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The conceptual design of the double-pin type blanket with hollow ceramic breeder pellets for fusion experimental reactors
ELSEVIER
Fusion Engineering and Design 24 (1994) 403-412
Fusion Engineering and Design
The conceptual design of the double-pin type blanket with hollow ceramic breeder pellets for fusion experimental reactors K. Shibata a, K. Maki a, M. Otsuka a, T. Amada u, y . Ozawa b, H. Takatsu c a Energy Research Laboratory, Hitachi Ltd., 7-2-10hmika-cho, Hitachi-shi, lbaraki-ken 319-12, Japan b Hitachi Works, Hitachi Ltd., 3-1-1 Saiwai-cho, Hitachi-shi, Ibaraki-ken 316, Japan c Naka Fusion Research Establishment, Japan Atomic Research Institute, Naka-machi, Naka-gun, Ibaraki-ken 311-01, Japan Received 28 September 1993 Handling Editor: Robert W. Conn
Abstract
Several ceramic breeder blanket concepts for fusion experimental reactors have been proposed. Satisfying the allowable temperature ranges of each material and enhancing the tritium breeding ratio (TBR) are indispensable to development of any blanket concept. The present paper proposes a ceramic breeder blanket concept which realizes these points. Lithium zirconate (Li2ZrO3) is adopted as the breeder along with double-cladding pins having hollow pellets. The pellets are placed in the co-axial center tubes, while beryllium pebbles fill the outer tubes and serve not only as a neutron multiplier but also as a thermal barrier between the breeder and coolant, which flows outside the pins. The pins can be shortened and straightened by setting them in the toroidal direction. Hollow pellets are used to unify the radii of the outer clad, and to reduce the volume fraction of stainless steel by making baffle plates unnecessary between the pin rows. The optimum ratio of the Li2ZrO3 volume fraction to the beryllium plus Li2ZrO3 one is investigated to maximize the TBR, and the pin radii and pin arrangement are determined. The proposed blanket concept is seen to satisfy the temperature ranges and enhance the local TBR to 1.3.
1. Introduction Several ceramic breeder blanket concepts have been proposed for fusion experimental reactors: layered [1,2], pebble bed [1,3], and breeder in tube (BIT) [ 1,4] concepts. In the blankets, lithium oxide (Li20), lithium aluminate (LiAIO3), lithium orthosilicate (Li4SiO4), or lithium zirconate (Li2ZrO3) were used as the ceramic breeder, beryllium as a neutron multiplier, stainless steel (SS) as structural material, supporting and non-
supporting material, and water or helium gas as coolant. Achieving a net tritium breeding ratio (TBR) about unity is one of the primary design goals for the experimental reactor blanket. Since the coverage of the experimental reactor blanket is restricted to 60-70%, meeting the design goal appears to be difficult. To enhance the TBR, therefore, is essential to develop a suitable blanket. However, there is an appropriate temperature range for each material. As for the breeder, when
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Li2ZrO3 is used, its minimum temperature is restricted to 370°C to provide for efficient tritium recovery, and its maximum is about 1000°C to prevent mass transfer of the ceramic material [1]. As for beryllium and the supporting SS, their maximum temperatures are restricted to 600 to 400°C, respectively, to prevent expansion by swelling [1]. In addition, the non-supporting SS is made of cold-worked SS, and its working temperature of 500°C is permitted if swelling is to be avoided [1]. It is not so important to enhance the operating temperature, hence the fusion experimental reactor does not aim at the power production. Then water, pressurized at l MPa, is applied for the coolant under operating temperatures below 100°C in order to keep the water from boiling. Since the minimum temperature of the breeder is much higher than the maximum temperature of the coolant, it is necessary to provide a thermal barrier between them. In the three concepts there are two methods for this. One is to use the beryllium not only as multiplier but also as the thermal barrier for the sintered block or pebble bed concepts. The major concern in this approach is the possible increase in beryllium and breeder temperatures due to deformation-induced separations at the interfaces [1]. The other is to use a helium filled gap for the BIT concept. The primary concerns for this have to do with fabricating the long bent concentric tubes, while maintaining close (0.1 ram) gap tolerances [1]. Advanced methods to satisfy the temperature ranges are expected. The purpose of the present paper is to propose a ceramic breeder blanket concept to satisfy the temperature ranges and to enhance the TBR. The principal design parameters of the target reactor are as follows [1]. The major and minor radius of the plasma are 6.0 and 2.15 m, respectively. The average neutron wall loading and surface heat flux are designed as 1.0 MW m -2 and 0 . 5 M W m 2 respectively, for nominal fusion power of 1 GW.
2. Design concept We chose LizZrO3 as a breeder material from among various kinds of lithium ceramic for the
following reason. The Li2ZrO3 exhibits the least swelling of all the candidate ceramics under irradiated conditions [5], and possesses the lowest level of retained tritium, that is, desirable characteristics in tritium recovery and safety. Most of the nuclear heating in the ceramic breeder is produced by the tritium breeding reaction. Nuclear heating in the ceramic breeder is removed by the coolant, through the thermal barrier as mentioned above. The cooling effect of the BIT concept is superior to that of the layered concept, since the surface area of the thermal barrier in contact with the coolant per heating material volume in the former is larger than that in the latter. In the layered concept, the thickness of the first breeding layer which fulfills the important role of enhancing the TBR is restricted by the heat removal ability. Enhancement of the TBR cannot, therefore, be expected by increasing the first breeder layer thickness. In the BIT concept blanket composed of pins, more breeder can be installed in the area adjacent to the first wall than in the layered one. Furthermore the configuration in which the heating material is surrounded by coolant has already been applied to fission reactors. Taking these results into consideration, we chose the pin concept to enhance the TBR. It is necessary to simplify the structure for the pin concept, i.e. not using quadruple tubes, which are too complicated to fabricate. Beryllium pebbles can be used not only as the multiplier but also as the thermal barrier. Applying this approach we do not require the helium filled gap, which complicates the structure in the BIT concept. The blanket design is shown in Fig. 1. The pins have co-axial double clads. The Li2ZrO3 pellets are placed in the inner clad. The beryllium pebbles fill the space between the inner and outer clads. Helium purge gas flows through pores in the Li2ZrO3 pellets and between the beryllium pebbles to recover tritium which is produced in the materials. The pins can be shortened and straightened by setting them in the toroidal direction. Setting them in this way allows the longitudinal thermal expansion to be mitigated and fabrication is easier than for the long bent pins used in the BIT concept. The coolant flows outside the pins and coolant manifolds are located at the top and bottom of the blanket segment.
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412
~on C o o ~ Vertical
Radial direction~
Outer
~
~
Li2Zr03 1/11 pellet "-~
~lnedr j ~
"
~
'~B~ ,"~l~'~,~)
I ]
EmptyZone
Expanded view of pin Fig. 1. Cut-away view of double-pin type blanket concept.
Inner Wall
Outer Wall
ltX3mm
Fig. 2. Expanded view of the purge gas header constructed by double wall of the double-pin type blanket concept.
