How Can Steam From A Nuclear Plant Be Radioactive?

[RitterSport]

@robby

That’s not what’s happening. At temperatures above 1000 deg C, the hot water (or steam) reacts with the zirconium alloy cladding on the fuel elements and produces hydrogen gas and zirconium oxide in an exothermic reaction.

The reaction is:

H[sub]2[/sub]O + Zr ------> 2H[sub]2[/sub] + ZrO[sub]2[/sub] + heat

The heat produced is about 6,500 kJ per kg of zirconium that reacts.

Cite (pdf).

Thanks! That’s just what I was looking for.

RS

[matt]

Another factor is that hydrogen is apparently deliberately added to the coolant water in BWRs in small amounts, to prevent oxygen formation by radiolysis. It’s an anti-corrosion measure. See: Integrated Publishing - Your source for military specifications and educational publications

“As noted several times previously, reactor coolant is maintained at a basic pH (in facilities other than those with aluminum components or those that use chemical shim reactivity control) to reduce corrosion processes. It is also important to exclude dissolved oxygen from reactor coolant for the same reason. As shown in the preceding section, however, a natural conse- quence of exposing pure water to ionizing radiation is production of both hydrogen and oxygen. The addition of a base to control pH has essentially no effect on this feature. To prevent the formation of oxygen in reactor coolant, hydrogen is added. Hydrogen suppresses the formation of oxygen primarily by its effect on the reactions that OH radicals, produced by Reaction (3-3), undergo.”

This was news to me; thought I’d spread it around.

[robby]

@matt

Another factor is that hydrogen is apparently deliberately added to the coolant water in BWRs in small amounts, to prevent oxygen formation by radiolysis. It’s an anti-corrosion measure. See: http://www.tpub.com/content/doe/h1015v2/css/h1015v2_23.htm

“As noted several times previously, reactor coolant is maintained at a basic pH (in facilities other than those with aluminum components or those that use chemical shim reactivity control) to reduce corrosion processes. It is also important to exclude dissolved oxygen from reactor coolant for the same reason. As shown in the preceding section, however, a natural conse- quence of exposing pure water to ionizing radiation is production of both hydrogen and oxygen. The addition of a base to control pH has essentially no effect on this feature. To prevent the formation of oxygen in reactor coolant, hydrogen is added. Hydrogen suppresses the formation of oxygen primarily by its effect on the reactions that OH radicals, produced by Reaction (3-3), undergo.”
t
This was news to me; thought I’d spread it around.

This rings a bell with me. IIRC, this is a fairly common practice with pressurized water reactors and boiling water reactors. However, I’m pretty sure that the amount of hydrogen added is fairly minimal, something like a few parts per million of dissolved hydrogen in the makeup water.

I think the hydrogen being produced via the reaction of hot water and/or steam with zirconium produces far more hydrogen (i.e. many orders of magnitude greater) than the small amount of hydrogen added as an oxygen scavenger.