The Beryllium Cell is a fluid cell added by TechReborn. You can get by using empty cell on Beryllium fluid block or from several machine recipes.
Industrial Electrolyzer recipe:
29xEmerald Dust + 18xEmpty Cell → 2xAluminium Dust + 3xBeryllium Cell + 6xSilicon Cell + 9xCompressed Air Cell
16xEnder Pearl Dust + 16xEmpty Cell → 5xNitrogen Cell + Beryllium Cell + 4xPotassium Cell + 6xChlorite Cell
Fluid Replicator recipe:
10xUU Matter → Beryllium Cell
Beryllium Cell used in the following crafting recipe:
Beryllium Cell used in the following Fusion Reactor recipe:
Beryllium Cell + Wolframium Cell → Platinum Dust