In a maneuver called “feed and bleed” Japanese engineers are performing a hail Mary style attempt at cooling the troubled Fukushima reactor that has at least partially melted down.
The technique involves feeding, or rather pumping, cold water into the reactor’s pressure vessel, but they can only do that for a short time. As the water hits the overheated fuel rods, it boils. The resulting steam raises the pressure inside the chamber to a point where no more water can be pumped in. To allow more cold water to be pumped in, the chamber must be bled. Bleeding is accomplished by allowing the superheated gases to escape through vents – these vents are described in a nirs.org fact sheet on boiling water reactors (BWR).
The vent is a reinforced pipe installed in the torus and designed to release radioactive high pressure steam generated in a severe accident by allowing the unfiltered release directly to the atmosphere through the 300 foot vent stack.
Sometimes the steam also carries Hydrogen in a gaseous form which is suspected of causing the explosions at Fukushima reactors #1 and #3. Since those explosions are outside the reactor chamber, no damage to the pressure vessel usually occurs.
One of the most important considerations to this procedure is the condition of the fuel rods, the New York Times posted the ramifications of compromised rods.
When the fuel was intact, the steam they were releasing had only modest amounts of radioactive material, in a nontroublesome form. With damaged fuel, that steam is getting dirtier.
Another potential concern is that some Japanese reactors (as well as some in France and Germany) run on a mixed fuel known as mox, or mixed oxide, that includes reclaimed plutonium. It is not clear whether the stricken reactors are among those, but if they are, the steam they release could be more toxic.
Here, the Old Grey Lady missed something. On day one of the disaster, Japanese officials released that Fukushima #3 used Plutonium MOX (mixed-oxide). Mixed-oxide is dirtier and could present a greater threat should the feed and bleed cause excessive venting of the pressure vessel.