Effect of hydrogen on dimensional changes of zirconium and the influence of alloying elements: First-principles and classical simulations of point defects, dislocation loops, and hydrides

Hydrogen-assisted irradiation growth may result in significant channel bow in addition to regular fluence gradient-induced bow in boiling-water reactor (BWR) fuel channels, especially at high exposures through "shadow corrosion," if hydrogen is picked up early in channel sides facing a control rod. This phenomenon may be responsible for recent high channel bow observations. To develop a better understanding of the effect of hydrogen on dimensional changes of channel materials, first-principles calculations combined with embedded-atom molecular dynamics simulations have been performed under EPRI's BWR channel distortion program. The simulations reveal that: (1) H dissolved in zirconium expands the lattice; (2) the volume effect of H in solution and as hydride is similar; (3) regions under tensile strain attract hydrogen; (4) near Ni atoms the binding of H is increased, and reduced near Sn and Nb; (5) 1 % Zr vacancies decrease the volume by 0.44 % and 1 % Zr self-interstitial atoms (SIAs) expand the volume by up to 1.2 %; (6) the bulk modulus of hydrides rises with increasing H concentration, the shear modulus of hydrides is similar to that of pure Zr, while Young's modulus decreases; (7) coalescence of isolated vacancies into dislocation loops releases up to 80 kJ/mol; (8) vacancy dislocation loops larger than 10-15 Å in diameter tend to collapse thereby shrinking the lattice; (9) interstitial hydrogen is attracted to isolated vacancies and vacancy loops and can retard or prevent their collapse; (10) the diffusivity of interstitial Zr is higher than that of interstitial H atoms, diffusion of vacancies is slower; (11) substitutional Fe and Cr atoms spontaneously swap with interstitial Zr atoms and diffuse rapidly in the c direction; and (12) Nb impedes the diffusion of Zr self interstitials, thus reducing the buildup of a-loops. These simulations confirm some trends observed in material test reactors followed by advanced transmission electron microscopy (TEM). Simulations can be used to help optimize the materials properties for development of future channel alloys to minimize their in-service distortion up to very high fluence.