Nuclear reactors

[egbert]

The containment barrier in question here are the steel vessel, not the zirconium cladding. That could be due to seawater corrosion

My critical thinking/skepticism is asking - if seawater is so corrosive that it can eat through the rather thick containment vessel in a mere 3 weeks, how come there are any WWII era ship wrecks left for divers to explore - shouldn't they have all corroded away long ago?
Just wondering....

or it could be due to a variety of other mechanisms, such as melt through of the primary containment due to decay heat. Much of the zirconium cladding was clearly breached early in the accident, or we wouldn't be seeing radiation levels nearly this high.

Obviously, the I 131 is a giveaway.

Of course, the seawater could have caused problems in other ways, such as by leaving salt residue in hot spots where it boiled away.

That's a separate issue from corrosion.

[Warren Dew]

My critical thinking/skepticism is asking - if seawater is so corrosive that it can eat through the rather thick containment vessel in a mere 3 weeks, how come there are any WWII era ship wrecks left for divers to explore - shouldn't they have all corroded away long ago?
Just wondering....

Corrosion is an extremely strong function of concentrations and temperature, both of which would have been significantly higher in the reactor than in the ocean. Certain forms of corrosion, such as stress corrosion cracking and some forms of pitting, happen only under higher concentrations of electrolytes.

Also, the plants were 40 years old, and it seems like TEPCo had at best average competence. My bet is that there were already some fairly corroded pipes or joints, or even some small leaks ready to get much bigger once the blowout discs blew out and the torus was flooded.

That's a separate issue from corrosion.

It's not a separate issue from my suggestion of containment barrier deterioration, of which corrosion was only one illustrative example.

[Warren Dew]

Sure, but with Iodine's half life of 8 days, and the fact that fission alledgedly stopped soon after the shut down, I would have thought there would not be too much left...

You might not have an appreciation for the orders of magnitude involved.

The radiation levels in the seawater are in the thousands of picocuries per liter.[1] On shutdown, my back of the envelope calculation indicates the reactor had hundreds of gigacuries of I-131 in it. The difference is a factor of 100,000,000,000,000,000,000, give or take a couple orders of magnitude. Even after accounting for the dilution in perhaps trillions (1,000,000,000,000s) of liters of seawater, we're still left with a spread of somewhere around eight orders of magnitude.

After a month of radioactive decay, that 8 orders of magnitude has been cut down to around 7 orders of magnitude because of radioactive decay, again, plus or minus a couple of orders of magnitude. That still suggests that less than one part in one million of the I-131 in the power plant has escaped into the sea, though. Even with the short half life of I-131, it's going to take a year or two before the left over I-131 in the reactor has decayed to the point where it's no longer a threat.

[1] Most recent measurement I can find reported are around 2000 pCi/l: http://nextbigfuture.com/2011/03/fukush ... ch-26.html

[JimC]

Sure, but with Iodine's half life of 8 days, and the fact that fission alledgedly stopped soon after the shut down, I would have thought there would not be too much left...

You might not have an appreciation for the orders of magnitude involved.

The radiation levels in the seawater are in the thousands of picocuries per liter.[1] On shutdown, my back of the envelope calculation indicates the reactor had hundreds of gigacuries of I-131 in it. The difference is a factor of 100,000,000,000,000,000,000, give or take a couple orders of magnitude. Even after accounting for the dilution in perhaps trillions (1,000,000,000,000s) of liters of seawater, we're still left with a spread of somewhere around eight orders of magnitude.

After a month of radioactive decay, that 8 orders of magnitude has been cut down to around 7 orders of magnitude because of radioactive decay, again, plus or minus a couple of orders of magnitude. That still suggests that less than one part in one million of the I-131 in the power plant has escaped into the sea, though. Even with the short half life of I-131, it's going to take a year or two before the left over I-131 in the reactor has decayed to the point where it's no longer a threat.

[1] Most recent measurement I can find reported are around 2000 pCi/l: http://nextbigfuture.com/2011/03/fukush ... ch-26.html

Good info...

Sometime in the future, when the real facts are known and documented, I hope there will be a solid article in, say, NewScientist that I can copy and give to my Year 11 Physics students to analyse. Getting them to do calculations involving half lives involving a real life scenario will be useful...