R. Kemp, G. Cottrell and H. K. D. H. Bhadeshia A classical thermodynamic approach to void nucleation in irradiated materials R. Kemp, G. Cottrell and H. K. D. H. Bhadeshia University of Cambridge EPSRC, Culham Science Centre

Helium in irradiated materials Alloy elements activated by neutron bombardment He created by a decay Sits at octahedral interstitial sites http://www.msm.cam.ac.uk/phase-trans/2004/Tempered.Martensite/tempered.martensite.html

Voids in irradiated materials Voids formed from excess vacancies Require He to form BUT Not to grow (Cawthorne and Fulton, Nature 216 (1967)) Mansur L K, Journal of Nuclear Materials 216 (1994) 97-123

Void nucleation He moves into vacancies Can we model this as a bubble? What is the equilibrium pressure? Adams and Wolfer, Journal of Nuclear Materials 166 (1989) 235-242

Gas potential Chemical potential per atom:

Solution potential Quasi-chemical model At equilibrium: McLellan and Dunn, J. Phys. Chem. 30 (1969) 2631-2637

Equilibrium fugacity He = 1 appm

Steady state He concentration Migration energy = 0.078 eV Trapped at dislocations and in vacancies Need estimate of He concentration actually in solution Ghoniem et al, Journal of Nuclear Materials 117 (1983) 96-105

Steady state He concentration He concentration of O(10-15 atom-1) QC effects significant from 10-9 atom-1 No contribution from QC model Equilibrium fugacity at 10-15 atom-1 is O(1036 Pa) T / K Nominal 100 appm He

Summary “Equilibrium pressures” much greater than yield stresses He may force creation of Frenkel pairs during desolution Nucleation must be dominated by available sites and arrival rates Assumption of fixed bubble density valid for many purposes