In breeder reactors, do the plutonium-rich rods have to be processed?

CCWilson

I understand that Plutonium 239 is produced when fast neutrons bombard the U-238 in the fuel rods of a breeder reactor. Does the plutonium immediately start producing neutrons to increase the controlled nuclear reaction, or do the rods have to be removed and processed for future use?

Also, do breeder reactors theoretically have a longer lifespan than conventional power plants, or does the deterioration of materials limit the lifespan to the current 40 plus years? In other words, do they produce more fissionable material, but it has to be moved to a new power plant in order to be used?

Is it possible to build nuclear plants that have virtually unlimited lifespans, and if so, which components would periodically require replacement?

Astronuc

Breeders use driver fuel and blanket fuel. The driver fuel can remain in the core for some time, and the blanket fuel would be removed for extraction of the bred Pu. FVR fuel is clad in special stainless steel cladding tubes. If it were to be used in an LWR, the MOX fuel would be removed and processed (fission products removed and fissile inventory Pu and TU blended with UO2) for use in an LWR. LWR cladding is typically a Zr-alloy.

Generally no. The fuel is limited to about 10 to 15% FIMA (fissioned initial metal atoms), or a burnup of 100 to 150 GWd/tHM, which is about 2 or 3 times the burnup in a conventional LWR. Growth and swelling of steel components is life-limiting.

No. The fluence on core structural materials can be limiting, in conjunction with chemical changes associated with leaching of alloying elements. Usually, the higher the temperature, the more limited the lifetime.

http://www.iaea.org/Publications/Mag...0604782938.pdf

http://www.world-nuclear.org/info/inf98.html

QuantumPion

Note that regular LWR's breed fuel. A typical PWR has a breeding ratio of ~0.5, meaning for every kg of U-235 consumed, 0.5 kg of Pu are created. About 30% of the output power of a commercial nuclear reactor comes from the plutonium that is continuously bred from U-238.

caldweab

Plutonium isn't made by uranium 238 absorbing a fast moving, uranium 238 is a fissionable material (it is also a fertile material) not a fissile material so by absorbing a fast neutron it would actually fission. Plutonium 239 is created by uranium 238 absorbing a slow moving neutron.

Astronuc

Not quite. In the neutron energy range from 0.1 to ~1.4 MeV, the capture cross-section is greater than the total fission cross-section.

Neutron energies in the 100 keV (0.1 MeV) range are considered fast.

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Astronuc

Here's a couple of figures that better illustrate U-238 the (n,γ) and (n,f) cross-sections, and the neutron energy spectrum in an LWR vs a fast reactor.

In the fast spectrum, U-238 must compete with Pu-239, the latter of which is more likely to capture a neutron and fission. After one or two collisions, fast neutrons (E > 1 MeV) drop below 1 MeV where they are more likely to be captured, rather than cause a fission, by U-238.

Fast neutrons are those neutrons with energy > 10 keV
http://ocw.mit.edu/courses/nuclear-e..._dep_neutr.pdf

MOX fuel can be reprocessed, but then one has to deal with the increased levels of Am, Cm isotopes, which undergo higher rates of spontaneous fissions. For those programs in which MOX is recycled, there are limits on burnup and Am, Cm limits.

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caldweab

I see, I was speaking specifically of breeding reactors and admittedly I'm still in the beginning of my actual nuclear engineering courses, I've done all the mechanical engineering course work for the most part and took a nuclear energy class. I'm currently in fundamentals of nuclear engineering and we just recently covered breeding reactors, it is true that those neutrons that come out due to fission are very fast moving and have to be slowed by the moderator in order to fission. Are you a nuclear engineer by the way? I see you reply on a lot of nuclear engineering threads, very helpful stuff

jim hardy

While you're here -
It is my understanding that neutrons are "born" at about 1 mev, is that so?

The graphs you posted show cross section for fission increasing in range 1-10 mev.
Some materials have a n,3n reaction in that range, hafnium for example.

Is there significant flux in that energy range before moderation?
One wonders if that's why "Bigten" works.

Pardon my ignorance - we didn't touch on fast reactions in the basic reactor physics course i took (1969).

old jim

Astronuc

Yes - I'm a nuclear engineer. Most of my work is in fuel design and analysis, core materials performance, and some experience in core design.

Fission neutrons are born in the MeV range. Here is a good reference:
http://neutron.kth.se/courses/transm...a/Spectra.html

See Figure 1: Fission neutron spectra for U-235 (red line) and Pu-239 (blue line),

and Figures 3 and 4. Note the difference in the energy weighted spectra. Part is due to oxide vs nitride, and part is due to the coolant. Nitrogen is a better moderator than oxygen, and carbon is a better moderator than nitrogen. There is a difference in n-spectrum is one uses carbide fuel as opposed to nitride or oxide. Most experience is with oxide fuel.

For fast reactors, (n, 2n) should be considered depending on core/fuel composition. I don't think n,2n and n,3n are important in the low MeV range and below. One will find reference to 14 MeV neutrons, for which n,2n and n,3n reactions would occur, but one does not find 14 MeV neutrons in LWRs or FRs.

Another useful resource: http://neutron.kth.se/courses/reacto...06_Fission.pdf
from http://neutron.kth.se/courses/reacto...urenotes.shtml

See also - http://neutron.kth.se/courses/

See also - Fast Reactor Physics and Core Design - www.ne.doe.gov/pdfFiles/FRPhysics.pdf
and - physor2012.org/Workshops/9.SFR-physics.pdf

jim hardy

Thanks Astro - your kindness is much appreciated.

law&theorem

316 stainless steel is always used as fuel clad in LMFBR.