ELSEVIER Journal of Nuclear Materials 225 (1995) 22-25 Critical parameters controlling irradiation swelling in beryllium V.I. Dubinko a V.R. Barabash b a Kharkov Institute of Physics and Technology, Kharkoc 310108, Ukraine b The D.I,(. Efreraot, Institute of ElectrophysicalApparatus, 189631 St. Peterburg, Russian Federation Abstract Radiation effects in beryllium can hardly be explained within a framework of the conventional theory based on the bias concept due to elastic interaction difference (EID) between vacancies and self-interstitial atoms (SIAs) since beryllium belongs to hexagonal close-packed metals where diffusion has been shown to be anisotropic. Diffusional anisotropy difference (DAD) between point defects changes the cavity bias for their absorption and leads to dependence of the dislocation bias on the distribution of dislocations over crystallographic directions. On the other hand, the elastic interaction between point defects and cavities gives rise to the size and gas pressure dependencies of the cavity bias, resulting in new critical quantities for bubble-void transition effects at low temperature irradiation. In the present paper, we develop the concept of the critical parameters controlling irradiation swelling with account of both DAD and EID, and take care of thermal effects as well since they are of major importance for beryllium which has an anomalously low self-diffusion activation energy. Experimental data on beryllium swelling are analyzed on the basis of the present theory. 1. Introduction The most important effects of neutron irradiation on beryllium are swelling, embrittlement and tritium retention. The current models of irradiation swelling in beryllium are based on the assumption that it is bubble rather than void swelling [1,2]. The arguments given for neglecting void swelling in beryllium are as follows: (a) prevalence of a-type dislocations together with their preferred absorption of vacancies; (b) high He/dpa ratio in both fission and fusion neutron irradiations. (c) low activation energy for self-diffusion in beryllium which means that it becomes significant at about 250°C under neutron irradiation, and for the case of DEMO blanket (where temperatures vary from 300 to 600°C) void swelling should not occur. Here we shall discuss these arguments briefly to justify a more general approach to beryllium swelling. (a) a-type dislocations are indeed biased for vacan- cies as compared to c-type dislocations, if vacancies migrate preferably in the basal plane. The latter seems to agree with the data on self-diffusion in Be [3]. However, according to Ref. [4], there is increasing evidence that in hcp metals with less than ideal c/a ratios, including beryllium, vacancy migration is only weakly anisotropic whereas the stable interstitial prob- ably takes a basal octahedrical configuration, which migrates preferably in the basal plane. If this is so, then the net bias of a-type dislocations is for intersti- rials, and it is stronger than that due to EID whereas c-type dislocations can be net-biased for vacancies. The experimental observations by Gelles and Heinisch [5] seem to be in agreement with this conclusion. After neutron irradiation of Be specimens at 400°C, they observed a high density of small loops with c(0002) c-type Burgers vectors and small bubbles (4 to 10 nm) on grain boundary dislocations. On the other hand, after irradiation at 500°C, a low density of dislocation tangles of 1/3a(1120) a-type Burgers vectors and no small loops were found. What is more, facets and sharp corners were identified on some of the largest cavities (about 20 nm in size), indicating (according to the authors) that they may have grown by preferential absorption of vacancies into voids with reduced helium pressures. (b) According to the theory of bubble-void transi- tion (see e.g. Eq. (45) in Ref. [6]), a rough estimate of the bubble swelling domination regime is that He/dpa 0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3 1 15(94)0044 1-2