The prototype fast breeder reactor “Monju” is a loop-type sodium-cooled fast neutron reactor with an electric power of 300MW developed in Japan. The initial criticality was achieved in 1994, and the reactor power reached 40% of the rated power in 1995. After a long-term shutdown period, the operation of Monju restarted in 2010. In 2016, Monju was transferred to decommissioning stage without achieving the full power operation. This chapter summarizes design features of Monju, R&D activities to establish the design, experiences in operation and maintenance, and technological achievements through the commissioning.

A fast neutron reactor (FNR) is a sustainable energy source that uses natural uranium resources very efficiently as a fission energy source. In addition to this, major long-lived radioactive fission products, that is, minor actinides (MAs), in spent fuels of nuclear power plants can be used as fissile material in FNR, yet are reduced through spent fuel recycling. A sodium-cooled fast reactor (SFR) is one of the most promising FNR systems. SFRs have been developed in many countries since the beginning of fission reactor development. Its sustainability significantly serves Japan’s energy industries relying on imported natural resources for energy production. Peaceful uses of nuclear and reduction of the volume and toxicity of radioactive waste are also key issues for Japan, the only nation that has experienced a nuclear bombing.

Following the development of the experimental reactor Joyo and the prototype reactor Monju, several design studies of large-scale SFR plants were conducted in Japan, which includes a demonstration plant design by the utilities and a 1500 MWe SFR that is named Japan sodium-cooled fast reactor (JSFR) as a commercial plant jointly by JAEA, the utilities, and the industry.1.Demonstration reactor project: A demonstration fast breeder reactor “DFBR” project was carried out primarily by industries, namely utilities and manufacturers. However, due to both domestic and international circumstances, the DFBR project was canceled.2.JSFR in FaCT project: As a commercial plant concept in the FaCT project, JSFR with a capacity of 1500MW was developed. MOX fuel was studied as a reference for the core, but metal fuel was also considered as an option. JSFR employs several innovative key technologies to achieve high safety and economic targets, and these innovative technologies were evaluated in terms of maturity before moving forward with further development steps such as conducting large-scale demonstration tests, basic design, and detailed design of a demonstration reactor. For the demonstration reactor, a comparison of reactor output between 500 MWe and 750 MWe was conducted and 750 MWe was selected. The AtheNa sodium test facility was constructed to conduct component and system demonstration tests for the large-scale demonstration test.3.Safety improvement: Since 2011, to contribute to the development of safety design criteria and safety design guidelines, in the framework of the Generation IV International Forum (GIF), a design study on the 750 MWe demonstration JSFR enhancing safety and maintainability was conducted, which included lessons learned from the TEPCO’s Fukushima Daiichi nuclear power plants accident (the 1F accident).4.Pool-type SFR: International collaboration on ASTRID was initiated in 2014, and a design study of a pool-type concept accumulating knowledge from the ASTRID collaboration was conducted showing feasibility even against Japanese severe seismic conditions.

This chapter describes the latest results and future prospects of the research and development activities in ten key technologies for the future sodium-cooled fast reactors (SFRs).1.Safety: Safety is the top priority for SFR development and design. In this section, research and development on the critical safety issues specific to SFRs are discussed, including safety design criteria and safety design guidelines (SDC/SDG), self-actuated shutdown system (SASS), severe accident, sodium combustion, sodium-water reaction, and source term.2.Sodium component development: Conceptual designing of Japan Sodium-cooled Fast Reactor (JSFR) has been conducted since 2006 and several innovative sodium component designs were established. The development plan for JSFR includes large-scale sodium component experiments and system demonstrations.3.Reactor core physics: JAEA has been developing a standard reactor core physics analysis method for fast reactors for many years. This analysis method was developed on the premise that integral experimental data are reflected in the core design using a kind of data assimilation methodology. This section outlines the features of this analysis method, including not only the analytical models and computer codes but also the experimental database.4.Fuel and materials: This section introduces the results of physical properties study, irradiation behavior evaluation and performance code development of MOX fuel, and long-life core material development such as ODS steel cladding that has been implemented in Japan for practically realizing fast reactors.5.Thermal-hydraulics: Various thermal-hydraulics phenomena should be evaluated accurately to ensure the safe and reliable operation of an SFR plant, mainly by computational fluid dynamics (CFD). These include plant dynamics, natural circulation, thermal-hydraulics in fuel assemblies, and the hot plenum of a reactor vessel.6.High-temperature structural materials and structural design: Elevated temperature design methods considering time-dependent deformation and failure by creep phenomena are required for design and construction of the SFRs operated at high temperatures. This section introduces the development and codification of the Japanese unique elevated temperature design codes.7.Seismic design: This section introduces some typical research activities related to the seismic design of SFRs. Establishing the seismic design technology of structures and mechanical components is a critical challenge in realizing SFRs in Japan that has many experiences of the severe earthquake disaster.8.Operation and maintenance: This section describes the fitness-for-service code and LBB assessment guidelines for sodium-cooled fast reactors that have been newly developed by the Japan Society of Mechanical Engineers. In addition, the development of an under-sodium viewer that is a unique technology for inspections in the opaque coolant is explained.9.Fuel cycle-related studies: JAEA has promoted the development of simplified pelletizing fuel fabrication and advanced aqueous reprocessing methods to establish the fuel cycle technology for fast reactors through the Fast Reactor Cycle Technology (FaCT) development project. In this section, the summary of the developments described before is introduced as the main issues in the fuel cycle-related studies.10.ARKADIA–Advanced reactor knowledge- and AI-aided design integration approach through the whole plant life cycle: JAEA has started to develop “Advanced Reactor Knowledge- and AI-aided Design Integration Approach through the whole plant life cycle (ARKADIA)” as one of the research and development infrastructures. ARKADIA offers the best possible solutions for any challenges that could arise in the plant life cycle, including plant design and safety measures, maintenance, and decommissioning.

The experimental fast reactor Joyo is the first liquid sodium-cooled fast reactor in Japan. The purpose of constructing Joyo was to obtain technical information about sodium-cooled fast reactors (SFRs) through experience with its design, construction, and operation, and to use the reactor as a fast neutron irradiation facility for the development of fuels, materials, and other components required for the SFR program. Through design, construction, testing, operation, and maintenance experience, Joyo has contributed much to the SFR development program. In addition to providing operating experience, many kinds of irradiation tests have been conducted for the development of fuels and materials under the conditions of higher fast neutron flux and temperature than those in light water reactors. Through its successful operation over a quarter-century, Joyo has demonstrated the safety and reliability of the sodium-cooled fast reactor.