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Home arrow Environment arrow Nuclear Back-end and Transmutation Technology for Waste Disposal

Impact on the Repository

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

Fig. 19.9 Result of FR+ADS scenario

Fig. 19.10 Potential radiotoxicity of all wastes and uranium ore

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.

Discussion

Table 19.11 summarizes the results of scenario analysis. In comparison between LWR-OT and LWR-PuT, reductions are observed in Pu amount, repository footprint, and decay time of toxicity, although they are not drastic. Important benefits of MOX utilization are Pu isotopic deterioration as a nuclear weapon and improved confinement of radionuclides by calcinations, as discussed by Nishihara et al. [7]. However, there remains 110 t of separated Pu that can raise concerns about proliferation.

In comparison between conventional and transmutation scenarios, significant reductions of TRU amount, repository area, and decay time of toxicity are observed. The remaining Pu undergoes several irradiations in the transmuter and is highly resistant to weapon utilization. Repository area is about one fifth and decay time is reduced by one tenth in the maximum case. To achieve such benefit, a total of 15–32 transmuters have to be introduced for 180–240 years with corresponding reprocessing and fabrication facilities. Cost and risk during operation of these facilities would be high compared to their reduction in the repository in the further future.

Comparing the transmutation scenarios, the number of units in the FR scenario is fewest owing to its high thermal output. However, transmutation performance is

Table 19.11 Summary of scenario analysis

Scenario

Transmuter

Period (years)a

Remaining Pu

Remaining MA

Repository

FR

ADS

Area (km2)

Toxicity (years)b

Conventional

LWR-OT

350

80

3.9

100,000

LWR-PuT

260

100

3.3

70,000

Transmutation

FR

15

240

40

17/16c

0.9

40,000

ADS

32

180

8

3/16c

0.8

10,000

FR+ADS

11

9

240

20

8/16c

0.8

20,000

aNecessary period for transmutation after closing LWRs

b Time to decay less than toxicity of corresponding uranium ore

cSeparated MA/vitrified MA

Fig. 19.12 Trans-uranium (TRU) amount available for transmutation in transmutation scenarios excluding minor actinides (MA) vitrified in waste

less than the ADS, resulting in longer transmutation era and larger remaining TRU. Figure 19.12 shows amounts of TRU excluding vitrified MA. An amount of the ADS scenario is reduced to 125 t after the first generation of the transmutation era from 2050 to 2110. In the FBR scenario, equal decrement is achieved after the second generation. Very high MA content up to 15 % in the FR fuel is also problematic. The decrement of the FR+ADS scenario is as same as that of the FR scenario until the second generation, but after that the decrement becomes faster because of ADS introduction.

Although transmutation performance of the FR is inferior to the ADS, cost including profit of electricity generation by transmuter would be much smaller than the ADS because the number of ADSs is doubled, accelerator cost is added, and thermal efficiency of the ADS is worse. Considering high MA content in the FR scenario, the FR+ADS scenario can be a modest solution, although the ADS scenario is preferable if rapid transmutation is required regardless of the cost.

Conclusion

With consideration of the phase-out option from NP utilization in Japan, an ADS for Pu transmutation was designed and scenario analysis introducing it was performed. The ADS was designed based on the existing design of the ADS for MA transmutation considering two options of Pu supply: pure Pu and a mixture of the same amount of Pu and U from the reprocessing plant for the LWR. After designing a one-batch core with large criticality drop, a six-batch core with a short operation day was analyzed. The criticality drop of the six-batch core was small enough in the equilibrium state. Several maintenance cases were assumed, and those effects on the transmutation half-life were surveyed. Finally, a core with pure Pu supply and 30-day fuel reloading was selected as the reference case for the scenario analysis. The transmutation half-life was estimated as 24.8 years, meaning that the amount of Pu is reduced to half after 24.8 years of operation, taking maintenance and cooling time of spent fuel into account.

In the scenario analysis, once-through scenario of LWR spent fuel was referred to as a conventional scenario. LWR-MOX utilization with reprocessing of LWR spent fuel was also considered. As the transmutation scenario, three cases of transmuters that are only-FR, only-ADS, and both-FR+ADS were analyzed. The numbers of necessary transmuters were obtained as 15 to 32 units, and the necessary period for transmutation as 180–240 years. Benefit to the repository by reduction of Pu and MA was reduction of repository area by a factor of five and of decay time of toxicity by one order of magnitude. It was shown that MA vitrified in the LWR reprocessing plant before introduction of the partitioning technology in 2025 considerably deteriorates both benefit. Therefore, early introduction of the partitioning process and retrievability of MA from vitrified waste should be investigated.

In comparison among transmutation scenarios, reduction of TRU in the ADS scenario is two times faster than that in the FR scenario. It was found that MA content in FR fuel in the FR scenario was 15 %, which is much higher than the design limit of 5 %. On the other hand, the cost of the FR scenario including profit of electricity generation by transmuter would be much smaller than that of the ADS scenario because the number of ADSs is double, the accelerator cost is added, and thermal efficiency of the ADS is worse. Considering high MA content in the FR scenario, the FR+ADS scenario can be a modest solution, although the ADS scenario is preferable if rapid transmutation is required regardless of cost.

The present scenario study revealed that the number of the transmuters and time necessary to transmute Pu and MA in the LWR legacy is considerably large. However, impact on the TRU amount in the repository related to the nonproliferation issue, repository size, and decay time of the potential radiotoxicity is also expected to be large. Assessments of increasing cost and risk to operate transmuters based on the present analysis are the next subject.

 
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