Some time before the expedition visited the planet so had Genii built a nuclear power stations, but abandoned it since. In 2008 it was Teyla and Ronon been on the planet after hearing from one of Teyla's contacts there was the old technology on the planet.
They had not discovered the city on the planet and met its people. Lieutenant Peter McRays team with Radek Zelenka visited the planet and discovered a lot of radiation, and it came from an abandoned Genii nuclear work which was close to the city that poisoned residents. The team researched the work and found some abandoned vehicles and the team were surprised by the Genii soldiers who were left on the planet. The entire team was killed by Genii soldiers and Zelenka had discovered that the plant's generators were overheating and could cause a meltdown and kill the whole town.
Argan and Zelenka managed to stop the meltdown, and the population of the planet was saved from disaster. (ATL: "Stargate Atlantis: Meltdown")