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 berylliu m tiles on a Russian Federation International Thermonuclear Experiment al Reactor (ITER)-relevant divertor mock-up were completed on the elec tron beam test system at Sandia National Laboratories. The beryllium t iles were diffusion bonded onto an oxygen-free high-conductivity coppe r saddle-block and a dispersion-strengthened copper alloy tube contain ing a copper porous coating. Thermal response rests were performed on the files to an absorbed heat flux of 5 MW/m(2) and surface temperatur es near 300 degrees C using 1.4 MPa wafer at 5 m/s flow velocity and a n inlet temperature of 8 to 15 degrees C. One tile was exposed to incr ementally increasing heat fluxes up to 9.5 MW/m(2) and surface tempera tures up to 690 degrees C before debonding at 10 MW/m(2). A second til e debonded in 25 to 30 cycles at <0.5 MW/m(2). However, a third file d ebonded after 9200 thermal fatigue cycles at 5 MW/m(2), while another debonded after 6800 cycles. Posttest surface analysis indicated that f atigue failure occurred in the intermetallic layers between the beryll ium and copper. No fatigue cracking of the bulk beryllium was observed . It appears that microcracks growing at the diffusion bond produced t he observed gradual temperature increases during thermal cycling. Thes e experiments indicate that diffusion-bonded beryllium files can survi ve several thousand thermal cycles under ITER-relevant conditions. How ever, the reliability of the diffusion-bonded joint remains a serious issue.