My Bishop is level 17, and I have 100 in Wizardry and Fire magic. When I leveled up, Nuclear Blast wasn't on the list. What am I missing?

I never class changed at all. I imported my bishop from Wiz 7.

Weird, i never encountered or read about that glitch
also know the game mechanics quite well,
based on that i could only imagine unlikely reasons
like corrupted files, ram error or OS realted issues
(e.g. when installing Wiz8 in \program files (x86) folder)

How i would try to resolve it:
1st: Backup the saves!

Use tha saves in a new install of the game,
preferably on another(working;) computer, preferably running Win XP,
that should exclude all hardware, install or OS related resons.

If this fails, it may indicate a corrupted save game.
In that case you would have the masters permission
to visit tk421.net/wizardry/files.shtml , get Wizardry 8 Editor(98K) and fix the issue.

Note that a corrupted save may cause more issues which may even be gamebraing.

Don't see a problem until he hits level 18... its may get a spell pick sooner not must and he has other casting picks i.e could have been offered a extra mind spell in stead.

p.s let the Bard cast it as he is better (more powerful) anyway.

Purist mages (bish included) get their 7th level spells at 18lvl. So totally not a bug, just wait until you lvlup. Hybrid classes get it even later - at 22lvl, but that's not the case with bishop. Besides you always can use save editor and simply add that spell to your spellbook.
Btw that spell is crap :P

Just in case someone else was interested, once I hit level 18, my bishop could pick NB. I don't know what the min Wiz and Fire are, as both were already at 100.

Thanks for the replies!

The skill requirement for a spell is (15 x level) - 15. (0 for 1st, 15 for 2nd, etc, up to 90 for 7th.)

The character's skill value is Highest Relevant Spellbook + (Realm/10).

90 in a spellbook will give access to every spell of that level, but a high Realm skill will give access to spells of that realm somewhat earlier. The latter matters more for bishops given that they have to split their casting between four books.