Spent Nuclear Fuel

In the previous previous article, I explained what happens to uranium inside nuclear reactors. This article concludes the series by describing what happens to nuclear waste after it’s removed from the reactor.

Most nuclear power plants use uranium-235 as fuel, requiring it to be enriched to 3-5%. The enriched uranium is formed into small (1cm diameter x 1cm long) uranium pellets, protected by a zirconium cladding. These pellets are assembled into rods (several meters long), and the rods are grouped into fuel assemblies. A reactor core can contain 100-200 fuel assemblies, each with hundreds of rods. Fuel rods are interspersed with control rods (made of silver, cadmium, etc.) and sometimes (depending on the reactor design) moderator bars like graphite (though water is typically used as the moderator). Inserting or removing these bars controls the reaction rate. Even after the reactor is shut down (SCRAM), the decay of fission products generates heat. Immediately after shutdown, this residual heat is about 7% of the reactor’s full power output, decreasing to around 0.2% after a week. Despite being less “hot” than when in the reactor, spent nuclear fuel needs cooling.

Because they require cooling and are highly radioactive, spent fuel rods are stored underwater in spent fuel pools, typically 10-15 meters deep. The water cools the rods and shields radiation. Boron is added to the water to prevent a chain reaction from restarting. Rods can be stored in these pools for decades, and the pools often have capacity for multiple times the reactor’s full fuel load. Every 12-18 months, about one-third of the fuel rods are considered spent and are removed from the reactor and placed in the spent fuel pool.
So, a fuel rod spends an average of 36-54 months in the reactor core. Moving rods is done remotely to minimize contamination risk.

After sufficient cooling, the spent fuel is moved from the pool to concrete storage facilities for further cooling in air. While the rods are underwater, they pose little risk. It’s even possible to swim in a spent fuel pool (carefully!), as long as you don’t get too close to the rods. The surface is safe, and the water temperature is kept comfortable (30-50°C). Just remember not to drink the water, as it might contain dissolved radioactive particles. The real danger arises if the cooling system fails. Every major nuclear power plant incident has been caused by cooling system failures.

If a spent fuel pool’s cooling fails, the water can boil off, exposing the rods to air. This can cause the rods to overheat further, potentially burning the zirconium cladding. The resulting oxidation can release hydrogen, which can lead to explosions and the dispersal of radioactive material. Maintaining water coverage over spent fuel is crucial during a nuclear incident.

Finally, a curious fact: the Chernobyl power plant continued operating for 14 years after the 1986 accident and was only fully shut down in 2000.

Staying cool is essential!