YIELD AND DEPTH OF BURIAL HYDRODYNAMIC CALCULATIONS IN GRANODIORITE:IMPLICATIONS FOR THE NORTH KOREAN TEST SITE

This paper reports on continued research toward establishing a consistent modeling framework for calculating nuclear explosions in earth materials. The model must be consistent with observed phenomena in the near-field by correctly 1) calculating the resulting explosive cavity radius for a given yield and depth of burial, 2) accounting for the correct energy deposition by partitioning it into internal (heat and plastic strain) and kinetic (e.g. radiated seismic) energy, 3) predicting the free-field displacement/velocities waveforms and 4) predicting the measured attenuation of the free-field peak velocity with distance. The model developed in the last year satisfies all of these criteria and has been exercised in the investigation of the 2009 North Korean nuclear test. The main findings reported in this paper are: a) the extension of the developed model to the analysis of scaled depth of burial and free surface effects in 2D Earth structure, and b) the improvement of the computational equation of state (EOS) for granite/granodiorite and some examples of the models self-consistency. Our study focuses on the North Korean test site and the May 2009 test. When compared to the Denny and Johnson (1991) and to the Heard and Ackerman (1967) cavity radius scaling models, the results presented in this paper show a clear preference to the statistical model developed by Denny and Johnson. In addition, comparative work between Patton (2011) and the model developed under this project provides a lower limit to the yield and depth of burial for the 2009 North Korean test. A series of sensitivity analysis comprising the variation of key material properties and the incorporation of topography is being produced at the time of writing this paper and will be presented at the conference. This extended analysis will provide additional bounds on the uncertainty for these estimates. A strong motion hydrodynamics code was used to investigate the dependence of cavity dynamics and final cavity radius on the main material properties (Young’s modulus, shear modulus, porosity, etc.). The material model developed was obtained by taking the Piledriver and the Hardhat nuclear test events as the main design references. The following features of the problem were identified when developing the material model: velocity profiles at given stations (near field), source modeling alternatives (iron pill, ideal gas, Hydses/SESAME), energy partition after the shot, peak velocity and peak displacement attenuation profiles and final cavity size as a function of the depth of burial. Previous attempts made with existing material models failed to comply with one or more of these features. The Tillotson type of equation of state combined with a shear plastic strain-dependent strength model was implemented and used to observe surface effects from various scaled depths of burial for a nuclear explosion. The developed material model was used in a set of 2D axially symmetric simulations with a flat free surface, i.e., no topography. Given the best estimates for the material parameters and the fact that there was no evidence of surface spall at the North Korean test site after the 2009 nuclear test, the calculations place a minimum yield and depth of burial of 5.7 kilotons and 375 meters for a uniform source region. Further refinement of these numbers will only be possible by introducing a more realistic topography profile corresponding to the North Korean test site.