The purpose of this paper is to report the results of an experimental study to determine the effects of previous irradiation history on the fast-flux neutron irradiation creep behavior of 20 percent cold-worked Type 316 stainless steel for liquid metal fast breeder reactor (LMFBR) application. Previous irradiation creep experiments have involved monitoring the irradiation creep strain of specimens held at constant stress while irradiated at approximately constant temperature and flux. Under these conditions, the irradiation creep rates have been observed to vary with neutron exposure. Application of these “creep curve” data to design and analysis of reactor core structural materials requires some knowledge of how the creep rate at a given point in time depends upon the previous stress history.

In the present experiment, in-reactor stress relaxation tests were performed on specimens which had previously been irradiated in the stress-free condition to various exposures ranging from 0 to 4 × 10²² (n)/cm² (E > 0.1 MeV) at 800 and 1000 °F (427 and 538 °C). It was found that the creep coefficients derived from this experiment were essentially the same as that obtained from the constant-load in-reactor test when allowance was made for thermal deformations. Thus it is concluded that, for the materials and conditions studied, the irradiation creep rate is essentially independent of previous stress history. This result may be rationalized if the microstructural development during irradiation is influenced predominantly by the irradiation flux and temperature variables and only to a minor extent by the irradiation creep deformation.

The sensitivity of the experimental technique to in-reactor effects coupled with supplementary thermal control studies led to a new insight into what has been called transient irradiation creep. The results of this experiment suggest that thermal effects can account for the initially higher creep rate portion of the total in-reactor behavior observed. The thermal component was observed to recur upon reloading, giving rise to a significantly enhanced creep rate for cyclic loadings.