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Problems and solutions for design and construction of a pin-type ceramic blanket module
Fusion Engineering and Design 17 (1991) 105-111 North-Holland
105
Problems and solutions for design and construction of a pin-type ceramic blanket module F. Z a c c h i a a, M. G r a t t a r o l a ", F. R o s a t e l l i a a n d G . S i m b o l o t t i b a Ansaldo Divisione Nuooe Tecnologie, Corso Perrone, 25, 16161, Genoa, Italy b ENEA, CRE Frascati, Via E. Fermi, 27, 00044, Frascatti Rome, Italy
In the frame of the conceptual design of ITER, Ansaldo DNT and ENEA Frascati have brought foreward the design of a ceramic blanket. At the present state of the design, a module consists of two tube bundles containing breeder rod segments; the tubes are topped by helium/water manifolds and constrained to the back-plate by ten supporting grids. One fundamental objective of the work was to study the feasibility, in the near term, of this pin-type blanket. In particular, the feasibility of the thermal barrier between the sintered pellets (LiA102) and the low-temperature cooling water, was a matter of uncertainty. To reach the aforecited objective means the possibility of realizing a component with technologies which are currently used in the nuclear field. The main problems and solutions related to the mechanical design and construction of the blanket subcomponents are now briefly presented. From a theoretical approach, the problem of breeder temperature control has been solved: relatively short straight cladding segments keep the pellets centered inside the internal water pipe with the help of inconel spacers. An investigation has shown that an industrial effort is needed to develop a technology able both to produce long thin pipes, which constitute the water channel, with very tight tolerances and to bent them to the desired radius. Two different versions of manifolds have been proposed. In each case, care was taken to reduce the number and length of welds needed to connect the tubes to the relative tube sheets. The supporting grids of the egg-crate type are such to minimize the pitch between the pipes, have a relatively high stiffness and make possible differential thermal elongation between the bundles and the back-plate. Their feasibility has already been assessed since similar grids have been used in steam generators.
1. Introduction A N S A L D O and E N E A are collaborating since years on the conception and the design of ceramic breeder blankets. A m o n g others, a concept with v-LiA10 2 as breeder in the form of pellets and beryllium as multiplier, cooled by water, was considered potentially interesting. It was largely investigated in the frame of the International Tokamak Experimental Reactor (ITER) Conceptual Design Activities (CDA) and represents, at present, the EC reference driver blanket for I T E R [1]. The idea of developing a PIN geometry was launched in E N E A to try to take the maximum advantage from the past experience with fission reactors: to demonstrate the feasibility with present nuclear technology was a fundamental goal. Advantages of the concept are: (1) a simple cylindrical geometry; (2) the minimization of the heat fluxes, since multiplier and breeder are positioned in such a way to be separately cooled by water; (3) a low beryllium working temperature. 0920-3796/91/$03.50
The most critical item remains the realization of the thermal barrier which has the role, in ITER, of maintaining lithium alluminate above 450°C, to make on-line tritium recovery possible. After detailed thermo-mechanical and sensitivity analyses, a solution has been proposed and out-of-pile tests are now on their way in order to validate design calculations.
2. Design aspects 2.1. Description of concept The following refers to the concept as it was developed for an outboard side segment of I T E R [2] (fig. 1). As known, such a component is entirely enclosed in a First-Wall Box (FWB) and the process fluids (helium and water) inlet and outlet are located at the top of each module. The blanket can be subdivided into three main parts.
© 1991 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. A l l r i g h t s r e s e r v e d
106
IE Zacchia et al. / Pin-type ceramic blanket module (b) T w o double manifolds
The upper and lower manifolds are made by two tanks, one for water and the other for helium. The pressure tube and the second cladding are fixed to the water and helium tube sheets respectively. The upper water tube sheet also plays the role of main bundle support.
