Improvement of SSR Core Design for ABWR-II

Transcription

1 GENES4/ANP003, Sep , 003, yoto, JAPAN Paper 108 Improvement of SSR Core Design for ABWR-II Masanao Moriwaki 1*, Motoo Aoyama 1, Hiroyuki Okada, Hideya itamura, oichi Sakurada 3 and Akira Tanabe 4 1 Hitachi, Ltd., Hitachi, Ibaraki, , Japan Tokyo Eectric Power Company, Tokyo, , Japan 3 Toshiba Corporation, awasaki, anagawa, , Japan 4 Goba Nucear Fue Japan, Yokosuka, anagawa, , Japan In order to enhance the spectra shift effect in the ABWR-II reactor, a nove core design to bring out better performance of spectra shift rods (SSRs) is studied. The SSR is a new type of water rod, in which the water eve deveops naturay during operation and changes according to the cooant fow rate through the channe. By using the SSR, the average moderator density, which is directy reated to core reactivity, can be controed over a wide range by the core fow rate. In the new SSR core design, two types of SSR bundes, in which settings for the SSR water eves are different, are utiized and oaded according to fow distribution in the core. This two-region SSR core design aows wide variation in the average SSR water eve, thus improving fue economy. Enhancement of SSR function in the two-region SSR core increases the uranium saving factor by about 5%, from the 6 % of the conventiona uniform SSR core to about 8%. EYWORDS: LWR, BWR, ABWR-II, Large bunde, Large attice, Spectra shift, SSR I. Introduction The next generation ABWR, the ABWR-II 1-9), has been under deveopment for more than a decade in Japan by Japanese utiities represented by TEPCO and three BWR pant vendors, Hitachi, Toshiba and GE, and a BWR fue vendor, GNF. This new ABWR wi offer improved pant economy, greater safety, ease of operation and maintenance, and fexibiity in fue cyce strategy. The start of commercia operation of the ABWR-II is expected in the ate 010s when repacement of Japan s first-generation nucear power pants is anticipated. Among various features newy adopted for the ABWR-II, a 1.5 times arger -attice bunde design is particuary notabe. This increases the number of fue rods per unit cross section, thus increasing pant output to 1700 MWe without increasing the average inear heat generation rate and the average heat fux. This merit of scae is expected to give the ABWR-II approximatey 15% ower power generation costs compared with the ABWR. Owing to the arger bunde design, the number of bundes is reduced to amost haf of that of the ABWR. This reduction is expected to contribute to shortening the refueing outage period. Aong with deveopment of the basic bunde design, an advanced bunde design taking advantage of the arger bunde is under deveopment to further improve fue economy and operabiity. The main feature of this bunde is use of spectra shift rods (SSRs) 10). The SSR is a new type of water rod, in which the water eve is naturay deveoped during operation and changed according to the cooant fow rate through the channe. Therefore the average moderator density, which is directy reated to the core reactivity, can be controed over a wide range by the core fow rate. An increase in the in-channe area of the ABWR-II bunde affords a arge voume for the SSRs, and hence it can * Corresponding author, Te , Fax , E-mai: masanao_moriwaki@pis.hitachi.co.jp enhance this effect. By incorporating the SSRs into the ABWR-II, it was shown that operation with a contro rods withdrawn throughout the entire operation cyce was feasibe without increasing the maximum core fow rate. 9) It is aso shown that the uranium saving factor of around 6% against the reference ABWR-II core using conventiona water rods can be expected due to the higher spectra shift effect. 9) In this paper, a nove core design to eicit better performance of the SSRs is studied as a means of further enhancing the spectra shift effect. II. ABWR-II Core Apping SSR 1. ABWR-II SSR core and bunde design The specifications of the ABWR-II core are isted in Tabe 1 and configuration of the core is dispayed in Fig. 1. The therma output of the ABWR-II core is 4960 MW, which is 1.6 times arger than that of the ABWR core. Athough the former has 44 bundes, which is 49% of that of the ABWR, there are 197 contro rods (CRs), about the same as in the ABWR. This is because the ABWR-II empoys the -attice, in which a bunde faces two CRs with a four faces, in order to maintain the cod shutdown margin (CSDM). Targeted operation cyce ength and average discharge burnup are set to be 18 months and 60 GWd/t, respectivey. The fow window is 0%, in which the maximum and minimum core fow rates are set to 10% and 100%, respectivey. The main characteristic of the ABWR-II bunde is 1.5 times arger bunde pitch compared to the conventiona BWR bunde. This arge bunde comprises four sub-bundes, which are separated by partitions, and each sub-bunde consists of an 8 x 8 fue arrangement. In the reference design 6), which is shown in Fig. (a), the

