Parametric Study on Fast Reactors with Low Sodium Void Reactivity by the Use of Zirconium Hydride Layer in Internal Blanket

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1 Jornal of Nclear Science and Technology ISSN: (Print) (Online) Jornal homepage: Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity by the Use of Zirconim Hydride Layer in Internal Blanket Masami RACHI, Toshihisa YAMAMOTO, Akshay Kmar JNA & Toshikaz TAKDA To cite this article: Masami RACHI, Toshihisa YAMAMOTO, Akshay Kmar JNA & Toshikaz TAKDA (1997) Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity by the Use of Zirconim Hydride Layer in Internal Blanket, Jornal of Nclear Science and Technology, 34:2, , DOI: 1.18/ To link to this article: Pblished online: 15 Mar 212. Sbmit yor article to this jornal Article views: 76 View related articles Citing articles: 4 View citing articles Fll Terms & Conditions of access and se can be fond at Download by: [ ] Date: 16 December 217, At: 13:42

2 Jornal of NUCLAR SCINC and TCHNOLOGY, Vol. 34, No. 2, p (Febrary 1997) TCHNICAL RPORT Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity by the Use of Zirconim Hydride Layer in Internal Blanket Masami RACHP 1, Toshihisa YAMAMOTOP, Akshay Kmar JNAt' and Toshikaz TAKDA Department of Nclear ngineering, Gradate School of ngineering, Osaka University* (Received May 13, 1996), (Revised Agst 28, 1996) Downloaded by [ ] at 13:42 16 December 217 A stdy on the sodim void reactivity redction of 1, MWe class fast reactors was performed. The target of this stdy is to materialize a core concept with a zirconim hydride layer in the internal blanket which satisfies the design criteria; that the possibility of the prompt critical by sodim voiding can be exclded at any time dring reactor operation; and that the brnp reactivity is acceptably low, which is less than 2.5%dk/kk'. A series of parametric srveys were performed on the radial size of the intera blanket _and he width of the zirconim hydride layer. The width of the layer was set to 3. em as the optimized specification which satisfies all the design criteria regardless of the fel materials; oxide, nitride and metal. Transient behaviors of the cores were investigated dring the nprotected loss of flow accident (LOF). The analyses showed that the oxide core was able to avoid sodim boiling. It was also observed that the cores with nitride and metal fels had larger tolerance against the nprotected LOF: The otlet temperatre were lower by 39 and 6 c compared with the oxide core. KYWORDS: fast reactors, mixed oxide fels, mixed nitridesfels, liqid metal fels, transient analysis, nprotected loss of flow, zirconim hydride layer, internal blankets, sodim void reactivity, brnp reactivity I. INTRODUCTION The se of sodim as coolant in the fast breeder reactors provides a benefit from the point of heat removal, however, the positive reactivity de to voiding have been considered as a drawback from the point of reactor safety. Many design stdies have been performed with the aim for redcing the positive sodim void reactivity. Among these, the introdction of the pper sodim plenm< 1 l and the se of zirconim hydride (ZrH1.7) as the moderator< 2 l have been spported by many athors as effective measres to redce void reactivity withot sffering specific drawbacks in economy. These measres, however, have also demerits in core characteristics. The former impedes the breeding in the pper axial blanket and reslts in poor breeding gain. The latter sffers from the same problem throgh the decrease in the rj vale de to spectrm softening. These problems are normally observed as increase of * Yamadaoka, Sita-shi 565. l1 Present address: Toden Software, Inc., Shinbashi, Minato-k, Tokyo 15. t 2 Corresponding athor, Tel , Fax , -mail: toshi@ncl.eng.osaka-.ac.jp t 3 Present address: Indira Gandhi Centre for Atomic Research, Kalpakkam, INDIA. brnp reactivity, and decrease in breeding ratio. For the latter, there is also a concern