1. 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

2. Helium in irradiated materials
Alloy elements activated by neutron bombardment
He created by a decay
Sits at octahedral interstitial sites

3. 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)

4. 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)

5. Gas potential
Chemical potential per atom:

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

7. Equilibrium fugacity
He = 1 appm

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

9. 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 atom-1 is O(1036 Pa)
T / K
Nominal 100 appm He

10. 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