Why is plutonium-240 less desirable than plutonium-239 for nuclear weapons?

Plutonium-240 is considered an undesirable contaminant in nuclear weapons compared to plutonium-239 due to several key factors that complicate weapon design and performance.12

• Spontaneous Fission: Plutonium-240 has a high rate of spontaneous fission, which releases neutrons. In a weapon assembly, these neutrons can initiate a chain reaction prematurely—before the device reaches its optimal supercritical state—resulting in a significantly lower explosive yield.13
• Critical Mass: Plutonium-240 possesses a larger critical mass than plutonium-239, meaning a greater amount of material is required to sustain a chain reaction.1
• Radiation Exposure: The presence of plutonium-240 increases the level of radiation exposure for personnel handling the material.1

Because of these issues, production reactors are operated with restricted fuel-residence times to limit the buildup of plutonium-240 to approximately 6 percent, ensuring the resulting material remains "weapons-grade". During the Manhattan Project, the challenges posed by plutonium-240's spontaneous fission ultimately led scientists to abandon the "gun-type" assembly method for plutonium in favor of the more complex implosion method.123