Complex sentence: A large but unknown number of these sources probably belong to the high security risk category; noteworthy are the very strong Russian beta emitting strontium-90 sources used as radioisotope thermoelectric generators for beacons in lighthouses in remote areas.
Split into simpler sentences: A large but unknown number of these sources probably belong to the high security risk category. Noteworthy are the Russian very strong beta emitting strontium-90 sources used as radioisotope thermoelectric generators for beacons in lighthouses in remote areas.