UPPER NANI
(c) The grids"
BACKPLM
A set of ten supporting grids is needed to constrain the bundles to the back plate. Grids of the egg-crate type have been chosen, where each pin is individually supported at four positions apart (see fig. 4): such a
FIR~
EOUATORIAL 5ECfION fRO~f ~UN~tt
Y TUBE B,
LOWERMANIFO
-
~
.J//
Fig. 1. Outboard side segment of ITER.
UPPER 5LCflO~
(a) The pin bundles
Each bundle, one front and one rear, is composed by pins arranged in a triangular pitch and having the same outer diameter (see fig. 2). The double pipe where water flows is the structural part of each pin. Ceramic pellets are contained inside straight cladding segments, the length of which depends on the double-pipe fabrication technology and on the poloidal coordinate of the segment itself. These segments are then kept centered inside the so called second cladding (see fig. 3) by special INCONEL spacers that must guarantee the minimum possible eccentricity of the helium gap during normal operation. Annular beryllium segments (3-20 cm high) are positioned outside along the pressure tube.
Fig. 2. Blanket equatorial and top cross-sections.
107
F. Zacchia et a L / Pin-type ceramic blanket module
design makes possible assembly as well as all the movements caused by the temperature differences inside the blanket and between pins and FWB. The grids transfer all the loads (weight, pressure on lower manifold, temperature) to the back plate, through a bolted frame. The tube-strap contact region has been suitably reinforced by positioning a collar around the pressure tube; around this collar a couple of half-rings gives the required flexibility to make assembly of the straps easier and, above all, to minimize friction between pipes and straps.
2.2. Thermo-mechanical analysis of a cladding segment 2.2.1. Aims and model
The analysis was performed on a first-row pin at an equatorial position. This case corresponds to the maximum volumetric heat deposition; this is 26 W / c m 3 for the breeder and 12 W / c m 3 for steel. The finite-element ANSYS code has been used, with thermal solid elements STIF70 and solid elements STIF45; the spacer was modelled with the elastic beam STIF3. r--- LiA[02 / $ ~ : ~ l s t cLodding
r,~,~
~
2ndcLadd/ng
~ s p a c e n
~
~sprin9
~
SECTION A
m
!6,C 1
20, 0 A 5ECFION B
~ 47
he[ium 9ap outen ctodding
beryLLium
pressure Lube wooer
2rid cLaddin9
~pacer
Ist cLaddin9 LML02 I
Fig. 3. Pin cross-sectionand blanketinternals.
108
F. Zaechia et al. / Pin-type ceramic blanket module
F-SPaCF~
~OUTERWATERPlP~ ~BERYLLJGM --ObIERCLADO!NG
BREEOINGPELLEF~ Ist CLAOOING-.~..~
/-ELASTIC SPACER
2~d C[APO!NG-JI..~._~"
GPlO51RAPJF...~-~-
05dPPOR/ /
//
i
•i
i I
L. _ Fig.
dPPERMAN!~O{D
~SJPP0~rlN00RIOS
_ 4~ 3 :
._
4. Egg-crate type grid.
Aims of this calculations are: to check the value of the breeder temperature window, focusing in particular on the spacer induced border effect; the precise determination of the structural material temperature; the assessment of stresses. The two important input parameters are related to heat transfer at interfaces. After a series of evaluations, the following averge values have been taken [3]: - thermal conductance b r e e d e r / c l a d d i n g : 3500 W / ( m 2 K); -thermal resistance s p a c e r / s e c o n d cladding: 980
K/W, 2.2.2. Results
lOWER MANIFOLD
Fig. 5. Constraint scheme of blanket.
