RESONANT CREEP ENHANCEMENT IN AUSTENITIC STAINLESS-STEELS DUE TO PULSED IRRADIATION AT LOW-DOSES

Creep responses of austenitic stainless steels to cyclic 10 MeV deuter on irradiation have been examined, for solution-annealed (SA) or 20% c old-worked (CW) 316 stainless steel (SS) and Fe-25Ni-15Cr alloy. After the strain rate reached a steady state under a long-term continuous i rradiation, the irradiation mode was switched to square waveforms (pul se width tau(p) = 10 ms-1000 s). Anomalous large creep enhancement due to the cyclic irradiation was observed at a particular pulse width ra nge around tau(p) = 100 s, even at a low damage rate of 2.0 x 10(-7) d pa s(-1) whereas the specimen gave zero or negative strain changes wit h respect to the steady state level at tau(p) < 1 s or tau(p) > 200 s. Consequently, the pulse width dependence of strain change had a reson ant feature with a peak around tau(p) = 100 s. The SA-316 SS and SA- a nd CW-Fe-2SNi-15Cr alloys exhibited strain increments of about 10(-4) for 10(-3) dpa, which were larger by one to two orders of magnitude th an the steady state creep. The mechanism of the large creep is ascribe d to pulse-induced point defect enrichment, by the aid of a rate theor y. It was thus demonstrated that first-wall materials in a plasma oper ation of about 10(2) s may suffer from unexpected transient creep and that the dynamic aspects for damaging effects on the first-wall materi als should be taken into account, especially for fusion devices to be operated at low temperatures such as ITER.