2 Tabe 1 Main Parameters for ABWR-II Core Item Unit ABWR-II ABWR Water rod Eectric power MWe Reactor therma power MWt Operating cyce ength months Water Box Average discharge burnup GWd/t Maximum core fow rate t/h 6.1x x10 3 Partition Active core height m Fue bunde pitch cm Number of contro rods (a) Reference bunde Number of fue bundes SSR Partition (b) SSR bunde Fig. ABWR-II bunde configurations Fue bunde (44) Fig. 1 ABWR-II core configuration Contro rod (197) four sub-bundes are separated by a partition and there is a arge water box in the center, formed by parts of the partition, which occupies 4 fue rod positions. In addition, there are eight sma water rods, each equivaent in size to the fue rod. There are severa bunde designs for SSR. 9) One of the designs, which is shown in Fig. (b), has 8 circuar SSRs, each of which occupies 4 fue rod positions. In this paper, we discuss the improvement of the SSR core design based on this SSR bunde design. Aso, the reference core design with the reference bunde is referred to in comparison.. Principe of SSR The SSR is a component intended to be used in ieu of the water rod, in which the water eve deveops naturay during operation and changes according to the recircuation fow rate through the channe. The configuration and mechanism of the SSR are iustrated in Fig. 3. The SSR consists of a arge ascending path and a narrow descending path. The cooant enters the SSR ascending path from beow the ower tie pate (), goes up to the top, comes down in the descending path, and goes out right above the. With this structure and a tight inet, the water fow rate into the SSR is so sma that the water heated by the irradiation of gamma rays and neutrons reaches the saturation temperature and bois in the SSR. Since the water veocity is very sma in the ascending path, a water eve deveops there. The static head of the water beow the water eve in the SSR, P S (Pa), can be expressed as = ρ g ( L), (1) P S where ρ = average water density (kg/m 3 ) g = gravitationa acceeration (m/s ) = height of the water eve from the SSR inet (m) L = height of the ascending path (m).

3 Fue Rod Ascending Path Descending Path Vapor V = cooant veocity at (m/s) A = fow area at (m/s). Eqs. (), (3) and (4) eads to W ρg A ρ, (5) Since the part from the water eve to the SSR exit is fied with vapor and its discharge veocity is quite sow, the pressure difference there is negigibe. Therefore, the pressure difference between the SSR inet and exit can be aso expressed by Eq. (1). On the other hand, the pressure drop outside of the SSR between the SSR inet and exit can be expressed by the sum of the oca pressure drop at the, P, and the pressure drop between the top surface of the and the SSR exit, P F. Thus the foowing equation is estabished: P P + P. () S F In this equation, if the SSR exit is ocated ow enough, P F can be ignored as P S P. (3) P is, on the other hand, expressed as foows: P Water Leve Density Head Baance Pressure Difference CONFIGURATION = ρv = Exit where = pressure drop coefficient ρ = cooant density (kg/m 3 ) W = channe fow rate (kg/s) Inet W A ρ, (4) γheat MECHANISM Lower Tie Pate Cooant Fig. 3 Configuration and operationa principe of SSR namey, the SSR water eve height is neary proportiona to the second power of the channe fow rate. Usuay it is desirabe that the water eve is highest, which means that the SSR is fu of water, at the maximum channe fow rate. This is because the reactivity at the end of cyce (EOC), when the channe fow rate becomes argest, can be maximized. Then, the desirabe pressure drop characteristics of the can be obtained in the foowing reationship with the maximum channe fow rate during the operation cyces, W MAX : A gl = ρ ρ. (6) W MAX Then, an SSR water eve change reative to the ength of the SSR height, r, is expressed with W MIN for the minimum channe fow rate during the operation cyces and MIN for the SSR water eve at that time as r MIN WMIN = 1 = 1. (7) L W MAX For instance, if the maximum and minimum