[ExI] Plutonium

[Keith Henson]

On Thu, Mar 27, 2008 at 2:36 PM, Tom Nowell <nebathenemi at yahoo.co.uk> wrote:
snip

>   The trouble for NASA is that getting the Pu-238 out
>  of the mix is expensive. When people were refining the
>  stuff to make weapons grade >92% Pu-239 (the report
>  gives handy definitions of different plutonium
>  grades), they separated out the Pu-238.

They never did, it's extremely hard to do it, darn close to
impossible.  Think of how hard it is to sort out U235 from U238 and
that's *three* atomic mass units.

What the Russians did was to sort out the Neptunium 237 when they
reprocessed fuel rods and stick that back in the reactor to absorb
another neutron.  From Wikipedia,

Pu-238

    Main article: Plutonium-238

There are small amounts of Pu-238 in the plutonium of usual
plutonium-producing reactors. However, isotopic separation would be
quite expensive compared to another method: when a U-235 atom captures
a neutron, it is converted to an excited state of U-236. Some of the
excited U-236 nuclei undergo fission, but some decay to the ground
state of U-236 by emitting gamma radiation. Further neutron capture
creates U-237 which has a half-life of 7 days and thus quickly decays
to Np-237. Since nearly all neptunium is produced in this way or
consists of isotopes which decay quickly, one gets nearly pure Np-237
by chemical separation of neptunium. After this chemical separation,
Np-237 is again irradiated by reactor neutrons to be converted to
Np-238 which decays to Pu-238 with a half-life of 2 days.

The article isn't entirely accurate but for this discussion it lays
out the method.  If anyone wanted more of it, the method I discussed
for making plutonium would work fine

Keith