Purge gas headers are located in both side walls in the blanket. Each header is constructed from a double-wall at each side of the blanket, as shown in Fig. 2. An outer wall constructs the vessel of the blanket, and the cooling channels for the first wall pass through the outer wall. An inner wall is connected to the outer clad of pins and constructs the boundary between the purge gas and the coolant. The header in one side supplies the purge gas to the areas of Li2ZrO3 and Be, that in the other side recovers the purge gas from these areas. The inner wall adjacent to the Li2ZrO3 areas has a hole for each pin to supply to purge gas to these areas and to recover it. It goes without saying that
405
the diameter of the hole is smaller than the breeder pellet diameter. The inner wall adjacent to the Be areas has a large number of pin holes to supply the purge gas to the area and to recover it. It goes also without saying that the diameters of the pin holes are smaller than the Be pebble diameter. With this configuration we can simplify the blanket structure over that of the BIT concept, but it is necessary to design the blanket vessel with an endurance to the 1 MPa coolant pressure. The nuclear heating rate in Li2ZrO3 adjacent to the end wall decreases in the radial direction by more than one order over that adjacent to the first wall. The rates at the top and bottom areas also decrease in the vertical direction by approximately one-half of that on the midplane. The pin radii and pin arrangement are determined so as to satisfy the temperature ranges according to the nuclear heating rate distribution. When the pin radii in each row in the radial direction are determined according to the nuclear heating rate distribution in Li2ZrO3, the pin radii adjacent to the first wall are small, and the pin radii become larger with decreasing of the nuclear heating rate in Li2ZrO3. Since the pins cannot be symmetrically arranged, the coolant spaces between them are irregular. This irregularity causes a problem in controlling the coolant flow in the blanket, that is, the coolant flows not only in the vertical direction but also in the radial direction irregularly. Baffle plates between each row of pins are indispensable to control the coolant flow. As the plates are made of SS, SS is needed not only for the claddings but also for the baffle plates. The increase in SS volume fraction reduces the TBR for the following reason. For fast neutrons of more than several megaelectronvolts, inelastic scattering reactions with iron interfere with the neutron multiplying reactions of beryllium. Moreover, the number of moderated neutrons is reduced due to absorption by SS, since there are many resonance peaks from the capture reaction on iron in the energy range from several hundreds of electronvolts to hundreds of kiloelectronvolts during the slowing down process. The moderated neutrons reacting on lithium make a main contribution to the TBR, since the cross-section spec-
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trum of the tritium production reaction on lithium obeys the l I E profile at energies lower than 100 keV. The decrease in moderated neutrons, therefore, results in the decrease in the TBR. If all radii of the outer clad are the same, we can array the pins in a lattice and eliminate the baffle plates. Consequently the SS volume fraction is less. In order to make the outer clad radii uniform, an empty zone was adopted at the center of the LiEZrO3 pellets, which leads to their designation as hollow pellets. Where the nuclear heating rate is high, the volume fraction of the empty zone should be large to reduce the heating volume. In order to keep the optimum volume fraction ratio which maximizes the TBR, as explained in the next section, in practice, not only the radius of the empty zone but also the inner clad radius in each pin must be adjusted. In this way the outer clad radii can be made uniform in the radial direction.
Neutron and gamma-ray fluxes were calculated by the one-dimensional discrete ordinates transport code ANISN [6] and nuclear group constant set Fusion-40 [7] based on the evaluated nuclear data file JENDL-3 [8]. The nuclear heating rates were estimated on the basis of the fluxes and nuclear heating responses of the K E R M A factor set [9]. As another method to calculate the nuclear heating rate, it is useful in the process of determining the pin radii and pin arrangement that the nuclear heating rate distribution can be directly predicted from only the material volume fractions in the blanket and a few parameters. Then we introduce the approximation as follows, and evaluate the accuracy in the next section. We investigated the relationship between the radial nuclear heating rate distribution and the material volume fractions. The radial nuclear heating rate Q was approximated by an exponential function of the distance r from the blanket surface adjacent to the first wall as follows Q = Qo e x p ( - k r )
3. Determination of pin radii and pin arrangement The nuclear heating rates of Li2ZrO3 and beryllium accomplish the important function of maintaining the temperature difference of the thermal barrier. As their nuclear heating rates depend on the volume fractions of Li2ZrO3, beryllium, SS, water and helium, the calculation process to determine the pin radii and pin arrangement is described as follows. At first the material volume fractions are assumed to calculate the nuclear heating rate. The blanket materials were assumed to be uniformly mixed. The nuclear heating rate distribution is estimated by a transport code or an approximation method. The pin radii in each row in the radial direction are calculated so as to satisfy the temperature ranges. Heat conduction equations on cylindrical coordinates are used to estimate the temperature distribution inside each pin. Next, the material volume fractions are redetermined considering the pin radii and pin arrangement given in the previous step. This is one cycle of the calculation process. The cycle is repeated until the material volume fractions converge.
(1)
where Q0 is the nuclear heating rate at r = 0, k is the attenuation constant. We surveyed the dependences of Qo and k on the material volume fractions. Qo and k are represented as linear combinations of the material volume fractions: Qo = ao + alL[Li + a2JBe + a3Jss + aaJh2o
(2)
k = bo + b,fLi + b2fRe + b3,fss + h4J'H~o
(3)
where fei is the volume fraction of Li2ZrO3, JBe is the volume fraction of beryllium, Jss is the volume fraction of SS, fn2o is the volume fraction of water, ai, bi are coefficients of linear combination. The coefficients at and bi (i = 0 - 4 ) were determined by means of a least-square approximation according to the set of data including Q0 and k acquired as the output and JLi, Jae, fSS and JH~o used as the input data of ANISN. We represented the TBR by only the ratio of the Li2ZrO 3 volume fraction to the beryllium plus Li2ZrO3 one. The solved ratio was chosen to maximize the TBR. Under this ratio, the nuclear heating rate distribution was estimated and the sets of the pin radii and pin arrangement were
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412
calculated so as to satisfy the temperature range. Among them we chose the solution so that the SS volume fraction would be minimized.
4. Results
1.4
0.4
At first we investigated the TBR dependence on the ratio of the Li2ZrO 3 volume fraction to the beryllium plus Li2ZrO3 one. The dependence was analyzed under the following conditions. The volume fractions of SS and water were fixed at representative values of 10% and 30%, respectively. The blanket thickness was fixed at 60 cm for the outboard segment. We investigated the dependence for two helium volume fractions which indicate representative values of the solid core and the hollow pellet type blankets. The TBRs were obtained in the blanket of the different volume fraction ratios as shown in Fig. 3. In this figure, closed and open circles indicate the TBRs for two helium volume fractions of 20% and 30%, respectively. The former correspond to the solid core pellet type blanket and the latter hollow pellet type one. The horizontal and vertical axes indicate the ratio of the LiEZrO3 volume fraction to the beryllium plus LiEZrO 3 one and the TBR, respectively. The ratio of the LizZrO 3 volume fraction to the beryllium plus Li2ZrO 3 one which has maximum TBR is about 0.1. This ratio is almost independent of the helium volume fraction.
1.4
,
.
,
.
,
.
,
-
,
.
~OeOOe
1.3
000
•
0
~ 1.2 m
0
1.1
,.%
•
=
,
i
,
0.2" 0 . 4 ' 0 . 6 ' 0.8" Li2ZrO3/(Li2ZrO3+Bc)
1.0
Fig. 3. TBR dependence on the ratio of Li2ZrO 3 volume fraction to Be plus LiEZrO 3 one. @, He 20% (solid core pellet type); ©, He 30% (hollow pellet type); other material volume fractions SS 10%, H20 30%.
i
o
i
i
0 0 0 0 0 0
o
n¢ en 1.o 0.8 0.6
4.1. Radial direction
i
1.2
407
o I
0
I
I
I
20 40 60 80 Blanket Thickness (cm)
100
Fig. 4. TBR dependence on blanket thickness. Material volume fractions: Li2ZrO3, 3%; Be, 27%; SS, 10%; HzO, 30%; He, 30% in representative hollow pellet type.
