Talk:Plutonium-240

Speed?

Does the speed of the neutron affect this?

Yes, neutron speed (energy) always impacts on which nuclear reactions you get, likevise energy of target. Energy also always impact on reaction probability (cross section) and what reaction you get! Seniorsag (talk) 15:04, 8 June 2015 (UTC)

"spontaneous fission?"

Hmm. What's the half-life of Pu-240 for this specific decay mode? Article seems to imply that about two out of three Pu-240 nuclei "spontaneously fission" a fraction of a nanosecond after they come into being, but that third one... doesn't. Except maybe after a delay that could be, if I am reading the article correctly (and I may not) seconds, or centuries. — Preceding unsigned comment added by 69.41.40.24 (talk) 18:26, 26 September 2012 (UTC)

The CRC lists decay modes as 76% α and 24% SF, with a half-life of 6561 years. That means after 6561 years, 12% of your sample of ²⁴⁰
Pu
will have undergone spontaneous fission and 38% α-decay. Tarl N. (discuss) 03:41, 18 July 2016 (UTC)

Dubious

I just flagged a sentence in the article as dubious: ²⁴⁰Pu has only about 1⁄3 as large a neutron absorption cross section as ²³⁹Pu.

According to my sources (and Wikipedia's article Isotopes_of_plutonium), the cross section of ²³⁹
Pu
is around 270 barns, while that of ²⁴⁰
Pu
is 287 barns. Effectively identical. What's different between the two isotopes is that 239 has only a 15% chance of undergoing spontaneous fission, while 240 has a 24% chance, and a shorter half-life as well (thus, by my calculations, resulting in a six-fold greater source of neutrons). Are my sources, busted, am I misunderstanding something, or is this deliberate disinformation? (smile) Tarl N. (discuss) 02:45, 18 July 2016 (UTC)

O.k. - I've copy-edited the statement which bothered me. The statement was technically not incorrect, as the reference was to ²³⁹
Pu
total capture cross-section, of 270+747 barns being 3 times greater than ²⁴⁰
Pu
total capture cross-section of 287+0.064 barns, but it was confusing. I also replaced the pu-xxx with {{SimpleNuclide2}} templates, I'm not sure the result is an improvement as far as readability. I'll address any comment here.

The other thing I flagged as dubious is the claim that ²⁴⁰
Pu
propensity for spontaneous fission renders its presence problematic in nuclear weapons. Given its half-life, it generated about six times more spontaneous fissions as ²³⁹
Pu
(incorrect calculation, see revised calculation below, it's five orders of magnitude higher) , but that strikes me as unlikely to be a major issue - more likely the presence of ²⁴¹
Pu
having a huge fission cross-section would be an issue. I don't have access to fast-neutron cross-sections for either isotope to answer what the issue is, so I suspect this particular issue will have to remain questionable. Tarl N. (discuss) 19:42, 22 July 2016 (UTC)

I know that plutonium-240 is problematic in nuclear weapons, so they usually like to keep the amount of ²⁴⁰
Pu
low. According to the table here, ²³⁹
Pu
emits 3.0 x 10^-2 neutrons per gram per second, whereas ²⁴⁰
Pu
emits 1.0 x 10³ neutrons per gram per second. That's 30,000 times as much. However, I also know that back in 1962 the Americans detonated a bomb using low-grade plutonium, just to prove that it could be done. Hawkeye7 (talk) 21:29, 10 September 2016 (UTC)

The numbers in that table don't add up, unless there are additional things going on behind the scenes. It claims ²³⁹
Pu
has a fission half-life of 10⁵ years, while ²⁴⁰
Pu
has a fission half-life of 10¹¹ years. That would make sense, even-even isotopes are much less likely to fission than even-odd isotopes, and would suggest ²⁴⁰
Pu
would generate 10^-6 as many neutrons. But that table nonetheless claims ²⁴⁰
Pu
generates 10⁵ more neutrons. That just doesn't add up. The straight decay half-life doesn't apply, that's production of alpha particles.

@Hawkeye7 and Tarl N.:Ideally, if i understand it correctly, the assembly need to be completed before the start of the chain reaction to maximise the initial supercriticality and hence maximize the resulting yield. The high rate spontaneous fission of Pu240 triggers the chain reaction too soon (within the first msec) and reduces the resulting yield leading to a fizzle. (quote from Los Alamos Primer:It seems one shoulds trive for a neutron background of 10000 neutron/sec or less and firing velocities of 3000ft/sec or more.Both of these are difficulty of attainment). I added few references to support this. Afernand74 (talk) 20:13, 11 September 2016 (UTC)

My source in the Project Y article gives a similar figure for spontaneous fission in ²⁴⁰
Pu
of 1.6 million fisions per gram per hour, which seems to match the figures above. We can calculate. It also says Segre counted 180 fissions per gram per hour in his X-10 plutonium. That makes 0.05 per gram per second. If we are assembling a 10 kg sphere of Pu, that makes 500 fissions per second. If our projectile is going at 1,000 m/s, then it will take 0.0001 seconds to travel 10cm. In that time we expect 0.05 fissions. Using Poission's approximation, the probability of zero fissions is exp (-0.05) = 0.95. So one chance in 20 of a fizzle. Hawkeye7 (talk) 21:37, 11 September 2016 (UTC)

Let's take the numbers we've seen.. According to Isotopes_of_plutonium#Table, ²⁴⁰
Pu
has a half-life of 6.56e3 years, almost all of it α decay. 5.7e-6 of the decays are spontaneous fissions, which translates to a spontaneous fission half-life of 1,15e9 years, or 3e16 seconds. This translates to 1.66e7 SFs per second per mole, or 69,000 per second per gram, which is *way* higher than the 1.6 million per hour mentioned from Project Y. However, the ²⁴⁰
Pu
is claimed to cause significantly more SFs than ²³⁹
Pu
does, by five orders of magnitude - I calculate 1.0266 fissions per second per gram for that isotope.

Ah! That's the error - in the table, they list a spontaneous fission half-life for ²³⁹
Pu
of 5.5e5 years, when the other numbers we have indicate the SF half-life should be 7.7e13 years. So, that does explain why ²³⁹
Pu
may fail to detonate on a simple compression without a neutron enhancer. Tarl N. (discuss) 22:34, 11 September 2016 (UTC)

I removed the mark "dubious" from the main article @Hawkeye7: Following one of your comments above, which US nuclear test used "low grade" Pu in 1962?

No idea. The U.S. announced in 1977 that it had successfully tested a nuclear weapon using reactor-grade plutonium back in 1962.[1] But the U.S. conducted 96 nuclear tests that year, and about 36 of them had a yield matching the details given about the test. [2] (quote: "Such statements are now finding their way to sources such as Wikipedia.") Hawkeye7 (talk) 16:00, 14 September 2016 (UTC)