The reactor probably 'ticks' exactly on the second in game. However there is no in game way of determining that alignment so the point is moot.
This statement is not entirely true. As sequencers are synced to game time. A 0.25 sec Sequencer would pulse each side once per second. Then you'd just have to determine which side coincides with the reactor spitting out items.
And, yes, I believe reactors pulse once per second [I think it's been mentioned], and I'm pretty sure that's when the items are spit out. Also I just tested a sequencer to be sure. It stayed on time with the reactor spitting out items, through world and chunk loading, so... yeah it's pretty solid. Also I got lucky and picked the right side when I placed the sequencer, happened to be the south side (if the sun rises in the east) though that may be different for each sequencer.
@glasstables and breeders:
I'm not sure if there's a specific post for it, though Albaka goes into the details in the nuke tutorial.
Basically how it works out is you can figure out the number of cycles needed to enrich uranium based on reactor heat / number of nearby uranium.
I'll just give you the ones for 3k heat (since I assume we're using this system)
with just 1 uranium next to the isotope cell, it would take 2 cycles to recharge it.
2 uranium next to isotope = 1 cycle
3 = 2/3 of a cycle
4 = 1/2 a cycle.
http://forum.industrial-craft.…ad&postID=13642#post13642 for more details.