THERMOMECHANICAL ANALYSIS OF THE ITER BREEDING BLANKET*

by

Saurin Majumdar and Herbert Gruhn Program Argonne National Laboratory 9700 S. Cass Avenue Argonne, IL 60439 USA

and

Yousry Gohar and Mark Giegerich FTER Joint Central Team Garching Joint Work Site c/o Max-Planck-Institut fur Plasmaphysik Boltzmannstrasse 2,85748 Garching bei Miinchen, Germany

DISTRIBUTION Of THIS DOCUMENT IS UNLIMITED o March, 1997

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Paper to be presented at the 4th International Symposium on Fusion Nuclear Technology (ISFNT-4), April 6-11, 1997, Tokyo.

* Work supported by Office of Fusion Energy, U.S. Department of Energy, under Contract No W-31-109-Eng-38. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. THERMOMECHANICAL ANALYSIS OF THE ITER BREEDING BLANKET

by

Saurin Majumdar and Herbert Gruhn Fusion Power Program Argonne National Laboratory 9700 S. Cass Avenue Argonne, IL 60439 USA

and

Yousry Gohar and Mark Giegerich HER Joint Central Team Garching Joint Work Site c/o Max-Planck-Institut fur Plasmaphysik Boltzmannstrasse 2, 85748 Garching bei Munchen, Germany

ABSTRACT

Thermomechanical performance of the ITER breeding blanket is an important design issue because it requires first, that the thermal expansion mismatch between the blanket structure and the blanket internals (such as, beryllium multiplier and breeders) can be accommodated without creating high stresses, and second, that the thermomechanical deformation of various interfaces within the blanket does not create high resistance to heat flow and consequent unacceptably high temperatures in the blanket materials. Thermo- mechanical analysis of a single beryllium block sandwiched between two stainless steel plates was carried out using the finite element code AB AQUS to illustrate the importance of elastic deformation on the temperature distributions. Such an analysis for the whole ITER blanket needs to be conducted in the future. Uncertainties in the thermomechanical contact analysis can be reduced by bonding the beryllium blocks to the stainless steel plates by a thin soft interfacial layer. 1.0 Introduction

In addition to performing all the shielding functions of the Basic Performance Phase (BPP), the UER breeding blanket [1] also provides the necessary tritium fuel to achieve the fluence goal during the Enhanced Performance Phase (EPP). To maintain compatibility with the ITER design of the BPP, zirconate (Li2Zr03) ceramic breeder and water coolant are have been proposed for the EPP. Enriched lithium and beryllium multiplier are utilized to enhance the tritium breeding ratio, to reduce the radial blanket thickness, to improve the shielding performance, and to reduce the breeder material fraction in the blanket. Both forms of beryllium material, blocks and pebbles are used at different blanket locations based on design requirements. As for the shielding blanket, Type 316LN austenitic steel is used as structural material.

The present paper is concerned with the problem of heat transfer across all interfaces of the inboard blanket and, in particular, thermomechanical interactions between the first wall and the beryllium block immediately behind it.

2.0 Description of Blanket Module

The inboard blanket modules have two breeder zones embedded in beryllium with three coolant panels (Fig. 1). The first wall has the first coolant panel to remove the surface heat flux and the nuclear heating from the front section of the breeding blanket. It consists of 13 mm thick Type 316LN stainless steel plate with built in rectangular coolant channels 4 mm x 10 mm separated by 4 mm walls. A 5 mm beryllium is used as coating material to protect the first wall from the plasma interaction. The second panel is located between the two breeder zones separated by beryllium. The last coolant panel is located at the back section of the module to remove the nuclear heating from the back section of the breeding blanket and the module structure.

A critical factor in the ITER breeding blanket design is the ability to predict heat transfer across various interfaces so that the temperature of the breeder can be kept within a prescribed range while at the same time keeping the maximum stress in the first wall due to contact pressure from the beryllium within reasonable limits. As will be illustrated in this paper, thermomechanical contact is an important design issue that may impact the blanket performance. An alternative design approach proposed by the European Union is to use a soft layer at the interfaces to reduce the uncertainty in the thermal resistance at the interfaces.

