N journalnf nuclear materials ELSEVIER Journal of Nuclear Materials 233-237 (1996) 974-978 Displacement rate dependence of irradiation creep as predicted by the production bias model C.H. Woo ~*, F.A. Garner b•~ a Atomic Energy qt'Canada Limited, Whiteshell l,ahoratories, Pinawa, MB, Canada b Pacifi c Northwest Laboratory, Richhmd, ~1,;4, U5;4 : Abstract Recently, it has been shown that the non-swelling component of irradiation creep of austenitic stainless steels is relatively independent of temperature but is sensitive to the displacement rate. An earlier model of Lewthwaite and Mosedale anticipated the sensitivity of displacement rate and attributed it to the flux sensitivity of point-defect recombination• The point-defect recombination process does not yield the observed temperature dependence, however, although it does predict the inverse dependence of the creep rate (in dpa ~) on the square root of the displacement rate that was experimentally observed at relatively low temperatures. The production bias concept of Woo and Singh provides for an improved irradiation creep model. It predicts the correct temperature and displacement rate dependence of the non-swelling-related component of creep, and also provides a good description of the swelling-enhanced component of creep. The dose rate dependence of the creep and swelling rates occurs as a result of the generation, during cascade damage, of primary clusters, the sink strengths of which are significant compared with those of sinks visible using electron microscopy. The primary clusters act as recombination centres for the single defects. As the dose rate increases, the number density and hence the sink strength of these primary clusters also increases, thus reducing their efficiency. This model leads to a prediction that the creep rate varies as the displacement rate, in close agreement with the observed square root behaviour. 1. Introduction In a recent review [1], it was shown that some earlier creep experiments were conducted in such a way that made it difficult to isolate the separate and possibly syner- gistic effects of temperature and displacement rate. From subsequent experiments [ 1 ] that allowed separation of these variables, it was found that the creep compliance, describ- ing creep in the absence of swelling, was relatively inde- pendent of temperature over most of the range encountered in fast reactors anti anticipated fusion reactors. When early experiments were conducted at relatively low tempera- tures, characteristic of reactors with inlet temperatures of 260-270°C, it appeared that there is a strong inverse square root dependence of the creep rate on the displace- • Corresponding author. E-mail:wooc@wl.aecl.ca. i E-mail: fa-gamer@ pnl.gov. 2 Operated for lhe US Deparnnent of Energy by Balelle Memo rial Institute under Contract DE-AC06-76RL0 1830. ment rate. Water-cooled fusion devices such as ITER will also operate in this temperature range over a range of displacement rates. Therefore, it is important to understand the sensitivity of irradiation creep to both temperature and displacement rate. In fusion devices the possible influence of laelium on creep has also been debated, but several studies have shown that the apparent dependence on helium/dpa ratio was misleading and was actually produced by tile differ- ences in displacement rote inherent in these studies [2-4]. These more recent studies also indicated that the inverse square root sensitivity of creep rate (in dpa i ) to the dose rate (in dpa/s) was maintained at higher temperatures as well. This can be seen, for example, in the results of Ref. [4], where the creep rate (in h I) as a function of close rate (dpa/h) is shown in a log-log plot of slope of 0.5 in Fig. 1. These findings, however, are in conflict with the predic- tions of an earlier accepted model. Shown in Fig. 2 is an early demonstration by Lewthwaite and Mosedale [5] of the dpa rate sensitivity of irradiation creep at relatively low temperatures, a dependence which has now been con- 0022-3115/96/$15.00 Copyright (c) 1996 Elsevier Science B.V. All rights reserved PII S0022-3115(96)00310 8