The TBR increases as the helium volume fraction decreases. To explain this fact, two causes are considered. One is that the decrease in the helium volume fraction causes increasing effective blanket thickness over 60 cm. The other is that the decrease in the helium volume fraction corresponds to the increase of the beryllium plus LizZrO3 one. This increase reduces the ratio of the SS volume fraction to the beryllium plus Li2ZrO3 one, which has more influence on the TBR than other materials such as helium and water. We investigated the relationship between the blanket thickness and the TBR in order to clarify the cause. The value of the TBR dependence on the blanket thickness was estimated as shown in Fig. 4. The volume fractions of all the blanket materials were fixed at values which were representative of the hollow pellet type blanket in this calculation. When the blanket thickness becomes larger than 60 cm, the TBR almost saturates at a value of 1.3. Increasing the blanket thickness is ineffective at enhancing the TBR. From these facts, the increase of the effective blanket thickness over 60 cm which is caused by the decrease in the helium volume fraction does not cause the TBR increase in Fig. 3. However, in another survey we showed that a 5% decrease in the SS volume fraction causes a 0.1 increase in the TBR. The decrease in the ratio of the SS volume fraction to the beryllium plus Li2ZrO 3 one which is caused by the decrease in helium volume fraction is about 5% and the increase in the TBR is about 0.1 in Fig. 3. Hence, we expect that the decrease in the ratio of the SS volume fraction to the beryllium plus Li2ZrO3 one
K. Shibutu et ul. / Fusion Engineering and Design 24 (1994) 403-412
408
Fig. 5. Vertical the blanket.
cross-sectional
view in the midplane
area
l .O:ANlSN ___ present approaimallon
of (
>
Fig. 6. Nuclear heating rate distributions in the midplane area of the blanket for the average first wall loading I MW mm ?.
causes the TBR increase in Fig. 3. It is necessary to reduce the volume fraction of SS for enhancement of the TBR. We emphasize that the ratio of the Li,ZrO, volume fraction to the beryllium plus Li,ZrO, one of about 0.1 gives the maximum TBR as shown in Fig. 3. Under this ratio we determined pin radii and pin arrangement. According to the nuclear heating rate distribution, the pin radii and pin arrangement were determined so as to satisfy the temperature ranges. Among them the solution to minimize the SS volume fraction was chosen. The solution of the midplane area is shown in Fig. 5. We split the blanket into two regions. The first and second regions included three pins and one pin, respectively. The reason why the blanket is constructed as in Fig. 5 is as follows. If the blanket consists of one region, the volume fraction of the empty zone in the pellet must be larger than that of two regions. The increase in the volume fraction of the empty zone causes an increase in the ratio of the SS volume fraction to beryllium plus Li,ZrO, one. Consequently, the TBR in the one region blanket becomes smaller than that in the two regions one. The local TBR in the case of the two regions is obtained as about 1.3. The nuclear heating rate distribution for this case is shown in Fig. 6. The dotted lines and circles represent, respectively, the
approximated values and the values calculated by ANISN based on the determined pin radii and pin arrangement. There is agreement within 10% which is the same order as the nuclear data uncertainty. The temperature in each material is expected to satisfy the temperature range within the 10% error. We think the approximation can then be used to determine the pin radii and pin arrangement 4.2. Vertical direction The nuclear heating rates in the vertical direction were predicted in proportion to the neutron wall loading under the condition that the pin radii and pin arrangement were constant. The nuclear heating rate distribution based on the neutron first wall loading is shown in Fig. 7. The horizontal axis presents the relative nuclear heating rate assuming that the value in the midplane area is unity, The vertical axis presents the distance from the midplane in the lower half of the blanket segment. The relative nuclear heating rates in the top and bottom areas of the blanket segment decrease by approximately one-half of that in the midplane area. The dotted line, indicated by Q, represents the relative nuclear heating rate at the
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412 Q
409
500 4O3
3
1
°
°
~.
30(} 200 100
00,5
0.6 0.7 0.8 0.9 1.0 Relalive nuclear beatiu8 rau~
Fig. 7. Relative nuclear heating rate distribution assuming a value of unity on the midplane.(Q is the nuclearheatingrate at the bottom of the blanket.) bottom of the blanket segment. The decrease in the nuclear heating rate in the vertical direction is much smaller than that in the radial direction. We then investigated how wide a region is covered with the same pin radii and pin arrangement as the midplane in the vertical direction. It is an essential requirement to keep the necessary temperature difference in the thermal barrier in order to satisfy the temperature ranges of the blanket materials. According to the heat conduction equations, the temperature difference in the thermal barrier is approximately proportional to the nuclear heating rate on condition that the pin radii are fixed. However, since the coolant flows from the bottom to the top, the coolant temperature in the lower half of the blanket segment is lower than that in the upper half. Then the necessary temperature difference in the lower half of the thermal barrier becomes larger than that in the upper half. Since the condition to satisfy the temperature range in the lower half is severer than that in the upper half, we investigate the lower half of the blanket segment. The correlation of the temperature difference in the thermal barrier to the nuclear heating rate is shown in Fig. 8. The horizontal axis presents the relative nuclear heating rate assuming that the value in the midplane area is unity. The vertical axis presents the temperature difference in the thermal barrier given by the nuclear heating rate and the pin radii. The dotted lines A and B present upper and lower limits of temperature differences in the thermal barrier, respectively. The pin radii and pin arrangement in the midplane area were estimated so as to satisfy the
0
0.0
region
~[~he.first'! Iregion
0.2 0.4 0.6 0.8 1.0 Relative nuclear heating rate
Fig. 8. The correlation of the temperature difference in the thermal barrier to the relative nuclear heating rate. (A, B, upper and lower limits in the temperature difference; C, D, the lines which show the correlation; P, limit of the nuclear heating rate in the first region; Q, nuclear heating rate at the bottom of the blanket.)
upper limit of the temperature difference in the thermal barrier. The temperature difference decreases along the line C, indicated in Fig. 8, as the nuclear heating rate decreases. At the crossing point of lines B and C, it is difficult to keep the lower limit temperature, therefore the pin radii and pin arrangement have to be changed, and estimated so as to satisfy the upper limit temperature. At this point the region with the same pin radii and pin arrangement as the midplane (indicated as the first region in Fig. 8) ends, and another region (indicated as the second region in Fig. 8) begins. The inclinations of lines A and B are caused by the increase in the coolant temperature from the bottom area to the midplane one. However, as for the upper half, the inclinations of lines A and B are to the contrary. The crossing point of lines B and C in the upper half is further from the midplane than that in the lower half. Then the first region in the upper half is larger than that of the lower half. The temperature difference in the thermal barrier decreases along line D with decreasing nuclear heating rate. Before line D crosses line B, the second region in the blanket segment is covered with the same pin radii and the same arrangement as that determined above. Thus the lower half of the blanket segment consists of two regions. The TBR in the region which consists of the same pins as the midplane area occupies about 95% of tri-
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K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403.412
tium production in the blanket. The rest (5%) of the tritium production can be considered small. If we neglect the contribution, we can fill up the rest of the region with shielding material.