3.0 An Illustration of Thermoelastic Contact

Thermoelastic contact between two solids is a complex nonlinear phenomenon [2-3] requiring iterative solutions between thermal and stress analyses. Detailed analysis of thermomechanical contact between a single block of beryllium held under pressure between two rigid stainless steel plates was presented in Ref. 4. It was shown that the maximum temperature drop through the beryllium block due to a surface heat flux could be significantly higher when thermoelastic deformations were taken into account as compared to when the contact is uniform.

To illustrate the thermoelastic contact problem, a two-dimensional iterative thermomechanical analysis using the finite element code ABAQUS was conducted for a symmetrically situated 127 mm thick and 100 mm wide beryllium block subjected to a volumetric heating rate of 30MW/m3. The block was sandwiched between two 9 mm thick clamped stainless steel plates internally cooled by water at 423K. To exaggerate the thermoelastic effects, cooling was deliberately chosen to be insufficient and all the thermophysical properties were assumed to be independent of temperature. Thus the problem does not represent a realistic design but is used only as an illustration. Analyses were conducted for two interfacial conditions. In the first case, the beryllium block was perfectly bonded to the stainless steel; the temperature gradient along the contact surface of the stainless steel is -100K (Fig. 2). The heat flow from the beryllium to the coolant holes in the stainless steel plates are approximately uniform along the width of the beryllium block. In the second case, the beryllium block was not bonded but was simply in contact with the stainless steel plates with the thermal conductance at the interfaces varying as a function of the gap created during deformation [5]. The effects of a change in interfacial pressure or radiation on interfacial heat transfer were ignored. As shown in Fig. 3, the resulting heat flow pattern changes drastically and a temperature gradient > 1000K is created along the contact surface of the stainless steel between the region of contact and the region with gaps. Although such extreme temperatures cannot occur in reality, thermoelastic deformations will very likely play an important role in determining the temperature distribution in the blanket. An attempt to solve a two-dimensional thermomechanical contact problem for the complete midplane section of the ITER inboard blanket by the finite element code AB AQUS is currently under consideration 4.0 Results of Analysis Using Soft Interface Layers

An alternative to bypass the uncertainties in heat transfer across contacting interfaces is to bond the beryllium blocks to the stainless steel plates by means of a soft interfacial layer. For the present purposes, each beryllium-to-stainless steel interface was assumed to be bonded by a 1 mm thick layer having the same thermal properties as stainless steel but with a reduced modulus of elasticity, which was varied to simulate layers with different stiffness. A simplified model of half of a mid plane section of an JLTER inboard blanket module was analyzed by the AB AQUS finite element code. The first wall was taken as a sandwich of a 3 mm and a 6 mm plates separated by 4 mm high coolant channels with a 5 mm beryllium tile. The beryllium blocks were slotted with 1 mm wide slots and 1 mm thick ligaments. Thermal conductivity, thermal expansion coefficient, and elastic modulus of each component (beryllium was assumed to be 85% dense) were input as functions of temperature. The surface heat flux on the first wall was 0.5 MW/m2 and a radially varying nuclear volumetric heating rate appropriate for ITER corresponding to an average neutron wall loading of 1 MW/m2 was used. The coolant temperature was 423K.

A temperature profile across the whole module cross-section is shown in Fig. 4. The temperature in the first breeder layer varied between 600K and 900K and that in the second breeder layer varied between 600K and 770K. For lithium zirconate breeder, these temperatures result in 100 gram tritium inventory in the breeder material for the whole reactor, which is quite satisfactory. Because of heating from both sides, the temperature profile across the first wall is approximately symmetric with a maximum ~570K (Fig. 5).

The maximum compressive toroidal stress in the first wall decreases significantly with decreasing stiffness of the interfacial layer although the maximum tensile stress increases somewhat (Fig. 6). Because of the assumption of generalized plane strain, the maximum poloidal stress stays relatively constant at 770 MPa (compressive). Since the beryllium blocks are segmented both poloidally and toroidally, this is an overestimate of the poloidal stress. A three-dimensional model is needed to obtain a better estimate of the poloidal stress.

