I think this would fix it:
src/main/java/shedar/mods/ic2/nuclearcontrol/crossmod/ics2/IC2ExpCross.java:29
Should be 10000, instead of 20000. See IC2 Wiki, it states that uranium cells last for 10000 damage which equals 20000 seconds.
Also, I do not see a case where MOX Fuel is considered. So, you might want to add a if ... instanceof ItemReactorMOX (which should also last for 10000 damage, unfortunately there is no information in the IC2 wiki)..

Okay, I just compiled this mod myself with the aforementioned "fix". Now the remaining time ranges from 0 to almost 3 hours, BUT only every second real second a second is being count down on the timer. Since there was an extended usage of the word second in the last sentence, I'll give you an example 😆 : If the time says 1:13:08 it takes two real time seconds until it reaches 1:13:07. My best guess is that there must be another line of code that is wrong. I suspect that "10000 damage equals 20000 seconds" was coded into the mod exactly the other way around. Because 20000 seconds equals approx. 5.5 hours, which is the value that was shown on the timer with the value of 20000 hardcoded. Nevertheless the timer stopped at 10000 seconds (approx 2.75 hours) instead of 0. And that means, the problem is the counting down itself.

Is getNuclearCellTimeLeft constantly called for an update of the timer or is it just called once at load up and the countdown is done manually?