The 3-D calculations have shown that breeder temperature is in the allowed temperature window, exactly 421 7 5 2 ° C at nominal power. The plug ang cladding temperature distribution is quite axisymmetric and does not present a cold spot on the spacer connection region. In the plug area, the net heat transferred by the spacers turns out to be negligible; as a matter of fact, loss of heat is mainly due to the dissipating fin effect of the plug itself. The second cladding experiences a hot spot (200°C) in correspondance of the spacer contact points. As far as the stress analysis is concerned, all components are well below the allowable limits. The highest stress is found in the spacer wire and is 76 MPa. The maximum stress in the plug is 40 and 28 MPa in the
F. Zacchia et al. / Pin-type ceramic blanket module cladding. The peak thermal stress in the second cladding, located in the hot spot zone, only reaches 14 MPa.
2.3. Mechanical analysis of the tube bundles 2.3.1. Blanket module constraints The purpose of this analysis was not only the verification of the mechanical behaviour of the double pipe and the grid straps during normal operation but also the optimization of the blanket constraint number and location. The initial layout of the grids was determined by imposing a limit to the vibration frequency of each pipe [3]. An optimization study has indicated that the best constraint scheme is the one illustrated in fig. 5. It can be noted, in particular, that at one over two grids, the front and rear tube bundles are discoupled. 2.3.2. Stress calculations The results of this analysis was an evaluation of the membrane and bending stresses in the double pipe, of the forces taken by the grid straps and of the general reaction loads transmitted from the grids to the backplate. Loading conditions are the temperature distribution along the pipes (60 ° C inlet - 90°C outlet), pressure, weight and fluido-dynamical forces due to the change in flow direction of the water (essentially at the lower manifold). Maximum stresses remain below 90 MPa in the pressure tube and below 80 MPa on the second cladding. The maximum forces taken by the straps are in the order of 200 N and are fully allowable. However, it can be noted that the highest stressed region is located at the counter-bent. It was shown that lower loads on the grids and lower stresses at the tubes/tube sheet contact zones would be obtained by setting the main blanket support (water tube sheet) horizontal and suppressing the counter-bent. This must be checked against positioning welds in a higher fluence region. 2.4. Design of manifolds The main problem of manifolding design is the high number of double pipes with a ratio pitch/diameter which is relatively small (48/41). As a consequence, the solution with cylindrical tubes cannot be adopted since space to weld and control the welds between tube and nozzle of the cylindrical manifold would be too small. The only envisageable solution seems then to be the use of a tube sheet with a welded bottom. Two configurations are then possible: the helium tank can be totally dipped inside the water tank or can constitute the second floor of a single component. This last case looks
109
preferable for loads distribution effects in the pipes, even if it might create some shielding problems. The reliability of the solution used to connect the tube extremity to the tube sheet is also a very important parameter in the iterative design process. For a given number of pins per segment, the major contribution to blanket unavailability comes from the manifolds. Therefore, the objectives have been: (1) to minimize the number of weldings, (2) to go for higher quality connections as expansions of pipe on the tube sheet. The feasibility of these solutions will be discussed in detail in the next chapter.
3. Feasibility aspects 3.1. Claddings Two aspects have been addressed: tolerances and holes. No special solution but grinding needs to be used to bring the internal and external diameters of a relatively short cladding segment to the tolerance of more or less 20 ~m. A good circularity tolerance is also required. As far as small holes through the cladding wall to help for tritium extraction, the problem must still be analysed in detail, since the hole diameters should be lower than 1 mm. Drilling looks then too expensive a solution.
3.2. Spacers Fabrication technology of the centering spring spacers must be considered in detail since they are the most critical items of the whole concept. The chosen material, inconel X-750 or 718, has already demonstrated good performances in nuclear environment. Fabrication from wires is easy and some spacers will soon be tested in an out-of-pile environment. However, such a solution cannot be adopted for a high number of spacers that must then be obtained by hot pressing.
