evaluate ailed thermal hydrauli harateristis. The suhannel analysis ode is ased on the SILFEED ode for BWR. The SCPR fuel assemly has many square-shaped ater rods. The fuel rods are arranged around these ater rods. The fuel rod pith and diameter are 11.2 mm and 10.2 mm, respetively. Sine oolant flo distriution in the fuel assemly strongly depends on the gap idth eteen the fuel rod and ater rod, the rational fuel assemly is designed y adjusting the gap idth. Suhannel analysis larifies that oolant flo distriution eomes uniform hen the gap idth is set to 1.0 mm. The highest fuel ladding surfae temperature as less than 600 ºC. The temperature margin of the fuel ladding inreases in the design. KEYWORDS: Superritial-ater ooled poer reator, Suhannel analysis, Fuel assemly I. Introdution The superritial-ater ooled poer reator (SCPR) is expeted to have high thermal effiieny ith a one-through diret yle 1, 2). Main harateristis of the SCPR are ompared in Tale 1 ith those of an advaned oiling ater reator (ABWR) 3). The plant system of the SCPR is shon in Fig. 1. The plant system design is ased on tehnologies of urrent light ater reators (LWRs) and superritial pressure fossil-fired poer plants (FPPs). The thermal effiieny is improved y 27 % over that of LWRs. The reirulation system, steam separators and dryers that are required for BWRs are unneessary. The ontainment volume is dereased and only to oolant lines are needed for the 1000 MWe lass poer reator. The poer ost ill e greatly redued ompared ith LWR osts. To make these advantages viale, a Japanese team onsisting of University of Tokyo, Kyushu University, Hokkaido University, Toshia Co. and Hitahi, Ltd. started the development projet of SCPR ith a national fund in fisal year Sine this reator onept as hosen as one of the next generation nulear systems y Generation-IV International Forum (GIF) in Septemer 2002, it is expeted that international development projets ill start in the near future. (In GIF, SCPR is knon as SCWR (SuperCritial- Water-ooled Reator system).) One of the prinipal advantages of the SCPR is high thermal effiieny otained y generating high pressure and high temperature steam, as a result of using superritial * Corresponding author, Tel , Fax , kazuaki_kitou@pis.hitahi.o.jp pressure ater as the oolant. The thermal effiieny of urrent LWRs is loer than that of FPPs, as urrent LWRs limit the main steam temperature to the saturation temperature. On the other hand, superritial pressure ater does not hange phase, so the steam temperature is not limited to the saturation temperature. On the one hand, it is neessary to are aout the folloing point from a viepoint of a fuel assemly thermal design. When oolant flo distriution in the fuel assemly is non-uniform (there is an area here oolant mass flux is small), it is possile that the fuel ladding surfae temperature inreases loally due to a synergy effet due to: 1. Coolant temperature inreases eause superritial pressure ater does not have speifi latent heat for oiling. 2. Heat transfer oeffiient dereases. In the present study, fuel assemly design is examined to derease the maximum fuel ladding surfae temperature y using suhannel analysis to evaluate ailed thermal hydrauli harateristis. Tale 1 Main harateristis of SCPR SCPR ABWR Plant system One-through diret yle Diret yle System pressure (MPa) Thermal poer (MWt) Eletri Poer (MWe) Core inlet / outlet temperature (ºC) 280 / / 284 Thermal effiieny (%) Main steam line numer 2 4

