The effects of alpha particle irradiation on stainless steel

Abstract

A Monte Carlo code was developed to calculate the alpha particle emission rate from WGPu. It yielded information pertaining to the alpha particle source strength at the WGPU and stainless steel interface as well as the damage production and He concentration depth profiles within the stainless steel. SRIM (The Stopping and Range of tons in Matter) was then used to determine the fluency equivalent of a 40 keV ⁴He⁺ beam that would result in the same total displacement production in the near-surface region of stainless steel as would be produced by alpha particles from WGPu. This value was found to be 4.45E13 ⁴He⁺ ions/cm² for each year of storage. Rutherford backscattering and channeling analysis was then used to analyze single crystal iron which had been implanted with nuances representative of various periods of storage. Analysis of the sample using a 140 keV proton beam directed along the <100> axis showed a significant increase in dechanneling between the fluences of 10¹⁵ and 10¹⁶ ⁴He⁺ ions/cm², corresponding to storage periods of 22.5 years and 225 years, respectively. This sharp rise in dechanneling may be attributed not only to lattice disorder produced by ion beam collisions, but also to lattice stress/strain due to bubble cluster formation. Given the sensitivity of RBS and channeling analysis, the possibility of bubble cluster formation cannot be eliminated; this is true even when SEM and TEM analyses do not provide clear evidence of bubble formation. According to additional work performed by colleagues at The University of Texas - Austin, neutron depth profiling analysis of stainless steel - 316 samples implanted with 140 ke ³He⁺ ions showed no broadening of the helium depth profile between the quinces of 10¹⁶ and 3E16 ³He⁺ ions/cm². These fluency levels correspond to storage periods of 107 and 321 years, respectively. This suggests that no micro-cracks formed between bubble sites which would allow diffusion of helium within the metal. Therefore, it can be assumed that a storage period of approximately 300 years may not result in compromising the integrity of the stainless steel shell. However, the impact of increasing lattice disorder resulting from alpha particle irradiation even in the early years of encapsulation on regions of the shell which may already be stressed remains unclear.