liquid metal lead, are that liquid sodium is far less dense and far less viscous(reduced pumping costs), is not corrosive(via dissolution) to common steels, and creates essentially no radioactive neutron activation byproducts. The disadvantages of sodium coolant, as opposed to lead coolant is that sodium is chemically reactive, especially with water or air (both typically found in abundance in reactors). Lead may be substituted for the eutectic alloy of lead and bismuth as used for coolant in Soviet Alfa-class submarines' reactors. Basic design decisions Metallic fuel Metal fuel with a sodium-filled void inside the cladding to allow fuel expansion has been demonstrated in EBR-II. Metallic fuel makes pyroprocessing the reprocessing technology of choice. Fabrication of metallic fuel is easier and cheaper than ceramic (oxide) fuel, especially under remote handling conditions. Metallic fuel has better heat conductivity and lower heat capacity than oxide, which has safety advantages. Sodium coolant Use of liquid metal coolant removes the need for a pressure vessel around the reactor. Sodium has excellent nuclear characteristics, a high heat capacity and heat transfer capacity, low density, low viscosity, a reasonably low melting point and a high boiling point, and excellent compatibility with other materials including structural materials and fuel. The high heat capacity of the coolant and the elimination of water from the core increase the inherent safety of the core. Pool design rather than loop Containing all of the primary coolant in a pool produces several safety and reliability advantages. Onsite reprocessing using pyroprocessing Reprocessing is essential to achieve most of the benefits of a fast reactor, improving fuel usage and reducing radioactive waste each by several orders of magnitude. Onsite processing is what makes the IFR integral. This and the use of pyroprocessing both reduce proliferation risk. Pyroprocessing (using an electrorefiner) has been demonstrated at EBR-II as practical on the scale required. Compared to the PUREX aqueous process, it is economical in capital cost, and is unsuitable for production of weapons material, again unlike PUREX which was developed for weapons programs. Pyroprocessing makes metallic fuel the fuel of choice. The two decisions are complementary. Summary The four basic decisions of metallic fuel, sodium coolant, pool design, and onsite reprocessing by electrorefining, are complementary, and produce a fuel cycle that is proliferation resistant and efficient in fuel usage, and a reactor with a high level of inherent safety, while minimizing the production of high-level waste. The practicality