THE RECENT sabre rattling by Vladimir Putin may have unwittingly done what the United States Congress has failed to do for decades: refocus attention—and billions of additional dollars—on overhauling America's nuclear arsenal. The $585 billion defence bill for the next fiscal year sailed through the House of Representatives last week with broad bipartisan support, and then did the same in the Senate on December 12th, despite all the fractious squabbling over the $1.1 trillion government funding measure.



More pertinently, the $11.7 billion request for the National Nuclear Security Administration (NNSA), a branch of the Department of Energy that oversees nuclear weapons, naval reactors and nonproliferation activities on behalf of the military, represents a 4% increase over the previous year. The biggest chunk of that—covering work on modernising the country’s nuclear weapons—is to increase by 7%. All this at a time when mandated “sequestration” cuts are supposed to be reducing military spending.



All told, the federal government intends allocating up to $1 trillion to upgrade the country’s missiles, bombers and submarines over the coming decades. A third of that is to be spent improving the country's nuclear weapons.



The average age of America’s stockpile of nuclear warheads is now 27 years. Many of the buildings used to store and refurbish them date back to the Manhattan Project of the second world war. The sprawling Pantex plant near Amarillo, Texas, where nuclear warheads are assembled and dismantled, has become so rat-infested that workers have to secure their lunch boxes. Meanwhile, sections of a concrete roof at the Y-12 National Security Complex at Oak Ridge, Tennessee, where America’s main supplies of highly enriched uranium are stored, collapsed in March into an assembly area. Following further collapses, workers have been issued with hard hats.



Practically all of America’s nuclear weapons facilities are showing their age. With the end of the Cold War, concerns about the need for investment were swept aside. But the increasing tension with Russia over Crimea, China’s more assertive territorial claims, Pakistan’s growing arsenal of nuclear weapons, and the ongoing nuclear threat from an unstable North Korea mean such issues are no longer being left on the back burner.

Right now, the NNSA’s main concern is the production of “pits”—the hollow fissile core of a nuclear warhead. These were made at the Rocky Flats plant built near Boulder, Colorado, until the facility was closed in 1989, following a raid by the Federal Bureau of Investigation for environmental and safety breaches. The plant has remained shut ever since, despite numerous attempts to reopen it.



The fabrication of pits, along with the manufacture of the dense beryllium tamper and reflector layers that surround a fissile core, was subsequently relocated to the Los Alamos National Laboratory in New Mexico, one of the two laboratories in the United States where nuclear weapons are designed (the other is the Lawrence Livermore National Laboratory in California). Los Alamos is now thought capable of producing 20 pits a year, though the NNSA sees 80 or more being made there annually.



In 1993, the cost of transferring the plutonium and beryllium work from Rocky Flats to Oak Ridge was put at $660m. A geological survey subsequently raised the relocation cost to $5.8 billion. With money scarce, many have questioned whether America really needs a whole new production line for manufacturing plutonium pits. To answer that it is necessary to examine how nuclear weapons are made.



Nowadays, most nuclear weapons are implosion types. A spherical charge of high explosive is used to compress a lump of fissile material (usually plutonium-239) at the core. When the chemical explosives are detonated, the implosion squeezes the fissile core, increasing its density enough to create a critical mass.



A burst of neutrons is then supplied to trigger as many chain reactions in the compressed fissile material as possible. The neutron source is usually a high-voltage vacuum tube containing a miniature particle accelerator that bombards a metal hydride target with deuterium and tritium ions. The result is a small fusion reaction producing neutrons that penetrate the pit.



The critical mass of a sphere of bare uncompressed metal is 50kg (110lb) for uranium-235 and 16kg for plutonium-239. Since the 1960s, pits have been made almost exclusively from plutonium, because it fissions faster, produces more neutrons and has a much smaller critical mass. To keep the warhead safe before being armed, the plutonium has to be maintained in a sub-critical state. That is hard to do with a solid ball of bare metal, but much easier if the fissile material is in the form of a hollow sphere. Modern hollow pits need only three kilograms or so of plutonium to go critical when crushed.



The layers of beryllium surrounding the pit serve to tamper (ie, delay) the expanding plutonium and to reflect neutrons back into the core, so more can be captured and drive the chain reaction further. Without the heavy beryllium cladding, the expanding plutonium that had begun to fission would quickly become less dense and lose its critical mass. In early nuclear weapons, only 20% or so of the fissile material actually fissioned, with the rest being scattered around.



