Most of the long lived radioactive components of nuclear waste are the actinides, shown in the list below. With the exception of U236 and U238, all these actinides tend to build up in thermal spectrum reactors but are readily fissionable in a fast spectrum reactor. There are certain aspects of using the LMFBR concept for this purpose that require further treatment. The table below shows the reproduction (neutrons produced per neutron absorbed) for the actinides of interest:1
|Isotope||η (LWR)||η (LMFBR)|
Table 12 Reproduction rates for actinides
Since the quantity η is a measure of the number of neutrons produced per absorption of the isotope in question, it is apparent from the table above that it might be feasible to fuel an LMFBR with minor actinides without any contribution from either uranium or plutonium, saving those fuels for the LWRs and LMFBRs that are not committed to actinide burning. However, doing so could possibly require that the actinide burner fuel form be metal.2
From the table above, it is reasonably clear that the minor actinides will tend to accumulate in LWRs while in LMFBRs they would be more likely to fission. It is instructive to consider how the isotopes in the table are formed in reactors. In the figure below, which is the portion of the chart of the nuclides of interest for this subject, long lived isotopes are shown in dark blue color while the shorter lived appear as successively lighter. The only significance of the isotope outlined in red (Cm246) is it is in the middle of the table. For example, Am244 has a half life of just about 10 hours, so it will not build up in the reactor but it will decay to Cm244 which has a half life of about 18 years. The Np237 is formed from double neutron radiative capture by U235, first to U236 then to U237 which decays with a 6.7 day half life to Np237. Another path to Np237 is through an (n, 2n) reaction of U238. About 60% of the minor actinides formed in LWRs are in the form of this one isotope.
Figure 54 Chart of the nuclides – region of interest
The americium and curium isotopes are formed by successive neutron captures followed by beta decay of Pu243 and Am244. About 35% of the minor actinides in LWRs are americium and less than 5% are curium. There will be trace amounts of berkelium and californium but not much since the curium isotopes are all long lived up to Cm249 so eleven neutron captures above Pu239 would be required for berkelium formation – an unlikely prospect.
All three of these minor actinides, neptunium, americium, and curium wind up in the waste stream following PUREX reprocessing, so means would need to be employed to remove them from the waste stream. Electrochemical methods have been developed for their separation and various chemical methods (e.g. TRUEX) have been proposed. Reprocessing spent fuel followed by removal of these three minor actinides results in waste stream radioactivity equal to the uranium from which it was originally derived after about 700 years in contrast to 250,000 years for un-reprocessed spent fuel. Thus the incentive for actinide burning becomes clear.3
It is well to have an idea of the magnitudes involved. Each atom percent burnup of LWR fuel contributes about 200 grams of minor actinides per metric tonne of LWR fuel. Another way of looking at this situation is at 33,000 MWD/MTU burnup of LWR fuel, the spent fuel composition is 96% uranium, 0.8% plutonium, 3.2% fission products and 0.05% minor actinides. Most LWRs operate at a burnup somewhat higher than this today. The scale up of minor actinides is somewhat less than linear due to burnup in the reactor but it is nearly linear for the curium isotopes. A fleet of 100 1000 MWe LWR reactors operating their fuel to a burnup of 50,000 MWD/MTU will generate about 1-1 ½ metric tonnes of minor actinides per year. This would be sufficient to fuel 2-3 equivalent sized LMFBRs.
An oxide fueled LMFBR running on this concoction alone would possibly not be capable of achieving criticality and would require some spiking with plutonium, probably on the order of 10-15 %. The neptunium would be, in essence, the fertile isotope, converting to Pu238, which is fissile in a LMFBR. Because these isotopes are all highly radioactive and biologically hazardous, fabrication of fuel would not be a trivial matter, but probably only somewhat more difficult than fabrication of plutonium bearing fuel. Since the difference in η between the fertile and fissile isotopes is nowhere near as large as is the case between U238 and Pu239, such reactors will be ineffective breeders, and will not be capable of operating for long periods without refueling. Because of this shortcoming, the attractiveness of committed actinide burners diminishes somewhat. Committed actinide burners would be possible, but they would likely require frequent (annual) refueling with all that implies to the plant design.
It may be preferable to mix relatively smaller amounts of the minor actinides into a uranium plutonium mix. This could also be accomplished by fabricating selective assemblies exclusively out of the minor actinides. Tailoring the reactor core with these substances could possibly offer opportunities that are not present in a reactor fueled with uranium and plutonium alone, particularly if the neptunium, americium, and curium streams are kept separate from one another. Since each of the minor actinides has different neutronics characteristics, if tailored core assemblies were fabricated it doesn’t require much of a stretch of imagination to conclude that each may be particularly well suited to a particular region of the core. One could, for example, place neptunium assemblies in the lowest flux regions of the internal blanket in the interest of flux flattening or americium in high flux fuel assembly locations for the same reason. The core designer would be suddenly granted several new degrees of freedom that did not theretofore exist. This might make an interesting topic for a PhD thesis.
1 Drawn from Characteristics of a Minor Actinide Fueled Reactor, FFTF Internationalization Symposium, Rockwell International, May 28, 1991
2 It should be pointed out that Table 11 assumed metal fuel was being used in the LMFBR, which would yield a harder spectrum than an oxide fueled system. The reproduction numbers are slightly lower for the softer spectrum associated with an oxide fueled reactor.
3 Further improvement can be obtained by separating the long lived fission products Tc99 and I129 and converting them through neutron irradiation.