The burnup of "burnable poison" isotopes also limits lifespan of a control rod. Fluoride salts dissolve poorly in water, and do not form burnable hydrogen. Reactivity is controlled almost entirely by control rods and burnable poisons. Gadolinium is also used in nuclear marine propulsion systems as a burnable poison. The raw coal has less than 25% of burnable substance. This absorption chain results in a long-lived burnable poison which approximates non-burnable characteristics. He had used a burnable connector that ran from candle to candle.