I went thru the Sith Academy to get the charges. Before using them on Uthar's door, I saved and doubled the quantity pl_thorium. The charges appeared in the inventory just below the other plot item, the sonic imprint sensor, quantity two. However, after using one of the charges on the door, there were NO Thorium charges in the inventory <_< . I reloaded, confirmed the two charges were back in the inventory, then finished the Academy and went to Dxun without doing the Sith Cave.
On Dxun, I went to the first cache and used one of the Thorium charges on the door. In this case, there was indeed still one Thorium charge in the inventory. It appears the logic is different on Korriban -- it apparently clears the inventory of charges regardless of the quantity. Makes me wonder what it would do if you had the "legit" charge from Dantooine plus the one from the locker on Korriban and then used "one" charge on Uthar's door -- would you lose your extra charge regardless?
Oh well, there was only junk in Uthar's room anyway, so it wasn't worth the trouble to cheat. At least the cache on Dxun had some good loot. But I don't think I can get into the Academy after fighting Sion when I go back to Korriban to finish the cave.