3.3. Double pipe The three aspects to be addressed are: (a) fabrication of 10 m long pipes with tight tolerances on the internal diameter, (b) bending of the pipes and (c) double pipe assembly, these two problems being closely related. Aspect (a) does not look like a problem, since up to 3 m long and 0.5 mm thick pipes are currently pro-
110
F Zacchia et aL / Pin-type ceramic' blanket module
duced. However, if an increase in length appears as straight foreward, the actual usual fabrication tolerances (_+ 120 Fm) are rougher than those requested on the second cladding inner diameter (+_70 ~m). Some technological development must therefore be foreseen in this area. Aspect (b) may have two solutions, depending on whether a polygonal can be used to approximate the original curve or not. In case of a positive answer, machines are presently available which would make possible to fit the theoretical curve of each pipe with good accuracy. Otherwise, hot rolling of pipes to such large radii (5-7 m) is not so accurate and, up to now, has not been used for series production. Aspect (c) is, nevertheless, the one which forces the choice of the usable bending technic. As a matter of fact, an "ad hoc" method to realize bending of the two pipes together must be developed. It is clear that if the second cladding is to be introduced inside the pressure tube after bending, the bending radius of each pipe must be constant, which is not presently the case. In particular, a counter bent would require a weld on the outer pipe, provided that both bent and counter-bent have a uniform radius. Eventually, manufacturing tests are needed to control the quality of the pipe wall in the elbow region, due to the small starting thickness.
~ILi ,rtJN i4Nr ()
i
i~ o q d p , r; e ~ p a n s , c:,t
"
vs rFn
~;i
" ~///A
rA,~,~
,~'\\\K
Fig. 6. Tubes/tube sheet contact zone.
distribution in the tube sheets would probably be the critical parameter. 3.5. Grids
3.4. Manifolds The main goal, as recalled in the paragraph on reliability, is to achieve two connections of the highest possible quality between the double pipe extremities and the relative tube sheets. This means choosing a solution like expansion and head sealing weld, see fig. 6. It is unlikely that a mandrin head will be able to operate between the tapered end of the second cladding and the pressure tube wall. The tapered end must, in this case, be welded after that the aforecited expansion has been performed. Unfortunately, the addition of a weld goes against the component reliability. However, such a circumferencial TIG welding is a well known technic and its control does not appear too difficult in this configuration. Brazing of pipes to the plate has also been considered. It would probably be an interesting solution to develop if the whole component had not been too large to be put in a vacuum furnace for high-temperature brazing. Local heating by induction looks, on the other end, a process too difficult to control: temperature
The grid straps are 25 mm high steels of a rectangular cross-section which can be cold extruded. The slots for coupling are punched on a depth of 12.5 mm. It is to be noted that for simple assembly reasons, the tolerances on the pitch between slots (50 Fro) cannot be accumulated. Since the angle between the straps differs from grid to grid, depending on their poloidal coordinate, preassembly on a jig is necessary for each kind of grid. The geometry of the tube bundle is then reproduced by a set of dowels that simulate the reinforced pipes. The correct length of each strap and the correct diameter of each elastic collar must be set during these preassembly operations.
4. Conclusion At the conclusion of the ITER CDA, the EC reference pin-type driver blanket has shown to be one of the most interesting candidates. In particular, the small research and development work needed to assess the concept feasibility mainly derives from the today technology choice: fast-breeder reactor technology for the
F. Zacchia et al. / Pin-type ceramic blanket module
pins and nuclear heat exchanger technology for the manifolds. Numerous analyses support the design of the helium gap with spring spacers, which turn out to be, at least theoretically, a valid method for breeder temperature control. However, if experiments or further studies should demonstrate the unreliability of this approach, the option of batch tritium recovery could be envisaged. Manufacturing of tube bundles, manifolds and grids has been investigated and does not represent a major problem for the concept feasibility. Beside this, some procedures like bending of the double pipes with large
111
radii and the bundle assembly must still be studied in detail and optimized.
References
[1] ITER Documents Series No. 16: ITER conceptual design final report, IAEA Report (Vienna, 1991). [2] ITER Documents Series No. 29: ITER blanket, shield and data base, IAEA Report (Vienna, 1991). [3] G. Simbolotti et al., EC contribution to ITER blanket workshop, NET Report N/R/0832, Summer Session 1990.
