They do if you don’t replenish the tritium in their booster cans. The amount you puff into the pit will let you control of the size of the minisun, aka dial-a-yield. The stuff has about 13 years half-life. Quite interesting stuff it is.
Still quite an improvement on the polonium based initiators that had to be replaced much more often.
I quite miss the aboveground tests. I got born too late to see any. 
