[Klaustrophobia]

[sigh]  I guess I'm either not communicating effectively or am talking about technical details most people don't know about.  Klaustraphobia, would you help me out a little?

I am not talking about radioactivity in general.  I am talking about specific short-lived byproducts that contribute a significant amount to the total heat output of a functioning fission reactor (I've heard 6 - 7% bandied about, but it's been awhile since I read up).  The fuel rods will be nasty and radioactive for uncounted years, yes, but their heat output will only be sufficient to cause them to melt until enough of the short-term fission byproducts have decayed.  When the rate of heat generated by decay drops down to the rate at which heat can be passively removed from the fuel rods (without an active cooling loop in motion) there is no longer a risk of the fuel rods melting.

Those pumps eventually get shut down during turnaround.  It isn't like the fuel rods must have water circulating over them indefinitely.

EDIT: Now reading I may be incorrect in this assumption.  The fuel rods may not cool off enough that they are passively safe from melt down for years.  Apparently spent fuel pools are actively cooled as well.  I did not think this was the case.

i understand you.  to be honest, i don't know the answer to that.  it's not something i've ever explicitly dealt with, but i might be able to hunt down an equation in my notes that will give the decay power as a function of time.  it does take quite a while before NO cooling is necessary, but long before that you can put it in a pool and let natural circulation take care of it.

flipside, the rods are NOT reacting any more.  they are just decaying.  the fission reaction was shut down as soon as the quake was detected.  yes, it can still melt if it becomes uncovered (because 7% of 1GW is 70 megawatts, 70 times more powerful than my school's reactor at full power), but it's not going to burn through the 8 inches of steel and however many feet of concrete below it.  it kindof turns into a puddle and re-solidifies into what they call "corium" in the lower internals of the reactor vessel. 

at this point, it doesn't make a whole hell of a lot of difference what happens as far as the end results.  it does seem that we're on the way to melting based on what i've heard (hearsay, i haven't been actively following to a high degree), but even if not, the core is a total loss.  it will never operate again after they put sea water in it.  the difference is that if it doesn't melt, they can eventually deactivate the plant like normal, pulling the intact fuel out and storing it, and mothball the plant.  if they REALLY wanted to, they could replace all the primary structure, but then you might as well just build a new plant.  with a meltdown, it's just going to sit there encased in it's containment festering for a long time, like TMI.

suongadon, you are also mistaken.  neutron poison has nothing to do with it at this point.  the reason for the boron added to the emergency coolant was as a fail-safe.  the fission reaction is and has been shut down since the beginning.  all we are concerned with at this point is keeping the fuel cool.  the amount of coolant flow has no bearing on the heat production of the reactor, all that matters is that you have at least enough to remove the heat being generated.  the ONLY thing controlling the rate of cooldown of the core is the decay of built-up fission products. 



EDIT:  i ran the numbers as best i could to get an estimate of the current power level.  i assumed a power history of 18 months at 3000 MW thermal.  i have no idea what the actual time of operation was, but 18 months is the standard operating cycle so the result will be conservative.  shutdown time of roughly 3.5 days give approximately 9.5 megawatts of heat still generated in the core. 

attached is a plot of the percent of reactor power vs. days shutdown.

[attachment deleted by ninja]

[kir2yar]

So, how long does it take for the radioactivity in the fuel rods to die down enough that active cooling (pumping) is no longer needed?  A couple weeks?

3~10 days. Depend on reactor type / fuel status...

[Astronomiya]

On another board, people were saying that no reactor containment has been breached by any of the three explosions.  The building shell is meant to keep the weather out, and is not part of the containment system, for those that don't know.  Latest word from the IAEA is that Reactors 1, 2, and 3 are in cold shutdown, and no longer a danger.  However, the vicinity of reactor 4 is now on fire.  This is bad.  You see, TEPCO did something very, very stupid:  the spent fuel cooling pond drained and/or evaporated away in the days since the earthquake, and they didn't monitor it at all after the earthquake hit (!?).  So, predictably, the rods heated up and ignited, spewing radioactive material into the atmosphere.  Jesus.  Latest radiation figure I could find was something like 8 Sv/hr (the IAEA says peak levels were around 400 mSv/hr).