that the fel adjacent to the moderator tends to have a local power peak. Recetly, we have proposed a new core concept which has the zirconim layer in an internal blanket<'l< 4 l. In the stdy, it was fond that the ZrH1.7 layer in this position not only has a large effect in redcing the sodim void reactivity bt also eliminates the local power peak problem. A parametric stdy was also done, however, only for the radial size of the internal blanket. The comparison of the core characteristics was limited to the static basis: sodim void reactivity and power distribtion. In this stdy, we attempt to perform a more extensive parameter srvey to optimize the core concept. The width of the ZrH1. 7 layer was also added as one of the parameters. In addition, some aspects were newly introdced in the optimization stdy. The first aspect is abot the pper limit of the sodim void reactivity. It normally becomes more positive with brnp. In the present stdy, we have set a goal to keep down the sodim void worth at the end of eqiblim cycle (OC) to less than 1$, so that the prompt critical does not occr by sodim voiding at any time dring reactor operation. This condition is necessary if one wishes to exclde the possibility of prompt criticality at the reactor conditions when Doppler feedback is 193

3 194 M. RACHI et al. Downloaded by [ ] at 13:42 16 December 217 not expected immediately after the positive reactivity insertion. The passage of large amont of gas throgh the core dring a short period at zero power criticality is sch an example. The second aspect is the se of advanced fels, sch as nitride and metal. It is widely known that these materials have better featres against nprotected LOF than oxide thanks to the high thermal condctivity. However, the se of these fels also has a potential of large positive sodim void reactivity and it may be more difficlt to achieve the goal set above. The third aspect is the well-balanced core design between the sodim void reactivity redction and other core characteristics: breeding ratio, brnp reactivity, and so on. The increase of brnp reactivity may reqire a large control rod worth which leads to an nrealistic nmber of control rods in the core. In this stdy, the pper limit of the bnrp reactivity was set to 2.5%dk/kk', which is the same level as the fast prototype reactor MONJU. The goal of the present stdy is to find the soltion of the core configration which satisfies all the three aspects of optimization by the se of ZrH layer in the internal blanket. The difference in tendency among the different fel material will be also discssed. II. CALCULATIONAL MODL 1. Basic Core Configration Figre 1 shows the basic core configration for the parametric stdy. The core height is 1 em and the core radis is 17 em. The sodim plenm is placed at the pper region of the core in order to redce sodim void reactivity. The internal blanket is placed at the axial level 5 em lower than the mid-plane: this configration allows the increase of netron flow into the internal blanket and the pper sodim plenm, and then the frther redction of sodim void reactivity<s). The ZrH layer is placed at the mid-plane of the internal Z(cm) 27. GasPlcmm J s.c... o f l Upper Sodim Plenm (1) - Ul Internal Blanket: - - HIO.O ' Oter Radial J Stainless Shielding Shielding Level... A (12'-) B f i, Axial Blanket &. f Gas Plenm. L... _.... _ _, B t l R(cm) Fig. 1 Basic core configration of parametric stdy and the three axial levels for the comparison of radial power distribtions blanket. By several athors, attempts have been made to redce sodim void reactivity by ptting the ZrH1. 