I'm pretty sure that's the water pump. It usually has the fan attached to it, but I'm guessing your 318 has an electric fan. The bearings probably gave out. No hoses ruptured? Right now, it's 50/50, it could be bad if the impeller fell to pieces inside, or maybe replacing the pump and pulley might be all you need to do. Did you notice some slight leaking a few days or weeks before this, like drops of coolant on the ground? Usually the pump will leak through the weep hole just before dying out, giving you some warning. But never count on that!