fow rate is 10% and 100%, respectivey, then the SSR water eve change becomes III. Towards an Improved SSR Core Design 1. Uniform SSR Core We assumed the unit channe in the previous chapter. However, we have uneven fow distributions in actua cores. In other words, W MAX and W MIN are different in every channe. The fow distribution is derived from the power distribution in a BWR; namey, a channe with higher power tends to have a ower fow rate due to the higher pressure drop in two-phase fow. As ong as uneven power distribution exits, uneven fow distribution cannot be avoided. Therefore, in actua core design, it must be determined which W MAX shoud be used when the design is based on Eq. (6). From the viewpoint of neutron economics, the reactivity at EOC shoud be maximized, thus, a of the SSRs shoud be fu of water at EOC. In this sense, W MAX in the channe that has the minimum channe fow rate in the core shoud be used for Eq. (6), because then the SSRs in any other channes become fu at EOC. We set this W MAX as W MAX,MIN,

4 yieding A ρglρ =. (8) W MAX, MIN If we appy this pressure drop characteristic to a s, which we ca a uniform SSR core hereafter, then the SSR water eve change for a channe N, W MiX,N, is expressed with the minimum fow rate for this channe during the operation cyces as rn, = 1 WMIN, N. (9) W MAX, MIN The maximum fow rate during the operation cyces for the channe N, W MAX,N, is aways greater than W MAX,MIN except for N=MIN, thus r,n is ess than the idea water eve change, r. Due to this effect, the average SSR water eve change tends to be much ess than the one expected from the core fow rate change during the operation cyces. We expain this effect fuy, based on the resut of an equiibrium core anaysis. Figures 4, 5 and 6 show the power, fow and SSR water eve distribution in the SSR core, respectivey, at the midde of cyce (MOC) and EOC when the core fow rate is minimum and maximum, respectivey. Here, we excude bundes facing the outer boundary in these figures and from further discussion, since we usuay use different inet orifice settings for them so that channe fow rates there are aways ow compared with others. Taking these bundes into consideration in this anaysis severey reduces the apparent benefit of the SSR, whie excuding them does not affect neutron economics due to their ow neutron importance, even though SSR water eves are aways ow there. Channe powers in the periphera region are ower than those in the center region throughout the operation cyce, which causes channe fow rates there to be kept high. The SSR water eves are set so that amost a of them are positioned at the top of the SSR at EOC as seen in Fig. 6(b). Because channe fow rates in the periphera region are high even when core fow rate is minimum, the SSRs are amost fu of water in the region at MOC as seen in Fig. 6(a). This resuts in very itte SSR water change in the periphera region as seen in Fig. 6(c). Normaized channe power Normaized channe power (a) Midde of cyce Normaized channe fow rate Normaized channe fow rate (a) Midde of cyce (b) End of cyce (b) End of cyce Fig. 4 Channe power distribution of uniform SSR core (1/4core w/o most periphera bundes) Fig. 5 Channe fow distribution of uniform SSR core (1/4core w/o most periphera bundes)

5 Normaized SSR water eve Normaized SSR water eve SSR water eve change (a) Midde of cyce (MOC) (b) End of cyce (EOC) (c) EOC - MOC Fig. 6 SSR water eve distribution of uniform SSR core (1/4core w/o most periphera bundes) Figure 7 dispays the SSR average water eve changes during the operation cyce. It aso dispays the additiona ine that represents the idea SSR water eve change, which is proportiona to the second power of the core fow rate. The anayzed SSR water eve change is imited to /3 of the idea one due to the periphera region effect, which means that the fu effect of the SSR is reduced.. Two-region SSR Core If we simpy adjust every channe s pressure drop characteristics for its maximum fow rate, we coud increase the SSR water eve change. Namey the pressure drop characteristics for the channe N woud become Average SSR water eve * (reative to the SSR height) A gl = ρ ρ W, N MAX N. (10) Idea water eve *:Average except most periphera bundes 0 Fig. 7 SSR water eve change on uniform SSR core However, an is attached to a bunde, and the bunde goes through severa operation cyces in its ifetime; namey when it is young, channe power is high and channe fow is ow in genera, and vice