Plutonium experts have two major concerns about aged plutonium: corrosion reactions and the results of self-irradiation.
As with the atoms of all metals, plutonium atoms form structures on scales as small as a billionth of a meter. These microstructures are constantly changing because of plutonium's radioactive nature. When an atom of plutonium-239 (the isotope of plutonium used in nuclear weapons) decays, it splits into an alpha particle--a helium nucleus with two protons and two neutrons--and an atom of uranium-235. The heavy uranium atom recoils, displacing other plutonium atoms and disrupting the surrounding microstructure. The buildup of gaseous helium atoms combined with other elements in the weapon's environment might gradually change the properties of the plutonium metal. Spontaneous decay creates a cascade of chaos. Most of the helium atoms return to their old homes, but some don't, leaving microscopic voids behind. When they find new homes, they cause the pit to swell ever so slightly and may change the dynamic mechanical properties of the pit material. Dislocations-which can be described as an extra half plane of atoms-can create sinks or sources for radiation damage. Over time, changes in the density, shape, and mechanical properties of the pit may affect the overall performance of the weapon.
If typical weapons-grade plutonium, plutonium-239, is spiked with some plutonium-238, which decays more quickly, the self-irradiation process dramatically picks up speed. If 5 percent of the plutonium-239 is replaced with plutonium-238, the sample will age 11 times faster than normal plutonium-239. Aging can be accelerated by a factor of 16 over normal aging processes if 7.5 percent of the sample is plutonium-238. A useful measure of acceleration aging is defined as the number of years required to reach a radiation dose that results in 10 displacements per atom. Weapons-grade plutonium normally takes 100 years to reach this dose but will need just 6.25 years if it is spiked with 5-percent plutonium-238.
The traditional method for manufacturing a pit includes casting a disk (blank) of plutonium, rolling and pressing it to the right size and overall shape, and machining it into its final shape. While effective at producing parts, this method was expensive, generated considerable waste, and required a large amount of plutonium to be recycled in the plant. An alternative approach being developed is to cast the parts to their near-final shape in a precision mold, which avoids the rolling, pressing, and extensive machining. This process also reduces waste generation in the machining process and thus the amount of plutonium that must be recycled.
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