FBaggins is correct.
ALL nuclear power reactors have plutonium in them ( assuming they've been operating ) whether or not you put MOX in them.
The Uranium fuel in a power reactor is only about 3% to 4% made of the fissile Uranium-235. U-235 is really the reactor's fuel. The other 96% to 97% of the Uranium in the reactor is non-fissile Uranium-238.
The Uranium-238 can be fissioned but only by very high energy or fast neutrons fresh from fission. The VAST majority of the neutrons in the reactor (>99%) are slow neutrons and Uranium-238 will NOT fission with slow neutrons.
However, when slow neutrons hit Uranium-238; it will capture the neutron, and then radioactively decay to Plutonium-239:
U-238 + n --> U-239 --> Np-239 --> Pu-239
Uranium-238 absorbs a neutron, temporarily becoming U-239; which then "beta" decays to Neptunium-239, which again "beta" decays to Plutonium-239.
That is happening all the time in an operating reactor. The operating power reactor is continually making plutonium, and it is both being burned / fissioned ( 40% of the energy you get from a power reactor comes from fissioning Pu-239 that was created in situ ), and some of that Pu-239 is left over in the spent fuel.
The spent fuel also has fissioned products which are removed when you make MOX.
EVERY Plutonium atom that is in MOX has been in a reactor before; that's where it got made.
FBaggins is correct; this whole folderol about Unit-3 being "special" because MOX was added is really nothing more than a "red herring". ALL those reactors had plutonium in them, whether MOX was added or not; and the reactors are designed to run perfectly well with plutonium in them.
As I stated above; nearly half (40%) of the energy you get from a commercial power reactor comes from burning plutonium.
PamW
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