5. Discussion It is important to examine that the blanket can work properly in fractional or overpower operation. The nuclear heating rate in the breeder pellet becomes less than that during the normal operation, when the fusion power is less than the normal fusion power of 1 GW. According to the decrease of the nuclear heating rate, the maximum temperature of the Li2ZrO 3, the Be and the SS during the fractional power operation become lower than that during normal operation. The temperature lowering does not deteriorate the strength of these materials. The temperature lowering of the Li2ZrO3 and the Be decreases the tritium recovery rate from them, and causes the tritium inventory to increase in the blanket. However, the tritium recovery rate is improved and the tritium inventory decreases when the fusion power reaches the normal operation power. Accordingly there is no problem in the case of fractional operation. However, overpower operation leads to an excess larger value of the nuclear heating rate in the breeder pellet than that during normal operation. Hence, the maximum temperatures of the Li2ZrO3, Be and SS during the overpower operation become higher than those during the normal operation. The maximum temperatures of these materials are restricted not to exceed the temperature limit that deteriorates the strength of these materials. The strength of them is discussed as follows. As concerns the SS, the outer clad forms the tritium boundary and supports the pin in the present pin-type blanket configuration. Hence the inner clad is not assigned to satisfy the temperature limit for the supporting SS to prevent expansion by swelling. The maximum temperature of the outer clad is 100°C during normal operation. It is much lower than the temperature limit of 400c'C for the supporting SS as described in the
previous section. Consequently, overpower operation does not become an issue for the SS. The maximum temperature of the Be is restricted to 600°C to prevent expansion by swelling, and the maximum temperature of the Be is 500~C during normal operation as mentioned in the previous section. Hence the temperature margin of 100~C is allowed for the excess power during the overpower operation. The excess power leads to the increase of the neutron flux in the Li2ZrO m, Be and SS areas, and to an increase of the nuclear heating rate of these materials. According to the heat conduction equation with cylindrical coordinates in the Be, the temperature difference between the temperature adjacent to the inner clad at 500°C and that adjacent to the outer clad at 100°C is proportional to the nuclear heating rate of the Li2ZrO3 and the Be as described in the previous section. Therefore, the allowable excess power ratio to the normal power is estimated at 25%, since the ratio of the allowable temperature margin of 100°C to the temperature difference of 400'C is 25%. According to the heat conduction equation with cylindrical coordinates in the Li2ZrO), the difference between the temperature adjacent to the empty zone and that adjacent to the inner clad is 2 7 0 C at the first row of pins in the blanket. The temperature of the inner clad is restricted to 500°C during normal operation. Hence the maximum temperature in the Li2ZrO3 is 770°C during normal operation. The maximum temperature is restricted to 1000°C to prevent mass transfer as described in the previous section. Therefore, the temperature margin of 23OC is allowed for the excess power during the overpower operation. The temperature difference between the temperature adjacent to the empty zone of 770c'C and that in the outer clad at 100:'C is proportional to the fusion power as described above. Therefore, the allowed excess power ratio to the normal power is estimated at 34%, since the ratio of the temperature margin of 230°C to the temperature difference of 670"C is 34%. Considering the allowable temperatures of the Li2ZrO3, Be and SS, the restricted excess power ratio to the normal one can be seen to be 25%.
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412
411
Two-dimensional thermal analysis is required to estimate the accurate temperature distribution in the pin and to evaluate the thermal stress of the materials. The present paper aims to show the concept of the double-pin type blanket with hollow ceramic breeder pellets. From above discussions this aim can be considered to be established. The saturated local TBR of the double-pin type blanket is about 1.3 for the sufficient breeding region. The net TBR can be obtained considering the thickness of the breeding region and three dimensional effects, i.e. port, side wall and rib effects on the TBR. They were previously evaluated for the typical fusion experimental reactor [3], and the scheme to evaluate the net TBR was shown in the previous report [3]. The net TBR of the double-pin type blanket is evaluated at 0.79 considering the thickness of the breeding region and these effects. The value is the same as that of the layered type blanket [3], and the TBR needs to be enhanced to achieve a net TBR beyond unity.
the ratio of the Li2ZrO 3 volume fraction to the beryllium plus LizZrO 3 one was investigated. The ratio which gives the maximum TBR is about 0.1. Using this ratio, we determined pin radii and pin arrangement in the midplane area. We investigated how wide a region is covered with the same pin radii and pin arrangement as the midplane in the vertical direction. This region supplies 95% of tritium production in the blanket. The results show that the blanket concept of the double-pin type with the hollow ceramic breeder pellets can satisfy the temperature ranges and realize the local TBR of 1.3. The net TBR of the double-pin type blanket is evaluated at 0.79 considering the thickness of the breeding region and three dimensional effects. As for the fractional or overpower operation, the fractional power operation does not become an issue, and the overpower operation does not become an issue within the allowable excess power ratio restricted to 25%.
6. Conclusions
Acknowledgments
We proposed the blanket concept to satisfy the blanket material temperature range and to enhance the TBR. The concept features the doublepin type blanket with hollow ceramic breeder pellets. Lithium zirconate is used as the ceramic breeder. Hollow pellets are placed in the center of the co-axial tube, and beryllium pebbles fill up the outer tube. The beryllium pebbles are used not only as the neutron multiplier, but also as the thermal barrier between breeder and coolant. The pins can be shortened and straightened by setting them in the toroidal direction. The coolant, 1 MPa pressurized water, flows outside the pins and coolant manifolds are located at the top and bottom of the blanket segment. We determined the pin radii and pin arrangement using heat conduction equations with the nuclear heating rate distribution. An approximation method was introduced to calculate the radial nuclear heating rate distribution, assumed as exponential decay. The TBR dependence on
We would like to thank Messrs. K. Sasaki, S. Satoh and other staff members of the Advanced Reactor Dept. of Hitachi Works for their support in the temperature distribution calculations and discussions. We would also like to thank Drs. H. Yoshida at Japan Atomic Energy Research Institute (JAERI) and P. Gierszewski at the Canadian Fusion Fuels Technology Project (CFFTP) for their useful comments on the layered concept and the thermal conductivity of beryllium pebbles.
References [1] ITER Doc. Ser. 29, ITER blanket, shield and material data base, IAEA, Vienna, 1991. [2] A. Ren~ Raffrayet al., Thermalcontrol of ceramicbreeder blankets, Fusion Technol. 23 (1993) 281-308. [31 T. Kuroda et al., Japanese contributionsto blanket design for ITER, JAERI-M 91-133, 1991, pp. 3-23.
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[4] F. Zacchia et al., Problem and solutions for design and construction of a pin-type ceramic blanket module, Fusion Eng. Des. 17 (1991) 105-111. [5] G.W. Hollenberg and D.L. Baldwin, The effect of irradiation on four solid breeder materials, J. Nucl. Mater 133 & 134 (1985) 242-245. [6] W.W. Engle, A users manual for ANISN, K-1693, ORNL, 1973. [7] K. Maki et al., Nuclear group ~onstant set Fusion-J3 for
fusion reactor nuclear calculations based on JENDL-3, JAERI-M 91-072, 1991. [8] T. Nakagawa et al., Curves and Tables of Neutron Cross Sections Japanese Evaluated Nuclear Data Library Version 3, JAERI-M 90-099, 1990. [9] K. Maki et al., Nuclear Heating Constant KERMA Lib r a r y - N u c l e a r Heating Constant Library for Fusion Nuclear Group Constant Set Fusion-J3, JAERI-M 91037, 1991.