7 layer at the core and blanket bondaries< 2 l-< 4 H 6 l. Or configration is sperior to those: the interference effect of the layer to the core is relieved, and the netron absorption in the internal blanket is enhanced only when the netron flow into the internal blanket increases de to core voiding< 3 l. This enables better core characteristics dring the normal operation. The inner core was divided into two sb-regions with different P enrichments: the lower inner core (LIC) and the pper inner core (UIC). The P enrichment of each fel region, LIC, UIC and oter core (OC) is so determined that the peaking factor of axial and radial power distribtions are both minimized. The procedre is described as follows. Firstly, a niform P enrichment in assigned to the three core regions and find the enrichment when the critical condition at the OC is satisfied. Secondary, the ratio of the enrichment in LIC and UIC is modified ntil the axial power peaking factor at the BOC is minimized. Thirdly, the enrichment of OC is adjsted so that the radial power peaking factor at BOC is minimized. As mentioned earlier, the se of moderator cases decrease in breeding ratio. The se of internal blanket can compensate for the loss to some degree, however, increase of the internal blanket volme cases decrease of fel inventory provided that the total core volme is constant. This directs to increase of brnp reactivity. On the contrary, the effect on sodim void reactivity redction increases as the internal blanket volme becase the netron flow into the internal blanket increases. So, the width of the internal blanket shold be so determined that the two opposite conditions are satisfied at the same time. In this stdy, the width of the internal blanket was fixed to 3 em, which was determined from the optimization stdy of the internal blanket size with no ZrH1. 7 layer<s). The radial size of the ZrH1.1 layer was set to the same size as the internal blanket. This choice was done from a previosly made parametric stdy on the radial size: the increase of the radis is effective for sodim void reactivity redction, however, not for brnp reactivity redction. Moreover, it was fond that the effect of the radial size increase satrates over 11 cm< 7 l. 2. Parametric Srvey Calclations The 7 grop cross sections were prodced sing the CASUP< > code from the fast reactor grop constant set JFS-3-J3.2 based on JNDL-3.2< 9 ). The effective micro cross sections for each core region was collapsed into 16 grop and 6 grop sing the region average flx obtained from a 7 grop RZ diffsion calclation. The grop strctre is shown in Table 1. The brnp calclations were done in 6 grop sing the CITATION< 1 > code. The P enrichment of JOURNAL OF NUCLAR SCINC AND TCHNOLOGY

4 Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity 195 Downloaded by [ ] at 13:42 16 December 217 Table 1 nergy grop strctres of 6 and 16 grop 6 grop 16 grop Upper Lower Lethargy energy energy width (MeV) 6.7 (MeV) (kev) (kev) (ev) (ev) l.ox each fel region was so adjsted that the criticality ( keff) at the end of cycle satisfies the condition 1.<keff<l.2. The brnp procedres were repeated 5 times ntil the eqilibrim core was obtained. The pltonim composition of P 239/4/41/42 is 58/24/14/4 (wt%). The parametric srvey was performed for each fel material, regarding the width of the ZrH1. 7 layer as the parameter. The parametric srvey on the radial size of the internal blanket was done only for oxide core. The sodim void reactivity was calclated based on the 16 grop diffsion exact pertrbation theory. 3. Unprotected LOF Analyses Transient behavior dring the nprotected LOF was investigated for the optimized configration obtained in the parametric srvey calclations. The flow rate dring the nprotected LOF was described with the following eqation< 11 l: W = Wo/(1 + tjt1), where Wo is the nominal flow rate and t 1 the flow-halving time, which is 5.5 s. As the feedback reactivity, the following components were considered; thermal expansion of fel and sbassembly strctres, Doppler reactivity, axial expansion of control rod drive line, and the change of coolant density. For the last component, axial distribtion of the sodim void reactivity was taken into accont. The axial distribtion for a given voiding rate is extrapolated sing a non-linear fitting eqation from the exact