2 Control rod Containment vessel Moisture separator reheater Intermediate pressure turine Safety relief valve High pressure turine Lo-pressure turine Generator Lo pressure ore injetion system ore Condenser Feedater heater Feedater pump Condensate ater storage tank Suppression pool Fig. 1 Plant system of SCPR II. Suhannel analysis ode A flohart of the suhannel analysis ode for SCPR is shon in Fig. 2. This ode onsists of to parts: a oolant flo distriution evaluation part and a fuel ladding surfae temperature evaluation part. The first part is ased on the SILFEED 4) (Simulation of Liquid Film Evaporation, Entrainment and Deposition) ode hih is a suhannel analysis ode for urrent BWR onditions. Coolant flo distriution and oolant temperature distriution in a fuel assemly are evaluated in this part. Pressure drop and transverse mass flux are evaluated ased on a ealth of experimental data under urrent BWR onditions. Auray of the first part is evaluated y omparing experimental data. At the present stage, there is no experimental data under superritial pressure onditions to ompare ith the analysis results. Therefore, analysis auray of this ode is verified under suritial pressure and single flo onditions 5). The shemati illustration of the test setion is shon in Fig. 3 and analysis results are ompared in Fig. 4 ith those of the experimental data. The test setion simulates a fuel assemly hih is omposed of 9 heating tues and a hannel ox. Analysis results of oolant mass flux distriution at the test setion outlet are ompared ith experimental data at orner, side and enter part. Although an experimental data at a orner part here oolant mass flux as aout 500kg/m 2 s had the relatively large error, almost errors eteen experimental and analysis results ere 10 % or less. Evaluated oolant flo and oolant temperature distriution are used as input data in the fuel ladding surfae temperature evaluation part. In this part, heat transfer oeffiient and fuel ladding surfae temperature are evaluated in eah node. Heat transfer oeffiient an e alulated y using plural formulas for superritial pressure ater, hih are Dittus-Boelter s formula, Watts s formula 6), Yamagata s formula 7) and Kitoh s formula 8). Dittus-Boelter s formula is used for only omparison, eause superritial pressure ater is outside the sope of this formula. Kitoh's formula as taken ith a 10 % margin aording to the desription outlined in the referene paper 8). The formulas are shon as follos. H s Nu λ (1) D h Dittus-Boelter s formula, R e P r Nu (2) Watts s formula, Nu Nu Nu Nu here, Nu Gr varp ρ T varp varp 0.021Re ( ρ ρ ) ρ dt T ρ T T, G r Re Pr 7000 Gr Re Pr µ Pr gd Cp h, T Cp dt h T T T T Cp µ Pr λ Gr Re Pr Gr Re Pr 0.35 ρ (3) ρ h T 10 >

3 Coolant flo distriution evaluation part (Based on SILFEED ode) Set oolant onditions of assemly inlet Set alulation onditions in eah node Kitoh s formula, Nu x Re Pr 0.9 here, x 0.015, y 0.85, z / q" q" 8 F F F y 200 G / q" 1.2 z 0.65/ q" / q" + F q" [ Enthalpy 0to1.5MJ / kg] [ Enthalpy 1.5to3.3MJ / kg] [ Enthalpy 3.3to4.0MJ / kg] (5) Evaluate pressure drop and ross flo Final node? No Corner part Side part Heating tue Yes Output oolant flo and temperature distriution Center part Fuel ladding surfae temperature evaluation part Input oolant flo and temperature distriution in eah node 14.5mm 18.7mm Fig. 3 Shemati diagram illustration of the test setion Calulate heat transfer oeffiient in eah node Evaluate fuel ladding surfae temperature Final node? End Yes No Analysis: mass flux (kg/m 2 s) Corner Side Center Base line +10% -10% Fig. 2 Flo hart of the suhannel analysis ode Yamagata s formula, Nu Re here, F F F n n 1 2 E 1.0 [ E 1.0] 0.05 n1 0.67PrCrit ( Cp / Cp ) [ 0 < E < 1] n2 ( Cp / Cp ) [ E 0] 0.77( 1+ 1/ PrCrit ) ( 1+ 1/ PrCrit ) ( T T )/( T T ) Crit 0.85 Pr 0.5 F (4) Experiment: mass flux (kg/m 2 s) Fig. 4 Auray of the suhannel analysis ode III. Analysis onditions The main harateristis of the hottest SCPR fuel assemly are summarized in Tale 2. The shemati of the SCPR fuel assemly is shon in Fig. 5. Total thermal poer and inlet oolant temperature of the