105
Problems and solutions for design and construction of a pin-type ceramic blanket module F. Z a c c h i a a, M. G r a t t a r o l a ", F. R o s a t e l l i a a n d G . S i m b o l o t t i b a Ansaldo Divisione Nuooe Tecnologie, Corso Perrone, 25, 16161, Genoa, Italy b ENEA, CRE Frascati, Via E. Fermi, 27, 00044, Frascatti Rome, Italy
In the frame of the conceptual design of ITER, Ansaldo DNT and ENEA Frascati have brought foreward the design of a ceramic blanket. At the present state of the design, a module consists of two tube bundles containing breeder rod segments; the tubes are topped by helium/water manifolds and constrained to the back-plate by ten supporting grids. One fundamental objective of the work was to study the feasibility, in the near term, of this pin-type blanket. In particular, the feasibility of the thermal barrier between the sintered pellets (LiA102) and the low-temperature cooling water, was a matter of uncertainty. To reach the aforecited objective means the possibility of realizing a component with technologies which are currently used in the nuclear field. The main problems and solutions related to the mechanical design and construction of the blanket subcomponents are now briefly presented. From a theoretical approach, the problem of breeder temperature control has been solved: relatively short straight cladding segments keep the pellets centered inside the internal water pipe with the help of inconel spacers. An investigation has shown that an industrial effort is needed to develop a technology able both to produce long thin pipes, which constitute the water channel, with very tight tolerances and to bent them to the desired radius. Two different versions of manifolds have been proposed. In each case, care was taken to reduce the number and length of welds needed to connect the tubes to the relative tube sheets. The supporting grids of the egg-crate type are such to minimize the pitch between the pipes, have a relatively high stiffness and make possible differential thermal elongation between the bundles and the back-plate. Their feasibility has already been assessed since similar grids have been used in steam generators.
1. Introduction A N S A L D O and E N E A are collaborating since years on the conception and the design of ceramic breeder blankets. A m o n g others, a concept with v-LiA10 2 as breeder in the form of pellets and beryllium as multiplier, cooled by water, was considered potentially interesting. It was largely investigated in the frame of the International Tokamak Experimental Reactor (ITER) Conceptual Design Activities (CDA) and represents, at present, the EC reference driver blanket for I T E R [1]. The idea of developing a PIN geometry was launched in E N E A to try to take the maximum advantage from the past experience with fission reactors: to demonstrate the feasibility with present nuclear technology was a fundamental goal. Advantages of the concept are: (1) a simple cylindrical geometry; (2) the minimization of the heat fluxes, since multiplier and breeder are positioned in such a way to be separately cooled by water; (3) a low beryllium working temperature. 0920-3796/91/$03.50
The most critical item remains the realization of the thermal barrier which has the role, in ITER, of maintaining lithium alluminate above 450°C, to make on-line tritium recovery possible. After detailed thermo-mechanical and sensitivity analyses, a solution has been proposed and out-of-pile tests are now on their way in order to validate design calculations.
2. Design aspects 2.1. Description of concept The following refers to the concept as it was developed for an outboard side segment of I T E R [2] (fig. 1). As known, such a component is entirely enclosed in a First-Wall Box (FWB) and the process fluids (helium and water) inlet and outlet are located at the top of each module. The blanket can be subdivided into three main parts.
© 1991 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. A l l r i g h t s r e s e r v e d
106
IE Zacchia et al. / Pin-type ceramic blanket module (b) T w o double manifolds
The upper and lower manifolds are made by two tanks, one for water and the other for helium. The pressure tube and the second cladding are fixed to the water and helium tube sheets respectively. The upper water tube sheet also plays the role of main bundle support.
