Any reactor leaks neutrons, and fast reactors rather more, adding an uranium (which is what spent fuel is still mostly made of) blanket around such source of otherwise "useless" neutrons will allow for some Pu breeding. It is the optimisation of the design that may allow to breed more than is burned.
You are confusing things. Indeed U238 fission without breeding Pu with fast neutrons. Now ask yourself : why are specifically breeder designs nearly all fast reactors? Where are the modern breeders LWR (I mean specifically aimed at breeding)? (yes I know about Windscale or UNGG, my point is that fast reactor are by far favored for breeding purposes)
I'm guessing OP question is whether you can put user fuel directly into the primary salt, which is a different can of worms.
The problem is what U-238 only fissions from fast neutrons, so a fast spectrum isn't great for breeding.
What you really need to look at is at the ratio of (macroscopic) cross-sections for U-238 neutron absorption, and the sum of Pu-239 absorption and fission (or absorption and fission of whatever fissile is powering the reactor). That number tells you the odds for a neutron to breed new fissile vs. being wasted by capture or consumed by fission. If the number is greater than one, you've got a breeder; if it's lower than one, you've got a burner.
Turns out that in a thermal spectrum, the microscopic absorption cross section of U-238 is two orders of magnitude lower than the combined absorption + fission cross section of Pu-239. But the concentration of U-238 in a reactor core is typically one order of magnitude higher than Pu-239, so the ratio of macroscopic cross sections is compensated by that. Still, in the end it's still below one, and thermal reactors in the U-Pu cycle run as burners.
In a fast spectrum the fission + absorption cross section of Pu-239 decrease dramatically, while absorption for U-238 doesn't decrease as much, so they become commensurate.
adding an uranium (which is what spent fuel is still mostly made of) blanket around such source of otherwise "useless" neutrons will allow for some Pu breeding.
I have this image in my mind of an Elysium designed with a blanket of used LWR fuel surrounding the core, soaking up neutron leakage before it finally gets added to the pot.
Considering that an Elysium started on reprocessed LWR fuel would require extraction of considerable uranium to get a mixture which can go critical in a fast spectrum, maybe use that instead. Anything to cut the doubling time!
Any reactor leaks neutrons, and fast reactors rather more, adding an uranium (which is what spent fuel is still mostly made of) blanket around such source of otherwise "useless" neutrons will allow for some Pu breeding. It is the optimisation of the design that may allow to breed more than is burned.
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You are confusing things. Indeed U238 fission without breeding Pu with fast neutrons. Now ask yourself : why are specifically breeder designs nearly all fast reactors? Where are the modern breeders LWR (I mean specifically aimed at breeding)? (yes I know about Windscale or UNGG, my point is that fast reactor are by far favored for breeding purposes)
I'm guessing OP question is whether you can put user fuel directly into the primary salt, which is a different can of worms.
You're right, I am confusing things
What you really need to look at is at the ratio of (macroscopic) cross-sections for U-238 neutron absorption, and the sum of Pu-239 absorption and fission (or absorption and fission of whatever fissile is powering the reactor). That number tells you the odds for a neutron to breed new fissile vs. being wasted by capture or consumed by fission. If the number is greater than one, you've got a breeder; if it's lower than one, you've got a burner.
Turns out that in a thermal spectrum, the microscopic absorption cross section of U-238 is two orders of magnitude lower than the combined absorption + fission cross section of Pu-239. But the concentration of U-238 in a reactor core is typically one order of magnitude higher than Pu-239, so the ratio of macroscopic cross sections is compensated by that. Still, in the end it's still below one, and thermal reactors in the U-Pu cycle run as burners.
In a fast spectrum the fission + absorption cross section of Pu-239 decrease dramatically, while absorption for U-238 doesn't decrease as much, so they become commensurate.
I have this image in my mind of an Elysium designed with a blanket of used LWR fuel surrounding the core, soaking up neutron leakage before it finally gets added to the pot.
Considering that an Elysium started on reprocessed LWR fuel would require extraction of considerable uranium to get a mixture which can go critical in a fast spectrum, maybe use that instead. Anything to cut the doubling time!