pertrbation reslts for the three voiding states; 1, 4 and 1%. The negative reactivity de to thermal expansion of core spport plate is considered to be large in magnitde. However, the time delay before this effect comes into action is strongly dependent on the primary cooling system design; ths general discssions wold be mch more difficlt. So in this stdy, the inlet temperatre dring the nprotected LOF was regarded as nchanged from the nominal. This condition is conservative for the nprotected LOF. The fel/coolant temperatres, reactivity, and reactor power were evalated based on the one point reactor approximation. ill. RSULTS AND DISCUSSIONS 1. Parametric Srvey Calclations In this stdy, the design targets are specified as follows: firstly, total reactivity is less than 2.5%dk/ kk' for one fll year operation, and thirdly, fast flx flence is less than 3x 1 23 n/cm 2 after the three years of irradiation, which is considered to be a realistic criterion for cladding materials available at present. The other design specifications were taken from the previos stdies<' 2 l<' 3 l. The principal parameters are listed in Tables 2 and 3. For nitride fel, the 15 N isotopic ratio was assmed as 1%. Fel bonding is sodim for metal, helim for the others. (1) Internal Blanket Radis This parametric stdy was done only for oxide core. The width of the ZrH1. 7 layer was fixed to 1. em. Table 4 shows the change of core characteristics de to the internal blanket size. Sodim void reactivity and breeding ratio decrease as the internal blanket size increases. On the contrary, brnp reactivity and fast flx flence show a different tendency: the both reveal minimm peaks arond the point when the radis is rv13 em. To make this point clear, the radial power distribtions were compared among the three different internal blanket radises; 7, and 165 em, which reslts are shown in Fig. 2. The comparison was made for the three axial levels of the core, which positions are marked in Fig. 1. In the case with 7 em radis, the power peak appears near the inner/ oter core bondary (Fig. 2(a)). As the radis increases, the location of the Table 2 Principal core parameters Reactor power (MWth) 2,6 Core height (em) 1 Axial blanket width (em) 3 Radial blanket width (em) 29.8 Nmber of sbassemblies Inner core 223 Oter core 132 Radial blanket 15 Control rod 24 Shielding 51 Operation cycle (days) 365 Residence time (cycles) Core 3 Blanket 4 VOL. 34, NO.2, FBRUARY 1997

5 196 M. RACHI et al. Table 3 Principal parameters of the core and blanket sbassemblies Item Oxide Nitride Metal Item Oxide Nitride Metal Core Radial Blancket Fel material (Uo.aPo.2)2 (U. 8 P. 2 )N U-15P-1Zr Fel material o2 UN U-1%Zr Theoretical Theoretical density (g/cm 3 ) density (g/cm 3 ) Smear density (%) Smear density (%) Nmber of pins Nmber of pins per sbassembly 271 <-- <-- per sbassembly <-- Pin diameter (mm) <-- Pin diameter (mm) <-- Clad thickness (mm).5.4 <-- Clad thickness (mm).4.5 <-- Pellet diameter (mm) <-- Pellet diameter (mm) <-- Volme fraction (vol%) Volme fraction (vol%) Fel <-- Fel <-- Strctre <-- Strctre <-- Coolant+Gap <-- Coolant+Gap <-- Downloaded by [ ] at 13:42 16 December 217 Table 4 Core characteristics as a fnction of internal blanket radis 1 IB radis Brnp reactivity Sodim void reactivity Breeding Fast flx fiencett (em) (%dk/kk') at BOC (%dk/kk') ratio (1 23 n/cm 2 ) Oxide core with 1. em ZrH1. 7 layer tt After 3 years irradiation.,., ,..._..._ Level B (ZrH 1.1) I..._..._..._ \ r---1 c: c: c:.2.2 Level A (UIC) - L- 1 ;;; Level A (UIC) \ I L-1 "" "" 1,.! ' : I... I --- :::. --r ' : :::. :::. _ :el C (LIC) '-- RB c: c:., c:.,., IC I oc -c -c Level B (ZrH) -c....,....,...., I I IC IC /?