A key uncertainty affecting our confidence in the long-term reliability of nuclear weapons is the stability of the plutonium in the core, or “pit.” The pit is a sphere or shell of plutonium that is compressed by conventional explosives to create the supercritical mass required to sustain a nuclear chain reaction. In hydrogen bombs, this first part is called the primary. The energy of the primary is used to compress the fusion part of a hydrogen bomb or secondary. For large bombs, the great majority of the energy comes from the secondary, but if the primary produces too little energy, the secondary will fail completely.
Designers of weapon ands those responsible for their long-term maintenance have worried that the natural radiation produced by the plutonium over the years might damage the material in the primary, causing the bomb to fail to explode. The Jasons were asked to investigate this question; they have now released their report and they believe that plutonium will be adequately stable in most cases for at least a century.
Each year, about one in thirty thousand atoms of plutonium-239 disintegrates naturally and turns into uranium. The resulting tiny change in composition is of no concern but the nuclear disintegration results in emission of a high energy alpha particle. As the alpha particle flies off in one direction and the plutonium atom, now converted to a uranium atom, flies off in the opposite direction, they disrupt the atoms nearby. Plutonium has a very complex set of crystal structures or “phases” and can, under some circumstances, shift from one phase to another. There was worry that the microscopic areas of damage to the crystal caused by nuclear disintegration might reach some threshold and induce a phase change in the bulk material of the primary. This could lead to cracks or other macroscopic faults in the bulk of the material. Additionally, the alpha particles are just helium nuclei; as they come to rest, they pick up two electrons from the surrounding material and become helium atoms. Therefore, another concern was that this helium might collect in tiny bubbles that would weaken the plutonium or otherwise change its properties. The Jason study shows that neither of these effects is going to change the properties of plutonium pits in any significant way for a century after their manufacture.
This finding should make a huge difference in the nation’s future plans for stockpile maintenance and replacement. It probably will not. As I have written elsewhere in this blog, the justification for the Reliable Replacement Warhead (RRW) has been a combination of uncertainty about the existing stockpile and the need to maintain an ongoing design and production base. Even before the recent Jason report, worries about the existing warheads were a weak justification for the RRW. The real purpose of the RRW program is not to replace existing warheads that do not need replacing, it is to maintain an operating nuclear weapons production base forever. What the Jason report will accomplish is to focus debate where it should be: the future of nuclear weapons and the nuclear weapons production facilities.