Nuclear Weapon High Explosives

In the first implosion bombs, a layer of explosive was formed from 32 specially shaped blocks. The inner center-facing surface of the blocks was spherical with a diameter equal to the outer diameter of the aluminum tamper layer. On the outer surface of the blocks of explosive were grooves shaped to provides for the insertion of 20 hexagon-shaped lenses and 12 pentagon-shaped lenses.

A 1/16 inch thick felt pad was placed between the surfaces of the explosive and the lenses perpendicular to the axis of the sphere, and the empty spaces between the radial contact surfaces was filled with blotting paper. The air gaps between the layer of explosive and the lenses did not exceed 1/32 inch, since bigger air gaps can contribute to either the slowing-down or the speeding-up of the detonation depending on the direction of these gaps.

The lenses were cast in special molds made from cellulose acetate. Each lens consisted of two types of explosive, one fast-detonating Composition-B [60% RDX, 39% TNT, 1% wax] and the other slow-detonating Baratol. Each lens had three pieces: two made of high velocity explosive, and one of low velocity explosive. The outermost piece of high velocity explosive had a conical cavity in its inner surface into which fitted an appropriately shaped piece of slow explosive. When the lenses were in place, the fast-detonating part of each lens touched the layer of explosive. The entire high explosive implosion system made a layer some 45 cm thick weighing at least 2500 kg.

The ideal high-energy explosive must balance different requirements. HE should be easy to form into parts but resistant to subsequent deformation through temperature, pressure, or mechanical stress. It should be easy to detonate on demand but difficult to explode accidentally. The explosive should also be compatible with all the materials it contacts, and it should retain all its desirable qualities indefinitely.

No such explosive existed in 1944. While using what was available to meet wartime demands, scientists at Los Alamos began to develop a high-energy, relatively safe, dimensionally stable, and compositionally uniform explosive. By 1947, scientists at Los Alamos had created the first plastic-bonded explosive (PBX), an RDX*-polystyrene formulation later designated PBX 9205. Although other PBXs have since been successfully formulated for a wide range of applications, only a handful have displayed the combination of adequate energy content, mechanical properties, sensitivity, and chemical stability required for stockpile nuclear weapons.

Los Alamos demonstrated the first use of plastic-bonded explosives in a nuclear explosion in 1956. This development allowed the shift from precision machined cast explosives to formulations containing high concentrations of high-energy density compounds with reduced sensitivity, more uniformity, and better mechanical characteristics. Pressed, plastic-bonded explosives are the key energetic materials in today's enduring stockpile. Since the 1960s, Livermore has been researching and developing safer HE for Livermore-designed weapons.

The plastic coating that binds the explosive granules, typically 5 to 20% of each formulation by weight, is what gives each PBX its distinctive characteristics. Pressing a PBX molding powder converts it into a solid mass, with the polymer binder providing both mechanical rigidity and reduced sensitivity to accidental detonation. The choice of binder affects hardness, safety, and stability.

Too brittle a PBX can sustain damage in normal handling and succumb to extreme temperature swings or thermal shocks, while too soft a PBX may be susceptible to creep and may lack dimensional stability or strength. To achieve safe and stable PBXs, the Laboratory uses two main charge explosives based on HMX and TATB. HMX is more energetic than RDX but retains good chemical and thermal stability, important for long-term storage and survival in extreme environments. Sensitivity of any PBX is a complex characteristic strongly affected by HE particle size distribution, viscoelastic properties, binder-to-HE wetting, and storage environment.

Only the TATB-based formulations of Livermore's LX-17 and Los Alamos's PBX 9502 are considered "insensitive" high explosives (IHE); others are termed "conventional."