Carrigan Charles R, Sun Yunwei, Antoun Tarabay
Lawrence Livermore National Laboratory, Livermore, CA, USA.
Sci Rep. 2022 Aug 1;12(1):13169. doi: 10.1038/s41598-022-16918-5.
Radioactive gas signatures from underground nuclear explosions (UNEs) result from gas-migration processes occurring in the subsurface. The processes considered in this study either drive or retard upward migration of gases from the detonation cavity. The relative importance of these processes is evaluated by simulating subsurface transport in a dual-permeability medium for the multi-tracer Noble Gas Migration Experiment (NGME) originally intended to study some aspects of transport from a UNE. For this experiment, relevant driving processes include weak two-phase convection driven by the geothermal gradient, over pressuring of the detonation cavity, and barometric pumping while gas sorption, dissolution, radioactive decay, and usually diffusion represent retarding processes. From deterministic simulations we found that over-pressuring of the post-detonation chimney coupled with barometric pumping produced a synergistic effect amplifying the tracer-gas reaching the surface. Bounding simulations indicated that the sorption and dissolution of gases, tending to retard transport, were much smaller than anticipated by earlier laboratory studies. The NGME observations themselves show that differences in gas diffusivity have a larger effect on influencing upward transport than do the combined effects of tracer-gas sorption and dissolution, which is consistent with a Sobol' sensitivity analysis. Both deterministic simulations and those considering parametric uncertainties of transport-related properties predict that the excess in concentration of SF[Formula: see text] compared to [Formula: see text]Xe as might be captured in small volumetric samples should be much smaller than the order-of-magnitude contrast found in the large-volume gas samples taken at the site. While extraction of large-volume subsurface gas samples is shown to be capable of distorting in situ gas compositions, the highly variable injection rate of SF[Formula: see text] into the detonation cavity relative to that of [Formula: see text]Xe at the start of the field experiment is the most likely explanation for the large difference in observed concentrations.
地下核爆炸（UNE）产生的放射性气体特征源于地下发生的气体迁移过程。本研究中考虑的过程要么推动要么阻碍气体从爆轰腔向上迁移。通过在双渗透率介质中模拟地下传输，对最初旨在研究来自地下核爆炸的某些传输方面的多示踪惰性气体迁移实验（NGME），评估了这些过程的相对重要性。对于该实验，相关的驱动过程包括由地热梯度驱动的弱两相对流、爆轰腔的超压和气压泵吸，而气体吸附、溶解、放射性衰变以及通常的扩散则代表阻碍过程。从确定性模拟中我们发现，爆轰后烟囱的超压与气压泵吸产生了协同效应，放大了到达地表的示踪气体。边界模拟表明，倾向于阻碍传输的气体吸附和溶解比早期实验室研究预期的要小得多。NGME的观测结果本身表明，气体扩散率的差异对向上传输的影响比示踪气体吸附和溶解的综合影响更大，这与索伯尔灵敏度分析一致。确定性模拟和考虑传输相关属性参数不确定性的模拟均预测，与小体积样品中可能捕获的[公式：见原文]Xe相比，SF[公式：见原文]浓度的过量应该比现场采集的大体积气体样品中发现的数量级差异小得多。虽然大体积地下气体样品的提取被证明能够扭曲原位气体成分，但在现场实验开始时，相对于[公式：见原文]Xe，SF[公式：见原文]向爆轰腔的注入速率高度可变，这是观测浓度存在巨大差异的最可能解释。