_ o,_ o,_ Distance from core center (em) Distance from core center (em) Distance from core center (em) (a) 7cm (b) 127.5cm (c) 165cm Fig. 2 Radial power distribtion at varios axial levels as a fnction of internal blanket radis peak moves to oter region and the magnitde of the power peak decreases (Fig. 2(b)). However, when the radis exceeds a certain point, the power peak moves to the core center and the magnitde increases again (Fig. 2(c)). This also explains the tendency of fast flx flence. From this fact, it may be conclded that the redction of brn p reactivity arond 13 em radis was broght by the flattened radial power distribtion. Figre 3 shows the magnitde of each component of sodim void reactivity at the BOC as a fnction of internal blanket radis. As the radis increases, the leakage component is enhanced while other components are almost constant. From this reslt, it is clear that the redction of sodim void reactivity is de to the increase of netron leakage into the internal blanket. The minimm radis which reveals the negative void reactivity falls arond 11 em. From Table 4, arond 13 em radis seems to have the best core characteristics. This conclsion is the same as the previos srvey on the same configration withot JOURNAL OF NUCLAR SCINC AND TCHNOLOGY

6 Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity 197 Downloaded by [ ] at 13:42 16 December 217..X: ' - ti!ii CJ) a: -o c; -2 > -3 ::J -4 C/) o Leakage --o--o.-o. _ -o IB Radis (em) ---o Fig. 3 Magnitde of each component of sodim void reactivity as a fnction of internal blanket radis the ZrH1.7 layer<sj. (2) ZrH1.7 Layer Width This stdy was performed for each fel material. The width of the ZrH1. 7 layer was changed from 1. to 4. em and the change of core characteristics were investigated. Strictly speaking, the optimized internal blanket radis is dependent on ZrH1.1 layer width, however, the radis was fixed to 13 em for simplicity. Table 5 smmarizes the reslts. Brnp reactivity increases and sodim void reactivity decreases as the increment of the width. The increase of brnp reactivity is explained as follows: the increase of the ZrH1.7 layer cases softer spectrm in core regions, which leads to low TJ vale and poor breeding ratio. The same reasoning can be applied to the explanation for the breeding ratio in Table 5. The tendencies described above are common to all the fel materials except their magnitde. For the nitride and metal fel, brnp reactivity is lower than the oxide becase of the higher heavy metal density. On the contrary, sodim void reactivity is larger becase of harder netron spectrm. The comparison of each component of sodim void reactivity is given in Fig. 4. Along with the leakage component, the spectrm component also changes as the width increases. The differences in the total reactivity among the fel materials are mainly attribted to the spectrm component. 2. Redction of Sodim Void Reactivity by the Use of ZrH1.7 Layer Table 6 shows the redction of sodim void reactivity by the se of the ZrH1.1 layer in the optimized core configration for each fel type. It is obvios that the se of the ZrH layer is effective for either fel material: the total void reactivity at the OC decreases by %dkjkk'. On the contrary, the brnp reactivity increases by.3-.81%dkjkk'. The metal fel shows the largest difference in both the sodim void reactivity Table 6 Change in sodim void reactivity and brnp reactivity by the se of 3. em thick ZrH1.1 layer 1 (a) Sodim void reactivity at BOC (%dk/kk') Oxide Nitride Metal With ZH1.1 layer (A) Withot ZH layer (B) Difference (A-B) (b) Sodim void reactivity at OC (%dk/kk') With ZH1.1 layer (A) Withot ZH layer Difference (B) (A-B) (c) Brnp reactivity (%dk/kk') With ZH layer (A) Withot ZH1.1 layer Difference (B) (A-B) t f3eff of oxide, nitride and metal are.459,.471 and.49% dk/kk'. Internal blanket radis is 13cm. Table 5 Core characteristics as a fnction of ZrH1.7 layer width 1 ZrH layer Fel Brnp reactivity Sodim void reactivity Breeding Fast flx flencett width (em) type (%dk/kk') at BOC (%dk/kk') ratio (1 23 n/cm 2 ) Oxide LO Nitride Metal Oxide Nitride Metal Oxide Nitride Metal Oxide Nitride Metal t IB radis is fixed to 13 em, tt After 3 years irradiation VOL. 34, NO.2, FBRUARY 1997

