Lawrence Livermore National Laboratory



Hai Le

Overview

There is currently no credible simulation capability to predict the ultraviolet (UV), visible light, and infrared (IR) emissions from a nuclear explosion at high altitude. The weapons-effects research community has identified the relevant physics, but credible simulations do not exist due to the inherent complexity. This feasibility study combined three existing modeling capabilities at the Lawrence Livermore National Laboratory to provide a first-estimate prediction. We integrated the NEQAIR molecular radiation transport code and the Chemk chemical kinetics code into the Cretin non-local thermodynamic equilibrium (LTE) radiation transport code to simulate a 750-kt nuclear explosion with a 70-km burst height. Atomic kinetics, x-ray absorption, and radiation transport were modeled self-consistently within the Cretin simulation framework. UV, visible light, and IR emissions (sometimes collectively referred to as the extended spectrum) were calculated using molecular and atomic opacities from NEQAIR. We produced a detailed radiative spectrum including the extended spectrum, present at an altitude of 200 km, along with the time-dependent total emissions from these spectral regions.

Background and Research Objectives

Simulating a nuclear explosion at high altitudes (70 km and above) is a challenging task as it requires a self-consistent treatment of the transport of particles and radiation. At low altitudes, particle transport is accurately described by the hydrodynamic equations (Euler/Navier-Stokes), which can be solved self-consistently with the radiation transport equation. Existing radiation hydrodynamic codes work well in this regime. At high altitudes, hydrodynamic models are not valid due to finite collision mean free paths. (The mean free path is the distance a particle will travel, on average, before experiencing a collision event.) However, full transport models are still very expensive due to the need to resolve the high-dimensional phase space of the particles. As the particle density decreases, collision rates also decrease, so the gas/plasma is no longer in LTE. The radiation transport calculation then requires a non-LTE treatment for both molecules and atoms present in the atmosphere.

Accurate prediction of light emissions in the extended spectrum from a high-altitude nuclear blast is a capability that does not exist at present. This study was a first attempt to examine the feasibility for the problem described in the figure below.


Figure 1.
Flowchart for predicting ultraviolet, visible light, and infrared emissions from a high-altitude nuclear blast. This feasibility study considers only the outlined (dashed line) pathway. Predicting nuclear bomb debris (red outline) is a significantly more complicated task requiring further study.


We only modeled the radiation transport of nuclear x rays and how they affect the atmosphere and subsequent light emissions. (Modeling the bomb debris is another significant task that was not undertaken during this feasibility study.) We integrated two existing capabilities, the NEQAIR and Chemk simulation codes, into Cretin code to simulate this problem. Cretin, a non-LTE atomic kinetics and radiation transport code developed at the Laboratory (Scott 2001), has been used extensively to simulate many laser-produced plasmas as well as inertial confinement fusion experiments at the National Ignition Facility. The atomic kinetics and opacities of all the relevant atoms (N, O, H, He, and Ar) and radiation transport were modeled with Cretin. Because Cretin does not include molecular opacities, which are the dominant contributions in the low-energy range of interest, we obtained the opacities for air molecules (N2, O2, and NO) using the NEQAIR molecular radiation transport code (developed at NASA's Ames Research Center) and simulated shock-induced radiation during reentry (Brandis and Cruden 2014). Although NEQAIR provided the required molecular radiative transitions, we needed to provide the molecular and atomic compositions, which are driven by the chemistry. We therefore employed a standalone chemical kinetics code to simulate an arbitrary set of user-specified reaction mechanisms. We applied the standard reaction mechanisms used in hypersonic reentry (Gnoffo et al. 1989) to model the air chemistry surrounding the nuclear explosion.

The goal of this project was to explore the feasibility of predicting the characteristics of the UV, visible, and IR emissions from a nuclear explosion. We focused on linking together existing capabilities (instead of developing new ones) to provide a first estimate prediction. As a result, we made assumptions to simplify other aspects of nuclear explosion physics missing from these codes while focusing on the ones relevant to emissions in the extended spectrum. Under these assumptions, we simulated the radiative output of a representative 750-kt nuclear explosion at 70 km. The objectives were time-varying UV, visible, and IR emissions at an arbitrary location a significant distance away from the blast.

Impact on Mission

Accurate prediction of the UV, visible light, and IR emissions of nuclear blasts provides a design-to-effect capability and a quantification of our current and future risk posture, both of which are key requirements for understanding our deterrence posture. This research supports the NNSA goal to provide expert knowledge for counterproliferation and addresses the Laboratory's mission focus areas of nuclear stockpile stewardship and multi-domain deterrence. This study also supports the Laboratory's core competency in nuclear, chemical, and isotopic science and technology.

Conclusion

By integrating three existing capabilities at the Laboratory, we demonstrated the feasibility of calculating time-dependent light emissions in the UV, visible, and IR spectrum from a nuclear explosion at high altitude. Our results set up future work to focus on improving the treatment of the molecular kinetics, opacities, and self-consistent coupling with radiation transport, similar to what was done for the atoms. Future research may also include modeling bomb debris and its interactions with the atmosphere, which requires extensions to conventional hydrodynamic models. At mid-high altitude (70–200 km), multifluid hydrodynamics simulations are a promising alternative to full kinetic simulations. This approach can capture some relevant kinetic (finite mean free path) effects missing from traditional models (e.g., plasma interpenetration and charge exchange reactions).

References

Brandis, A. M., and B. A. Cruden. 2014. "NEQAIRv14.0 Release Notes: Nonequilibrium and Equilibrium Radiative Transport Spectra Program." NASA Technical Report, accessed 15 January 2019, https://ntrs.nasa.gov/search.jsp?R=20150000832.

Gnoffo, P. A., et al. 1989. "Conservation Equations and Physical Models for Hypersonic Air Flows in Thermal and Chemical Nonequilibrium." NASA Technical Paper 2867, accessed 15 January 20119, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890006744.pdf.

Scott, H. A. 2001. "Cretin—A Radiative Transfer Capability for Laboratory Plasmas." Journal of Quantitative Spectroscopy and Radiative Transfer 71(2–6): 689–701. doi: 10.1016/S0022-4073(01)00109-1.