It looks like I may have spoken too soon. It appears that, although the pre-nuclear power tech (1948) makes the 6th nuclear reactor available, that the Productionsite Nucelarweapons tech (1945), even though it makes only the 5th nuclear reactor available, also introduces the chance that the Nuclear Waste Bomb tech may become available for researching. I assume that having 6 reactors, as opposed to 5, increases the chances of being presented with the chance of researching Nuclear Waste Bomb. I have no idea what the relative probabilities are, but it seems logical that a player has to get at least a little lucky to get an a-bomb in '45 (with only 5 reactors). But at least there's some possibility.
Also, as far as I can tell, it doesn't seem to make any difference in the game whether the nuclear weapons researchable (event 8011), is triggered by tech 5540 or 5610. I ran the game both ways and it didn't seem to make any difference. That seems a little strange to me, but what do I know.
Thanks again for your replies.
CB