versa. Therefore it is difficut to adjust the pressure drop characteristics to suit each operation cyce. In the ABWR-II core, on the other hand, a minimum shuffing strategy is adopted in order to minimize the refueing time, in which a bunde stays in the same position during the first three operation cyces, and it is shuffed ony for the fourth operation cyce (and the fifth cyce for some of the bundes). The non-shuffing area in this anaysis is iustrated in Fig. 8. In this minimum shuffing strategy, bundes oaded in the Minimum shuffing zoning n :Non-shuffing area (first three cyces) :Thrice burned bunde (shuffed) :Four-times burned bunde (shuffed) :Most periphera bundes (excuded from the discussion in this study) setting 4 :High power channe region 4 :Low power channe region Fig. 8 Core oading map on 1/4 core C L C L

6 periphera region experience ow channe power throughout their ifetime. Athough they are shuffed eventuay, they are then too od to produce high channe power. Hence, our approach to improving the SSR core is to estabish two regions in the core, a ow channe power region in periphera ocations and a high channe power region esewhere, and set different SSR water eve settings according to their maximum channe fow rates. More specificay, if we set W MAX,MIN,L and W MAX,MIN,H for the owest channe fow rates at EOC in the ow and high channe power regions, respectivey, we can use the foowing equations to set the pressure drop characteristics for both regions: ρglρ = A L WMAX, MIN, L for the ow channe power region, (11) ρglρ = A H WMAX, MIN, H for the high channe power region. (1) Based on this approach, the SSR water eve change in the ow channe region can be expected to expand, thus it increases the SSR average water eve change in the core. In this paper, we ca this approach a two-region SSR core. IV. Characteristics of Improved SSR Core In order to show the improvement of the two-region SSR core for the ABWR-II, the equiibrium core performances were evauated and compared with the uniform SSR core and the reference core design. An identica oading pattern was appied to a of the cores to remove the infuence of the oading pattern difference. The SSR region setting used for the two-region SSR core is aso shown in Fig.8. Tabe summarizes core characteristics for three designs. The bunde design for each core is briefy summarized as we. The enrichment distribution and Gd rods arrangement for the SSR bundes were set so that their oca peaking factors coud stay in the aowabe range considering the maximum inear heat generation rates (MLHGR). Here, the maximum enrichment was set to be 5.9 wt% in order to bring future higher burnup designs within the scope of the present study. Consequenty, average enrichment vaues of the SSR bundes for which oca peaking factors tended to be higher, were ower than that of the reference bunde. Athough the arrangement of Gd rods is the same between SSR bundes, the Gd density distribution was adjusted so as to obtain the controabe reactivity provided by the recircuation fow in each design. Figure 9 shows the variations fow rate that coud maintain criticaity throughout the operation cyce with a contro rods withdrawn from the SSR cores. The fow rate variations in both cores showed amost the same curve and stayed in the range of the 0% fow window, which meant circuation fow coud contro the core reactivity without the hep of CR operation. Figures 10 (a) and (b) show the SSR water eve distribution for the two-region core at MOC and EOC, respectivey. They can be directy compared with Fig. 6, which is for the uniform SSR core. In the two-region core, the SSR water eves in the periphera region at MOC stayed ow compared with those in the uniform core, which resuts in increasing the water eve change in the region as shown in Fig. 10 (c). In order to see this effect more ceary, we ooked at the trend of the average SSR water eve in the ow channe power region during the operation cyce, which is shown in Fig. 11. It was found that the average SSR water eve change in that region with the two-region SSR approach Tabe Core Cacuation Resuts Item Unit Reference core Uniform SSR core * Two-region SSR core * Average enrichment ** wt% Average Gd enrichment / # of Gd rods wt% / - 4.3/60 4.7/5 4.5/5 Discharge burnup GWd/t Uranium saving factor *** % (base) 6 8 CSDM %Δk MLHGR kw/ft *: Operation with a contro rods withdrawn **: Incuding upper and ower banket regions. ***: Cyce ength effect is considerd.

