It's mentioned in the article as well

"Another good use for nuclear batteries is to supply power in remote locations on Earth. Beginning in the 1970s, for example, the Soviet Union deployed over 1,000 RTGs in northwestern Russia to power its uncrewed lighthouses, radio beacons, and weather stations. Most of these batteries ran on strontium-90, and each weighed about 2,000 kg. The United States has deployed hundreds of similar systems for remote power both on land and on the ocean floor, particularly for remote monitoring sites in the Arctic."