In addition to post-irradiation creep tests, in-pile creep tests were perfo
rmed using newly developed technology with in situ measurement under differ
ent neutron spectra. The in-pile creep properties of type 304 stainless ste
el at 550 degreesC appear to depend on neutron spectrum, but a spectral eff
ect on post-irradiation creep properties is not clearly seen. The rupture t
ime of in-pile creep under a high thermal neutron flux condition is the sho
rtest. The order of the rupture time following the high thermal flux condit
ion is post-irradiation creep, in-pile creep with a thermal neutron shield
condition and finally creep of unirradiated material, all in increasing ord
er. It is suggested that the acceleration of creep deformation and fracture
observed in irradiation creep tests may be related to enhancement of therm
al creep in terms of FMD increased under a high thermal neutron flux in add
ition to increased helium embrittlement. (C) 2000 Elsevier Science B.V. All
rights reserved.