7 198 M. RACHI et al. 3 :;.: -o- o-- :;.: 3 ---o--- :;.: 4 "" Spectrm "" ::;;.:: 2 Spectrm ::;;.:: "" ::;;.:: 3 ' ' 2 ' Absorption '!!.. 2 Absorption 8=- :-::_-:--=8=-7 - -=8=-...:::B. ;.::_ Fission - > ::..._-:---:8=--;-_--;-...::B=:::- ;-:---=8 : ----o;: FISSIOn - - «S -=-o---- ==o-- «S --o «S a: -1 Total=o a: -1 Total a: -1 '5-2 '5-2 '5-2 > > > ::l ::l ::l ' ' Leakage ' o- -!-a_gg_ Leakage rjj -4 --o rjj -4 o------o------o o rjj o o------o ZrH1.7 Width (em) ZrH1.7 Width (em) ZrH1.7 Width (em) (a) Oxide (b) Nitride (c) Metal Fig. 4 Magnitde of each component of sodim void reactivity as a fnction of ZrH1.7 layer width with different fel materials Downloaded by [ ] at 13:42 16 December 217 and the brnp reactivity. It may be worth noting that the sodim void reactivity at the OC is positive bt less than 1$ for each fel material. 3. Choice of Optimized Core Specification The sodim void reactivity at OC increases by. 7-.8dk / kk' for all the fel materials and the criterion for the sodim void reactivity at OC becomes crcial for the metal core. As a matter of fact, the criterion was cleared for all the fel type only when the width is 3. em or more. On the contrary, the brnp reactivity of oxide satisfies the 2.5%dk/kk' criterion only when the width is 3. em or less. For the fast flx flence, metal does not exceed the pper limit only when the width is 1. and 3. em. As a conclsion, the choice of the 3. em width satisfies all the design conditions regardless of the fel material and this specification was sed in the nprotected LOF analyses for each fel material. The pltonim enrichment of fel zones are different for each fel material becase they are so determined that the peaking factors of radial and axial power distribtions are minimized for each case (Table 7). 4. Unprotected LOF Analyses The nprotected LOF analyses were performed for the core with the optimized internal blanket radis (13 em) and ZrH1.7 layer width (3. em), and the pltonim enrichments determined for each fel material. The analyses were all made at the BOC condition except oxide, of which OC condition was also analyzed for comparison. As seen in Table 6, the sodim void reactivity at the OC is sbstantially positive for all the fel rna- Table 7 P enrichment of the optimized cores Oxide Nitride Metal UIC LIC oc terials. This leads to higher otlet temperatre at the nprotected LOF becase the reactivity de to the sodim temperatre rise becomes more positive. On the contrary, Doppler coefficient becomes smaller at the OC: this leads to lower otlet temperatre becase the total amont of reactivity prodced dring the power decrease is smaller, ths it becomes easier to compensate with the inherent negative feedback effect of the core. These two tendencies determine whether the safety characteristic is more crcial at the BOC or OC. Table 8 smmarizes the major reactor parameters sed in the transient analysis. Figre 5 shows the comparison of the total reactivity. The nitride and metal are larger in absolte vale than oxide by.21 and.23%dk/kk', respectively. For the oxide, the reactivity became more negative at the OC. Figre 6 describes the major components of the reactivity; Doppler and fel expansion, core radial expansion, and coolant expansion. The other effects, i.e., thermal expansion of cladding and wrapper tbe, and axial expansion of control rod drive line, were small by abot one order of magnitde and were omitted from the figre. The largest difference among fel materials is observed in Doppler and fel expansion effect. The difference from oxide is as large as.84%dk/kk' for Table 8 Major parameters sed in the transient analyses t Doppler and fel expansion coefficient (core total) Oxide Nitride Metal Radial expansion coefficienttt Sbassembly strctre expansiontt Cladding Wrapper tbe Total t Unit: XlO 6 dkjkk'(c tt Core total; oxide at the BOC JOURNAL OF NUCLAR SCINC AND TCHNOLOGY