UPPER NANI
(c) The grids"
BACKPLM
A set of ten supporting grids is needed to constrain the bundles to the back plate. Grids of the egg-crate type have been chosen, where each pin is individually supported at four positions apart (see fig. 4): such a
FIR~
EOUATORIAL 5ECfION fRO~f ~UN~tt
Y TUBE B,
LOWERMANIFO
-
~
.J//
Fig. 1. Outboard side segment of ITER.
UPPER 5LCflO~
(a) The pin bundles
Each bundle, one front and one rear, is composed by pins arranged in a triangular pitch and having the same outer diameter (see fig. 2). The double pipe where water flows is the structural part of each pin. Ceramic pellets are contained inside straight cladding segments, the length of which depends on the double-pipe fabrication technology and on the poloidal coordinate of the segment itself. These segments are then kept centered inside the so called second cladding (see fig. 3) by special INCONEL spacers that must guarantee the minimum possible eccentricity of the helium gap during normal operation. Annular beryllium segments (3-20 cm high) are positioned outside along the pressure tube.
Fig. 2. Blanket equatorial and top cross-sections.
107
F. Zacchia et a L / Pin-type ceramic blanket module
design makes possible assembly as well as all the movements caused by the temperature differences inside the blanket and between pins and FWB. The grids transfer all the loads (weight, pressure on lower manifold, temperature) to the back plate, through a bolted frame. The tube-strap contact region has been suitably reinforced by positioning a collar around the pressure tube; around this collar a couple of half-rings gives the required flexibility to make assembly of the straps easier and, above all, to minimize friction between pipes and straps.
2.2. Thermo-mechanical analysis of a cladding segment 2.2.1. Aims and model
The analysis was performed on a first-row pin at an equatorial position. This case corresponds to the maximum volumetric heat deposition; this is 26 W / c m 3 for the breeder and 12 W / c m 3 for steel. The finite-element ANSYS code has been used, with thermal solid elements STIF70 and solid elements STIF45; the spacer was modelled with the elastic beam STIF3. r--- LiA[02 / $ ~ : ~ l s t cLodding
r,~,~
~
2ndcLadd/ng
~ s p a c e n
~
~sprin9
~
SECTION A
m
!6,C 1
20, 0 A 5ECFION B
~ 47
he[ium 9ap outen ctodding
beryLLium
pressure Lube wooer
2rid cLaddin9
~pacer
Ist cLaddin9 LML02 I
Fig. 3. Pin cross-sectionand blanketinternals.
108
F. Zaechia et al. / Pin-type ceramic blanket module
F-SPaCF~
~OUTERWATERPlP~ ~BERYLLJGM --ObIERCLADO!NG
BREEOINGPELLEF~ Ist CLAOOING-.~..~
/-ELASTIC SPACER
2~d C[APO!NG-JI..~._~"
GPlO51RAPJF...~-~-
05dPPOR/ /
//
i
•i
i I
L. _ Fig.
dPPERMAN!~O{D
~SJPP0~rlN00RIOS
_ 4~ 3 :
._
4. Egg-crate type grid.
Aims of this calculations are: to check the value of the breeder temperature window, focusing in particular on the spacer induced border effect; the precise determination of the structural material temperature; the assessment of stresses. The two important input parameters are related to heat transfer at interfaces. After a series of evaluations, the following averge values have been taken [3]: - thermal conductance b r e e d e r / c l a d d i n g : 3500 W / ( m 2 K); -thermal resistance s p a c e r / s e c o n d cladding: 980
K/W, 2.2.2. Results
lOWER MANIFOLD
Fig. 5. Constraint scheme of blanket.
The 3-D calculations have shown that breeder temperature is in the allowed temperature window, exactly 421 7 5 2 ° C at nominal power. The plug ang cladding temperature distribution is quite axisymmetric and does not present a cold spot on the spacer connection region. In the plug area, the net heat transferred by the spacers turns out to be negligible; as a matter of fact, loss of heat is mainly due to the dissipating fin effect of the plug itself. The second cladding experiences a hot spot (200°C) in correspondance of the spacer contact points. As far as the stress analysis is concerned, all components are well below the allowable limits. The highest stress is found in the spacer wire and is 76 MPa. The maximum stress in the plug is 40 and 28 MPa in the
F. Zacchia et al. / Pin-type ceramic blanket module cladding. The peak thermal stress in the second cladding, located in the hot spot zone, only reaches 14 MPa.
