How do you get Plutonium from Uranium?

[marcusl]

Hi all,

I am trying to study fission, and my crude understanding has some gaps.

a) U235 fissions when it absorbs a thermal neutron because U236 is energetically favorable, and the energy difference is larger than the activation barrier to splitting. The activation barrier in the simple model arises from the larger surface-tension energy of two small nuclei compared to the one larger one. So it's U236 that splits, right?

b) Can a fast neutron also induce the reaction? I'm thinking yes since bombs fission and must have mostly fast neutrons, not thermal...

c) Would that imply that the purpose of moderators in reactors is to absorb excess neutrons to keep the fuel from going critical?

c) If U238 absorbs a thermal neutron it becomes Pu239. Where do the two extra protons come from?

Thanks in advance.

 

[mathman]

a) and b) You're right (I'm not completely sure of the details for a, but you've got the general idea).

c) U235 fission cross-section is much higher for thermal than for fast neutrons, so reactors can work that way. Moderator primarily is to slow neutrons down. Control is by rods containing nuetron absorbing material to keep criticality exactly 1.

d) U238 absorbs neutron to become U239, beta decay to Np239, beta decay to Pu239.

 

[Astronuc]

c) Would that imply that the purpose of moderators in reactors is to absorb excess neutrons to keep the fuel from going critical?

Just adding to what mathman wrote - the purpose of the moderator is to slow fast (fission) neutrons to thermal energies so that they have a greater probability of causing fission. The ideal moderator would not absorb neutrons. H readily absorbs neutrons to become D, which has a much lower absorption cross-section (probability).

To control the nuclear reaction, control rods containing neutron absorbers, like B-10 (in carbide), Hf (not good since if absorbs hydrogen and swells), Dy (in Dy titanate), and Ag-In-Cd (not used in BWRs), are used in certain types of reactors, namely Boiling Water Reactors (BWRs).

In Pressurized Water Reactors, boric acid (with LiOH or KOH) is added to the primary coolant to control reactivity of the fuel. Soluble boron cannot be used in BWRs because the boiling (in core) would cause localized depositions. In PWRs, the control rods are usually fully withdrawn during operation, and are generally used only for shutdown.

In addition to contorl rods and soluble boron, neutron absorbing material is added directly to the fuel, as a solid solution (oxides of gadolinia or erbia) or as coating Zr-diboride on the surface of fuel pellets.

Most commercial plants (e.g. LWRs, CANDU, VVER, FBR) use ceramic UO₂ as fuel, although oxides of U,Pu are also used. The pellets are clad in an appropriate Zr-alloy in most cases. Fast reactors use stainless steel cladding.