**1. Introduction**

The worldwide nuclear power industry is currently dominated by light water reactor technology. However, U-235 fissile material resource utilization challenges are likely to drive the need for non-light water reactor technologies when one considers timelines extending beyond the next half century. Many alternative reactor technologies that are capable of addressing the resource constraints of light water reactors are currently being pursued.

It is frequently worthwhile to look to the past as a means of guiding the path for the future. The first demonstration nuclear power plant was influenced by the expectation that limited supplies of fissile material will necessitate breeding fissile material. The Experimental Breeder Reactor I (EBR-I) achieved initial power production operation on December 20, 1951 (see **Figure 1**) and produced the first significant amounts of electrical energy generated by nuclear fission. EBR-I was a sodium-potassium cooled fast neutron spectrum reactor capable of breeding more fissile material than it consumed. The reactor was part of a power plant design that included steam generation and a turbine/generator system.

Following the success of EBR-I, the Experimental Breeder Reactor II (EBR-II) was constructed near EBR-I on the high-altitude arid Snake River Plain of

**Figure 1.** *Chalk message at EBR-I [1].*

southeastern Idaho in the western United States. Like EBR-I, EBR-II was a complete power plant demonstration, and it also included an attached fuel cycle facility to reprocess spent fuel using a melt refining process (see **Figure 2**). The reactor was a sodium cooled fast reactor (SFR) capable of producing more fissile material than it consumed. EBR-II achieved initial criticality in 1964 and operated until 1994. The reactor produced 19 MWe and supported decades of sodium cooled fast reactor development activities. The success of EBR-II provides insight into the potential benefit of future widespread use of sodium cooled fast reactors as a means of addressing fissile material resource limitation issues. It should also be noted that numerous other sodium cooled fast reactors have been developed including, but not limited to, Fermi I and the Fast Flux Test Facility in the US, Phénix and Super Phénix in France, Joyo and Monju in Japan, BN-350 in Kazakhstan, BN-600 and BN-800 in Russia, as well as sodium cooled fast reactors in India and China.

From an industry perspective, there is a resurgence of interest into sodium cooled fast reactors. Two commercial entities have proposed the use of sodium cooled fast reactors. The TerraPower company is pursuing a sodium cooled fast reactor coupled with a molten salt heat storage capability. The reactor is capable of producing 345 MWe as well as boosting the output to 500 MWe by using heat stored in molten salt. The reactor is called Natrium, which is Latin for sodium. In October 2020, the US Department of Energy awarded TerraPower funding to demonstrate the Natrium technology. TerraPower is targeting 2023 for submission of a construction permit from the US Nuclear Regulatory Commission. The planned location for the reactor will be one of four prospective sites in the state of Wyoming in the western United States. Furthermore, the Oklo Power Company has a sodium cooled fast reactor design which produces 4 MWth and integrates significant inherent safety

**Figure 2.** *Experimental breeder reactor II [2].*

features into the design. Oklo Power submitted the first-ever combined construction and operation license application to the US Nuclear Regulatory Commission in March 2020.

From a US Government perspective, the US Department of Energy is pursuing the Versatile Test Reactor (VTR). The VTR is a sodium cooled fast reactor that will operate at 300 MWth. The purpose of the VTR is to provide a very high neutron flux (4 x 1015 n/cm2 sec) which will be used to test fuels and components for a wide range of advanced reactor concepts. The VTR project received Critical Decision–1 approval in September of 2020, allowing the project to proceed to preliminary design.

With this information in mind, it is worthwhile to reflect on the design and performance EBR-II since it provides tremendous knowledge and potential direction for sodium cooled fast reactors moving forward.
