It appears that Mariculture's titanium is not registered as a coil material.

It looks like the error is here: https://github.com/erogenousbeef/BigReactors/blob/master/erogenousbeef/bigreactors/common/BigReactors.java#L738
"blockPlatinum" appears twice, but one has the comment "Mariculture". Looking downwards in that method, titanium appears as a reactor interior block. I suspect this just got copy/pasted and wasn't updated to change the entry to titanium.

Derp, copy/paste error. Thanks, I'll fix this in the next build.