How a sodium fast reactor works

Sodium melts at 98 °C and does not boil until 883 °C, so at the 550 °C core outlet it is still liquid at atmospheric pressure. There is no pressurizer and no thick-walled vessel: in the BN-800 the pool, about 13 meters in diameter, is a stainless steel tank a few centimeters thick, wrapped in a second guard vessel that would catch any leak. Above the metal surface, a blanket of argon keeps it away from air.

In the pool design the whole primary circuit, core, pumps and heat exchangers, is submerged in hundreds of tonnes of sodium: an enormous thermal mass that takes hours to heat up if cooling fails. Sodium conducts heat a hundred times better than water, and being a metal, it does not moderate neutrons. Its two flaws are that it is opaque, which forces blind refueling, and that it burns on contact with air and reacts violently with water.

In a thermal reactor the water slows the neutrons until almost any collision with a U-235 causes fission. Here there is no moderator: neutrons keep the energy they are born with, around 1 MeV, and at that speed the probability of fissioning a nucleus is much lower. To compensate, the fuel is far richer in fissile material, mixed uranium-plutonium oxide with 20 to 25% Pu, or uranium enriched to 17 to 26% as in the BN-600, and the core is compact: in the BN-800 it is under a meter tall and about two and a half in diameter for 2,100 MW thermal.

The fuel assemblies are hexagonal, with steel cladding instead of Zircaloy, and are cooled by fast-flowing sodium. The boron carbide control rods enter from above through a rotating plug which, as it turns, positions the refueling machine over each location. The fast spectrum has an important physical consequence: if the sodium boiled in the center of the core, reactivity would rise, so the design must guarantee that never happens.

Surrounding the core, in the radial blanket and at the ends of each rod, there is depleted uranium, the leftover from enrichment. A fast neutron escaping the core can be captured by a U-238, which through two beta decays, of 23 minutes and 2.3 days, turns into Pu-239: new fuel. Since each plutonium fission with fast neutrons releases almost three neutrons, enough are left over for the reactor to produce more fissile material than it consumes.

That breeding ratio reached 1.2 in Superphénix. With reprocessing, the closed cycle uses more than 60% of natural uranium, against under 1% in a thermal reactor. The same spectrum also fissions americium, neptunium and curium, the actinides that keep high-level waste dangerous for hundreds of thousands of years. Arms control keeps its eyes on that almost pure plutonium from the blanket: the BN-800 operates without a radial blanket, with steel reflectors, and programs such as Astrid and Natrium are conceived as burners, not breeders.

The sodium passing through the core becomes activated: some of the Na-23 captures neutrons and turns into Na-24, an intense gamma emitter with a 15-hour half-life. That sodium never leaves the pool. Inside it, the intermediate heat exchangers, six in the BN-800, hand the heat to a second, clean sodium circuit that does leave the reactor building and goes to the steam generators.

The intermediate circuit exists for one reason: if a steam generator tube breaks, sodium and water react to produce hydrogen and caustic soda, with a pressure spike. Hydrogen detectors and rupture discs relieve the reaction, but it is essential that it happens with non-radioactive sodium and far from the core. The price is one more loop, with its pumps, trace-heated piping so the metal never freezes and an extra temperature drop of about 40 °C.

The steam generator is once-through: water enters at the bottom, runs through tubes bathed in sodium at 505 °C and leaves as superheated steam at about 490 °C and 14 MPa, coal-plant parameters. The turbine is that of a conventional thermal plant and the cycle reaches 40 to 42% efficiency, against 33% for a light water reactor. The BN-600 drives three 200 MW turbines; the BN-800, one of 800 MW.

With two sodium barriers between the core and the water, the turbine hall is a clean area. The exhausted steam is condensed with water from a third circuit that is cooled in towers or in a reservoir, like Beloyarsk's. Recent designs such as Natrium insert a molten-salt store: the reactor heats the salt at constant power and the turbine discharges when the grid asks for it, up to 500 MW for five hours.

The world's first nuclear electricity came from a fast reactor: EBR-I lit four light bulbs in Idaho in December 1951. France built Phénix, 250 MWe from 1973 to 2009, and Superphénix, at 1,240 MWe the largest fast reactor in history, closed in 1998 after a decade of sodium leaks, protests and low availability. Monju, in Japan, suffered a sodium fire in 1995 months after starting up and was decommissioned having barely operated.

Russia is the exception: Beloyarsk's BN-600 has generated since 1980 and the BN-800, next to it, since 2016, today with a full MOX core. China has operated the small CEFR since 2011 and is raising the CFR-600s at Xiapu. India began loading the 500 MWe PFBR at Kalpakkam in 2024, with start-up stretching into 2025. In the United States, TerraPower is building the 345 MWe Natrium at Kemmerer, Wyoming. After seventy years, the technology remains the only one proven at commercial scale for multiplying uranium.