Proceedings of ITC/ISHW2007

Design and Optimization of Support Post for Cryogenic
Components in the FFHR
Hitoshi TAMURA, Akio SAGARA and Shinsaku IMAGAWA
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan

FFHR is a design of a steady state fusion reactor which has been studied to demonstrate LHD-type fusion power
plant. The weight and the deformation of the cryogenic components are sustained by support posts. A folded multiplate type post which was adopted in the LHD could be used for the FFHR. The dimensions of the post were
decided according to buckling load estimation against a gravitational load. Using this fundamental design, aflexibility and stress distribution of the post were calculated by using FEM analysis. The flexibility against a radialdisplacement was 24kN/mm and the maximum stress was 159MPa for CFRP plate and 390MPa for stainless steel
plate, respectively. These were under allowable level for the materials. The heat loads were also calculated and10.5kW to 80K and 0.34kW to 4K were obtained and the result showed that the head load to 4K was almost 1/20
compare with the post which was all made of stainless steel. The natural frequency was analyzed to estimate
safeness against external load such as an earthquake. Those results showed that the LHD-type support post waspractical for the FFHR in mechanical and thermal points of view.
Keywords: FFHR, fusion reactor, superconducting coil, cryogenic component, support post, modal analysis

1. Introduction
FFHR is a concept design of a steady state fusion
reactor which has been studied to demonstrate LHD-type
fusion power plant [1-3]. Fig. 1 shows the schematic drawof the cryogenic components in the FFHR. Total weight ofsuperconducting coils and supporting structure exceeds
16,000 tons. Since the cryogenic components can bemainly made of stainless steel, the thermal contraction
between room temperature and the cryogenic temperature
is about 0.3%. For example, the maximum displacement

Fig. 1 Schematic draw of the cryogenic components inthe FFHR.
author’s e-mail:tamura@nifs.ac.jp

caused by the thermal contraction is almost 55mm at 18m
in radial position from the center of the device where theouter poloidal field coils are set. The weight and thedeformation are sustained by support posts which are set
on a base plate of a cryostat vessel whose temperature isroom temperature. The support post must have following
functions; support the weight of the cryogenic components,
reduce heat leak to lower temperature side, absorb thethermal contraction, maintain a cyclic deformation, give asafe warranty against an external load such as anearthquake.
The Large Helical Device (LHD) has 3.9m of major
radius and 850 tons of cold mass. The LHD adopted a“folded multi plates” type post [4,5]. The post consisted ofcarbon reinforced plastic (CFRP) and stainless steel (SS)plates. The SS plates were cooled down to 80K to be athermal anchor region and the plates were connected to theCFRP plates through the thick SS blocks. To minimize aheat leak to low temperature side, the FRP plates were used
between room temperature and the thermal anchor region,and the thermal anchor region to the cryogenic temperature.
The maximum thermal contraction at the post position was
13mm and the post absorbed the contraction by its
flexibility against the radial direction of the components.
The CFRP was chosen since it has much larger
compressive strength than another low heat conductive
materials such as glass fiber reinforced plastic. This type ofProceedings of ITC/ISHW2007
post can be also valid for the support post of the FFHR. A
conceptual design and specification of the folded multiplate type post is introduced and stress distribution under
several kind of operational situation are shown in this paper.
The deformation mode and natural frequency of thecryogenic components including the post are also
calculated and safeness against an earthquake is discussed.

2. Basic geometry of the folded multi plates
The cryogenic components which are cooled down tocryogenic temperature are superconducting helical coils,superconducting poloidal coils, and the coil supporting
structure. Total weight of the cryogenic components in
FFHR has been estimated 16,000ton so far. Considering asymmetrical and homogeneous distribution of thegravitational load, number of the posts were decided 30; 20
posts were located under the outer poloidal coil and other10 posts were set under the inner poloidal coil. The span ofeach outer and inner post was 5.7m and 6.2m, respectively.
The CFRP plates in the folded multi plate type post are
subjected to a compressive load while the SS plates are
subjected to a tensile load. The geometrical dimensions of
the CFRP plates were decided according to buckling load
estimation against a gravitational load with bent longcolumn model. The buckling load in this model can be
describes as follows.

Pk =π 2 EI

4 L2
bh 3
I=,12,(1)

where, E is Young’s module of the longitudinal direction, I
is the flexural rigidity, b is the width, h is the thickness, andL is the length of the plate. We defined that the safety
factor against the buckling load was over 5 times larger
than the nominal weight; (16,000ton)/(30posts)*(safetyfactor 5)=2667tons. The cross sectional dimensions of theCFRP plate were assumed 1200mm in width and 80mm inthickness considering a setting space. Fig. 2 shows thebuckling load against plate length with the parameters of
the number of the plate. Since the post must have flexibility,longer plate was desirable but an increase of number of theplate makes the size and weight bigger. 1.2m of the length
was chosen and 5 plates and 3 plates were adopted for thecenter column and outer column, respectively. For thestainless steel plate, 1.2m in width, 30mm in thickness,
1.0m in length, and 3 plates for both side were chosenconsidering the tensile load and the strength of SS material.
Fig. 3 shows the setting up of the support post and its
dimension. The weight of a post itself was 10 tons in thiscase.

Fig. 2 Buckling load of the CFRP plate with the crosssection 1200mm x 80mm using the bent longcolumn model.

Fig. 3 Schematic draw of the support post for the FFHR.

