Numerical study of the influence of tungsten recrystallization on the divertor component lifetime

The thermonuclear fusion reaction between deuterium and tritium could become an alternative, sustainable and safe way to generate electricity in demand. Plasma facing components will ensure the mechanical integrity of the reactor internal walls and the heat extraction and must be compatible with the plasma operation to not compromise its exploitation. ITER and WEST (W -for tungsten- Environment in Steady-state Tokamak) components will be exposed to particles fluxes during transient loading up to 20 MW/m^2 . To withstand such loading, components are actively cooled and are made with pure tungsten as material facing the plasma. Although this technology fulfills ITER’s requirements, cracks appear in tungsten block under cyclic high heat flux at 20 MW/m^2 . Such cracks propagate from the exposed surface to the cooling pipe. Even if the appearance of this crack does not immediately affect the component heat exhaust capability, it could limit the reactor operation. In literature, numerical models were developed and tungsten recrystallization was discussed as playing a significant role on the component lifetime. To improve the numerical predictions, this paper deals with the development of a Finite Element model dedicated to the lifetime prediction of tungsten-based Plasma Facing Components when subjected to very high heat fluxes. Recrystallized tungsten is known to offer lesser resistance to thermal fatigue damage than as-received rolled or forged tungsten. The main novelty of this paper lies in the modelling of tungsten recrystallization that happens after a given number of pulses. This fully coupled model predicts recrystallization, the induced change in mechanical properties and the resulting progressive damage. This final model, named RXMAT, is implemented as a user subroutine in the finite elements code ANSYS. Fueled by the tungsten recrystallization kinetics and by up to date as-received rolled tungsten and recrystallized tungsten elastic-viscoplastic constitutive laws, RXMAT links the tungsten recrystallization fraction evolution to a mechanical stress and strain fields for the first time. This was never done for tungsten employed as armoured material in experimental fusion reactors. This approach paves the way to develop a multiphysics numerical tool able to take into account the entire tokamak environment constraints (neutron irradiation, chemical reactions, thermal loads, etc) to predict the lifetime of tungsten-based Plasma Facing components.