Modeling and TEM Investigation of Helium Bubble Growth in RAFM Steels Under Neutron Irradiation

摘要

A model is developed to describe homogeneous nucleation and growth of helium bubbles in reduced activation ferritic/martensitic (RAFM) steels under neutron irradiation for timescales relevant to operation of a future fusion reactor. The model is based on kinetic rate equations for helium clustering, which are numerically solved by using a Fortran code. Model calculations are performed with helium production rates and helium contents adapted to ARBOR irradiation experiment, where helium effects were studied by irradiation of EUROFER97 based experimental steels doped with different contents of natural boron and separated ~(10)B-isotope. The calculations, which are performed with effective helium diffusivity as a free parameter, yield the time evolution of the size distribution of helium clusters. The simulation results are very sensitive to the model parameters used. The simulated peak bubble diameter at the end of the irradiation experiments for different boron doped steels is between 2 and 8 nm depending on the model parameter. The transmission electron microscopy (TEM) investigation of a neutron irradiated boron doped steel revealed a homogeneous distribution of helium bubbles with no sign of preferential nucleation at microstructural sinks. Predominantly spherical appearance of the bubbles indicates a low helium density in the bubbles. By fitting the parameters the model yields a proper magnitude of peak bubble diameter and peak bubble density, though it still fails to properly reproduce the shape of the bubble size distribution. Possible reasons for this observation are discussed.