8 Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity 199 Downloaded by [ ] at 13:42 16 December 217 Fig. 5 1 Comparison of total reactivity dring the nprotected LOF among different fel materials nitride, and.92%dk/kk' for metal: they claim abot 4% of the total difference of the reactivity in Fig. 5. At the OC, the reactivity is redced to abot half of the BOC. This tendency is more distinctive than expected from the difference of Doppler coefficient at OC which is abot 2% less than BOC. The rest part is considered to be broght from the fel temperatre profile as will be described later. The radial expansion effect shows the same tendencies among the fel types except the magnitde and the sign of the reactivity. The difference from oxide is.32%dk/kk' for nitride, and.39%dk/kk' for metal. As for coolant expansion, oxide at the OC reveals negative reactivity in spite of the fact that total sodim void worth is positive. This can be explained as follows. Figre 7 shows the axial profile of the sodim void reactivity of the oxide core at BOC. It is clearly observed that the pper sodim plenm has large negative void reactivity. The coolant expansion effect is proportional to local change of sodim density. The pper plenm which is located at the pper part of the core is most sensitive to the otlet temperatre rise and ths its negative void worth has the largest weight in the coolant expansion effect. As a reslt, the negative component exceeded the positive component mainly broght from the core regions. Figre 8 compares the reactor power and flow rate dring the transient. This reslt directly reflects the total reactivity in Fig. 5. It was observed that the OC case was somewhat lower than the others dring the first 2 s. This reason may be attribted to the slight difference in reactivity at early stage which appears in Fig. 5. The oxide was always larger than the rest dring the transient. Figre 9 shows the fel temperatre change dring the nprotected LOF. It is clear from the reslt that the change of the fel temperatre is small in nitride and metal: this reslt is attribted to the thermal condctivity of the materials which are abot ten times larger than oxide. The fel temperatre of oxide is higher than nitride and metal at nominal power and the temperatre change is also larger when the reactor power is spontaneosly redced dring the nprotected LOF. For nitride and metal, not only the fel temperatre decrease de to the power change is very small, bt also the average coolant temperatre rise dring the nprotected LOF tends to impede the fel temperatre decrease. The metal reslt in Fig. 9, which shows slight increase dring the first 2 s, sggests this phenomenon. For oxide where the fel temperatre always exceeds the coolant temperatre, this phenomenon is not observed. These tendencies in fel temperatre explains well the behaviors of Doppler and axial expansion effect of each fel material: the reactivity change showed in Fig. 6 directly reflects the profile of temperatre change in Fig. 9. For the OC case, the tendency is explained from the reactor power profile in Fig. 8. Figre 1 describes the change of otlet temperatre dring the nprotected LOF. The maximm temperatres appeared at abot 5-6 s and the peak temperatre was 741, 72 and 681 c for oxide, nitride and metal, respectively. The peak temperatre did s;- '" " 6 ::::. ':::- Metal > ;:; : -1 \\ Nitride '.::: t- ' b!::: : a: (a) Doppler and fel expansion (b) Core radial expansion (c) Coolant expansion Fig. 6 Comparison of major reactivity components dring the nprotected LOF among different fel materials* (*: BOC except the oxide (OC) case) VOL. 34, NO.2, FBRUARY 1997

