By Dr. Nick Touran, Ph.D., P.E., 2023-03-13 , Reading time: 4 minutes
In the 1950s, hordes of nuclear reactor designers were rushing to find out which combination of coolant, moderator, and fuel types would be the most economical for commercial power production. Along the way, someone came up with the idea of using sodium metal coolant with graphite moderator. It’s a sodium-cooled slow reactor (as opposed to the more common sodium-cooled fast reactor). The idea here was to capture the following benefits:
Sodium metal coolant can be brought to higher temperature than high-pressure water, allowing higher thermal efficiency (more heat can be converted to electricity when the heat is higher temperature). Thus, you can get more revenue for the same amount of nuclear heat and equipment, and therefore be more economical.
Sodium metal coolant remains at low pressure even at these high temperature. Thus, the thickness of pipes and protection equipment to deal with high-pressure leaks in water-cooled reactors can be reduced/eliminated.
Graphite moderator allows reactors to work with very little fissile mass and enrichment. This allows operation with natural or low-enriched uranium, as opposed to sodium-cooled fast reactors, which require ~12% or higher enrichment (or fissile concentration) to start up and maintain a neutron chain reaction.
The first two benefits are shared with the far more well-known sodium-cooled fast reactor (SFR) design, but the last one is what’s so special about SGRs. When high-fissile fuel such as HALEU or HEU or Plutonium is a little hard to come by, SGRs shine. This condition exists in any country that is not aggressively reprocessing its spent nuclear fuel.
The first vocal champion of the SGR design was Chauncey Starr, best known for later founding the non-profit, Electrical Power Research Institute (EPRI). While at Atomics International, he edited an entire book about SGRs that was released globally at the 2nd Atoms for Peace convention in 1958 and can be read in full today for free.
As was typical in the history of reactor development, we developed, built, and operated two SGRs:
As was common in the pioneering days of nuclear, these reactors were not without issues. SRE had a coolant issue that led to melted fuel, and Hallam had issues with the moderator cans leaking, allowing sodium to contact the graphite and cause swelling. The vendor, Atomics International, figured out solutions to these issues but did not make enough progress to convince Consumers Public Power District (the utility partner) to purchase Hallam at that time. Thus, as was the deal in the AEC’s Power Demonstration Reactor Program (PDRP), the AEC would decommission the plant.
Consumers entered into the experimental reactor program knowing they needed the power for their customers one way or another. Because sodium-cooled reactors can generate steam of the exact same quality as a coal plant, they decided to build a coal-fired plant right next to the HNPF and hook it up to the exact same turbine-generator set. When the nuclear plant would be down, the coal plant could fill the gap, and the customers could have reliable power. The plant as a whole was (and still is) called Sheldon Power Station.
As far as we’re aware, this was the only time in history that a nuclear and coal plant have fed steam into a shared turbine.