Thermodynamic properties of helium bubbles in aged plutonium

Abstract

A qualitative model of the thermal properties of helium gas bubbles in aged plutonium metal is developed. Elasticity theory and plasticity are used to calculate the gas pressure inside the bubble. The chemical potential of a gas atom inside a bubble or a vacancy is estimated relative to an interstitial position. It is shown that the bubble expansion is controlled by plastic yield, with the gas pressure of the order of the yield stress σ y, in particular p=2/3 σ y. When the metal is at low temperatures (at or below 300 K), surface tension is not operative, since there is no diffusion to move material. When the metal is heated, surface tension becomes operative, but the bubble does not shrink, because elastic forces are reversed. Helium atoms are readily incorporated into existing bubbles. They also have a strong tendency to enter vacancies. This increases the equilibrium vacancy concentration as vacancy–helium complexes. While insignificant at room temperature (300 K), this tendency is important when the metal is heated: when the helium atom concentration increases, helium can migrate into bubbles. The effective pressure of 10 000 atm is then large enough to enable bubbles to grow, overcoming surface tension for bubbles of 5 nm diameter. When the metal is at room temperature, heterogeneous nucleation is required, most likely at grain boundaries.