Simulation of cracks in tungsten under ITER specific heat loads
Forschungszentrum Karlsruhe, Institute for Pulsed Power and Microwave Technology
P.B. 3640, D-76021, Karlsruhe, Germany
The problem of high tritium retention in co-deposited carbon layers on the walls of ITER vacuum chamber motivates investigation of materials for the divertor armour others than carbon fibre composite (CFC). Tungsten is most probable material for CFC replacement as the divertor armour because of high vaporisation temperature and heat conductivity. In the modern ITER design tungsten is a reference material for the divertor cover, except for the separatrix strike point armoured with CFC. As divertor armour, tungsten should withstand severe heat loads at off-normal ITER events like disruptions, ELMs and vertical displacement events.
Experiments on tungsten heating with plasma streams and e-beams have shown an intense crack formation at the surface of irradiated sample [1,2]. The reason for tungsten cracking under severe heat loads is thermostress. It appears as due to temperature gradient in solid tungsten as in resolidified layer after cooling down. Both thermostresses are of the same value, but the gradiental stress is compressive and the stress in the resolidified layer is tensile. The last one is most dangerous for crack formation and it was investigated in this work.
The thermostress in tungsten that develops during cooling from the melting temperature down to room temperature is ~8-16GPa. Tensile strength of tungsten is much lower, < 1 GPa at room temperature, and at high temperatures it drops at least for one order of magnitude. As a consequence, various cracks of different characteristic scales appear at the heated surface of the resolidified layer.
For simulation of the cracks in tungsten the numeric code PEGASUS-3D  has been applied. Originally the code has been developed for simulation of brittle destruction in CFC and graphites. It has been tested against plasma gun experiments and proved reliability of its predictions. Now it has been modified to simulate crack formation in tungsten using a model for crack generation in the resolidified surface layer and propagation of cracks in the bulk. The model assumes that initially the resolidified layer is stress-less at the melting temperature and then the tensile stress develops in the layer during its cooling down. First results of the simulations are reported. The simulations reproduce tungsten crack morphology and predict the crack densities and the cracks depth.
 V.I. Tereshin, A.N. Bandura, O.V. Byrka et al. Repetitive plasma loads typical for ITER type-I ELMs: Simulation at QSPA Kh-50. PLASMA 2005. ed. By Sadowski M.J., AIP Conference Proceedings, American Institute of Physics, 2006, V 812, p. 128-135.
 J. Linke. Private communications.
 S. Pestchanyi and I. Landman. Improvement of the CFC structure to withstand high heat flux. Fusion Engineering and Design v. 81/1-7 pp. 275-279