Pu from spent fuels can't be used for weapons?

[middlephysics]

Pu from spent fuels can't be used for weapons??

Is this true?? I've always heard that Pu from spent fuel was a major major concern!

"The high spontaneous fission rate of 240Pu acts as a kind of poison in the core of nuclear bombs. More than about 7% 240Pu and the likelihood that the warhead won’t work increases, and the dangers of handling the more radioactive 240Pu become too great.

The 7% limit is reached in a typical nuclear reactor after about 90 days (depends on the reactor design and the 235U enrichment). Typical commercial power reactor fuel cycles are around two years. If the fuel cycle is shorter the electricity becomes uneconomic.

All this means that the plutonium than can be extracted from the fuel rods of a commercial nuclear power reactor is not suitable for making nuclear weapons."http://theconversation.com/debunking-myths-on-nuclear-power-its-not-for-making-bombs-20013

 

[etudiant]

It may be that the concern is more about using Pu to make dirty bombs than proliferation in the classic sense.
Still, one does wonder why the nuclear energy discussion is so dominated by half truths such as the plutonium issue.

 

[SteamKing]

Plutonium has several isotopes. Pu-239 is the material used in bomb devices, and is made by bombarding U-238 with neutrons. Pu-240 has a high rate of spontaneous fission, meaning that it emits neutrons as it fissions. This property is undesirable for use in a nuclear weapon which must remain in storage for lengthy period of time.

http://en.wikipedia.org/wiki/Isotopes_of_plutonium

 

[etudiant]

Too high a rate of spontaneous fission is also an impediment to effective bomb design. Unless properly allowed for, it can result in a fizzle, essentially a premature partial explosion with drastically lower yield.
It is however less of an issue for Mox fuel.

 

[RocketSci5KN]

I suppose you can chemically isolate the Pu, then subject it to isotope enrichment as is done with natural U, but it would be a terribly radioactive process. Easier to just remove the fuel after slight burn-up to limit the amount of Pu240. If you can guarantee a suspect country would not remove the fuel before it's normal 2-3 year cycle, its much less of a concern. However, I would not care to have even a 'fizzling' device go off anywhere near me.

 

[sf1001]

I know Pu239 produced in a thermal reactor is difficult or impossible to use for nuclear weapons if its fuel burn up is much greater than ~5 GW-d/tHM. But, could weapons grade Pu be produced from fast reactors w/ burn up > 30 GWd/t, since Pu 239 fission to absorption cross section ratio is much greater for fast neutrons than for thermal neutrons?

 

[etudiant]

Fast reactors lost headway in the US partly because of such proliferation concerns.
Admittedly, serious technical failures also contributed, not just in the US, but also in France and Japan.
Russia is currently the world leader in fast reactors, with the large BN 800 under construction and the smaller BN 600 operational since 1980. So the issue is not considered insuperable by some.

 

[etudiant]

It is interesting that Russia, which has had by far the most experience with liquid metal cooled reactors, sees that design as viable for a commercial fast reactor.
The US, France and Japan have all failed to achieve as much. Indeed, the ghostly video of the space suited Japanese technicians walking through areas of the Monju facility draped in sodium stalactites after the accident made it very clear that sodium cooling is not a routine technology.
The performance to date suggests that Russia will have world leading nuclear power technology available by the end of this decade.

 

[mheslep]

The first US plutonium bomb suffered from very high levels of Pu240 made in the "B pile" at Hanford, the first Pu production reactor. Per the Rhodes history, the Manhattan physicists knew about the spontaneous fission issue from Pu240, but did not anticipate the high levels coming out of the Hanford reactor, nor was their time to rework Hanford and remake purer Pu239. The issue when discovered was initially considered catastrophic to the Pu weapon effort, as spontaneous fission is fatal to the "gun" type of weapon then under consideration, since the slug fired towards the remaining Pu mass would melt, expand and fizzle in the microseconds before assembly. It was first thought this problem left the US program with only the U235 path, requiring the colossally expensive, gram-by-gram gas diffusion enrichment of Uranium.

However, the implosion approach was just proving itself feasible at the time, and an implosion design is far less susceptible to spurious neutrons, and far more complex than a gun. Manhattan literally doubled the 1944-45 workforce at Los Alamos to get implosion working.

I can only guess that spontaneous fission may well cause other problems for shelf life as suggested, but the answer to the OP question is yes, Pu contaminated with high levels of Pu240 can be used to make a weapon. The Fat Man weapon that destroyed Nagasaki in 1945 is evidence that, with sophisticated implosion design, a weapon can be made from 6.2 kg of Pu so contaminated.

 

[sf1001]

The plutonium used for modern nuclear bombs, which only use implosion type devices (similar to Fatman & Trinity), still needs to have <~7% Pu240 contamination, which can only be accomplished with <~90 days burn-up in a thermal nuclear reactor. Civilian (thermal) nuclear reactors typically have 6-18 mo. refueling cycles, so Pu from such reactors would have too much Pu240 for implosion type bombs as well. Also, according to http://en.wikipedia.org/wiki/Neutron_cross_section#Typical_cross_sections, Pu239 fission to capture cross section ratio is ~40 for fast neutrons and >1400 for thermal neutrons. So I guess Pu from U238 irradiated w/ same fast neutron flux for same amount of time has more Pu240/Pu239 than Pu from U238 irradiated w/ same thermal neutron flux for same amount of time?

 

[mheslep]

I would think that the higher than 7 percent 240/239 ratio would force a lower sub critical mass in weapon construction which would lower yield, but not rule out an implosion weapon usable by rogue states or terrorists.

 

[jim hardy]

Now I'm understanding why they didn't teach us much about fast reactors in my undergrad reactor physics course... in 1968...

 

[sf1001]

@ mheslep
Pu 240 has higher bare sphere (&reflected) fast critical mass than Pu239. But, the problem w/ Pu240 is that it lowers the time constant to become critical once mass has the right geometric configuration (I think because of Pu240's much shorter half life); the problem w/ Pu240 is that it causes the weapon to become critical too early. I think the smallest critical mass possible for various isotopes (after enough time has passed?) has more to do w/ neutron capture & fission cross sections (& maybe neutrons produced per fission event) than spontaneous fission rates for the different isotopes, which I think affect mostly time to become critical once appropriate conditions exist.

 

[Morbius]

Fast Reactors can be designed to be proliferation resistant..

I know Pu239 produced in a thermal reactor is difficult or impossible to use for nuclear weapons if its fuel burn up is much greater than ~5 GW-d/tHM. But, could weapons grade Pu be produced from fast reactors w/ burn up > 30 GWd/t, since Pu 239 fission to absorption cross section ratio is much greater for fast neutrons than for thermal neutrons?

sf1001,

Fast Reactors can be designed to be proliferation resistant. That was the case with the Integral Fast Reactor (IFR) researched by Argonne National Lab in the '80s and early '90s. See the following interview with Dr. Charles Till, then Associate Director at Argonne National Lab:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

Q: So it would be very difficult to handle for weapons, would it?

A: "It's impossible to handle for weapons, as it stands.

It's highly radioactive. It's highly heat producing. It has all of the characteristics that make it extremely, well,
make it impossible for someone to make a weapon."

Greg