Spent fuel, reprocessing and dry storage

A fuel assembly that leaves the core after three to six years can no longer sustain the reaction, but it is still intensely radioactive and hot: freshly shut down it releases about 6.5% of the thermal power it had at full load, and that fraction falls fast, to 1.5% after an hour and 0.4% after a day, though it never reaches zero. Burnup, the energy that fuel extracted per tonne of uranium, sets how much heat and radioactivity it carries: typical PWR values rose from 40–45 to 55 gigawatt-days per tonne, with 70 in sight. That is why all spent fuel first spends time in the plant's own pool, under water that cools and shields it, for at least a year and in practice five to ten.

From there the path splits. Many countries move the cooled fuel to dry casks: steel cylinders filled with helium inside concrete or steel overpacks, cooled only by natural air convection, with no pumps or electricity. Others reprocess it. At La Hague, France, Orano has operated since 1976 a plant with a capacity of 1,700 tonnes a year that has already reprocessed about 30% of the 400,000 tonnes of commercial spent fuel generated worldwide: it dissolves the fuel in concentrated nitric acid and separates the still-usable uranium and plutonium from the fission products by solvent extraction, the PUREX process. In Russia, Mayak's RT-1 plant has a nominal capacity of 400 tonnes a year, though in practice it reprocesses between 110 and 130.

Japan shows the opposite case: the Rokkasho plant, designed on La Hague's technology for 800 tonnes a year, began construction in the 1990s and has racked up more than twenty postponements of its startup date, still not operating commercially. It is proof that reprocessing is, besides a technical decision, an industrial undertaking of enormous scale and regulatory complexity.

Argentina does not reprocess: it goes straight from the pool to dry storage. Embalse has stored its spent fuel that way since the 1990s, and Atucha II is building its own system, ASECG II, in Zárate, because its decay pools are projected to reach capacity around December 2027. The project, led by Nucleoeléctrica Argentina, already has the high-strength concrete base for the casks in place and is advancing on fabricating the containers and armored lids; the design includes passive ventilation that keeps the elements' temperature in range without electricity or human intervention.