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Message: <blockquote data-quote="Phil Graham" data-source="post: 24070" data-attributes="member: 430">Re: Attempting to bring clarity to the nuclear problems facing JapanIn the event of a LOCA (loss of coolant) accident, nuclear plants are designed to be instantaneously hermetically sealed off from the turbines and external heat exchangers. For the GE BWR designs, the steam produced is then routed to the suppression pools, where the cooler water in the pool condenses the steam and keeps the internal pressure of the containment structure from growing too high. Further, in a plant that has backup power, the suppression pool water is flooded at a great rate into the reactor pressure vessel. This then generates steam, which is then further condensed by the suppression pool, and the process repeats. The suppression pool is cooled by outside water sources, and more cold water can be brought to the pool if needed. This again requires pumps, valves, and outside energy.The pressure buildup here is almost assuredly inside the reactor container. The fire trucks I assume they use to pump this water likely cannot supply as strong an overpressure as the cooling system pumps, to combat the internal steam pressure. You would then enter a cycle of vent, fill, vent, fill. If you do this well, only a minor amount of radioactive gasses, coupled with short live nitrogen-16 and hydrogen isotopes actually escape to the environment.  Vent/fill/vent/fill is what they want to do in Japan till the fuel cools.The spent fuel pools also have substanstial thermal mass, but they need to be kept filled, so they must keep adding water to these pools to keep the SNF rods cool.Hydrogen comes from several locations in a plant. There's hydrogen in the turbine "halls" but they aren't the concern here. The hydrogen they have seen is most likely from water that, at high enough temperatures, reacts with the fuel rod cladding and disassociates (splits). The cladding becomes an oxide (gains oxygen) and the hydrogen becomes a gas. This is why we generally don't use water to fight metal fires. Usually that it is out of an overabundance of caution, as water is thermochemically stable up to pretty high temperatures (800+ C). In this case zirconium's reactivity helps pull the oxygen away from the water. Hydrogen may also arise from overheated spent fuel rods. Spent fuel, again, is the big problem here. Some substances are "activated" by exposure to alpha or beta radiation, and some aren't. Hydrogen in steam can become radioactive, creating tritium, steam can also convect other radioactive substances into the air. The latter is more likely what is occurring.</blockquote>

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