Irradiation growth in deformed zirconium

Single crystals and polycrystals of pure zirconium were irradiated at 353 K and 553 K with fast neutrons to fluences up to 8.1 × 10²⁵ n/m² to determine the effects of various deformation-induced defect structures on irradiation growth. An a-axis single crystal that was swaged 18% grew linearly at a very high rate (˜1.4 × 10⁻²⁸ m²/n) as a result of the massive aligned twins and dislocations (with a-type and c-component Burgers vectors) introduced by deformation. Annealing the swaged crystal at 673 K for 1 h prior to irradiation resulted in a much lower growth rate due to the recovery of the dislocation structure generated by deformation. Annealing the swaged crystal at 823 K instead also retarded growth but to a lesser extent. Grain boundaries, caused by partial recrystallization, were found in the 823 K specimen only and were believed to be responsible for the observed effect. An a-axis crystal that was strained 5% in tension showed little change in length, suggesting that the effect of a small increase in a-type dislocations is relatively minor. Growth was suppressed in the polycrystalline zirconium specimens that were pulled 5%. This is attributed to radiation-enhanced stress relaxation. It also confirms that a high dislocation density is essential to giving a high growth rate in zirconium.