Modeling Dynamic Fracture of Cryogenic Pellets
- General Atomics, San Diego, CA (United States)
This work is part of an investigation with the long-range objective of predicting the size distribution function and velocity dispersion of shattered pellet fragments after a large cryogenic pellet impacts a solid surface at high velocity. The study is vitally important for the shattered pellet injection (SPI) technique, one of the leading technologies being implemented at ORNL for the mitigation of disruption damage on current tokamaks and ITER. The report contains three parts that are somewhat interwoven. In Part I we formulated a self-similar model for the expansion dynamics and velocity dispersion of the debris cloud following pellet impact against a thick (rigid) target plate. Also presented in Part I is an analytical fracture model that predicts the nominal or mean size of the fragments in the debris cloud and agrees well with known SPI data. The aim of Part II is to gain an understanding of the pellet fracturing process when a pellet is shattered inside a miter tube with a sharp bend. Because miter tubes have a thin stainless steel (SS) wall a permanent deformation (dishing) of the wall is produced at the site of the impact. A review of the literature indicates that most projectile impact on thin plates are those for which the target is deformed and the projectile is perfectly rigid. Such impacts result in “projectile embedding” where the projectile speed is reduced to zero during the interaction so that all the kinetic energy (KE) of the projectile goes into the energy stored in plastic deformation. Much of the literature deals with perforation of the target. The problem here is quite different; the softer pellet easily undergoes complete material failure causing only a small transfer of KE to stored energy of wall deformation. For the real miter tube, we derived a strain energy function for the wall deflection using a non-linear (plastic) stress-strain relation for 304 SS. Using a dishing profile identical to the linear Kirchkoff-Love profile (for lack of a rigorously derived profile) we derived the strain energy associated with the deflection and applied a virtual work principle to find a relationship between the impact (load) pressure to the measured wall deflection depth. The inferred impact pressure was in good agreement with the expected pressure for oblique cryogenic pellet impacts where the pellet shear stress causing cleavage fracture is well above the yield stress for pure shear. The section is concluded with additional discussion on how this wall deformation data lends further support to the analytical fracture model presented in Part I. In Part III we present three different size distribution models. A summary, with a few brief suggestions for a follow on study, is provided at the end of this report.
- Research Organization:
- General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE Office of Nuclear Energy (NE)
- DOE Contract Number:
- AC05-00OR22725
- OSTI ID:
- 1344852
- Report Number(s):
- GA-A28352
- Country of Publication:
- United States
- Language:
- English
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