Finland's Onkalo nuclear waste repository is set to become the world's first operational facility for permanently disposing of spent nuclear fuel, with authorities expected to grant an operational license within months. The 1 billion euro ($1.2 billion) facility, located on the island of Olkiluoto, has been under construction since 2004.

The underground repository sits 430 meters (1,411 feet) below ground in 1.9-billion-year-old migmatite-gneiss bedrock, chosen for its stability and low earthquake risk. Radioactive fuel rods will be sealed in copper canisters and buried in tunnels more than 400 meters underground, surrounded by water-absorbing bentonite clay. The facility can store 6,500 tons of spent nuclear fuel and is designed to operate until the 2120s before permanent sealing.

According to the International Atomic Energy Agency, nearly 400,000 tons of spent nuclear fuel have been produced globally since the 1950s, with most currently in temporary surface storage. Sweden began construction on a similar facility in Forsmark last year, expected to open in the late 2030s, while France's Cigéo project has yet to begin construction.

Experts acknowledge uncertainties in long-term nuclear waste storage. Edwin Lyman of the Union of Concerned Scientists noted that while geological disposal represents the "least bad option," copper canisters will eventually corrode, though the timeline remains scientifically debated. The hope is that radioactive decay will occur faster than container degradation.

The project reflects Finland's 1994 policy requiring all radioactive waste generated domestically to be handled within its borders. Posiva, the company managing the facility, estimates it will take hundreds of thousands of years for radioactivity levels to return to natural background levels. The long timeframe has prompted development of "nuclear semiotics" - research into creating warning systems comprehensible to humans tens of thousands of years in the future.