Fusion Engineering and Design 24 (1994) 403-412
Fusion Engineering and Design
The conceptual design of the double-pin type blanket with hollow ceramic breeder pellets for fusion experimental reactors K. Shibata a, K. Maki a, M. Otsuka a, T. Amada u, y . Ozawa b, H. Takatsu c a Energy Research Laboratory, Hitachi Ltd., 7-2-10hmika-cho, Hitachi-shi, lbaraki-ken 319-12, Japan b Hitachi Works, Hitachi Ltd., 3-1-1 Saiwai-cho, Hitachi-shi, Ibaraki-ken 316, Japan c Naka Fusion Research Establishment, Japan Atomic Research Institute, Naka-machi, Naka-gun, Ibaraki-ken 311-01, Japan Received 28 September 1993 Handling Editor: Robert W. Conn
Abstract
Several ceramic breeder blanket concepts for fusion experimental reactors have been proposed. Satisfying the allowable temperature ranges of each material and enhancing the tritium breeding ratio (TBR) are indispensable to development of any blanket concept. The present paper proposes a ceramic breeder blanket concept which realizes these points. Lithium zirconate (Li2ZrO3) is adopted as the breeder along with double-cladding pins having hollow pellets. The pellets are placed in the co-axial center tubes, while beryllium pebbles fill the outer tubes and serve not only as a neutron multiplier but also as a thermal barrier between the breeder and coolant, which flows outside the pins. The pins can be shortened and straightened by setting them in the toroidal direction. Hollow pellets are used to unify the radii of the outer clad, and to reduce the volume fraction of stainless steel by making baffle plates unnecessary between the pin rows. The optimum ratio of the Li2ZrO3 volume fraction to the beryllium plus Li2ZrO3 one is investigated to maximize the TBR, and the pin radii and pin arrangement are determined. The proposed blanket concept is seen to satisfy the temperature ranges and enhance the local TBR to 1.3.
1. Introduction Several ceramic breeder blanket concepts have been proposed for fusion experimental reactors: layered [1,2], pebble bed [1,3], and breeder in tube (BIT) [ 1,4] concepts. In the blankets, lithium oxide (Li20), lithium aluminate (LiAIO3), lithium orthosilicate (Li4SiO4), or lithium zirconate (Li2ZrO3) were used as the ceramic breeder, beryllium as a neutron multiplier, stainless steel (SS) as structural material, supporting and non-
supporting material, and water or helium gas as coolant. Achieving a net tritium breeding ratio (TBR) about unity is one of the primary design goals for the experimental reactor blanket. Since the coverage of the experimental reactor blanket is restricted to 60-70%, meeting the design goal appears to be difficult. To enhance the TBR, therefore, is essential to develop a suitable blanket. However, there is an appropriate temperature range for each material. As for the breeder, when
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Li2ZrO3 is used, its minimum temperature is restricted to 370°C to provide for efficient tritium recovery, and its maximum is about 1000°C to prevent mass transfer of the ceramic material [1]. As for beryllium and the supporting SS, their maximum temperatures are restricted to 600 to 400°C, respectively, to prevent expansion by swelling [1]. In addition, the non-supporting SS is made of cold-worked SS, and its working temperature of 500°C is permitted if swelling is to be avoided [1]. It is not so important to enhance the operating temperature, hence the fusion experimental reactor does not aim at the power production. Then water, pressurized at l MPa, is applied for the coolant under operating temperatures below 100°C in order to keep the water from boiling. Since the minimum temperature of the breeder is much higher than the maximum temperature of the coolant, it is necessary to provide a thermal barrier between them. In the three concepts there are two methods for this. One is to use the beryllium not only as multiplier but also as the thermal barrier for the sintered block or pebble bed concepts. The major concern in this approach is the possible increase in beryllium and breeder temperatures due to deformation-induced separations at the interfaces [1]. The other is to use a helium filled gap for the BIT concept. The primary concerns for this have to do with fabricating the long bent concentric tubes, while maintaining close (0.1 ram) gap tolerances [1]. Advanced methods to satisfy the temperature ranges are expected. The purpose of the present paper is to propose a ceramic breeder blanket concept to satisfy the temperature ranges and to enhance the TBR. The principal design parameters of the target reactor are as follows [1]. The major and minor radius of the plasma are 6.0 and 2.15 m, respectively. The average neutron wall loading and surface heat flux are designed as 1.0 MW m -2 and 0 . 5 M W m 2 respectively, for nominal fusion power of 1 GW.
2. Design concept We chose LizZrO3 as a breeder material from among various kinds of lithium ceramic for the
following reason. The Li2ZrO3 exhibits the least swelling of all the candidate ceramics under irradiated conditions [5], and possesses the lowest level of retained tritium, that is, desirable characteristics in tritium recovery and safety. Most of the nuclear heating in the ceramic breeder is produced by the tritium breeding reaction. Nuclear heating in the ceramic breeder is removed by the coolant, through the thermal barrier as mentioned above. The cooling effect of the BIT concept is superior to that of the layered concept, since the surface area of the thermal barrier in contact with the coolant per heating material volume in the former is larger than that in the latter. In the layered concept, the thickness of the first breeding layer which fulfills the important role of enhancing the TBR is restricted by the heat removal ability. Enhancement of the TBR cannot, therefore, be expected by increasing the first breeder layer thickness. In the BIT concept blanket composed of pins, more breeder can be installed in the area adjacent to the first wall than in the layered one. Furthermore the configuration in which the heating material is surrounded by coolant has already been applied to fission reactors. Taking these results into consideration, we chose the pin concept to enhance the TBR. It is necessary to simplify the structure for the pin concept, i.e. not using quadruple tubes, which are too complicated to fabricate. Beryllium pebbles can be used not only as the multiplier but also as the thermal barrier. Applying this approach we do not require the helium filled gap, which complicates the structure in the BIT concept. The blanket design is shown in Fig. 1. The pins have co-axial double clads. The Li2ZrO3 pellets are placed in the inner clad. The beryllium pebbles fill the space between the inner and outer clads. Helium purge gas flows through pores in the Li2ZrO3 pellets and between the beryllium pebbles to recover tritium which is produced in the materials. The pins can be shortened and straightened by setting them in the toroidal direction. Setting them in this way allows the longitudinal thermal expansion to be mitigated and fabrication is easier than for the long bent pins used in the BIT concept. The coolant flows outside the pins and coolant manifolds are located at the top and bottom of the blanket segment.
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412
~on C o o ~ Vertical
Radial direction~
Outer
~
~
Li2Zr03 1/11 pellet "-~
~lnedr j ~
"
~
'~B~ ,"~l~'~,~)
I ]
EmptyZone
Expanded view of pin Fig. 1. Cut-away view of double-pin type blanket concept.
Inner Wall
Outer Wall
ltX3mm
Fig. 2. Expanded view of the purge gas header constructed by double wall of the double-pin type blanket concept.
