Intense Explosion Image
The reflection of a blast wave by a very intense explosion (Proceedings A of the Royal Society Publishing)

A Lawrence Livermore National Laboratory (LLNL) team has taken a closer look at how nuclear weapon blasts close to the Earth’s surface create complications in their effects and apparent yields. Attempts to correlate data from events with low heights of burst revealed a need to improve the theoretical treatment of strong blast waves rebounding from hard surfaces.

This led to an extension of the fundamental theory of strong shocks in the atmosphere, which was first developed by G.I. Taylor in the 1940s. The work represents an improvement to the Lab team’s basic understanding of nuclear weapon effects for near-surface detonations. The results indicate that the shock wave produced by a nuclear detonation continues to follow a fundamental scaling law when reflected from a surface, which enables the team to more accurately predict the damage a detonation will produce in a variety of situations, including urban environments.

The work demonstrates that the geometric similarity of Taylor’s blast wave persists beyond reflection from an ideal surface. Upon impacting the surface, the spherical symmetry of the blast wave is lost but its cylindrical symmetry endures. The preservation of axisymmetry, geometric similarity and planar symmetry in the presence of a mirror-like surface causes all flow solutions to collapse when scaled by the height of burst (HOB) and the shock arrival time at the surface. The scaled blast volume for any yield, HOB and ambient air density follows a single universal trajectory for all scaled time, both before and after reflection.

The team used the Miranda code and the Ruby supercomputer to compare theory against numerical simulations and verified that Miranda reproduces Taylor’s similarity solution for a strong blast wave in an ideal atmosphere.

“Before gathering data and collecting results, we performed convergence studies by refining the grid until the answer did not change,” said researcher Andy Cook. “Then we performed a series of simulations at the converged resolution for different nuclear yields, heights of burst and ambient air densities. We found that the scaled blast volume in each case fell onto the same nondimensional curve. The simulations covered scales from a few millimeters to several kilometers. The largest simulations utilized 3,136 processors and ran for a week.”

The Strategic Consequence Assessment (SCA) air blast team uses the Miranda code to simulate nuclear blasts in non-ideal environments. “Non-ideal air blast” refers to anything more complicated than the Nevada desert, for example, blasts over mountainous terrain or over water or in the presence of rain or snow. These environments change the blast wave in operationally significant ways, which need to be characterized through accurate simulations. High-fidelity blast simulations enable weapons designers to assess the effectiveness of particular designs for specific scenarios.

The team said that understanding nuclear weapon blasts close to the Earth’s surface is important to the nation.

“Having the capability to accurately predict the damage of a high-yield device in a wide array of cases, urban settings in particular, is of paramount interest to our national security,” Spriggs said. “This information enables us to pre-compute damage and guide emergency response personnel in the event that the United States is attacked or in case of a catastrophic accident, such as the recent Beirut explosion.”