2.3. Mechanical analysis of the tube bundles 2.3.1. Blanket module constraints The purpose of this analysis was not only the verification of the mechanical behaviour of the double pipe and the grid straps during normal operation but also the optimization of the blanket constraint number and location. The initial layout of the grids was determined by imposing a limit to the vibration frequency of each pipe [3]. An optimization study has indicated that the best constraint scheme is the one illustrated in fig. 5. It can be noted, in particular, that at one over two grids, the front and rear tube bundles are discoupled. 2.3.2. Stress calculations The results of this analysis was an evaluation of the membrane and bending stresses in the double pipe, of the forces taken by the grid straps and of the general reaction loads transmitted from the grids to the backplate. Loading conditions are the temperature distribution along the pipes (60 ° C inlet - 90°C outlet), pressure, weight and fluido-dynamical forces due to the change in flow direction of the water (essentially at the lower manifold). Maximum stresses remain below 90 MPa in the pressure tube and below 80 MPa on the second cladding. The maximum forces taken by the straps are in the order of 200 N and are fully allowable. However, it can be noted that the highest stressed region is located at the counter-bent. It was shown that lower loads on the grids and lower stresses at the tubes/tube sheet contact zones would be obtained by setting the main blanket support (water tube sheet) horizontal and suppressing the counter-bent. This must be checked against positioning welds in a higher fluence region. 2.4. Design of manifolds The main problem of manifolding design is the high number of double pipes with a ratio pitch/diameter which is relatively small (48/41). As a consequence, the solution with cylindrical tubes cannot be adopted since space to weld and control the welds between tube and nozzle of the cylindrical manifold would be too small. The only envisageable solution seems then to be the use of a tube sheet with a welded bottom. Two configurations are then possible: the helium tank can be totally dipped inside the water tank or can constitute the second floor of a single component. This last case looks
109
preferable for loads distribution effects in the pipes, even if it might create some shielding problems. The reliability of the solution used to connect the tube extremity to the tube sheet is also a very important parameter in the iterative design process. For a given number of pins per segment, the major contribution to blanket unavailability comes from the manifolds. Therefore, the objectives have been: (1) to minimize the number of weldings, (2) to go for higher quality connections as expansions of pipe on the tube sheet. The feasibility of these solutions will be discussed in detail in the next chapter.
3. Feasibility aspects 3.1. Claddings Two aspects have been addressed: tolerances and holes. No special solution but grinding needs to be used to bring the internal and external diameters of a relatively short cladding segment to the tolerance of more or less 20 ~m. A good circularity tolerance is also required. As far as small holes through the cladding wall to help for tritium extraction, the problem must still be analysed in detail, since the hole diameters should be lower than 1 mm. Drilling looks then too expensive a solution.
3.2. Spacers Fabrication technology of the centering spring spacers must be considered in detail since they are the most critical items of the whole concept. The chosen material, inconel X-750 or 718, has already demonstrated good performances in nuclear environment. Fabrication from wires is easy and some spacers will soon be tested in an out-of-pile environment. However, such a solution cannot be adopted for a high number of spacers that must then be obtained by hot pressing.
