Thermal Fatigue Testing of a Diffusion-Bonded Beryllium Divertor Mock-Up Under ITER-Relevant Conditions

Thermal response and thermal fatigue tests of four 5-mm-thick beryllium tiles on a Russian Federation International Thermonuclear Experimental Reactor (ITER)-relevant divertor mock-up were completed on the electron beam test system at Sandia National Laboratories. The beryllium tiles were diffusion bonded onto an oxygen-free high-conductivity copper saddle-block and a dispersion-strengthened copper alloy tube containing a copper porous coating. Thermal response tests were performed on the tiles to an absorbed heat flux of 5 MW/m² and surface temperatures near 300°C using 1.4 MPa water at 5 m/s flow velocity and an inlet temperature of 8 to 15°C. One tile was exposed to incrementally increasing heat fluxes up to 9.5 MW/m² and surface temperatures up to 690°C before debonding at 10 MW/m². A second tile debonded in 25 to 30 cycles at <0.5 MW/m². However, a third tile debonded after 9200 thermal fatigue cycles at 5 MW/m², while another debonded after 6800 cycles. Posttest surface analysis indicated that fatigue failure occurred in the intermetallic layers between the beryllium and copper. No fatigue cracking of the bulk beryllium was observed. It appears that microcracks growing at the diffusion bond produced the observed gradual temperature increases during thermal cycling. These experiments indicate that diffusion-bonded beryllium tiles can survive several thousand thermal cycles under ITER-relevant conditions. However, the reliability of the diffusion-bonded joint remains a serious issue.