9 2 M. RACHI et al. 6 r Upper Sodim Plenm Downloaded by [ ] at 13:42 16 December Axial Blanket LIC Internal Blanket Axial location (em) Fig. 7 Axial profile of sodim void reactivity of the optimized oxide core at BOC 1.2 2l OS 1.,... s,...,....8., , (J OS.4 > '+l OS a; p::.2. UIC 2 ---Oxide } Oxide (OC) Reactor Power Nitride & Metal 5 1 Fig. 8 Comparison of reactor power and flow rate change dring the nprotected LOF among different fel materials not exceed the boiling point (881 'C at 1 atm), ths the coolant boiling was avoided for all the cases. The difference of peak temperatres from oxide is 39 and 6 'C for nitride and metal. For the OC case, the peak temperatre was 718 'C, which is lower by 24 'C than at the BOC. This fact, however, does not mean that the nprotected LOF is always most crcial at the BOC: the reslt will be strongly dependent of the tendency of Doppler and coolant expansion effects as already discssed. IV. CONCLUSIONS A parametric stdy was made on the sodim void reactivity redction of fast reactors with oxide, nitride or metal fel. The se of zirconim hydride layer in the internal blanket was proved to be an effective measre to satisfy low sodim void reactivity and acceptable brnp reactivity at the same time regardless of the fel type. The design criteria in this stdy were; that the possibility of the prompt critical by sodim voiding can be exclded at any time dring reactor operation; and that the brnp reactivity is acceptably low, which is less JOURNAL OF NUCLAR SCINC AND TCHNOLOGY

10 Parametric Stdy on Fast Reactors with Low Sodim Void Reactivity Ql :s co iii. 1 5q) Oxide ---II /_Nitride - / Oxide (OC) -M; Fig. 9 Comparison of hot spot fel temperatre dring the nprotected LOF among different fel materials* (*: BOC except the oxide (OC) case) p- 7 Ql :s co iii. 6 5q) Oxide (OC) Oxide I l _ / --- -: ,'>--Nitride / Metal i i 1 Fig. 1 Comparison of otlet temperatre dring the nprotected LOF among different fel materials Downloaded by [ ] at 13:42 16 December 217 than 2.5%dk/kk'. Throgh the parametric srveys on the width of the zirconim hydride layer, the optimized width of the layer was fond to be 3. em, where all the fel materials satisfy the design criteria. Transient analyses were performed on the reactor response dring the nprotected LOF. It was shown that coolant boiling was avoided for the oxide case, which revealed the largest otlet temperatre among the fel materials. The comparison was also made between the BOC and OC. As for the core configration treated in the stdy, the BOC was more crcial for the nprotected LOF inspite of the fact that the sodim void reactivity is more positive at the OC. This reslt may have been broght from the axial profile of the sodim void reactivity of the present core which has the pper sodim plenm. Finally, it was confirmed that the nitride and metal fels were able to keep the otlet temperatre considerably low, which were 39 and 6o c lower than the oxide, ths have larger safety margin against the nprotected LOF accident. -RFRNCS- ( 1 ) Matveev, V.I., et al.: Core concept of fast power reactor with zero sodim void reactivity, Proc. Int. Fast Reactor Safety Mtg., Ag , 199, (199). ( 2) ( 3) ( 4) ( 5) ( 6) ( 7) ( 8) ( 9) (1) (11) (12) (13) Jevremovic, T., et al.: J. Ncl. Sci. Technol., 3[6], (1993). Jena, A.K., et al.: Technol. Rep. Osaka Univ., 45[222], (1983). Kroishi, T., et al.: J. Ncl. Sci. Technol., 31[7], (1994). Takeda, T., et al.: J. Ncl. Sci. Technol., 3[5], (1993). Chang, Y.l., et al.: Core concepts for "Zero-sodimvoid-worth core" in metal felled fast reactor, Proc. Int. Conf. on Fast Reactor and Related Fel Cycles, Kyoto, Japan, Vol.II, 14.5 (1991). Kroishi, T.: Private commnications, (1994). Ono, S., et al.: Technol. Rep. Osaka Univ., 33[178], (1983). Nakagawa, T., et al.: J. Ncl. Sci. Technol., 32[12], (1995). Fowler, T.B., et al.: Nclear reactor core analysis code: CITATION, ORNL-TM-2496, (1971). Lewis,..: Nclear Power Reactor Safety, Chap.7, John Wiley & Sons, (1977). Lyon, W.F. et al.: Advancing liqid metal reactor technology with nitride fels, Proc. Int. Conf. on Fast Reactor and Related Fel Cycles, Kyoto, Japan, Vol.II, 14.8 (1991). Wakabayashi, T., et al.: Stdy on core characteristics of advanced LMFBR fels, Proc. Int. Conf. on Fast Reactor and Related Fel Cycles, Kyoto, Japan, Vol.II, 14.7 (1991). VOL. 34, NO.2, FBRUARY 1997