F O R U M O N P H Y S I C S & S O C I E T Y
of The American Physical Society 
January 2006 
Vol. 35, No. 1



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4S (Super Safe, Small and Simple LMR)


The 4S (Super-Safe, Small and Simple) reactor is a small sodium-cooled fast reactor that is being developed to serve as a dispersed energy source for the global market, such as for remote areas with high electricity costs or developing countries with small electricity consumption. To meet the needs of this market, the 4S reactor has been designed on the principle of simple operation, simplified maintenance in­clud­ing refueling, higher safety and improved economic features. The 4S reactor is a metallic fueled sodium cooled fast reactor. The electrical output is 10-50 MW. CRIEPI (Central Research Institute of Electric Power Industry) started work on the 4S project with Toshiba late in the 80’s and the 4S has been presented to IAEA (International Atomic Energy Agency) as an energy source for producing potable water from a seawater, which is nuclear desalination.

Design Requirements for 4S

Top level design requirements for 4S reactor include the following ten items;

        1. No refueling for 10 or more years,

        2. Simple core burn-up control without control rods and driving mechanism,

        3. Removal of control and adjustment components from the reactor system,

        4. Quality assurance and short construction period based on shop fabrication,

        5. Load following without operation of reactor control system,

        6. Minimum maintenance and inspection of reactor components,

        7. Negative reactivity temperature coefficients including coolant void reactivity,

        8. No core damage in any conceivable initiating events without reactor scram,

        9. Safety system not dependent on the emergency power and active decay heat removal system,

        10. Complete containment of reactivity under any operational conditions and decommissioning.

Based on above design requirements, 4S reactor has incorporated a high level of passive safety characteristics. In the design of 4S reactor, a neutron reflector is selected for the control of the core reactivity in place of neutron absorber rods used in the existing reactors. The reflectors are driven from outside of the reactor vessel and move very slowly.  Electromagnetic pumps are applied to the primary pumps. These design features reduce moving parts and decrease component failures and many maintenance requirements.

Safety and Reliability

The safety and reliability features in 4S design are as follows;

        (1) Negative reactivity temperature coefficients including coolant void reactivity,

        (2) No core damage in any conceivable initiating events without reactor scram,

(3) Safety systems are not dependent on the emergency power and active decay heat removal,

(4) Complete containment of radioactivity under any operational and accident condition and decommissioning.

All temperature reactivity coefficients are designed to be negative, which strongly helps to realize the passive safety features. It also enables simplification of the power control system so that only feed water control can be used to regulate reactor power. The other safety features of 4S include:

        (5) Simple core burn-up control without control rod and its rod driving mechanism,

        (6) Quality assurance and short construction period based on factor assembly,

        (7) Minimum maintenance and inspection of reactor components

Figure 1

Fig. 1 Reactor building of 4S

Reactor Size and Market

4S reactor is a small sodium-cooled fast reactor in which intensive efforts are concentrated with an aim at meet­ing the global power source market. To correspond to the global market, 4S reactor has been designed on the principle of simple operation, simplified maintenance in­clud­ing refueling, higher safety and improved economic features.

  To supply stable and reliable energy source to various remote populated areas is one of the most important tasks of the nuclear technology for sustained growth of mankind in the future. These areas may be remote islands or deep interior regions where sophisticated technological infrastructure is not expected and whose power demands are generally modest. The benefits of nuclear energy will best be brought to these communities by a small and simple power generation.

4S will be applied to supply the electricity and also will be applied to the nuclear desalination as one of the energy sources. 1 MWe can produce about 4,000 m3/day by the reverse osmosis desalination system. 10 MWe will produce 40,000 m3/day. If the required per capita fresh water is 0.5 m3/day, which is an average in Tokyo, Japan, the population of 80,000 can be supplied fresh water. If the required fresh water is 1/5 compared with Tokyo, fresh water can be provided to a population of about 400,000. There are many developing countries, such as in the Middle East, along the Mediterranean Sea and that would benefit from such a system. Then, 10 MWe of small reactor has a big potential to provide fresh water for the world.

Figure 2

Fig. 2 Application of 4S to provide electricity and fresh water

Fuel cycle options

A metallic fuel is used in 4S core, taking account of its inherent safety and pyro-reprocessing to recycle and minimize the volume of high level waste. The reactor is designed to operate in a closed fuel cycle with reprocessing of fuel. The reactor core operates for 10 or more years without refueling and reshuffling of fuel. The spent fuel will be sent to the regional or national center for the reprocessing following refueling.

A fast reactor technology using a metallic fuel cycle (pyro-process of spent fuel) is the most developed recycling approach. The technology is valuable because it has the potential to simplify reprocessing, fuel fabrication process and nuclear waste disposal, and it also reduces the fuel cycle cost.