5. Conclusions

The heat flow across the interface and the temperature distribution of two contacting bodies can be significantly affected by thermoelastic deformation of the bodies. Therefore, it is recommended that the JTER blanket design be validated by testing in a fission reactor to simulate the nuclear heating in the different materials and the heat transfer across all interfaces correctly.

An alternative is to bond the beryllium blocks to the stainless steel plates by a thin and soft bond layer. The maximum toroidal stress in the first wall can be reduced significantly by reducing the stiffness of the bond layer. However, a three-dimensional model is needed to properly evaluate the poloidal stresses. Further, the integrity of the bond layer under shear loading has to be demonstrated.

References

1. Y. Gohar, M. Billone, A. Cardella, I. Danilov, W. Danner, M. Ferrari, M. Giegerich, K. Ioki, T. Kuroda, D. Lousteau, P. Lorenzetto, S. Majumdar, R. Mattas, K. Mohri, R. Parker, R. Raffray, Y. Strebokov, H. Takatsu, and E. Zolti, Design and Analysis of the ITER Breeding Blanket, 19th Symposium on Fusion Technology, Lisbon, Portugal, September 16-20, 1996.

2. J.R. Barber, Indentation of the Semi-Infinite Elastic Solid by a Hot Sphere, Int. J. Mech. Sci., Vol. 15, 1973, p. 813.

3. J.R. Barber, Contact Problems Involving a Cooled Punch, J. Elasticity, Vol. 8, 1978, p. 409.

4. A. Stojimirovic and S. Majumdar, Analysis of the Thermomechanical Contact Between Beryllium Blocks and Stainless Steel Plates in ITER Design, in Fusion Tech. Vol. 23, 1993, pp. 309-315.

5. U.S. Technical Report for the FTER Blanket/Shield, 1TER-TN-BL-55-0-3, ITER Team, 1990. Figure 1. ITER inboard blanket cross section NTH 1 +5.21E+02 2 •6.20E*02 3 •7.19E+02 4 •8.18B+02 5 +9.16E+02 6 +1.01E+03 7 +1.11E+03

Figure 2 Temperature (in K) distribution in one of two cooled stainless steel plates symmetrically bonded to a central beryllium block which is subjected to volumetric heating. The plates are deliberately cooled insufficiently to exaggerate the difference between the bonded and the thermomechanical contact cases. NTH VALUE

-— '.AU-— i \ ( R577T r • ii —i — m

•"' I. •. ———^—— 1^ "——^«

Figure 3 Temperature (in K) distribution in one of two symmetrical cooled stainless steel plates in thermoelastic contact with a central beryllium block which is subjected to volumetric heating. The plates are deliberately cooled insufficiently to exaggerate the difference between the bonded and the thermomenhanical contact cases. Maximum value = 892.3 at node 3707 Minimum value = 413.6 at node 2007 NTH VALUE . ii. • +4.19E+02 LJ- +4.98E+02 IHL +5.76E+02 ^^HHL. 4-6 .55E+02 ^^^H— +7 .34E+02 ^^^H— +8 .13E+02 ^^™— +8 .92E+02

a o • a n n • o a a

Figure 4 Temperature (in K) distribution in the ITER inboard blanket for the case where the beryllium blocks are bonded to the stainless steel plates. NTH

M&XUDUSt value = 793.6 at node 2518 Minimum value = 423.0 at node 2466

2

3 1

Figure 5 Temperature (in K) distribution in the ITER inboard blanket first wall subjected to a surface heat flux of 0.5 MW/m2 for the case where it is bonded to the beryllium blocks. 200 CO Q. 100 CO CO Maximum tensile 2 55 stress (0 •JO "gS -100

I -200 Maximum compressive stress iZ -300 10 100 1000 E(steel)/E(layer)

Figure 6 Variation of the maximum tensile and compressive stresses in the first wall with decreasing stiffness of the interface layer.