Dimensional stability of neutron irradiated molybdenum

Polycrystalline molybdenum specimens were irradiated to fast neutron fluences between 1.1 x 1019 neutrons (n)/cm2 and 1.0 x 1022 n/cm2 at a temperature range of 70 to 4800C. The irradiation-induced length changes and subsequent changes during postirradiation annealing were measured using a high vacuum dilatometer. The effects of neutron fluences, at reactor ambient temperature of approximately 700C, were to increase the length changes up to the neutron fluence of approximately 1020 n/cm2 and then level off at the higher fluences. The length change data were compared with the resistivity data and the ratio of resistivity to length changes was calculated as 2.7 x 10-3 Ω-cm. Specimens irradiated to very high neutron fluence, 1 x 1022 n/cm2 at 4300C, showed very large length changes which were about 15 times those of low fluence specimens. This was attributed to the formation of voids in these specimens. The bulk density measurements of the specimens irradiated to 1 x 1022 n/cm2 at the temperature region of 430 to 10000C are presented along with the void swelling data. A postirradiation anneal study of the irradiation induced length changes was made in the temperature region of 70 to 16000C. A total of five different recovery stages was observed at the temperature regions of: 0.16 T/sub m/ (Stage III), 0.22 T/sub m/ (Stage IV), 0.31 T/sub m/ (Stage V), 0.36 T/sub m/ (Stage VI), and 0.54 T/sub m/ (Stage VII), respectively. The specimens irradiated at 700C revealed four recovery stages ranging from Stage III to VI. Those specimens irradiated to 1 x 1022 n/cm2 at 4300C revealed an additional recovery stage at 0.54 T/sub m/, and this recovery stage coincided with the thermal annealing of voids