ESTABLISHED after the halt of underground nuclear testing, the Stockpile Stewardship Program is a critical element of the National Nuclear Security Administration’s mission to ensure the safety, security, and effectiveness of the U.S. nuclear deterrent. As part of this program, scientists are tasked with guaranteeing nuclear weapons’ survivability—that is, ability to function—under hostile conditions, such as those that could result from a pre-emptive strike by an adversary, or when a conventional war may be fought in a nuclear environment.
In the absence of nuclear testing, high-energy-density (HED) facilities, such as Lawrence Livermore’s National Ignition Facility (NIF), have become a key tool for testing the survivability of nuclear weapon components. NIF allows researchers to subject nonnuclear parts, such as electronics and other materials, from weapon systems to intense radiation from x rays and neutrons and to probe material properties at extreme pressures and temperatures—conditions that mimic what systems may face in a real-world nuclear environment. Brent Blue, the National Security Applications program manager at NIF, says, “Survivability is a chess game. The moves an opponent will make are unknown as are their capabilities. Trying to predict what an adversary might do now, let alone 30, 40, or 50 years out, during the lifetimes of these systems, is incredibly challenging. An effective survivability strategy requires one to account for myriad possible ‘what-if’ scenarios both now and in the future.”
Blue is part of a team at NIF that is implementing innovative methods for improving survivability experiments. For example, new target designs are serving as more intense sources of neutrons and x rays for maximizing the energy fluence through a test sample. In addition, specialized systems have been implemented for fielding samples closer to radiation sources, and novel diagnostics are recording essential data for determining how test materials are affected. These capabilities are providing new insights into weapon systems’ durability and are also enhancing several national security programs.
Targets Produce Intense Radiation
After a nuclear blast, an enormous flux of high-energy neutrons and x rays are emitted by the fusion reaction. NIF enables researchers to recreate this environment in a laboratory setting. “NIF is the most energetic laser in the world, which makes it capable of generating exceedingly bright sources of x rays. NIF is also the only facility that can generate intense sources of 14 megaelectronvolt (MeV) neutrons, which are characteristic of deuterium and tritium fusion,” says Blue. The 14 MeV neutrons penetrate deep into materials and deposit large amounts of energy that heat materials throughout. X rays, on the other hand, are used to rapidly heat the surface of a test material, generating a shock wave that travels through to the rear of the sample.
The effects of neutrons and x rays on weapon components are studied in separate experiments that use different types of targets. This approach allows the source of either product to be optimized for generating the maximum fluence—that is, the number of neutrons or x rays flowing through a sample per unit area.
In neutron-generating experiments, scientists use direct-drive targets—tiny plastic spheres ranging in size from 2 to 4 millimeters that contain room temperature deuterium–tritium gas. NIF’s powerful laser beams are focused directly onto the target to “drive” the fusion reaction by rapidly heating the capsule’s outer surface. Compared to other experiments that use an indirect means of heating the capsule, direct-drive targets are easier to field and can be positioned closer to a test sample, which allows higher fluence levels to be achieved.