Performance estimation of beryllium under ITER relevant transient thermal loads

Abstract

The plasma facing first wall in ITER will be armored with beryllium. During operation, the armor has to sustain direct plasma contact during the start-up and ramp-down of the plasma. On top, transient thermal loads originating from a variety of plasma instabilities or mitigation systems are impacting the 8–10 mm thick beryllium tiles. In this work, possible armor thickness losses caused by the expected transient heat loads are reviewed. Applying conservative assumptions, vertical displacement events can cause locally a melt layer with a thickness of up to 3 mm. However, cracks after solidification/cool down are confined to the melt layer and the connection between melt layer and bulk remains strong. Radiative cooling mechanisms can be applied to significantly decrease the melt and evaporation layer thickness. To mitigate the critical damage potential of plasma disruptions, massive gas injections or shattered pellet injections can be deployed to transform the stored plasma energy into radiation, which implies a much more homogeneous distribution of energy to the plasma facing components. For a full power plasma discharge in ITER, these radiative loads can cause temperatures exceeding the melting temperature of beryllium. Experiments have demonstrated that a thickness of 340 µm at the entire first wall armor can be affected by these mechanisms over the lifetime of ITER. Edge localized modes with expected characteristics obtained by fluid model simulations caused fatigue cracks with a depth of up to 350 µm in experimental simulations. The critical heat flux factor FHF above which inflicted damage accumulates with each subsequent pulse has been determined to be in the range of FHF ≈ 9–12 MW m−2 s0.5. The damage from thermal loads below this threshold saturates between 104 and 106 pulses. Neutron irradiation has a deteriorating effect on the thermomechanical properties of beryllium, which strongly influence its resistance against thermally induced damages. The rather low neutron fluence over the lifetime of ITER is expected to reduce the material strength and thermal conductivity by a few tens of percents. If the thickness losses are affected to a similar extent, a sufficient margin of armor thickness will remain. Overall, the damage imposed by radiative loads from massive gas injections or shattered pellet injections is expected to be the dominant force influencing the condition of the first wall armor, at least if all disruptions can be successfully mitigated and the number of vertical displacement events can be constrained to a few occurrences over the service time of ITER. Keywords: ITER, First wall, Plasma facing materials, Beryllium, Erosion, Transient thermal load, ELMs, Massive gas injections, Shattered pellet injections