At this point, Fukushima's design, and TEPCO's administration of it, seems pretty bad.  Not because of anything in the reactor itself, but in the support structure around it.  From what I've heard, someone had the bright idea to stick all the backup diesel generators in a low spot on the plant grounds, causing to all die in the tsunami.  This wouldn't be so bad, since the battery backups all then engaged, and did give enough time for them to bring new diesels and other portable power sources in.  Only problem?  The plugs weren't the right type (!?!?).  If you haven't facepalmed yet, you should.  Then TEPCO concentrated so much on saving the damn reactors (ain't gonna do that without sufficient power, guys), they seem to have clean forgot that, oh hey, the spent fuel is really ****ing radioactive and gives off a lot of heat!  So now we have this mess.

Despite all this, we should remember that even with all the stupidity, plus the fact that the facility was hit with a earthquake far stronger than it was designed for, and then got smacked by a ~10 m tsunami, for the reactors themselves, the worst we're looking at is a partial meltdown; indeed the main concern was whether the reactors would remain viable as power generators!  The hydrogen explosions did jack and **** to the containment vessels, the safety systems all kicked in as designed.  This is no Chernobyl.  Not even close.

On a more somber note, it looks like the death toll in Japan is going to climb higher because of this incident.  Because the quake's effects have been so widespread, much of the power grid in northeastern Honshu is offline, and it is now very difficult to effectively bring food, water, and shelter to the affected population due in part to the lack of electric power.  Already, many people there are going into their fourth night without potable water, food, or heating.  Bodies are washing ashore, and they can't burn them fast enough.  At this point, if the death toll stops at 50,000 it'd be a blessing.

[Flipside]

Yeah. I probably used the wrong phrase with 'still reacting'

It's now been announced that the last explosion has thrown health-threatening amounts of radioactive material out..

http://www.bbc.co.uk/news/world-asia-pacific-12741133

[perihelion]

@ Klaustrophobia, thanks for the detailed insight.

As for everything else going on, I am at a loss for words.

[kir2yar]

Russian politicians have offered to relocate the Japanese people in Russia.
Sorry, only RU link: http://top.rbc.ru/special/japan/13/03/2011/558417.shtml

"The Voice of Russia" is helping in the emergency area
EN link: http://english.ruvr.ru/2011/03/15/47462956.html

Recall that “The Voice of Russia” is one of a few media available to the affected regions of Japan. We broadcast there on short and medium waves in Japanese, Russian, English and Chinese. We decided to take part in the search for those who are missing. Everyone who does not know anything about their relatives and friends in Japan can leave messages with the number +7 (495) 950 64 84 or write a letter to the e-mail address [email protected]. We will broadcast your information. The same number and address can be used for expressing your support to the Japanese people. Such letters are coming from all over Russia now.

[kir2yar]

Kind of assistance to Japan offered by Somali pirates. The sum of ransom for captured Japanese ships before they will be reduced by 10%.
 

[Androgeos Exeunt]

Well, that's an unlikely discount...

A Singaporean businesswoman has donated S$1M to the Japanese embassy here. I hope that this money, along with the government's S$500K and everybody else's donations, will be used to provide food and aid for the homeless and contribute to the rebuilding of Japan.

[SypheDMar]

In related news, China ran out of salt. http://www.monstersandcritics.com/news/health/news/article_1626731.php/Worried-Chinese-panic-buy-salt-iodine

[Herra Tohtori]

Interestingly, it's apparently possible that there could be sufficient amount of used nuclear fuel rods in those cooling pools that if it all melts and forms one puddle on the bottom of the dried cooling pool, the mass could go critical.

http://www.bbc.co.uk/news/science-environment-12762608

Fun and games.

I think the cooling pools are within the reactor containment building, but obviously not within the reactor pressure vessel.

I don't think it's necessary to state that this would complicate things quite significantly.

[Androgeos Exeunt]

The whole situation seems to be waffling back and forth between being under control and being out of control, so which is it now?

P.S. Thanks for enhancing my vocabulary, Goober! Much obliged!

[Mongoose]

I've read several lengthy posts by people who work in the nuclear power field, and based on what they've said, I'm not really sure what to think about the entire situation anymore.  I can understand the spent fuel pools being a major concern, since there's far less containment around them (which seems to be a bit of a flaw in this type of reactor's layout), but the reactors themselves have so many fail-safe cooling systems, and are so well contained, that it's hard to see how they could pose any sort of significant radiation threat, even in a partial-meltdown scenario.  Besides that, it's been long enough since reactor shutdown occurred that the decay heat in the reactors should have decreased dramatically by now.  I'm just not sure exactly what the helicopter water drops are accomplishing; we don't have nearly enough information available.