7 Core fow rate (reative vaue) Upper fow imit Lower fow imit 95 Fig. 9 Core fow change on SSR core operations with a contro rods withdrawn was more than twofod compared with the uniform SSR core. Conversey, the average SSR water eve in the high channe power region was not affected by this approach as seen in Fig. 1, because Eq. 11 is essentiay equa to Eq. 8. As a resut, the average SSR water eve change in the whoe core increased except in the bundes facing the outer boundary as seen in Fig. 13. Owing to the enhancement of SSR function, the excess reactivity, which is defined as obtainabe reactivity at the maximum fow rate, was arger in the two-region SSR core than in the uniform SSR core as seen in Fig. 14. This ed to the two-region SSR bunde design that needed ower Gd density to contro the excess reactivity compared with the uniform SSR core as shown in Tabe. Compared with the reference core, the cyce exposure in the SSR cores coud be proonged due to the spectra shift effect of the SSRs. By taking account of initia enrichment and cyce ength differences, the uranium saving factor for the uniform SSR core against the reference core was estimated to be 6%. On the other hand, that for the two-region SSR core was estimated to be 8%, which was about a 5% increase. The enhanced SSR spectra shift effect and ower Gd density design gave the two-region SSR core a superior uranium saving factor compared with the uniform SSR core. To study the feasibiity of the two-region SSR core, other important core characteristics were evauated and are summarized in Tabe in comparison to the uniform SSR core. For the CSDM, vaues of more than % k, which is the design criterion, were confirmed for the both SSR cores. The two-region SSR setting did not affect the channe peaking factor curves as seen in Fig. 15. On the other hand, the axia power peaking factor for the two-region SSR core became greater than that for the uniform SSR core. Axia power distributions at BOC and EOC are shown in Fig. 16. At BOC and MOC, the power peak was ocated at the bottom haf of the core for the SSR cores because axia heterogeneity of the moderator density woud be enhanced by the SSRs. For this reason, burning of the upper portion was deayed, and at EOC, when the water eve rose to the top of the SSRs, the power peaking in the upper portion increased. This effect was arger in the two-region SSR core due to the enhanced SSR effect. However, MLHGR for the two-region SSR core stayed the same as the uniform SSR core, which was beow 1 kw/ft and had sufficient margin for the design criterion, which was 13.4kW/ft. From a viewpoint of manufacturabiity, a fundamenta mechanica design study showed that both of the pressure drop characteristics in two SSR regions were feasibe by adjusting the size of hoes in the s. In summary, it was found that the two-region SSR core has a potentia to improve fue utiization effectivey during operation with a contro rods withdrawn for an ABWR-II core. Normaized SSR water eve Normaized SSR water eve SSR water eve change (a) Midde of cyce (MOC) (b) End of cyce (EOC) (c) EOC - MOC Fig. 10 SSR water eve distribution of two-region SSR core (1/4core w/o most periphera bundes)

