Helium Filled Bubbles in Solids : Nucleation, Growth and Swelling

Abstract: When nuclear fuel, fabricated for the purpose of transmuting spent fuel is irradiated, significant amounts of He is produced from alpha particles mainly emitted when 242Cm decays into 238Pu. From irradiation experiments it is known that the presence of He in the solids alters the swelling behaviour of the material. The thesis presents the theoretical background from which nucleation models of He bubbles can be formulated. Such models are presented for He in metals, and the case of He in Mo is studied as an example. MgO, which together with Mo is suggested as a matrix material in transmutation fuel is also studied and the stability of He containing bubbles in this material is discussed.By calculating parameters for a rate theory model derived from atomistic modelling, it is shown that He can stabilise vacancy clusters and cause cluster growth at temperatures and irradiation doses where nucleation and growth would not otherwise occur. At the initial stages of nucleation He can stabilise small bubbles while larger bubbles are unstable. This results in an incubation time of swelling, which implies that He does not always cause increased swelling, but can at certain irradiation conditions slow down the growth of large vacancy clusters and thereby delay swell-ing beyond the time of the irradiation.When comparing the behaviour of bubble nucleation in Mo and MgO, it is found that He has a significant impact even at very low concentrations in Mo. In contrast, the concentration of He has to be considerably higher in MgO to affect the swelling behaviour. For an inert matrix fuel, designed for transmutation purposes, this implies that the Mo matrix will have a tendency to swell considerably at rather high temperatures due to He stabilised vacancy clusters. If operated at lower temperatures, the swelling could instead be reduced due to the incubation time. In a MgO matrix, the swelling behaviour will instead depend largely on the production rate of He. For a low production rate, the material will have a swelling behaviour similar to the one seen when He is not present in the material. A high production rate implies that He will remain in vacancy clusters, thereby stabilising the clusters and enhancing the growth and swelling.