And of course, as the media goes crazy over the "scary nuclear catastrophe," it's somehow overshadowed the fact that more than 10,000 people are dead, hundreds of thousands are now homeless, and tens of billions of dollars in damage have occurred across a wide swath of northern Japan.  That's the true catastrophe here.

[Klaustrophobia]

The whole situation seems to be waffling back and forth between being under control and being out of control, so which is it now?

P.S. Thanks for enhancing my vocabulary, Goober! Much obliged!

well it really IS waffling back and forth.  they get it under control for the most part, until something happens like the hydrogen explosion or the fuel pool ignition. 

herra, i haven't read the link yet, but i'm highly skeptical of a melted spent fuel critical mass.  first and most importantly, if the fuel in the pools melts, they have BEYOND screwed up to the point where it's just not believable.  spent fuel in pools is still decaying, but at a slow enough rate all you have to do is have some changeout of warm water with cold to keep the pool temp down.  i'm guessing what happened initially is that the quake knocked out the heat exchanging system on the pools and it got overlooked because of the core worries.  second, it is spent fuel.  it was nearing the edge of sustaining criticality in the actual core, mixed with once and twice burned fuel.  it's going to be hard as **** to make a critical mass with pure thrice-burned.  third, a puddle of fuel is a non-optimal geometry with very high neutron leakage.  even if you have a "critical mass," you won't get criticality unless it's arranged right.  criticality accidents happen with things like sphere halves of plutonium dropped together or fuel in a solution collected in a bucket.  the same bucket spilled over the floor wouldn't be a problem.

that said, let me now go read the article and see if i need to stick my foot in my mouth. 

EDIT: yeah, they are basically just saying there is an outside chance.  which there is, but it's REALLY outside. 

[karajorma]

In related news, China ran out of salt. http://www.monstersandcritics.com/news/health/news/article_1626731.php/Worried-Chinese-panic-buy-salt-iodine

They at least have a reason to be worried. Although it is unlikely to hit much of China if a significant release did occur North East China isn't that far from Japan. This is much more mystifying.

[headdie]

* headdie then face palms then then and finally

[Sololop]

I hope people know dropping iodine won't really help you when nuclear fallout wafts into town.

[achtung]

I hope people know dropping iodine won't really help you when nuclear fallout wafts into town.

Correct. You must use vodka instead.

[Kosh]

And of course, as the media goes crazy over the "scary nuclear catastrophe," it's somehow overshadowed the fact that more than 10,000 people are dead, hundreds of thousands are now homeless, and tens of billions of dollars in damage have occurred across a wide swath of northern Japan.  That's the true catastrophe here.

What's even worse is that the only real solution to kicking coal IS nuclear, but this is going to cause people to pointless start chasing windmills again.

[Kolgena]

Do you see over yonder, friend Sancho, thirty or forty hulking giants?

That's pretty much how ridiculous I think the media is for crying excessively about nuclear. Even if all four reactors in Japan melt down, it doesn't make the technology any less legitimate.

[Astronomiya]

Interestingly, it's apparently possible that there could be sufficient amount of used nuclear fuel rods in those cooling pools that if it all melts and forms one puddle on the bottom of the dried cooling pool, the mass could go critical.

http://www.bbc.co.uk/news/science-environment-12762608

Fun and games.

I think the cooling pools are within the reactor containment building, but obviously not within the reactor pressure vessel.

I don't think it's necessary to state that this would complicate things quite significantly.

Klaustrophobia already covered this, but only the pools at No. 4, 5, and 6 have a hope in hell of doing that; the rest contain only spent fuel, which cannot go critical inside the reactor. The reactor has a geometry designed to encourage criticality, since that is what generates most of the heat.  And if it's so poisoned by fission products it can't go critical in a place where it is encouraged to do so, how can it do so laying in a pool?

Although pools 5 and 6 are at higher temperatures than normal (60 C instead of 25), I wouldn't be too worried about them yet.  The big question is pool 4, which may have gone completely dry (apparently the NRC seems to think so).  Certainly, some of the rods there were exposed for several hours (the two fires).

They at least have a reason to be worried. Although it is unlikely to hit much of China if a significant release did occur North East China isn't that far from Japan. This is much more mystifying.

The prevailing winds blow away from China across the Pacific.  The Chinese have no reason to be worried at all.  And America is full of anti-nuclear hysteria on the best of days; believe me, I've encountered a lot of people like that.