Purpose and Outline | Research Group for Nuclear Transmutation System

Purpose and Outline

Research Group for Nuclear Transmutation System (NTS) is studying the transmutation technology of Minor Actinides (MAs) and Long-Lived Fission Products (LLFPs) included in High-Level Waste (HLW) discharged from nuclear power plants.

One of major problems for the HLW disposal is that a very long period of over ten thousand years is necessary for the radio-toxicity of HLW to decrease to the level of the natural uranium. If it is possible to reduce this period, the burden for the geological disposal of HLW will also decrease. It is also required to reduce the amount and the emplacement area for HLW so that the efficient utilization of the repository site becomes possible

To explore these possibilities, NTS is investigating the transmutation technology by using an Accelerator-Driven System (ADS).

Partitioning and Transmutation (P&T)

In the HLW, there are not only the MAs and the LLFPs but also strontium and cesium, which were euthermic fission products, and other useful elements. These elements are partitioned according to their features (Partitioning).

The MAs and the LLFPs, which are main sources of the radio-toxicity, are irradiated by neutrons.

By the irradiation, fission and/or neutron capture reactions occur and the MAs and the LLFPs are transmuted to other nuclides which will have low radio-toxicity or a short life time nuclide (Transmutation) (Fig. 1).

There are mainly two methods for the transmutation; (1) By using Fast Reactor (2) By using Accelerator-Driven System (ADS). In our group, the method by using ADS is mainly being investigated.

Fig.1 Concept of nuclear fuel cycle introduced the transmutation cycle.

Accelerator Driven System (ADS)

ADS is a hybrid system of an accelerator and a subcritical core fueled with MA. The ADS (800 MW thermal power) investigated in our group is able to transmute about 250 kg MA per year. This amount is corresponding with the MA amount discharged from 10 units of 1 GW_e Light Water Reactors (LWR) per year.

Fig.2: Concept of ADS

Issues and Challenges for ADS

Since the ADS is a hybrid system of the accelerator and the nuclear reactor, there are many technical issues and challenges to be solved for

High intensity proton accelerator
Subcritical core
Technology to use Lead-Bismuth Eutectic (LBE) as a coolant and a spallation target

NTS is conducting research activities mainly on "the subcritical core" and "the technology to use LBE"

Subcritical Core

For the research of the subcritical core, a design study is being carried out to develop a feasible ADS concept.

Core design to perform an efficient transmutation
Power flattening to decrease a power peaking
Safety assessment at unprotected accidents
Estimation of uncertainties caused by nuclear data
Structural analysis for a beam window

Technology to use LBE

LBE is assumed to be used as a coolant and a spallation target for the ADS. However, the LBE is supposed to be corrosive at high temperature such as above 550 °C. Hence, the research for usable conditions (temperature, oxygen concentration, etc.) and the development of materials which have the resistibility against the corrosion by the LBE are in progress.

Effect of Partitioning and Transmutation

The effect of the partitioning and transmutation (P&T) technology is also being investigated in parallel with the R&D of the ADS described above.