Plutonium-240
isotope of plutonium
Plutonium-240 (240Pu or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron without undergoing fission. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.
As with the other major plutonium isotopes, the normal decay leads to a more-stable isotope of uranium (236U) and in effect no further decay chain on human timescales. Over geologic time it would follow the thorium series.
240Pu undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of 240Pu limits plutonium's use in a nuclear bomb, because the neutron flux from spontaneous fission initiates the chain reaction prematurely, causing an early release of energy that physically disperses the core before full implosion is reached (a "fizzle").
Nuclear properties
About 62% to 73% of the time when 239Pu captures a neutron, it undergoes fission; the remainder of the time, it forms 240Pu. The longer a nuclear fuel element remains in a nuclear reactor, the greater the relative percentage of 240Pu in the fuel becomes.
The isotope 240Pu has about the same thermal neutron capture cross section as 239Pu (289.5±1.4 vs. 269.3±2.9 barns), but only a tiny thermal neutron fission cross section (0.064 barns).
Begin with the source’s own compact description: “Plutonium-240” is isotope of plutonium. The dossier treats that line as a proposition to test through Plutonium-240, isotope and plutonium, not as a finished interpretation.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Apr 6, 2026. The linked authority identifier is Q2330653. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1944.
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This entry incorporates text from “Plutonium-240” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.