Purge gas headers are located in both side walls in the blanket. Each header is constructed from a double-wall at each side of the blanket, as shown in Fig. 2. An outer wall constructs the vessel of the blanket, and the cooling channels for the first wall pass through the outer wall. An inner wall is connected to the outer clad of pins and constructs the boundary between the purge gas and the coolant. The header in one side supplies the purge gas to the areas of Li2ZrO3 and Be, that in the other side recovers the purge gas from these areas. The inner wall adjacent to the Li2ZrO3 areas has a hole for each pin to supply to purge gas to these areas and to recover it. It goes without saying that
405
the diameter of the hole is smaller than the breeder pellet diameter. The inner wall adjacent to the Be areas has a large number of pin holes to supply the purge gas to the area and to recover it. It goes also without saying that the diameters of the pin holes are smaller than the Be pebble diameter. With this configuration we can simplify the blanket structure over that of the BIT concept, but it is necessary to design the blanket vessel with an endurance to the 1 MPa coolant pressure. The nuclear heating rate in Li2ZrO3 adjacent to the end wall decreases in the radial direction by more than one order over that adjacent to the first wall. The rates at the top and bottom areas also decrease in the vertical direction by approximately one-half of that on the midplane. The pin radii and pin arrangement are determined so as to satisfy the temperature ranges according to the nuclear heating rate distribution. When the pin radii in each row in the radial direction are determined according to the nuclear heating rate distribution in Li2ZrO3, the pin radii adjacent to the first wall are small, and the pin radii become larger with decreasing of the nuclear heating rate in Li2ZrO3. Since the pins cannot be symmetrically arranged, the coolant spaces between them are irregular. This irregularity causes a problem in controlling the coolant flow in the blanket, that is, the coolant flows not only in the vertical direction but also in the radial direction irregularly. Baffle plates between each row of pins are indispensable to control the coolant flow. As the plates are made of SS, SS is needed not only for the claddings but also for the baffle plates. The increase in SS volume fraction reduces the TBR for the following reason. For fast neutrons of more than several megaelectronvolts, inelastic scattering reactions with iron interfere with the neutron multiplying reactions of beryllium. Moreover, the number of moderated neutrons is reduced due to absorption by SS, since there are many resonance peaks from the capture reaction on iron in the energy range from several hundreds of electronvolts to hundreds of kiloelectronvolts during the slowing down process. The moderated neutrons reacting on lithium make a main contribution to the TBR, since the cross-section spec-
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trum of the tritium production reaction on lithium obeys the l I E profile at energies lower than 100 keV. The decrease in moderated neutrons, therefore, results in the decrease in the TBR. If all radii of the outer clad are the same, we can array the pins in a lattice and eliminate the baffle plates. Consequently the SS volume fraction is less. In order to make the outer clad radii uniform, an empty zone was adopted at the center of the LiEZrO3 pellets, which leads to their designation as hollow pellets. Where the nuclear heating rate is high, the volume fraction of the empty zone should be large to reduce the heating volume. In order to keep the optimum volume fraction ratio which maximizes the TBR, as explained in the next section, in practice, not only the radius of the empty zone but also the inner clad radius in each pin must be adjusted. In this way the outer clad radii can be made uniform in the radial direction.
Neutron and gamma-ray fluxes were calculated by the one-dimensional discrete ordinates transport code ANISN [6] and nuclear group constant set Fusion-40 [7] based on the evaluated nuclear data file JENDL-3 [8]. The nuclear heating rates were estimated on the basis of the fluxes and nuclear heating responses of the K E R M A factor set [9]. As another method to calculate the nuclear heating rate, it is useful in the process of determining the pin radii and pin arrangement that the nuclear heating rate distribution can be directly predicted from only the material volume fractions in the blanket and a few parameters. Then we introduce the approximation as follows, and evaluate the accuracy in the next section. We investigated the relationship between the radial nuclear heating rate distribution and the material volume fractions. The radial nuclear heating rate Q was approximated by an exponential function of the distance r from the blanket surface adjacent to the first wall as follows Q = Qo e x p ( - k r )
3. Determination of pin radii and pin arrangement The nuclear heating rates of Li2ZrO3 and beryllium accomplish the important function of maintaining the temperature difference of the thermal barrier. As their nuclear heating rates depend on the volume fractions of Li2ZrO3, beryllium, SS, water and helium, the calculation process to determine the pin radii and pin arrangement is described as follows. At first the material volume fractions are assumed to calculate the nuclear heating rate. The blanket materials were assumed to be uniformly mixed. The nuclear heating rate distribution is estimated by a transport code or an approximation method. The pin radii in each row in the radial direction are calculated so as to satisfy the temperature ranges. Heat conduction equations on cylindrical coordinates are used to estimate the temperature distribution inside each pin. Next, the material volume fractions are redetermined considering the pin radii and pin arrangement given in the previous step. This is one cycle of the calculation process. The cycle is repeated until the material volume fractions converge.
(1)
where Q0 is the nuclear heating rate at r = 0, k is the attenuation constant. We surveyed the dependences of Qo and k on the material volume fractions. Qo and k are represented as linear combinations of the material volume fractions: Qo = ao + alL[Li + a2JBe + a3Jss + aaJh2o
(2)
k = bo + b,fLi + b2fRe + b3,fss + h4J'H~o
(3)
where fei is the volume fraction of Li2ZrO3, JBe is the volume fraction of beryllium, Jss is the volume fraction of SS, fn2o is the volume fraction of water, ai, bi are coefficients of linear combination. The coefficients at and bi (i = 0 - 4 ) were determined by means of a least-square approximation according to the set of data including Q0 and k acquired as the output and JLi, Jae, fSS and JH~o used as the input data of ANISN. We represented the TBR by only the ratio of the Li2ZrO 3 volume fraction to the beryllium plus Li2ZrO3 one. The solved ratio was chosen to maximize the TBR. Under this ratio, the nuclear heating rate distribution was estimated and the sets of the pin radii and pin arrangement were
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412
calculated so as to satisfy the temperature range. Among them we chose the solution so that the SS volume fraction would be minimized.
4. Results
1.4
0.4
At first we investigated the TBR dependence on the ratio of the Li2ZrO 3 volume fraction to the beryllium plus Li2ZrO3 one. The dependence was analyzed under the following conditions. The volume fractions of SS and water were fixed at representative values of 10% and 30%, respectively. The blanket thickness was fixed at 60 cm for the outboard segment. We investigated the dependence for two helium volume fractions which indicate representative values of the solid core and the hollow pellet type blankets. The TBRs were obtained in the blanket of the different volume fraction ratios as shown in Fig. 3. In this figure, closed and open circles indicate the TBRs for two helium volume fractions of 20% and 30%, respectively. The former correspond to the solid core pellet type blanket and the latter hollow pellet type one. The horizontal and vertical axes indicate the ratio of the LiEZrO3 volume fraction to the beryllium plus LiEZrO 3 one and the TBR, respectively. The ratio of the LizZrO 3 volume fraction to the beryllium plus Li2ZrO 3 one which has maximum TBR is about 0.1. This ratio is almost independent of the helium volume fraction.
1.4
,
.
,
.
,
.
,
-
,
.
~OeOOe
1.3
000
•
0
~ 1.2 m
0
1.1
,.%
•
=
,
i
,
0.2" 0 . 4 ' 0 . 6 ' 0.8" Li2ZrO3/(Li2ZrO3+Bc)
1.0
Fig. 3. TBR dependence on the ratio of Li2ZrO 3 volume fraction to Be plus LiEZrO 3 one. @, He 20% (solid core pellet type); ©, He 30% (hollow pellet type); other material volume fractions SS 10%, H20 30%.
i
o
i
i
0 0 0 0 0 0
o
n¢ en 1.o 0.8 0.6
4.1. Radial direction
i
1.2
407
o I
0
I
I
I
20 40 60 80 Blanket Thickness (cm)
100
Fig. 4. TBR dependence on blanket thickness. Material volume fractions: Li2ZrO3, 3%; Be, 27%; SS, 10%; HzO, 30%; He, 30% in representative hollow pellet type.
