Fatigue cracking of a bare steel first wall in an inertial confinement fusion chamber

• Pulsed fusion X-rays cause a compressive stress spike on first walls, induced by thermal expansion.
• Between fusion pulses, the plasticity induced during compression results in tension on surface micro-cracks.
• Transient thermal–mechanical calculations show likely crack propagation as a result of fatigue loading.
• Cracks should arrest within 100 μm because the driving force decays rapidly away from the front surface.

Inertial confinement fusion power plants will deposit high energy X-rays onto the outer surfaces of the first wall many times a second for the lifetime of the plant. These X-rays create brief temperature spikes in the first few microns of the wall, which cause an associated highly compressive stress response on the surface of the material. The periodicity of this stress pulse is a concern due to the possibility of fatigue cracking of the wall. We have used finite element analyses to simulate the conditions present on the first wall in order to evaluate the driving force of crack propagation on fusion-facing surface cracks.Analysis results indicate that the X-ray induced plastic compressive stress creates a region of residual tension on the surface between pulses. This tension film will likely result in surface cracking upon repeated cycling. Additionally, the compressive pulse may induce plasticity ahead of the crack tip, leaving residual tension in its wake. However, the stress amplitude decreases dramatically for depths greater than 80–100 μm into the fusion-facing surface. Crack propagation models as well as stress-life estimates agree that even though small cracks may form on the surface of the wall, they are unlikely to propagate further than 100 μm without assistance from creep or grain erosion phenomena.