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1. Introduction<br />

The management of the high-level radioactive wastes (HLW) is one of the important key issues in<br />

nuclear society at present. Various concepts for transmuting long-lived radioactive nuclides contained<br />

in HLW to shorter-lived or stable nuclides have been proposed to reduce the risk from long-term<br />

toxicity. Reactors with a hard neutron spectrum have capability to burn minor actinides (MA) such as<br />

neptunium, americium and curium which dominate the long-term toxicity of spent fuels.<br />

Recycling the all actinides and some long-lived FPs into fast breeder reactor (FBR) and the<br />

accelerator driven system (ADS) to close the fuel cycle from a view point of actinide confinement is,<br />

therefore, one of the promising options to be considered in solving the problem.<br />

A new concept of nitride fuel cycle system based on pyrochemical reprocessing has been<br />

proposed, and excellent core performance of the lead cooled nitride FBR could provide design<br />

flexibility of reactor systems for energy production and/or reactors for burning or transmuting<br />

long-lived radioactive nuclides [1,2]. For utilising the nitride fuel, the effect of 15 N enrichment on<br />

nuclear characteristics and the evaluation of toxicity of 14 C generated from 14 N was appeared, and<br />

excellent performance for the minor actinide (MA) transmutation was shown [3]. Furthermore, the<br />

current status for the liquid heavy metal Pb-Bi technologies were investigated [4].<br />

The symbiosis concept of the fuel cycle system based on nitride fuelled fast reactors consists of<br />

base load reactors. The base load reactor produces electric power and extra plutonium fuel if needed,<br />

and the MA (about 1 wt%) generated by themselves are recycled [2]. The MA transmuter designed in<br />

the present study has simultaneously a role of incineration for Pu, MA and 129 I generated from UO 2<br />

and<br />

MOX fuelled LWRs, and FBRs systems. A part of the recovered Pu and minor actinides by<br />

pyrochemical reprocessing are recycled into the ADS transmuters to adjust to the excess or remained<br />

plutonium and to transmute the MA and iodine.<br />

Furthermore, accumulation and transmutation of MA and 129 I based on future symbiosis recycle<br />

system are investigated for introducing the accelerator driven system (ADS) with 800 MWt. It is<br />

shown that in the scenario of nuclear plant capacities for maximum 140 GWe, which consists of<br />

LWRs and FBRs, the introduction of ADS can play a significant role as “Transmuter” in the back-end<br />

of fuel cycle.<br />

A conceptual design study based on the experimental program for development and<br />

demonstration of accelerator driven transmutation technology under the project plan of the High<br />

Intensity Proton Accelerator and the OMEGA Programme at JAERI has been done. And the current<br />

activities of the design studies for the experimental facilities for ADS technology demonstration are<br />

reviewed.<br />

2. Design study of ADS<br />

Transmutation of MA and iodine was studied by using a lead-bismuth cooled ADS with<br />

800 MWt. MA (Am, Cm and 237 Np) are most dominant contributors for long-term potential hazard in<br />

spent fuel. On the other hand, long-lived fission product nuclide 129 I will be recovered as AgI<br />

formation in reprocessing system for spent fuels. And the iodine is soluble in water and one of the<br />

most troublesome nuclide on the geological disposal technology, though its potential hazard is smaller<br />

than those of MA Thus transmutation for this nuclide is strongly expected as one of troublesome<br />

isotopes from a viewpoint of waste disposal. The iodines are loaded axially and radially with the form<br />

of NaI around the MA-fule core in ADS, and it was shown that an ADS can transmute the MA and<br />

iodines generated from 9 or 10 units of LWR with 33 GWd/t per year as shown in Table 1. The<br />

conceptual design of the ADS is shown in Figure 1.<br />

534

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