4 fuel assemly are set to 24.7 MW and 280 ºC, respetively. Average oolant outlet temperature of the fuel assemly is ontrolled to ithin 500 ºC y adjusting the inlet oolant flo rate. The SCPR fuel assemly has 36 square-shaped ater rods. The fuel rods are arranged around the ater rods. The total numer of fuel rods is 301 hih inludes 1 instrumentation rod positioned at the enter of the fuel assemly. The fuel rod pith and diameter are 11.2 mm and 10.2 mm, respetively. In the present study, it is assumed that loal peaking is 1.0 and axial poer distriution is osine distriution. heked eforehand that alulated oolant flo distriution and oolant temperature distriution using 1/8 model ere the same as that using the full ross setion model. Channel ox Heat insulator Fuel rod Water rod Tale 2 Main harateristis of the SCPR fuel assemly Thermal poer (MW) 24.7 Inlet oolant temperature (ºC) 280 Outlet oolant temperature (ºC) 500 Numer of fuel rods *1 301 Numer of ater rods 36 Fuel rod diameter (mm) 10.2 Fuel rod pith (mm) 11.2 Gap idth eteen the fuel rod and ater rod (mm) *2 0.5, 0.75, 1.0, 1.25 Loal peaking *3 1.0 Axial poer distriution *3 Cosine distriution *1 Inlude 1 Instrumentation rod *2 Design parameter *3 Used assumption in the present study A Instrumentation rod A (a) 1/8 ross setion model Large oolant mass flux area Channel ox Heat insulator Fuel rod Water rod Small oolant mass flux area Design parameter 281mm Instrumentation rod 1.0mm 11.2mm 10.2mm 6.1mm () Enlarged illustration of the orner part Fig. 6 Analysis model Fig. 5 Shemati of SCPR fuel assemly The analysis model is shon in Fig. 6. Figure 6 () is an enlarged illustration of the orner part of Fig. 6 (a). The 1/8 ross setion model as used in the present analysis. It as The preliminary analysis larified that oolant mass flux as high near the hannel ox and eame loer near the ater rod all. This oolant flo distriution strongly depended on the gap idth eteen the fuel rod and the ater rod. Therefore, in the present study, the rational fuel

5 assemly as designed y adjusting the gap idth. Four ases of 0.5, 0.75, 1.0 and 1.25 mm gap idth ere examined. The hange of the gap idth as ahieved y adjusting the ater rod size. The influene of hanging ater rod size is small for nulear design due to small variation. IV. Analysis results Analysis results of oolant mass flux distriution in the fuel assemly vertial setion ith gap idths of 0.5mm and 1.0mm are shon in Fig. 7 (a) and (). The lateral diretion of Fig. 7 is the setion shon y A-A in Fig. 6 (a). The right side in Fig. 7 (a) and () is the enter of the fuel assemly and the left side is the hannel ox. The ottom in Fig. 7 is the fuel assemly inlet and the top is the fuel assemly outlet. as expanded as it reahed the upper part. Loer oolant mass flux areas ere near the ater rod all. For the 1.0 mm gap idth, oolant mass flux as loer near ater rod all as ell. Hoever, the lateral diretion flo distriution as muh loer than that of 0.5 mm gap idth, and oolant mass flux distriution as not expanded. An area of large oolant mass flux in the upper right part in Fig. 7 is an effet of the instrumentation rod. Standard deviations of oolant mass flux in eah gap idth are summarized in Tale 3 and results of frequeny distriution of oolant mass flux at the fuel assemly outlet are shon in Fig. 8. The horizontal axis of Fig. 8 is standardized oolant mass flux distriution, and the vertial axis is a ratio of the flo hannel ross setion. From Tale 3 and Fig. 8, it as larified that the ase of 1.0 mm gap idth had the most uniform oolant mass flux distriution. In this ase, there as no suhannel in hih standardized oolant mass flux as loer than 90 %. For gap idths of 0.5 mm and 0.75 mm, some suhannels near the ater rod all had the loest oolant mass flux. For the 1.25mm gap idth, suhannels near the ater rod all shoed large oolant mass flux, adversely. Tale 3 Standard deviations of oolant mass flux Gap idth (mm) Standard deviations (%) (a) 0.5mm gap idth () 1.0mm gap idth Fig. 7 Coolant mass flux distriution in fuel assemly vertial ross setion Ratio of flo hannel ross setion (%) Gap idth 0.5mm Gap idth 0.75mm Gap idth 1.0mm Gap idth 1.25mm Coolant mass flux distriution (%) In analyzing the result of 0.5 mm gap idth, lateral diretion flo distriution as non-uniform and distriution Fig. 8 Frequeny distriution of oolant mass flux

