How long until the material in the reactors is "cool?"

[Downtown Hound]

I heard some talking head on the news earlier say that the one good thing about the fact that the crisis goes on and on (with no meltdown YET) is that the rods lose their potential energy expenditure exponentially as time goes on. In other words, due to the fact that the plants were shut off right after the quake, the rods are cooling on their own. The question I'm wondering, and to which I haven't been able to find an answer for via google, is how long does it take before they actually cool down to the point of no longer being a threat? Is there some point at which if they can just hold off a meltdown long enough that they will eventually cool off on their own? (with the aid of pumped in water obviously) And if so, how long does it take? Days? Weeks? Months? Years? Anybody have any insight on this?

[Bonobo]

1. I'd like to know that too. Anyone know anything? nt

[Lastactiongyro]

2. You misunderstand, there is no "cool" the rods have to continually kept under cool water to

stay "cool" it's a catch 22 situation the rods has to be continually under water. Otherwise they heat up again, and since there is a partial meltdown you have a pile of radioactive lava in places. Uncontrolled fission.

[Downtown Hound]

3. But when does the fission reaction run dry?

With no source powering it, when the the reaction stop?

[Lastactiongyro]

9. well as long as the rods are in close proximity to each other there will be fission..like years.

The time it lasts is dependent on the fuel. I think Reactor 3 or 4 has Plutonium that could be up to 10 years or more if I recall right. Meltdown has occurred so there is no separating anything it's now radioactive lava, a self-sustaining fission constantly heating up.

[Lucinda]

11. The fission stopped when it went offline - scrammed (if I understand correctly) The problem

is coming from residual heat from the radioactive decay.

[Lastactiongyro]

15. Two separate issues. Scrammed refers to the active Nuclear Pile

Scrammed means that the rods have been shoved back into the pile ceasing fission.

Separate from this is the spent fuel rod Storage pool OUTSIDE the containment facility. This needs a constant suppliy of water. That has now everyone concerned. Because the heat is building up there. And possible meltdown is occurring there too. Hopefully cricticalcality will not be reached. Basically you have the potential of the worlds biggest dirty bomb sitting there.

[Tesha]

19. The self-sustaining chain reaction stopped when they "scrammed".

But uranium still spontaneously fissions and
some amount of "chain reaction" still occurs;
it's just kept well-below self-sustaining levels
through the use of spacing of the rods and boron
neutron absorbants in the fuel pond cooling water.

That's one of the big concerns if the borated
ccoling water leaks out and other water is used
to replace it; a self-sustaining chain reaction
*COULD* occur because the spent fuel in the
cooling ponds is packed so closely nowadays.

Tesha

[JVS]

12. Unless they were unable to shut the reactor off (and I've heard NOTHING to make me suspect that)....

what we have now is known as decay heat. Decay heat means that even though your reactor is off, the pile is still generating some heat because what was formerly a sustaining or even increasing chain reaction has still left some fission action going on. Once the reactor is shut off the pile still generates about 7% of its previous heat. It decreases fairly quickly from there. Here is a chart. The problem is that this isn't the temperature, this is the rate of energy production, so without cooling it will heat up the fuel to the point where it can melt. The chain reaction will not get started again though.

[JVS]

13. Source for graphic is wikipedia

http://en.wikipedia.org/wiki/Decay_heat

[Lastactiongyro]

16. Pronlem is there has been partial meltdown in several of the reactors already

[gateley]

5. Thanks for the info/education, and welcome to DU!

:hi:

[Lastactiongyro]

10. Thanks!

[Kip Humphrey]

4. per expert on MSNBC, the spent rods need 10-15 years to cool sufficiently to be casketed.

[Lucinda]

6. There is a point in time when they can move from water cooled into cask storage

I'm not sure how long that cooling process takes though.

[elias49]

7. Spent fuel rods are hot for 5 years

if I heard Rachel Maddow correctly last night. She did a very nice 'primer' on nuclear technology.
Not a very optimistic forecast.
Anyone else pick that up?

[PA Democrat]

8. There is a great website called All Things Nuclear

that has information on cooling the reactors as well updates.


http://allthingsnuclear.org/

Check out this graph which "shows the typical rate at which heat from a shut-down reactor core boils away water when the cooling systems are not functioning. The vertical axis shows the boil-off rate in gallons per minute. The horizontal axis shows the time, in days, since the reactor was shut down. Even a week after being shut down, the heat from a reactor core boils water at a rate of nearly 60 gallons per minute. The boil-off rate declines with time while the rate of radioactive release decreases."





These figures are for fuel rods in the CORE of the reactor. I have no idea what the rates would be for rods in the spent fuel pools which I assume would vary depending on the number of rods as well as their age.

[Downtown Hound]

14. That's a great graph, just what I was looking for! Thanks!

So even a month after the shutdown, they will still be burning almost 40 gallons per minute. Damn. Better get those workers some coffee!

[PA Democrat]

17. You're welcome. BTW, here's a direct link to the article on reactor cooling

http://allthingsnuclear.org/post/3859682324/reactor-core-cooling

[Tesha]

18. Technically speaking, they never "cool off".

It's not like you taking a spoon out of boiling water
and having it cool down to room temperature; the
spoon hasn't got any internal source of heat so it
fairly-quickly "equilibrates" (comes to equillibrium)
with the room's temperature.

But the fuel rods *DO* have an internal source of
heat: radioactive decay. Even with the uranium
fission chain reaction stopped, radioactive decay
of the nuclear waste products puts out (initially)
about 6% of the normal heat output of the reactor.
And it can't be turned off!

It does exponentially run down, of course, as each
isotope in the waste decays at its own individual
rate based upon its half-life. Some very radioactive
isotopes decay fast, others decay more slowly.

In roundish numbers, the rods need to be actively
water-cooled for maybe a year or two. After that,
they're not producing enough heat to melt themselves
and they can be "dry casked" (stored in air inside
big concrete coffins).

But they'll be palpably hot for decades or centuries.

Tesha