Waste Management and Environmental Impacts

The total high level waste from 4S will be reduced compared with the conventional reactors and the minor actinides will be consumed in the reactor. If 4S is used for the production of fresh water for the plantation in desert regions or similar places, carbon dioxide will be removed by the new plant growth. The present 4S design is not a breeder reactor.  However, spent fuel from 4S will be reprocessed and Pu will be extracted along with minor actinides for use in recycled fuel.

Proliferation resistance

4S has a high level of proliferation resistance due to the long life core without frequent refueling. Because of the long period (more than 10 years) between refueling it may be possible eliminate any need for on-site refueling equipment and a lengthy period of on-site storage of the fuel. Refueling of the 18 fuel assemblies can be accomplished in a very short time and with special shipping casks removed from the site to the recycling facilities. Also, the fact that access to the nuclear system is unnecessary during normal operation it means that access to the fuel and source of neutrons is restricted and easily monitored.

Cost and economy

The simplified design features in 4S are necessary to support installation of plants at remote locations in the developing or in developed countries. As the simplicity of the reactor is further advanced by having these features, it was found that the materials weight per output of 4S reactor structure is lower than that of a large reactor. Costs for the design, production facility, plant construction and operation for nuclear reactors are leveled by the number of production units. The major part of cost for mass products are the costs of the materials and inspection, and the cost is ultimately determined by the material cost if the automation of inspection work is advanced. Therefore, it is characteristic that the reduced volume of bulk material weight directly governs the economic feasibility of the reactor. As a result, the construction cost will be reduced, under the condition that 4S are manufactured at a rate of 10 units a year continuously for 10 years by a plant exclusively designed for the purpose, compared with the construction costs in a case where only one reactor is manufactured. An additional cost merit of small reactors is that the total development cost for commercialization is dramatically smaller than that of large reactor.

The cost study of 4S with 50 MWe in JPFS (Joint Preliminary Feasibility Study) shows the busbar cost is around 4 cents/kWh, which was estimated by the same way for S-PRISM and ENHS.


The target power for a standard 4S design is 50 MWe with 30 years long life core. However, the fundamental design can be implemented at power levels from 10 to 50MWe to meet the power requirements in developing countries or remote areas. Recently, the village of Galena, Alaska, US, has expressed an interest in a 10MWe >sized 4S in order to avoid the high electricity cost and to lead development for potential other applications in the State. Also other small communities in Alaska are interested in small nuclear power plant with very small capacity, such as 0.5-2 MWe.

Because of this interest, CRIEPI and Toshiba are going to request a pre-application review of the 4S with the US-NRC with the objective of future commercialization.


The author wishes to thank Mr. N. Brown, LLNL, USA, for his valuable comments on this report.


1) S. Hattori, N. Handa, “Use of Super-Safe, Small and Simple LMRs to Create Green Belts in Desertification Area”, Trans. ANS., Vol.60 (1989)

2) A. Minato, N. Handa, “Small LMR Electric Power for Small Grid”, 3rd International Conference on Nuclear Option in Countries with Small and Medium Grids, Croatia Nuclear Society, June, 2000

3) “Small Modular Nuclear Reactors”, US-DOE-NE Report to Congress, May 2001

4) N. Ueda, I. Kinoshita, Y. Nishi, A. Minato, T. Yokoyama, Y. Nishiguchi, “Current Design Status of Sodium Cooled Super-Safe, Small and Simple Reactor”, ICONE10-353, 10th International Conference on Nuclear Engineering, April, 2002, Arlington, USA,

5) D. Wade, N. Brown, J. Choi, E. Greenspan, W. Halsey, A. Minato, C. Smith, D. Vogt, “Liquid Metal Cooled Reactors and Fuel Cycles for International Security”, 11th International Conference on Nuclear Engineering, April 20-23, 2003, Tokyo Japan,

6) N. Brown, D. Wade, A. Minato et al., “Joint Preliminary Feasibility Study”, UCRL-TR-201726 (Controlled Distribution), December, 2003

7) C. Smith, D. Crawford, M. Cappiello, A. Minato, J. Herczeg, “The Small Modular Liquid Metal Cooled Reactor: A New Approach to Proliferation Risk Management”, 14th Pacific Basin Nuclear Conference,New Technologies for a New Era”, March 21-25, 2004, Honolulu, Hawaii, US

8) O. Okcek, P. Babka, G. Pavlenco, K. Kemmerer, “1994 ALMR Capital Busbar Cost Estimats”, GEFR-00940, March 1995

Akio Minato

Senior Research Scientist

Sector, Advanced Reactor Syatem

Nuclear Research Laboratory

Central Research Institute of Electric Power Industry (CRIEPI)

2-11-1, Iwado-kita ,Komae-shi, Tokyo 201-8511






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