8 Average SSR water eve * (reative to the SSR height) *:Average in ow channe power region 0 Fig. 11 Comparison of SSR water eve change in ow channe power region Average SSR water eve * (reative to the SSR height) *:Average in high channe power region 0 Fig. 1 Comparison of SSR water eve change in high channe power region Average SSR water eve * (reative to the SSR height) *:Average except most periphera bundes 0 Fig. 13 Comparison of average SSR water eve change (except most periphera bundes) Excess reactivity (% k) * *:Reactivity obtained by maximum fow rate 0.0 Fig. 14 Controabe reactivey by SSR V. Concusion This paper examined how a nove core design can improve the performance of spectra shift rods (SSRs) by enhancing the spectra shift effect in an ABWR-II reactor. In the conventiona uniform SSR core design, bundes paced in periphera ocations, excuding ones that faced the outer boundary, had ower power and higher fow rate compared with other bundes. This made the SSR water eve at these ocations remain at a higher position no matter how the core fow changed; thus SSR water eve changes caused by the fow tended to be imited to within a sma range there. In the proposed new SSR core design, two types of SSR bundes, in which settings for the SSR water eves were different, were utiized and oaded according to fow distribution in the core. This two-region SSR core design aowed the average SSR water eve to change widey, thus its fue economy was improved whie operation with a contro rods withdrawn was aowed. Enhancement of SSR function on the two-region SSR core increased uranium saving factor by about 5%, which was about 8% compared to 6 % of the conventiona uniform SSR core. This nove SSR core design is expected to increase the attractiveness of the ABWR-II as the next generation nucear reactor. For the future deveopment pan, therma-hydrauic tests for the SSR under actua high temperature and high pressure conditions are being schedued. Acknowedgment The authors wish to acknowedge the support and guidance of the foowing companies for making this study possibe: Tohoku Eectric Power Company, Chubu Eectric Power Company, Hokuriku Eectric Power Company, Chugoku Eectric Power Company, Japan Atomic Power Company and Eectric Power Deveopment Company.

9 Channe peaking factor Axia Position (Reative to core height) Fig. 15 Comparison of channe peaking factor variations Normaized Axia Power EOC BOC Fig. 16 Comparison of axia power distributions References 1) A. Omoto, The Japanese Utiities Requirements of Next Century BWR, Proc. 3rd Int. Conf. on Nucear Engineering (ICONE-3), yoto Japan (1995). ) L. E. Fennern et a., Core and Transient Design for a BWR of the Next Century, Proc. 3rd Int. Conf. on Nucear Engineering (ICONE-3), S08-, yoto Japan (1995). 3) R. Yoshioka et a., Core and Transient Design of a BWR for the Next Century, Proc. 4th Int. Conf. on Nucear Engineering (ICONE-4), Vo., p109, New Oreans LA. (1996). 4) M. Aoyama et a., Optimization of Core Design for the Next Generation BWR, Proc. 5th Int. Conf. on Nucear Engineering (ICONE-5), No-636, Nice France (1997). 5) M. Aoyama et a., Toward Enhanced Fexibiity in the Fue Cyce For ABWR-II Core Design, Proc. 6th Int. Conf. on Nucear Engineering (ICONE-6), No-656, San Diego CA. (1998). 6). Yamada et a., Core and Dynamic Characteristic Design for ABWR-II, Proc. 7th Int. Conf. on Nucear Engineering (ICONE7), No.-746, Tokyo Japan (1999). 7) T. Anegawa et a., The Status of Deveopment Activities of ABWR-II, Proc. 9th Int. Conf. on Nucear Engineering (ICONE9), No.-377, Nice France (001). 8) M. Moriwaki et a., ABWR-II Core Design with Spectra Shift Rods for Operation with A Contro Rods Withdrawn, Proc. Int. Congress on Advanced Nucear Power Pant (ICAPP), Session 6.0, Hoywood FL. (00). 9) H. itamura et a., ABWR-II Deveopment: The Economicay Competitive 1700MWe BWR with Large Bunde Concept, Proc. 003 Int. Congress on Advanced Nucear Power Pant (ICAPP 03), Session 5, Córdoba Spain. (003). 10) O. Yokomizo et a., Spectra shift rod for the Boiing Water Reactor, Nucear Engineering and Design, 144, 3-36 (1993).