3.3. Double pipe The three aspects to be addressed are: (a) fabrication of 10 m long pipes with tight tolerances on the internal diameter, (b) bending of the pipes and (c) double pipe assembly, these two problems being closely related. Aspect (a) does not look like a problem, since up to 3 m long and 0.5 mm thick pipes are currently pro-
110
F Zacchia et aL / Pin-type ceramic' blanket module
duced. However, if an increase in length appears as straight foreward, the actual usual fabrication tolerances (_+ 120 Fm) are rougher than those requested on the second cladding inner diameter (+_70 ~m). Some technological development must therefore be foreseen in this area. Aspect (b) may have two solutions, depending on whether a polygonal can be used to approximate the original curve or not. In case of a positive answer, machines are presently available which would make possible to fit the theoretical curve of each pipe with good accuracy. Otherwise, hot rolling of pipes to such large radii (5-7 m) is not so accurate and, up to now, has not been used for series production. Aspect (c) is, nevertheless, the one which forces the choice of the usable bending technic. As a matter of fact, an "ad hoc" method to realize bending of the two pipes together must be developed. It is clear that if the second cladding is to be introduced inside the pressure tube after bending, the bending radius of each pipe must be constant, which is not presently the case. In particular, a counter bent would require a weld on the outer pipe, provided that both bent and counter-bent have a uniform radius. Eventually, manufacturing tests are needed to control the quality of the pipe wall in the elbow region, due to the small starting thickness.
~ILi ,rtJN i4Nr ()
i
i~ o q d p , r; e ~ p a n s , c:,t
"
vs rFn
~;i
" ~///A
rA,~,~
,~'\\\K
Fig. 6. Tubes/tube sheet contact zone.
distribution in the tube sheets would probably be the critical parameter. 3.5. Grids
3.4. Manifolds The main goal, as recalled in the paragraph on reliability, is to achieve two connections of the highest possible quality between the double pipe extremities and the relative tube sheets. This means choosing a solution like expansion and head sealing weld, see fig. 6. It is unlikely that a mandrin head will be able to operate between the tapered end of the second cladding and the pressure tube wall. The tapered end must, in this case, be welded after that the aforecited expansion has been performed. Unfortunately, the addition of a weld goes against the component reliability. However, such a circumferencial TIG welding is a well known technic and its control does not appear too difficult in this configuration. Brazing of pipes to the plate has also been considered. It would probably be an interesting solution to develop if the whole component had not been too large to be put in a vacuum furnace for high-temperature brazing. Local heating by induction looks, on the other end, a process too difficult to control: temperature
The grid straps are 25 mm high steels of a rectangular cross-section which can be cold extruded. The slots for coupling are punched on a depth of 12.5 mm. It is to be noted that for simple assembly reasons, the tolerances on the pitch between slots (50 Fro) cannot be accumulated. Since the angle between the straps differs from grid to grid, depending on their poloidal coordinate, preassembly on a jig is necessary for each kind of grid. The geometry of the tube bundle is then reproduced by a set of dowels that simulate the reinforced pipes. The correct length of each strap and the correct diameter of each elastic collar must be set during these preassembly operations.
4. Conclusion At the conclusion of the ITER CDA, the EC reference pin-type driver blanket has shown to be one of the most interesting candidates. In particular, the small research and development work needed to assess the concept feasibility mainly derives from the today technology choice: fast-breeder reactor technology for the
F. Zacchia et al. / Pin-type ceramic blanket module
pins and nuclear heat exchanger technology for the manifolds. Numerous analyses support the design of the helium gap with spring spacers, which turn out to be, at least theoretically, a valid method for breeder temperature control. However, if experiments or further studies should demonstrate the unreliability of this approach, the option of batch tritium recovery could be envisaged. Manufacturing of tube bundles, manifolds and grids has been investigated and does not represent a major problem for the concept feasibility. Beside this, some procedures like bending of the double pipes with large
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radii and the bundle assembly must still be studied in detail and optimized.
References
[1] ITER Documents Series No. 16: ITER conceptual design final report, IAEA Report (Vienna, 1991). [2] ITER Documents Series No. 29: ITER blanket, shield and data base, IAEA Report (Vienna, 1991). [3] G. Simbolotti et al., EC contribution to ITER blanket workshop, NET Report N/R/0832, Summer Session 1990.