One of the impacts on the repository by transmutation is reduction of potential radiotoxicity, which is defined as total ingestion dose of the waste. Because waste is isolated from the public in the underground in reality, such direct ingestion never occurs and it is considered to be hypothetical, but it can represent the potential danger of waste. This toxicity of waste can be compared to that of uranium ore consumed for electricity generation causing radioactive wastes. Figure 19.10 illus­trates those toxicities corresponding to whole operation of LWRs and transmuters. Consumed natural uranium is 370,000 t.

When wastes are generated, the toxicity becomes higher than corresponding uranium ore by three orders of magnitude. Fission products such as Sr and Cs are dominant in the early several hundreds of years, although actinides contribute to toxicity after that. Toxicity in the LWR-OT scenario decays to the level of uranium ore after 100,000 years. By reducing Pu in the LWR-PuT scenario, the decay time becomes shorter, to 70,000 years. In the transmutation scenarios, shortening of decay time depends on the remaining amount of TRU. The decay time is about 10,000 years in the ADS scenario in which the remaining TRU is approximately 30 t, including vitrified wastes. In comparison between the LWR-OT scenario and the ADS scenario, the amount of TRU is reduced by one order of magnitude, so toxicity is also reduced by same order. If MA in the vitrified wastes is retrievable, the amount of TRU will be reduced to around 10 t, which implies toxicity is reduced to 1/30 and the decay time is around 2,000 years. Thus, the impact on toxicity by transmutation is significantly affected by MA in the vitrified wastes. Early intro­duction of MA partitioning to the RRP and R&D for retrievability from the glass wastes is of importance in this aspect.

Another impact on the repository is reduction of repository size by partitioning and transmutation of heat-generating nuclides in the wastes. Repository size is represented by a repository footprint, which is defined as an area devoted for waste excluding aisles, ducts, utility area, surface facility, and other.

In the LWR-OT scenario, the footprint corresponding to 45,000 t spent fuel reaches almost 4 km2, which is double the typical repository design for the glass

Electoricity generation capacity (GWe)
MA inventory (t)
	1970 2000 2030 2060 2090 2120 > b 2150 2180 2210 2240 2270
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2300

waste corresponding to 40-year operation of the RRP, because the spent fuel assembly occupies more area and heat generation from the Pu in it also contributes.

In the LWR-PuT scenario, two kinds of waste form are produced: 37,000 glass waste forms containing FP and MA, and spent fuel assembly of MOX of 4,000 t. Each occupies 1.6 km2, and the total is 3.3 km2. Although an amount of MOX spent fuel is smaller than that of UO2 spent fuel in the LWR-OT scenario by a factor of 11, it contains more heat-generating actinides such as Am and Pu, and its footprint is significant.

In the early several hundreds of years, 90Sr and 137Cs, whose half-life is around 30 years, are dominant for the footprint. They are separated in the RRP after 2025 as well as MA in the transmutation scenarios. They are absorbed by adsorbents such as zeolite and calcined to the waste form. Because half-life is rather short and the repository footprint is almost proportional to heat generation, long-term storage of the calcined waste is effective [9]. After 300 years of storage, an accumulated layout for the TRU wastes that is low heat generating and with long-term radioac­tive wastes becomes available. The footprint of this layout is smaller by two orders of magnitude than a typical layout for the vitrified waste. After separating 90Sr and 137Cs, 241Am, whose half-life is 432.2 years, becomes dominant, but this nuclide is transmuted in the transmutation scenarios. Heat generation from other fission products that are vitrified quickly decays to the level of the TRU waste.

As result of the long-term storage and transmutation, the footprint becomes almost constant after 2025 (Fig. 19.11). The glass waste form that is produced before 2025 and contains MA occupies 0.5 km2. In the ADS scenario, partitioning and long-term storage of Sr and Cs in the wastes produced from reprocessing of ADS spent fuel is not assumed because the impact is small. As a result, the footprint gradually increases to 0.8 km2. Technologically, separation is possible in the reprocessing for ADS, and it will be applied if the increase becomes significant. Steps observed in 2230 and 2330 are caused by wastes of remaining TRU that will

Fig. 19.11 Repository footprint when wastes are produced be diluted to the glass waste, considering heat generation. The remaining TRU of the FR scenarios are more than that of the ADS scenario.

In the transmutation scenarios, the final footprint is around 0.8 km2, which is a fifth of the LWR-OT scenario. As is the case of radiotoxicity, the time of introduc­ing partitioning is significant because more than half of the repository is occupied by glass waste forms with MA.