Construction of the Sodium Reactor Experiment

Me @whatisnuclear
2024-05-18 00:00:00 -0700

The Sodium Reactor Experiment (SRE) was a prototype reactor 27 miles NW of LA that served as a technology demonstration of sodium-cooled, graphite-moderated reactors (aka SGRs). In the 1950s, it was thought that the combination of high-temperature, low-pressure liquid metal sodium coolant with highly fuel-conservative neutron moderation would form a likely candidate for low-cost commercial nuclear power plants.


Then, as today, the best way to verify that your economic suppositions are anywhere near correct was to build and operate a prototype. Originally the SRE was going to dump all its heat to air, but they ended up hooking it up to generator and putting electricity on the local grid.

A big hole dug in the ground
Some concrete structures in the hole

Here's the reactor building being erected using the tilt-up concrete slab method.


The Santa Susana Field Laboratory was Atomics International's nuclear reactor skunk works. They developed and tested all kinds of reactor components and reactors there.


The liquid metal sodium is pumped into the lower plenum and enters the core through the grid plate. It moves up through fuel element channels in each moderator. Helium gas at 3 psig is maintained above the upper sodium.


To keep the surrounding concrete cool during operation, this cavity liner was included, which has coolant lines filled with an organic fluid called tetralin all around it...


This is the bottom grid plate. The moderator can pedestals sit upon each hole for positioning. Coolant from the lower plenum flows through the holes and into the fuel bundles.


To slow down the neutrons, moderators of graphite were used. They were surrounded by dimpled hexagonal zirconium cans to prevent the sodium from interacting with the graphite. Some of the moderator corners were scalloped to allow room for experimental channels and control rods. With moderated neutrons, SGRs can run on low-enriched or possibly even natural uranium, whereas fast-neutron reactors require higher enrichments (e.g. HALEU/MEU) or plutonium.


Loading the last moderator into position.


Because sodium chemically reacts with air, you can't just open up the top to move fuel or do maintenance. Instead, SRE pioneered a rotating face shield (right) placed inside a stationary ring shield (left). Both were filled with dense concrete. Today this would be called the reactor head.


Loading a fuel bundle into the SRE core. These were 7-pin bundles where the outer 6 pins were helically wrapped with wire for spacing and coolant mixing. Each bundle hangs down from a plug in the loading face. The pins contained uranium enriched to merely 2.78% U-235.


The SRE core loading map, showing location of fuel, control, experiments, etc.


With the fuel and moderators loaded, they just needed to lower the 5-ft thick concrete shield blocks over the sodium piping to top and left and they would be ready to come to power.


Here's the SRE control room.


This huge fuel handling machine could be coupled to the loading face. A fuel bundle would be picked up from the plug and lifted all the way up into the shielded cask. Then it could be moved out and replaced with a fresh assembly. In this sense, SRE pioneered a lot of the modern thinking and mechanisms in sodium-cooled reactor fuel handling.


It is imperative to keep oxygen levels low in sodium-cooled reactors. Cold traps that precipitate out sodium oxide and hot traps that absorb oxygen into zirconium were developed to keep it below 2 ppm. Pictured here is a guy loading in a huge coil of corrugated zirconium into a hot trap.

Me @whatisnuclear 2024-05-18 00:00:00 -0700

Much was learned at SRE, and a full-scale demonstration reactor of the SGR type was built and operated at Hallam, Nebraska based on its experience. The SRE became famous when tetralin leaked and clogged a channel, leading to fuel melt, and the SGR concept became unpopular. However, the capabilities of SGRs are strong. I found the pamphlet that all these photos came from on ebay and scanned them at 600 dpi. You can see dozens more pictures with good captions here.



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