Well, its also hard to determine what is used for anything in 40k, because we only have Solar Elements to use as a basis. For example, we have no idea what Adamantium is (Its actualy a derivitive of the greek word Adamant, which was Diamond). It may be an alloy, or another Element altogether. For the same reason, its impossible to know what the Tau use as a fuel source. However, i think it's safe to say that the Pulse Rifle functions, in principal, like a Plasma Rifle, just on a smaller scale with adiquate safeguards.