Irradiation creep is defined as the difference in dimensional changes between a stressed and an unstressed sample irradiated under identical condi­tions. Irradiation creep is important for structural materials for nuclear services as it is a major contributor to the dimensional instability of irra­diated materials at temperatures where thermal creep is negligible. However, studies on irradiation creep of SiC(-based materials) have so far been very limited, although it is of high importance for the behavior of the SiC TRISO shell.

Price published the result of the irradiation creep study on CVD SiC in 19 7 7.59 In this work, elastically bent strip samples of CVD SiC were irra­diated in a fission reactor, and the steady-state creep compliance was estimated to be in the order of 10-38 (Padpam-2 (E> 0.18 MeV))-1 at 1053— 1403 K. Scholz and coworkers measured the transient creep deformation of SCS-6 CVD SiC-based fiber, which was torsionally loaded under penetrating pro­ton or deuteron beam irradiation.70-73 They reported several important observations including the linear stress and flux dependency of the tangential primary creep rate at 873 K, and the negative temperature dependence of primary creep strain at the same dose. Recently, Katoh etal. determined the bend stress relaxation (BSR) creep in Rohm and Haas CVD SiC and Hoya monocrystalline 3C-SiC during irradia­tion in HFIR andJMTR at 673-1353 K.74 The results reported for CVD SiC are summarized in Table 1.

In the BSR irradiation creep experiment by Katoh et al, the creep strain for CVD SiC exhibited a weak temperature dependence at <0.7 dpa in the ^673-^ 1303 K temperature range, whereas a major transition at higher doses likely exists between 1223 and 1353 K. Below 1223 K, the creep strain appeared highly nonlinear with neutron fluence because of the

early domination of the transient irradiation creep. The transient creep is speculatively caused by the rapid development of defect clusters and the structural relaxation of as-grown defects during early stages of irradiation damage accumulation. At 1 353 K, irradi­ation creep mechanisms, which are common to metals, are likely operating.

In metals, steady-state irradiation creep rates are generally proportional to the applied stress and neutron (or other projectiles) flux, f,75,76 and there­fore, irradiation creep compliance, B, has been con­veniently introduced75:

eic = s(Bf + DS)

where S is void swelling and D is a coefficient of swelling-creep coupling. Ignoring the swelling — creep coupling term (valid in the saturable swelling regime), preliminary estimations of the steady-state irradiation creep compliance of CVD SiC were given to be 2.7 ± 2.6 x 10-7 and 1.5 ± 0.8 x 10-6 (MPa dpa)-1 at ^873-^1223 K and 1353 K, respectively. If linear-averaged, creep compliances of 1-2 x 10-6 (MPa dpa)-1 were obtained for doses of 0.6-0.7 dpa at all temperatures. Monocrystalline 3C-SiC samples exhibited a significantly smaller transient creep strain by 0.7 dpa and a greater subsequent deformation when loaded along <011 > direction.

To better define the irradiation creep behavior of SiC and the underlying physical mechanisms, it will be essential to further examine the stress depen­dence, dose dependence, effect of crystallographic orientation, microstructures of the crept samples, and the coupling between irradiation creep and swelling.