The TBR increases as the helium volume fraction decreases. To explain this fact, two causes are considered. One is that the decrease in the helium volume fraction causes increasing effective blanket thickness over 60 cm. The other is that the decrease in the helium volume fraction corresponds to the increase of the beryllium plus LizZrO3 one. This increase reduces the ratio of the SS volume fraction to the beryllium plus Li2ZrO3 one, which has more influence on the TBR than other materials such as helium and water. We investigated the relationship between the blanket thickness and the TBR in order to clarify the cause. The value of the TBR dependence on the blanket thickness was estimated as shown in Fig. 4. The volume fractions of all the blanket materials were fixed at values which were representative of the hollow pellet type blanket in this calculation. When the blanket thickness becomes larger than 60 cm, the TBR almost saturates at a value of 1.3. Increasing the blanket thickness is ineffective at enhancing the TBR. From these facts, the increase of the effective blanket thickness over 60 cm which is caused by the decrease in the helium volume fraction does not cause the TBR increase in Fig. 3. However, in another survey we showed that a 5% decrease in the SS volume fraction causes a 0.1 increase in the TBR. The decrease in the ratio of the SS volume fraction to the beryllium plus Li2ZrO 3 one which is caused by the decrease in helium volume fraction is about 5% and the increase in the TBR is about 0.1 in Fig. 3. Hence, we expect that the decrease in the ratio of the SS volume fraction to the beryllium plus Li2ZrO3 one
K. Shibutu et ul. / Fusion Engineering and Design 24 (1994) 403-412
408
Fig. 5. Vertical the blanket.
cross-sectional
view in the midplane
area
l .O:ANlSN ___ present approaimallon
of (
>
Fig. 6. Nuclear heating rate distributions in the midplane area of the blanket for the average first wall loading I MW mm ?.
causes the TBR increase in Fig. 3. It is necessary to reduce the volume fraction of SS for enhancement of the TBR. We emphasize that the ratio of the Li,ZrO, volume fraction to the beryllium plus Li,ZrO, one of about 0.1 gives the maximum TBR as shown in Fig. 3. Under this ratio we determined pin radii and pin arrangement. According to the nuclear heating rate distribution, the pin radii and pin arrangement were determined so as to satisfy the temperature ranges. Among them the solution to minimize the SS volume fraction was chosen. The solution of the midplane area is shown in Fig. 5. We split the blanket into two regions. The first and second regions included three pins and one pin, respectively. The reason why the blanket is constructed as in Fig. 5 is as follows. If the blanket consists of one region, the volume fraction of the empty zone in the pellet must be larger than that of two regions. The increase in the volume fraction of the empty zone causes an increase in the ratio of the SS volume fraction to beryllium plus Li,ZrO, one. Consequently, the TBR in the one region blanket becomes smaller than that in the two regions one. The local TBR in the case of the two regions is obtained as about 1.3. The nuclear heating rate distribution for this case is shown in Fig. 6. The dotted lines and circles represent, respectively, the
approximated values and the values calculated by ANISN based on the determined pin radii and pin arrangement. There is agreement within 10% which is the same order as the nuclear data uncertainty. The temperature in each material is expected to satisfy the temperature range within the 10% error. We think the approximation can then be used to determine the pin radii and pin arrangement 4.2. Vertical direction The nuclear heating rates in the vertical direction were predicted in proportion to the neutron wall loading under the condition that the pin radii and pin arrangement were constant. The nuclear heating rate distribution based on the neutron first wall loading is shown in Fig. 7. The horizontal axis presents the relative nuclear heating rate assuming that the value in the midplane area is unity, The vertical axis presents the distance from the midplane in the lower half of the blanket segment. The relative nuclear heating rates in the top and bottom areas of the blanket segment decrease by approximately one-half of that in the midplane area. The dotted line, indicated by Q, represents the relative nuclear heating rate at the
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412 Q
409
500 4O3
3
1
°
°
~.
30(} 200 100
00,5
0.6 0.7 0.8 0.9 1.0 Relalive nuclear beatiu8 rau~
Fig. 7. Relative nuclear heating rate distribution assuming a value of unity on the midplane.(Q is the nuclearheatingrate at the bottom of the blanket.) bottom of the blanket segment. The decrease in the nuclear heating rate in the vertical direction is much smaller than that in the radial direction. We then investigated how wide a region is covered with the same pin radii and pin arrangement as the midplane in the vertical direction. It is an essential requirement to keep the necessary temperature difference in the thermal barrier in order to satisfy the temperature ranges of the blanket materials. According to the heat conduction equations, the temperature difference in the thermal barrier is approximately proportional to the nuclear heating rate on condition that the pin radii are fixed. However, since the coolant flows from the bottom to the top, the coolant temperature in the lower half of the blanket segment is lower than that in the upper half. Then the necessary temperature difference in the lower half of the thermal barrier becomes larger than that in the upper half. Since the condition to satisfy the temperature range in the lower half is severer than that in the upper half, we investigate the lower half of the blanket segment. The correlation of the temperature difference in the thermal barrier to the nuclear heating rate is shown in Fig. 8. The horizontal axis presents the relative nuclear heating rate assuming that the value in the midplane area is unity. The vertical axis presents the temperature difference in the thermal barrier given by the nuclear heating rate and the pin radii. The dotted lines A and B present upper and lower limits of temperature differences in the thermal barrier, respectively. The pin radii and pin arrangement in the midplane area were estimated so as to satisfy the
0
0.0
region
~[~he.first'! Iregion
0.2 0.4 0.6 0.8 1.0 Relative nuclear heating rate
Fig. 8. The correlation of the temperature difference in the thermal barrier to the relative nuclear heating rate. (A, B, upper and lower limits in the temperature difference; C, D, the lines which show the correlation; P, limit of the nuclear heating rate in the first region; Q, nuclear heating rate at the bottom of the blanket.)
upper limit of the temperature difference in the thermal barrier. The temperature difference decreases along the line C, indicated in Fig. 8, as the nuclear heating rate decreases. At the crossing point of lines B and C, it is difficult to keep the lower limit temperature, therefore the pin radii and pin arrangement have to be changed, and estimated so as to satisfy the upper limit temperature. At this point the region with the same pin radii and pin arrangement as the midplane (indicated as the first region in Fig. 8) ends, and another region (indicated as the second region in Fig. 8) begins. The inclinations of lines A and B are caused by the increase in the coolant temperature from the bottom area to the midplane one. However, as for the upper half, the inclinations of lines A and B are to the contrary. The crossing point of lines B and C in the upper half is further from the midplane than that in the lower half. Then the first region in the upper half is larger than that of the lower half. The temperature difference in the thermal barrier decreases along line D with decreasing nuclear heating rate. Before line D crosses line B, the second region in the blanket segment is covered with the same pin radii and the same arrangement as that determined above. Thus the lower half of the blanket segment consists of two regions. The TBR in the region which consists of the same pins as the midplane area occupies about 95% of tri-
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K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403.412
tium production in the blanket. The rest (5%) of the tritium production can be considered small. If we neglect the contribution, we can fill up the rest of the region with shielding material.
