Honestly, for a nuclear plant siren, the best option is...
...wait for it...
...ANY siren that gets 100% coverage inside the 85dB curve for the entire EPZ with the network installed, and convinces people to go inside and turn on the TV or radio to find out what the hell just went wrong and what they need to do. (Apologies for the language, but that's a case where I think it's appropriate.)
Frankly, any kind of siren can easily do that, so long as you test it frequently enough for public education purposes (publicizing the tests in advance in local media, with information on what to do if the siren goes off unexpectedly), but NOT so often that hearing it becomes routine--monthly, at the most frequent, but probably more than the federally-required annual test. In areas that aren't prone to severe weather, any tone would be effective, so long as the public knows what it sounds like from testing. In areas where sirens *are* used for severe weather, you'd need to coordinate with local officials to make certain that the tone you use is *not* used for severe weather warnings (though this might be simple, if you offer the use of the EPZ sirens for severe weather warnings with an alternate signal).
Whoop *is* a great tone for electronics to use for a nuclear incident, as is the rare Yelp (basically the opposite of Whoop); if you're going to have a mechanical and/or mixed system, I'd say that Alternate Wail (pulsed or hi-lo) would be effective as all hell, be it a 3T22-style continuous pulse/hi-lo, or a Thunderbolt-style one where it pulses/hi-los while chopper power is on, and goes steady for the wind-down. Highly distinctive, and really gets your attention. The only problem with this is that most modern electromechanicals don't have solenoid options available...
Thirty minutes(!) of Federal Signal SS2000 Yelp:
https://www.youtube.com/watch?v=90e0wJ5sDrw (couldn't find an outdoor siren Yelp video)
EOWS/Siratone pulsed wail:
https://www.youtube.com/watch?v=L3vYmXIvduk
Thunderbolt alternate wail (with a slightly glitchy flasher):
https://www.youtube.com/watch?v=bH7h4YQTVGI
Modulator alternate wail (closest I could find to a 3T22 alternate wail):
http://youtu.be/tNQxXuJIW10?t=1m29s
Mod alternate wail ambiance:
https://www.youtube.com/watch?v=Yy_oX6SURRE