What is "beta-m"

Some Soviet era lighthouses and beacons were unique, as they were powered by radioisotope thermoelectric generators called the Beta-M. It was capable of generating 10 watts of electricity, almost enough to power a compact fluorescent light bulb with light output equivalent to a 60 watt bulb. Those generators are still in use and are well past the point at which they should be retired and recycled, prompting environmental safety concerns.