The Effects of Alpha Particle Irradiation on Stainless Steel (ANRCP-1999-21)

The storage of Weapons Grade Plutonium (WGPu) pits has prompted this study of the effects of alpha irradiation on stainless steel. Energetic alpha particles emitted from the decay of plutonium isotopes continually introduce helium into the metal cladding that surrounds the pits. The continuous introduction of helium into a metal leads to the degradation of its mechanical properties. The integrity of the encapsulating material may then be compromised. 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 Ions in Matter) was then used to determine the fluence equivalent of a 40 keV He 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 He ions/cm for each year of storage. Rutherford backscattering and channeling analysis was then used to analyze single crystal iron which had been implanted with fluences representative of various periods of storage. Due to the high iron content in stainless steel, iron is a suitable surrogate material. 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 10 and 10 He ions/cm, 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 keV He ions showed no significant broadening of the helium depth profile at a fluence of 10 He ions/cm as compared to the depth profile prediction by SRIM. This fluence level corresponds to a storage period of 107 years. This result suggests that there was no significant propagation of possible microcracks formed between bubble sites which would allow diffusion of helium within the metal. Therefore, it can be assumed that a storage period of approximately 100 years may not result in compromising the integrity of the stainless steel. However, the impact of increasing lattice disorder resulting from alpha particle irradiation even in the early years of encapsulation on regions of the stainless steel, which may already be stressed remains unclear. This page intentionally left blank.