I snooped around a bit in your code and found this:

https://github.com/erogenousbeef/BigReactors/blob/master/erogenousbeef/bigreactors/common/BigReactors.java#L792

BRRegistry.registerReactorInteriorFluid("pyrothuem",    0.66f, 0.90f, 1.00f, IHeatEntity.conductivityIron);

pyrothuem - Typo?

If not, please disregard ...

Yes, I noticed this late last night and made a note to fix it.