Post by Roger Clifton on Sept 24, 2017 10:15:09 GMT 9.5
Elsewhere on BNC we have discussed the recycling of ordinary (slow neutron) reactor fuel for reuse in fast reactors as metal fuel. A project in Russia is closing the cycle by developing the reverse process, where used fast reactor fuel is reused in ordinary reactors as MOX.
MOX is a mixture of the recycled plutonium with topped-up uranium oxides. In ordinary reactors, the plutonium 239 content in MOX burns down relative to the plutonium 240 until the fuel is replaced. The used fuel can still be burned in a fast neutron reactor because there the plutonium 240 fissions as well. However, at the same time, plutonium 239 is being regenerated from the uranium matrix, restoring the viability of the material as the next cycle of MOX fuel in an ordinary reactor.
Said like that, it would seem that the slow neutron reactor runs on 239 and the fast reactor runs on 240, but that ignores the burning of the top-up uranium. More accurately, the cycle through the slow neutron reactor burns all the odd-numbered isotopes including 241, thus inhibiting breeding of the unwelcome higher actinides in the next cycle through the fast neutron reactor. With more breeding neutrons available, breeding of 239 increases.
As long as the initial fuel contained the required minimum proportion of 240 to 239, all subsequent stages of the fuel cycle would be unattractive for misappropriation.
Post by Roger Clifton on Sept 26, 2017 9:27:22 GMT 9.5
Their (third) proposed cycle is speculative in that it refers to a world where there are many fast reactors. It is remarkable in that it goes to completion while inhibiting the accumulation of higher actinides, by alternating the fuel burns in fast and slow neutron reactors. The quoted 2% may be higher actinides.
The (second) REMIX cycle referred to is appropriate for the more immediate future, where the slow VVER reactors are being exported all over the world. The remarkable part of this cycle is that the top-up uranium has 20% 235, the maximum permitted in the "low enriched uranium" (LEU) category. The high enrichment allows a much higher proportion of the excess 238 to be tucked away as depleted uranium, which is innocuous and easy to store, unlike reprocessed uranium. In the remix cycle used fuel from slow reactors, containing increased 239, is cleaned of its fission products and higher actinides, topped up with LEU uranium and returned to the slow reactors. As long as the initial charge contains a little 240, the plutonium 239/240 ratio is never rich enough for bomb-makers to want to steal. Because it only involves slow reactors, the cycle accumulates 240 and so must be set aside after perhaps three or four cycles.
Post by Roger Clifton on Sept 26, 2017 16:54:53 GMT 9.5
IFRs? The IFR concept described in "Plentiful Energy" is a metal-fuelled fast reactor integrated with an on-site electroprocessing unit. Used fuel would be electrolysed from the anode into a chloride melt, where the actinides (U,Np,Pu,Am etc have similar electronegativity) would be pulled down together into the cathode, leaving the fission products behind. The actinides are freed from the cathode, then extruded as fuel pellets.
The IFR would be somewhat better suited to burning someone else's higher actinides (Am,Cm,Bk,Cf etc). Oxide fuels such as MOX and nitride are crystalline, so minority elements tend to diffuse towards the grain boundaries(*) and eventually emerge onto the surface of the fuel as a nasty sweat. More hospitably, the metal fuel of the IFR is rather like a solder in that it is a collection of microcrystals in a matrix near to its eutectic point, so the minority elements would merge in. Volatiles form bubbles rather than straining the crystal.
The IFR plant operator would probably prefer to leave most of the higher actinides in the melt by only progressing the voltage so far as to capture most of the uranium and plutonium only. The Russian reprocessing system manages to separate out the uranium and plutonium together but separate from the fuel's higher actinides. So to that extent the systems are probably similar. However the IFR operator may be put under contract to return the higher actinides into the cycle to be burnt up.
(*) Rejection from the crystal matrix is not inevitable. "Synroc" is designed to trap minority elements in favourable crystal structures, such as phosphate and orthosilicate cages. I would be interested to hear from experts on the subject.
Post by Roger Clifton on Jan 21, 2018 16:25:36 GMT 9.5
WNN reports that a project for a "fuel complex", to demonstrate a closed nuclear fuel cycle is being built in Tomsk, in Siberia. Firstly a module for production of uranium-plutonium nitride fuel is being built along with the generating module, a Brest OD-300 fast reactor. Lead-cooled and small, the 300 MW reactor builds on experience in the Cold War submarine fleet. It is a precursor to a commercial scale lead-cooled 1200 MW fast reactor. Russia already runs the world's first commercial scale fast reactors in the BN series, which are sodium-cooled and MOX (oxide) fueled. BN oxide fuel has been, or is being, redesigned for the BN1200.
The breeding performance of nitride fuel is superior to that of oxide fuel. It is also denser than oxide fuel, allowing a tighter, smaller core, with higher neutron efficiency. The WNN article speaks of a subsequent "used fuel re-fabrication module" to complete the cycle and complex. It will be interesting to find out how the used nitride fuel is to be cleaned of FPs and re-fabricated into nitride fuel, to close the cycle.