An accelerator study of void production in molybdenum

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Abstract
This paper describes results obtained during an investigation of irradiation damage in molybdenum at high temperatures, the displacement damage being produced by 2 MeV nitrogen ions from a Van de Graaf accelerator. Part of the study was aimed specifically at obtaining void data in molybdenum and results are given of an examination of the temperature range over which void formation becomes appreciable together with an examination of the variation of void swelling with dose. These results are extrapolated to neutron environment dose rates and indicate that molybdenum is particularly resistant to void swelling in the operating range of the sodium cooled fast breeder reactor. The high void concentrations found in molybdenum are thought to be important in this context and a simple theory is developed to demonstrate this point. Other experimental results are described and include a brief account of the void superlattice formation, a study of the dislocation substructures as a function of temperature and dose including some unexpected vacancy loop observations, a note on the production of voids during post-irradiation annealing, some qualitative results of the effect of high helium concentrations on void swelling, and a short account of self-ion irradiations with a result demonstrating one of the possible drawbacks of such irradiations. In addition some general comments are made on accelerator irradiations and the role of the bombarding ion with particular reference to the nitrogen-molybdenum system used in this work.