Sodium Fast Reactor (SFR)

From the initial conception of nuclear energy, it was recognized that full realization of the energy content of uranium would require the development of fast reactors. In the current once-through fuel cycle, enriched uranium is utilized as LWR fuel and over 99% of the energy content of the initially mined uranium remains in the residue from the enrichment process and used LWR fuel. Conversely, the favorable neutron balance in a fast spectrum sustains the fissile material. This behavior improves the performance of both once-through (breed/burn and long-lived core concepts) and recycle strategies, by enabling extended fuel burnups from a fixed fissile inventory.

For closed fuel cycle applications, a thermal spectrum leads to the generation of higher actinides that complicate subsequent recycling. Conversely, fission is favored in a fast spectrum limiting higher actinide generation. This behavior allows full recycle enabling complete consumption of uranium and transuranic elements, while eliminating the need for uranium enrichment. Significant waste management benefits can also be realized by excluding the long-term heat production elements (actinides) from the waste stream. 

A wide variety of actinide management strategies can be deployed in Generation-IV SFR reactors. The uranium loading can be varied to operate in different modes:

• A conversion ratio less than 1 (“transmuter” mode) which means that there is a net consumption of transuranics. Here, “transmute” means to convert transuranics into shorter-lived isotopes.
• A conversion ratio near 1 (“converter” mode) which provides a balance in transuranic production and consumption. This mode results in low reactivity loss rates with associated control benefits.
• A conversion ratio greater than 1 (“breeder” mode) which means there is a net creation of transuranics. This approach creates additional fissile materials, but requires the inclusion of extra uranium in the SFR and fuel cycle. 

An appropriately designed fast reactor has flexibility to shift between these operating modes; and the desired actinide management strategy will depend on a balance of waste management and resource extension considerations.

Note: The conversion ratio is defined as the ratio of the transuranic production rate to the transuranic destruction rate, whereas, the breeding ratio is a similar ratio for the fissile material.

For the commercialization of the SFR system, it is important to achieve a level of economic competitiveness that enables system installation in accordance with market principles.  For this purpose, an important goal is to achieve energy costs (unit cost of power generation) competitive with alternate future energy sources. For this purpose, the reduction of the plant capital costs is crucial. A number of innovative SFR design features have been proposed to improve SFR capital costs including:

• Configuration simplifications. These include reduced number of coolant loops by improving the individual loop power rating, improved containment design, refined (and potentially integrated) component design, and possibly elimination of the intermediate coolant loop.
• Improved Operations & Maintenance (O&M) technology. Innovative ideas are being considered for in-service inspection and repair. Remote handling and sensor technology for use under sodium are being developed, including ultra-sonic techniques. In addition, increased reliability for sodium-water steam generators (e.g., by using double tube configuration with leak detection) is being pursued by advanced detection and diagnostic techniques.
• Advanced reactor materials. The development of advanced structural materials may allow further design simplification and/or improved reliability (e.g., low thermal expansion structures and greater resistance to fatigue cracking). These new structural materials need to be qualified, and the potential for higher temperature operation evaluated.
• Advanced energy conversion systems. The use of a supercritical CO2 Brayton cycle power generating system offers the potential for surpassing 40% efficiency; a more compact design may also be possible. Cost and safety implications must be compared to conventional Rankine steam cycle balance-of-plant design.
• Fuel Handling. Techniques and components employed in previous fast reactors were reliable, but very complicated and expensive. Recent design innovations may simplify the fuel handling system, but require the development and demonstration of specialized in-vessel handling and detection equipment.

Important safety features of the Generation-IV SFR system include a long thermal response time, large margin to coolant boiling, a primary system that operates near atmospheric pressure, and an intermediate sodium system between the radioactive primary sodium and the power conversion systems. Technology development efforts focus on two general areas: assurance of passive safety response, and techniques for evaluation of bounding events. Advanced SFR designs exploit passive safety features to increase safety margins and to enhance reliability. The system behavior will vary depending on system size, design features, and fuel type. R&D for passive safety will investigate phenomena such as axial fuel expansion and radial core expansion, and design features such as self-actuated shutdown systems and passive decay heat removal systems. The ability to measure and verify the performance of these passive features must be demonstrated. Associated R&D will be required to identify bounding events for specific designs and investigate the fundamental phenomena necessary to prevent severe accident progression.
The favorable passive safety behavior of fast reactors is expected to virtually exclude the probability of severe accidents with potential for core damage. Nevertheless, design measures to mitigate the consequences of severe accidents are being considered. This approach is consistent with the defense-in-depth philosophy of providing additional safety margin beyond the design basis. A common safety approach incorporating the physical and chemical characteristics of the materials handled in the reactor (chemical activity and radio-toxicity, etc.) and unique SFR design features and phenomena should be established. The goal is to render the risk of installing SFR systems much lower than the risk of energy alternatives. Achieving this level of safety should result in licensing and regulatory simplifications that may in turn result in reduced system cost. To do this, probabilistic safety evaluations will be needed to identify design tradeoffs that assure very high levels of public health and safety.
 

Development of new SFR technology also provides the opportunity to design modern safeguards directly into the planning and building of new nuclear energy systems and fuel cycle facilities. Incorporating safeguards into the design phase for new facilities will facilitate nuclear inspections conducted by the International Atomic Energy Agency (IAEA). The goal of this oversight is to always have an accurate grasp of the current inventory through the utilization of advanced technologies to verify the characteristics of the security system (accountancy, detectability and promptness) and the physical protection characteristics (physical protection measures, the monitoring level and security measures) and for ensuring robust design to guarantee these characteristics.  It is also necessary to maintain transparency and openness in terms of information to more effectively and efficiently monitor and verify nuclear material inventories.