6 Analysis results of oolant temperature distriution, in hih gap idths ere 0.5 mm and 1.0 mm in the fuel assemly vertial setion, are shon in Fig. 9 (a) and (). For 0.5 mm gap idth, lateral diretion oolant temperature distriution as not uniform due to the non-uniformity of the oolant mass flux distriution as shon in Fig. 7 (a). Coolant temperature inreased loally at the upper part of the fuel assemly. On the other hand, there as no area here oolant temperature as loally high for 1.0 mm gap idth. idth. Tale 4 Standard deviations of oolant temperature Gap idth (mm) Standard deviations (ºC) Ratio of flo hannel ross setion (%) Average oolant temperature (500 ºC) Gap idth 0.5mm Gap idth 0.75mm Gap idth 1.0mm Gap idth 1.25mm Coolant temperature distriution ( o C) Fig. 10 Frequeny distriution of oolant temperature (a) 0.5mm gap idth () 1.0mm gap idth Fig. 9 Coolant temperature distriution in fuel assemly vertial ross setion Standard deviations of oolant temperature in eah gap idth are summarized in Tale 4 and results of frequeny distriution of oolant temperature at the fuel assemly outlet are shon in Fig. 10. The average oolant temperature at the fuel assemly outlet is 500 ºC hih is marked ith a roken line as shon in Fig. 10. In most suhannels, oolant temperature differene to average temperature as less than 20 ºC for 1.0 mm gap Fuel ladding surfae temperature hange for 1.0 mm gap idth along the height diretion is shon in Fig. 11. The horizontal axis is the standardized fuel assemly height and the vertial axis is fuel ladding surfae temperature. Fuel ladding surfae temperature as evaluated y the four heat transfer formulas. In Fig. 11, the maximum ladding temperature appeared near the fuel assemly outlet. Yamagata s formula, Kitou s formula and Dittus-Boelter s formula shoed almost the same maximum ladding temperature. The maximum ladding temperature using Watts s formula as higher than that alulated ith the other formulas. From these results, it is expeted that Watts s formula is the most onservative in this ase. Fuel ladding surfae temperatures for 1.0 mm gap idth as ompared in Fig. 12 ith those for 0.5 mm gap idth. Fuel ladding surfae temperature as evaluated using Watts s formula hih shos the highest temperature in Fig. 11. The highest fuel ladding surfae temperature for 1.0 mm gap idth as elo 600 ºC. This temperature as aout

7 70 ºC loer than that for 0.5 mm gap idth. It is expeted that the temperature margin of the fuel ladding inreases for 1.0 mm gap idth. Temperature ( o C) Temperature ( o C) Dittus-Boelter Watts Yamagata Kitoh Standardized height (-) Fig. 11 Fuel ladding surfae temperature hanges for 1.0mm gap idth of the height diretion Gap idth 0.5mm Gap idth 1.0mm Standardized height (-) Fig. 12 Comparison of the fuel ladding surfae temperatures y differene in gap idth IV. Conlusion The fuel assemly of the superritial-ater ooled poer reator as designed y using the suhannel analysis ode hih as ased on the SILFEED ode for BWR. This ode onsists of to parts: a oolant flo distriution evaluation part and a fuel ladding surfae temperature evaluation part. Pressure drop and transverse mass flux are evaluated ased on a ealth of experimental data under urrent BWR onditions. Fuel ladding surfae temperature an e evaluated y using plural formulas for superritial pressure ater, hih are Dittus-Boelter s formula, Watts s formula, Yamagata s formula and Kitoh s formula. The SCPR fuel assemly has 36 square-shaped ater rods and 301 fuel rods hih inludes 1 instrumentation rod. The fuel rod pith and diameter are 11.2 mm and 10.2 mm, respetively. In the present study, the rational fuel assemly as designed y adjusting the gap idth eteen the fuel rod and the ater rod from 0.5 mm to 1.25 mm. Suhannel analysis larified that oolant flo distriution eame almost uniform for gap idth set to 1.0 mm. The highest fuel ladding surfae temperature evaluated y Watts s formula, hih shoed the most onservative heat transfer oeffiient, as elo 600 ºC. It as expeted that the temperature margin of the fuel ladding inreased in the design. Nomenlature Cp Isopiesti speifi heat (J/kgK) D h Hydrauli diameter (m) G Mass flux (kg/m 2 s) g Aeleration of gravity (m/s 2 ) H s Heat transfer oeffiient (W/m 2 K) h Enthalpy (J/kg) Nu Nusselt numer Pr Prandtl numer q Heat flux (W/m 2 ) Re Reynolds numer T Temperature (ºC) λ Thermal ondutivity (W/mK) µ Visosity (Pa-s) ρ Density (kg/m 3 ) Susript Bulk temperature Wall temperature Crit Pseudo-ritial temperature Deterioration Aknoledgment This SCPR development projet is funded y the Institute of Applied Energy (IAE), Ministry of Eonomy, Trade and Industry (METI), Japan.

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