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 divertor mock-up were completed on the Electron Beam Test System at Sandia National Laboratories. The beryllium tiles were diffusion bonded onto an OFHC copper saddleblock and a DSCu (MAGT) tube containing a 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 5.0 m/s flow velocity and an inlet temperature of 8-15 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 third tile debonded after 9200 thermal fatigue cycles at 5 MW/m2, while another debonded after 6800 cycles. In all cases, fatigue failure occurred in the inter metallic layers between the beryllium and copper. No fatigue cracking of the bulk beryllium was observed. During thermal cycling, a gradual loss of porous coating produced increasing sample temperatures. These experiments indicate that diffusion-bonded beryllium tiles can survive several thousand thermal cycles under ITER relevant conditions without failure. However, the reliability of the diffusion bonded joint remains a serious issue. (orig.)