The hollow plutonium pits in some warheads have a channel for injecting a mixture of deuterium and tritium gas—to boost the fission process and thereby reduce the actual amount of fissile material needed. Most modern warheads also use a two-stage arrangement, with the primary stage being imploded by chemical explosives, while the secondary stage is imploded by the x-ray energy from the primary stage. Boosting the pit with deuterium and tritium can double the weapon’s energy release. Adopting two (or more) stages raises the output still further, while actually making the warhead more compact.



So much for the physics, now for the practicalities. Manufacturing plutonium pits is one of the most daunting tasks in engineering. Casting and machining plutonium is especially tricky. Not only is the metal highly toxic, but it also has many crystalline forms (allotropes). As it cools from the molten state, its various phase changes can cause components to distort and crack. Adding a small amount of gallium helps to stabilise the cooling, but it makes it harder to extract the plutonium from decommissioned weapons to turn it into fuel for nuclear reactors.



To fabricate the actual pit, a hollow sphere of a suitable structural material, usually in the form of two half shells, is lined with precisely machined hollow hemispheres of plutonium. The whole assembly is then welded around the equator, and the beryllium cladding providing the tamper and reflector layers attached to the outer surface.



Because machining the plutonium metal produces a lot of waste, the eventual aim is to cast (or even 3D-print) the plutonium shells directly. But with nuclear testing no longer permitted, the metallurgical differences between cast versus machined plutonium make theoretical simulations difficult. So far, little progress seems to have been made with direct casting, let alone 3D-printing, of plutonium pits.



With the beryllium and plutonium production now located to Los Alamos, some 12,000 spare pits have been placed in storage at the Pantex plant. The question, then, is why build a new plutonium production line at Los Alamos? It is not as though the thousands of existing pits in storage are rotting away. An independent advisory panel found that pits in storage “have credible minimum lifetimes in excess of 100 years.”



The NNSA’s answer has been that the new pit facility is needed for the proposed Reliable Replacement Warhead (RRW), a design for a whole new family of weapons that were supposed to be simpler, more reliable and easier to maintain than existing devices. The project, initiated in 2004, became the centrepiece for the NNSA’s plans to rebuild America’s ageing nuclear-weapons infrastructure.



Of the two competing RRW designs, the Lawrence Livermore proposal won out initially over the Los Alamos offering. The reason given was because it was based on a previous design which, though never deployed, had been thoroughly tested before the moratorium on underground testing went into effect in 1992. Subsequently, it emerged that the RRW would be a combination of both designs.



Despite all the lobbying for the RRW, Congress declined to fund it. In 2009, the Obama administration called for work on the RRW to cease. Nevertheless, the project’s ghost continues to shape the NNSA’s thinking.



If truth be told, the RRW was little more than a make-work programme for nuclear weapons laboratories. With an eye on keeping the ageing workforce employed, the Obama administration subsequently proposed a $60 billion plan to update some of the NNSA’s facilities, and to start work on a more modest nuclear device. This so-called Interoperable Warhead would combine the plutonium pit from an existing weapon with the neutron source and various core components from another.



The White House claims such a warhead would save money by reducing the number of weapon types from seven to five. But never having been tested, such a Frankenbomb could prove unreliable. Besides, many of the criticisms aimed at the RRW would apply equally to the Interoperable Warhead. Embarking on a new generation of nuclear weapons would certainly alarm allies as much as potential enemies. And it would undo much of the hard work done to further global cooperation on nonproliferation—one of the few (perhaps the only) means for keeping negotiations alive with rogue emerging nuclear powers like North Korea and Iran.



As it is, America has all the nuclear triggers it requires, and then some. Many would argue that the last thing it needs is a new production line for still more plutonium pits. The upgraded B61 gravity bomb—now being equipped with controllable tail fins that turn it into a precision guided weapon for the air force—will be part of the F-35 multi-role stealth fighter’s complement of armaments for decades to come. Meanwhile, the upgraded W76 warhead for the navy’s Trident- and Ohio-class missile submarines is expected to extend the weapon’s life from 20 to 60 years.



The thing to remember about nuclear deterrence is that it works best when enough of it is visible to scare the enemy into behaving sensibly. It begins to fail when an over-abundance of bomb-making bits at home sends shivers through the country’s own citizenry. Right now, the NNSA’s biggest task ought to be not upgrading its plutonium production lines, but downgrading the huge surplus supply of weapons-grade plutonium stored in Texas, so it can be burned in nuclear reactors to produce useful energy.