5. Discussion It is important to examine that the blanket can work properly in fractional or overpower operation. The nuclear heating rate in the breeder pellet becomes less than that during the normal operation, when the fusion power is less than the normal fusion power of 1 GW. According to the decrease of the nuclear heating rate, the maximum temperature of the Li2ZrO 3, the Be and the SS during the fractional power operation become lower than that during normal operation. The temperature lowering does not deteriorate the strength of these materials. The temperature lowering of the Li2ZrO3 and the Be decreases the tritium recovery rate from them, and causes the tritium inventory to increase in the blanket. However, the tritium recovery rate is improved and the tritium inventory decreases when the fusion power reaches the normal operation power. Accordingly there is no problem in the case of fractional operation. However, overpower operation leads to an excess larger value of the nuclear heating rate in the breeder pellet than that during normal operation. Hence, the maximum temperatures of the Li2ZrO3, Be and SS during the overpower operation become higher than those during the normal operation. The maximum temperatures of these materials are restricted not to exceed the temperature limit that deteriorates the strength of these materials. The strength of them is discussed as follows. As concerns the SS, the outer clad forms the tritium boundary and supports the pin in the present pin-type blanket configuration. Hence the inner clad is not assigned to satisfy the temperature limit for the supporting SS to prevent expansion by swelling. The maximum temperature of the outer clad is 100°C during normal operation. It is much lower than the temperature limit of 400c'C for the supporting SS as described in the
previous section. Consequently, overpower operation does not become an issue for the SS. The maximum temperature of the Be is restricted to 600°C to prevent expansion by swelling, and the maximum temperature of the Be is 500~C during normal operation as mentioned in the previous section. Hence the temperature margin of 100~C is allowed for the excess power during the overpower operation. The excess power leads to the increase of the neutron flux in the Li2ZrO m, Be and SS areas, and to an increase of the nuclear heating rate of these materials. According to the heat conduction equation with cylindrical coordinates in the Be, the temperature difference between the temperature adjacent to the inner clad at 500°C and that adjacent to the outer clad at 100°C is proportional to the nuclear heating rate of the Li2ZrO3 and the Be as described in the previous section. Therefore, the allowable excess power ratio to the normal power is estimated at 25%, since the ratio of the allowable temperature margin of 100°C to the temperature difference of 400'C is 25%. According to the heat conduction equation with cylindrical coordinates in the Li2ZrO), the difference between the temperature adjacent to the empty zone and that adjacent to the inner clad is 2 7 0 C at the first row of pins in the blanket. The temperature of the inner clad is restricted to 500°C during normal operation. Hence the maximum temperature in the Li2ZrO3 is 770°C during normal operation. The maximum temperature is restricted to 1000°C to prevent mass transfer as described in the previous section. Therefore, the temperature margin of 23OC is allowed for the excess power during the overpower operation. The temperature difference between the temperature adjacent to the empty zone of 770c'C and that in the outer clad at 100:'C is proportional to the fusion power as described above. Therefore, the allowed excess power ratio to the normal power is estimated at 34%, since the ratio of the temperature margin of 230°C to the temperature difference of 670"C is 34%. Considering the allowable temperatures of the Li2ZrO3, Be and SS, the restricted excess power ratio to the normal one can be seen to be 25%.
K. Shibata et al. / Fusion Engineering and Design 24 (1994) 403-412
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Two-dimensional thermal analysis is required to estimate the accurate temperature distribution in the pin and to evaluate the thermal stress of the materials. The present paper aims to show the concept of the double-pin type blanket with hollow ceramic breeder pellets. From above discussions this aim can be considered to be established. The saturated local TBR of the double-pin type blanket is about 1.3 for the sufficient breeding region. The net TBR can be obtained considering the thickness of the breeding region and three dimensional effects, i.e. port, side wall and rib effects on the TBR. They were previously evaluated for the typical fusion experimental reactor [3], and the scheme to evaluate the net TBR was shown in the previous report [3]. The net TBR of the double-pin type blanket is evaluated at 0.79 considering the thickness of the breeding region and these effects. The value is the same as that of the layered type blanket [3], and the TBR needs to be enhanced to achieve a net TBR beyond unity.
the ratio of the Li2ZrO 3 volume fraction to the beryllium plus LizZrO 3 one was investigated. The ratio which gives the maximum TBR is about 0.1. Using this ratio, we determined pin radii and pin arrangement in the midplane area. We investigated how wide a region is covered with the same pin radii and pin arrangement as the midplane in the vertical direction. This region supplies 95% of tritium production in the blanket. The results show that the blanket concept of the double-pin type with the hollow ceramic breeder pellets can satisfy the temperature ranges and realize the local TBR of 1.3. The net TBR of the double-pin type blanket is evaluated at 0.79 considering the thickness of the breeding region and three dimensional effects. As for the fractional or overpower operation, the fractional power operation does not become an issue, and the overpower operation does not become an issue within the allowable excess power ratio restricted to 25%.
6. Conclusions
Acknowledgments
We proposed the blanket concept to satisfy the blanket material temperature range and to enhance the TBR. The concept features the doublepin type blanket with hollow ceramic breeder pellets. Lithium zirconate is used as the ceramic breeder. Hollow pellets are placed in the center of the co-axial tube, and beryllium pebbles fill up the outer tube. The beryllium pebbles are used not only as the neutron multiplier, but also as the thermal barrier between breeder and coolant. The pins can be shortened and straightened by setting them in the toroidal direction. The coolant, 1 MPa pressurized water, flows outside the pins and coolant manifolds are located at the top and bottom of the blanket segment. We determined the pin radii and pin arrangement using heat conduction equations with the nuclear heating rate distribution. An approximation method was introduced to calculate the radial nuclear heating rate distribution, assumed as exponential decay. The TBR dependence on
We would like to thank Messrs. K. Sasaki, S. Satoh and other staff members of the Advanced Reactor Dept. of Hitachi Works for their support in the temperature distribution calculations and discussions. We would also like to thank Drs. H. Yoshida at Japan Atomic Energy Research Institute (JAERI) and P. Gierszewski at the Canadian Fusion Fuels Technology Project (CFFTP) for their useful comments on the layered concept and the thermal conductivity of beryllium pebbles.
References [1] ITER Doc. Ser. 29, ITER blanket, shield and material data base, IAEA, Vienna, 1991. [2] A. Ren~ Raffrayet al., Thermalcontrol of ceramicbreeder blankets, Fusion Technol. 23 (1993) 281-308. [31 T. Kuroda et al., Japanese contributionsto blanket design for ITER, JAERI-M 91-133, 1991, pp. 3-23.
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[4] F. Zacchia et al., Problem and solutions for design and construction of a pin-type ceramic blanket module, Fusion Eng. Des. 17 (1991) 105-111. [5] G.W. Hollenberg and D.L. Baldwin, The effect of irradiation on four solid breeder materials, J. Nucl. Mater 133 & 134 (1985) 242-245. [6] W.W. Engle, A users manual for ANISN, K-1693, ORNL, 1973. [7] K. Maki et al., Nuclear group ~onstant set Fusion-J3 for
fusion reactor nuclear calculations based on JENDL-3, JAERI-M 91-072, 1991. [8] T. Nakagawa et al., Curves and Tables of Neutron Cross Sections Japanese Evaluated Nuclear Data Library Version 3, JAERI-M 90-099, 1990. [9] K. Maki et al., Nuclear Heating Constant KERMA Lib r a r y - N u c l e a r Heating Constant Library for Fusion Nuclear Group Constant Set Fusion-J3, JAERI-M 91037, 1991.