The long-term radioactivities of neptunium, americium, and curium in the high-level reproces­sing wastes from the uranium-fueled water reactor are shown in Fig. 8.7. Except for 241 Am and e7Np, these curves are also applicable to unprocessed discharge fuel. The curves 241 Am and M7Np have been calculated for 0.5 percent of the plutonium in discharge fuel to appear in the wastes, so that there is not sufficient 241 Pu to significantly increase the amounts of 241 Am and

»’This effective cross section is greater than the cross section for thermal neutrons because of resonance absorption in 236 U.

231Np during the decay periods. The high activities of americium persist for thousands of years and are greater than the fission-product activity after a few hundred years of storage.

The radioactivities of the plutonium radionuclides in the high-level wastes from fuel reprocessing are shown as a function of storage time in Fig. 8.8 [Р1]. Because the initial plutonium quantities are due only to the small fraction, e. g., 0.5 percent, of the plutonium that is lost to these wastes in reprocessing, larger quantities appear after a few years due to the decay of americium and curium. The 238Pu increases with time because of the decay of 24201 Am and 242 Cm, 239 Pu increases from the decay of 243 Am and 243 Cm, and 240Pu increases due to the decay of 244 Cm. Therefore, even though the total actinide activity in these wastes is dominated by plutonium after the americium has decayed, the plutonium in the wastes at this time is due mainly to the earlier decay of americium and curium and not to the small fraction of plutonium lost to the wastes in fuel reprocessing.

The ingestion toxicity indices of the actinides in the wastes are shown as a function of decay time in Fig. 8.9 [P2]. Because the actinides are nonvolatile and because the wastes are expected to be geologically isolated, ingestion toxicity is probably a more important measure than inhalation toxicity. During the first 600 years the total toxicity index is controlled by the fission products, mainly 90 Sr. It is thereafter controlled by 241 Am and 243 Am, followed by

Figure 8.7 Radioactivity in cur­ium, americium, and neptunium as a function of decay time. (Amount in the wastes produced annually by reprocessing fuel dis­charged from a 1000-MWe uran­ium-fueled PWR.) these wastes. In Fig. 8.10 the toxicity indices are shown relative to the ingestion toxicity of the ore [P2]. The ore toxicity is due mainly to the 236 Ra, which is in secular equilibrium. Also shown are the relative toxicity indices for the uranium mill tailings, which contain 230Th and 226 Ra separated from the uranium ore, and for the depleted uranium from isotope separation, neglecting the likely later use of this uranium as fuel for breeder reactors. Because the uranium ore ingestion toxicity is dominated by 226 Ra, all of this toxicity is transferred to the mill tailings and is preserved for over 100,000 years because of the long half-life of 230Th. The tailings toxicity then decays to a lower value due to the residual uranium, e. g., about 5 percent, which remains with the mill tailings.

The ingestion toxicity of the high-level waste decays to a level below that of the initial ore after the fission-product period of about 600 years, and it ultimately decays to a toxicity that is a fraction of a percent of the toxicity of the original ore consumed to generate these wastes.

Because in the LWR fuel cycle most of the uranium in the ore appears in the depleted uranium from isotope separation, this depleted uranium if not used as breeder fuel, will slowly build up its decay daughters and 226Ra toxicity. Ultimately, a toxicity level within a few percent of that of the original ore will be reached.

The toxicity indices are not measures of hazards, in part because they take no account of the barriers that isolate these wastes from the biosphere or of the behavior of different radioactive elements with respect to these barriers. However, the long-term toxicities of the high-level reprocessing wastes are due to radium, which is the same element that controls the ore toxicity. The long-term radium toxicity of the reprocessing wastes is considerably less than the radium toxicity of the ore. It seems reasonable that high-level wastes can be geologically

Figure 8.8 Radioactivity in pluton­ium in high-level wastes as a func­tion of decay time (in wastes pro­duced annually by reprocessing fuel discharged from a 1000-MWe uranium-fueled PWR).

isolated so that the waste material has less access to the environment than the radium in the natural ore. Therefore, it is likely that the longer-term hazards from geologically isolated high-level wastes will be less than those already experienced due to the naturally occurring uranium minerals. The period of greatest importance in high-level waste management is probably the earlier, 600-year period of high fission-product toxicities.