3. FEM analysis
Using the basic geometrical design of the folded
multi plates, a flexibility and stress distribution of thepost were calculated by using FEM analysis. The FEmodel of the support post was prepared by using 3D
beam element for a rigidity estimation and 3D shellelement for a stress distribution analysis. A thermal
condition was also investigated by 3D shell model. Theentire 3D model included cryogenic components. Since
the cryogenic components were rigid enough comparedwith support post, we prepared a simple torus shaped
structure having not only the same weight of thecomponents but also the same geometrical location of thecenter of gravity. This torus shaped structure was set on
30 support posts. The natural frequency of this structure
was calculated by using this model. Physical properties ofthe stainless steel used in the analysis were from database software [6]. The property of the CFRP material wasan experimental data of the support post of LHD
measured by a manufacture. ANSYS was used forcalculation.

Proceedings of ITC/ISHW2007

3.1 Static analysis
The flexibility against a radial direction was
22.9kN/mm and the maximum stress when 55mm offorced displacement was subjected to the top of the post
was 155MPa for CFRP plate and 544MPa for stainlesssteel plate, respectively. These were under allowable level
for the materials and they could be reduced by setting thepost to the deforming direction in advance. For example,
if the post bent 27.5mm to the outward direction and thenthe cryogenic components were set on the post, the final
position of the top of the post could be 27.5mm bent to
the inward direction since the thermal contraction of thecryogenic components forcibly acted the post. The stresslevel caused by the thermal contraction could be reduced
to almost the half by this philosophy. The circumferential
direction was more than 10 times as rigid as the radial
direction.
The heat loads calculated by using the same model
were 10.5kW to 80K and 0.34kW to 4K were obtained
and the result showed that the head load to 4K was almost
1/20 compare with the post which was all made ofstainless steel. On the other side, heat load to 80K wasFig. 4 The distribution of von Mises stress in the CFRPplates (above) and the SS plates (bottom) when
the gravity load was only subjected.

about a half of the all SS made post.
Here we introduced three operating situations andcalculated the stress distribution for each case.
(1) Gravity onlyThis situation simulated a normal condition before
cooling down. The top of the post was bent 27.5mm tothe outward and no thermal contraction. In this case, themaximum von Mises stress appeared in CFRP plates andSS plates were 147MPa and 340MPa, respectively. The
contour expression of the stress distributions are shown infig. 4.
(2) Gravity + cool down + 0.2G of transverse acceleration.
This situation simulated 0.2G of transverse seismic
load is subjected to the device while the cryogenic
components are cooled down to 4K. The maximum vonMises stress appeared in CFRP plates and SS plates wereboth increased and the values were 159MPa and 390MPa,
respectively as shown in fig. 5.
(3) Gravity + cool down + 1.0G of transverse acceleration.
In case of 1.0G of transverse acceleration wassubjected, the maximum von Mises stress in CFRP plates
was 429MPa and 881MPa in the SS plates.

Fig. 5 The distribution of von Mises stress in the CFRP
plates (above) and the SS plates (bottom) when
the gravity and 0.2G of transverse acceleration
were subjected together with the thermalcontraction.

Proceedings of ITC/ISHW2007

Fig. 6 Results of modal analysis. Natural vibrations mode and their frequency.
3.2 Modal analysis
The natural frequencies of the structure were in arange of 3Hz and 8Hz. Fig. 6 shows the first four modes ofnatural vibrations and their frequencies. The first mode wasa horizontal vibration and the second one was rotational
deformation on its axis. The third and fourth modes wereconcerning with an up-down movement. The higher
natural frequencies exceeded 15Hz and the deformation
modes were mainly in the cryogenic components.

4. Discussion
The safeness of the structure against an earthquake
depends on a frequency and a magnitude of a seismic
acceleration. Since a typical earthquake has the range of
0.5Hz to 20 Hz frequency, a structure which has a naturalfrequency near this range may resonate. From a response
spectrum analysis of the typical earthquake such as El
Centro, TAFT, etc, the response acceleration of this range
would be 1.0 to 1.5G. The analytic result of 1.0G oftransverse acceleration load seems that this case is a limitof transverse load since the maximum von Mises stress
was slightly larger than permissible stress. This transverse
load limit could be a criterion for a design of the building.

5. Conclusions
The “folded multi plates” support post was designed
for the FFHR and obtained following conclusions.
The flexibility against a radial deformation was24kN/mm. The circumferential direction was more than 10times as rigid as the radial direction.
The total heat load to 4K was estimated not to exceed
340W. It was almost 1/20 value compare with the post
which was all made of stainless steel.
The maximum von Mises stress was 159MPa for
CFRP plate and 390MPa for stainless steel plate,respectively when 0.2G of transverse acceleration wassubjected to the components under cooled down situation.
Those stress level were allowable value.
Several natural frequencies of the structure closed tothe frequencies in a seismic vibration. To keep the safenessof the device, the acceleration subjected to the structure
should not be exceeded 1.0G from the results of staticanalysis.

Acknowledgement
This work was supported by National Institute for
Fusion Science (07ULAA117).

References
[1]
[2]
[3]
[4]
[5]
[6]

O. Motojima et al. Fusion Eng. Des. 81 2277 (2006).
Sagara A et al. Fusion Eng. Des. 81 2703 (2006).
Sagara A et al. Nucl. Fusion 45 258 (2005).
H. Tamura et al. Fusion Technology 1996 1019 (1996).
H. Tamura et al. Adv. in Cryogenic Eng. 45 753 (2000).
Cryocomp software, Eckels Engineering and Cryodata Inc.
(1997).