Phenomenology of baneberry nuclear event revisited with 3-D finite element transient simulation

Abstract

This paper highlights a three-dimensional (3-D) transient numerical simulation of the Baneberry event of December 18, 1970, with a 10-kT yield and a 278-m source depth, conducted at the Nevada Test Site. This site has complex geological features with preexisting faults and layered geological strata characterized by a hard Paleozoic layer below the source, and saturated tuff on the west side of the source and clay-rich tuff toward the east side, both overlaid by top alluvial layers. In addition, a layer of 50% montmorillonite is sandwiched between two layers of 20% montmorillonite on the east end. This event is reported to have vented because of fault rupture and shock-wave reflections from a closer hard Paleozoic layer near the source. Here, the shock-induced slip along the preexisting fault plane has an important bearing on the containment efficiency of this event. None of the earlier reported simulation studies address the above slip phenomenon and the influence of variation in geological strata in the presence of the preexisting fault in a 3-D framework for underground nuclear events. The paper describes the capabilities of the SHOCK-3D finite element code for simulating short-time shock-wave propagation, fault rupture leading to sliding along the fault plane, and subsequent crater formation at ground zero with a long-duration transient computation to study the quasi-static behavior of the Baneberry event. Precise modeling schemes of the composite geological strata and fault system demonstrate that a dip-slip mechanism had developed for this event, leading to final venting. The present numerical computation results with SHOCK-3D are in excellent agreement with site observations. In addition, the limitations of earlier reported simulation results from the TENSOR two-dimensional axisymmetric code presented by Terhune et al. have also been overcome.