Neutron activation of most materials results in some generation of both helium and/or hydrogen. Other cases of helium generation are encountered in materials containing tritium, and in materials which are radioactive and undergo alpha-decay. Finally, materials in accelerators, spallation neutron sources, and nuclear fusion devices are subjected to energetic beams of protons or helium ions. In all these cases, helium may be retained within the materials employed, and helium bubbles form and grow. With the exception of tritium decay, radiation damage usually accompanies the helium formation. The rates of helium formation and radiation damage are traditionally stated in terms of the ratio of appm He to dpa (displacement per atom). When this ratio approaches or exceeds about 10 appm He/dpa, a profuse population of helium clusters and bubbles appears in the material when the temperature is high enough for helium migration to occur, with or without a concomitant formation of voids.
We present here kinetic models to describe the formation and growth of helium bubbles. We find that under constant helium generation rates, the nucleation occurs very rapidly over a very short period of time during which the helium concentration in solution peaks sharply. After this initial nucleation burst, the helium concentration in solution remains low as bubbles already present quickly capture newly generated helium. The evolution of the bubble density and average radius is shown to approximately follow a universal curve when a dimensionless time and dimensionless density are introduced. The scaling relationships involved in mapping experimental bubble densities onto the universal curve provide the means to extract a helium diffusion coefficient.
In nickel-containing alloys, the helium generation rate builds up gradually, delaying the formation of helium bubbles to higher doses. Depending on the helium to dpa ratio, void swelling can intervene in bubble formation and even suppress it.