Creep of molybdenum irradiated with fission fragments at low temperature

Length and resistivity changes of uniaxially stressed molybdenum samples have been recorded during irradiations by fission fragments, in the temperature range 20-30 K. Eleven deformation versus dose curves are shown. They were obtained with cold-rolled and annealed molybdenum ribbons irradiated under fixed or variable stress up to fluences of about 1.5 d.p.a. At low temperature there is no steady-state creep prior to 1.2 d.p.a. Because of a long primary creep period, one can follow the build up of the dislocation network from the first displacement cascades to its saturation. The primary creep strain is neither linear with stress nor with fluences. Stress oriented nucleation of the dislocation loops is the predominant primary creep mechanism. A comparison of the creep rates corresponding to cold-rolled and annealed samples shows that primary creep is very sensitive to internal stresses produced by cold-working prior to irradiation. (author)