[Enthalpy]

At Fukushima they need to inject many m3 a day of sea water in the reactors to keep them cool despite the huge radioactivity.

Unfortunately, sea water contains dissolved gas that will evolve if water gets hot but won't dissolve back, and I suppose this contributes to raise the pressure in the reactor vessel.

So if you find an idea, workable on the field, to degas sea water in amounts of many m3 a day, do tell us!

[enahs]

It is relatively easy to degas the water. But the volume from dissolved gas is inconsequential as the volume change of liquid water to gas water. It would not make a difference at all.

[Enthalpy]

Evolved gas has a much smaller volume than vapour, agreed, but vapour can be condensed by the suppression torus designed for this purpose at the base of the Mark 1 reactor, and by additional built-in means. In contrast, evolved gas doesn't efficiently dissolve back, hence the question.

http://www.engineeringtoolbox.com/air-solubility-water-d_639.html
evolved gas would make at STP up to 2% of sweet water's volume, hence isn't very critical indeed, even if the vessel is replenished a few times.

Solved, thanks!

[Enthalpy]

The amount of sea water already injected makes the volume of gas far from negligible. New data:
http://edition.cnn.com/2011/WORLD/asiapcf/03/24/japan.nuclear.seawater/index.html?hpt=T2
"For almost two weeks now, we've been injecting about 100 gallons a minute of seawater into these reactors"

which makes 6dm3/s or 7000m3 of sea water, potentially releasing 140m³ of dissolved gas. This nears the total volume of the steel vessel.

So the query was founded after all:
how would you deaerate sea water in such amounts and under difficult conditions?
Wikipedia suggests heating it just under +100°C, but easier methods would be welcome!

A vacuum pump maybe? At the top of a tank 10m high? Or just a Venturi (=convergent-divergent section) followed by a large flat open tank?

[By the way, removing salt would be even better...]