Delayed signatures of underground nuclear explosions

Understanding the transport of radionuclides following underground nuclear explosions is a key challenge in nuclear verification. Prior research has shown that radionuclide signals are affected by subsurface geology, but the specific mechanisms remain unclear. Established knowledge includes the influence of fractures and groundwater flow on radionuclide migration. However, the delayed emergence of certain isotopes has not been fully explained. This gap motivated the need to study transport properties in real-world conditions. No prior work had resolved how containment structures affect the timing and isotopic composition of surface signals. The absence of field data on post-detonation gas migration limited predictive accuracy. This paper contributes by offering field-scale insights into radionuclide transport dynamics. It introduces a novel approach to interpreting delayed radionuclide signatures.

This study aims to evaluate transport properties of a former underground nuclear explosion site using tracer experiments. The specific problem is to understand how radionuclide signals evolve and reach the surface. The motivation stems from the need to improve detection and verification capabilities for clandestine nuclear tests. The study focuses on the role of containment structures in delaying and modifying radionuclide signals. It seeks to clarify how gas migration through fractured rock affects detectability. The goal is to develop a model that links subsurface transport to surface observations. This work addresses a critical gap in nuclear verification science. It provides a framework for interpreting delayed radionuclide signatures in real-world scenarios.

The study uses a field-scale tracer experiment to assess transport properties at a former underground nuclear explosion site. It involves injecting tracer gases into the post-detonation chimney to simulate radionuclide migration. Surface monitoring tracks the movement of gases under realistic transport conditions. Radon signals are measured to detect changes in cavity pressurization. Computer simulations are then used to model the transport of radioxenon isotopes. These simulations incorporate transport properties derived from the field experiment. The simulations track isotopic evolution within the chimney and their migration to the surface. The model is validated using atmospheric observations from a 2013 nuclear test in the DPRK.

The field experiment reveals that background radon signals respond strongly to cavity pressurization. This suggests that radon anomalies may indicate clandestine nuclear explosions. Simulations show that the chimney behaves as a leaky chemical reactor under fractured containment. This behavior introduces a new dependence on nuclear yield in isotopic evolution. The model successfully replicates observed radioxenon signals from the 2013 DPRK test. It matches isotopic observations with nuclear yields comparable to seismic estimates. The simulations confirm that containment structures delay and modify radionuclide signals. These findings support the use of radionuclide transport models for nuclear verification.

The study confirms that containment structures significantly delay and modify radionuclide signals from underground nuclear explosions. The field experiment provides transport properties that improve predictive models. Simulations using these properties accurately replicate observed radioxenon signals. The chimney's behavior as a leaky chemical reactor introduces a new yield dependence. This finding aligns with atmospheric observations from the 2013 DPRK test. The model supports the use of radon anomalies as potential indicators of clandestine explosions. It offers a framework for interpreting delayed radionuclide signatures in verification efforts. The authors suggest that these results enhance the accuracy of nuclear test detection systems.