Aftermath: Population Zero view of unattended nuke plants

[joema]

"Aftermath: Population Zero" view of unattended nuke plants

The National Geographic TV special called "Aftermath: Population Zero" explored the consequences if everyone on Earth vanished, particularly from the standpoint of suddenly unattended technical and industrial systems, and especially nuclear power plants.

Sci-Fi plots ("I Am Legend", etc) often depict a desolate but largely habitable earth.

However "Aftermath" depicted each nuclear plant as undergoing a somewhat Chernobyl-type failure after about 1 week, which resulted in a virtual "doomsday shroud" of radiation over large geographic areas.

The reasoning was each plant's on-site "spent fuel storage pool" (not the reactor itself) would lose coolant after about 1 week, thus causing an uncontained fire and release of material.

They didn't go into detail about the reactor itself and physical/thermal integrity if totally unattended for long periods. But the fire and billowing clouds from each wet storage area was depicted as Chernobyl-like.

My question is how accurate/reasonable is the scenario?

This must have been officially studied -- not from the Sci Fi standpoint -- but because certain disasters (nuclear attack, etc) could result in total breakdown of civil order hence some nuclear plants being unattended for long periods.

What is currently known about the failure modes and consequences of nuclear power plants being unattended, esp the on-site wet storage pools?

http://channel.nationalgeographic.com/channel/aftermath/

 

[mgb_phys]

Astronuc is the expert but I wouldn't have though that onsite wet storage holds that much material and I would hope the pools wouldn't empty from leaks within 1 week.
It might heat up enough to melt the fuel rods and burn but certainly wouldn't be enough to explode as Chernobyl.

The reactors themselves presumably have shutdown systems that operate in a total power failure, either gravity fed control rods or dumps of moderator material.

 

[Homer Simpson]

Plants keep a supply of trained monkees to fill in for the operators, just in case. :)

Can a plant maintain itself shutdown safely while unmanned?

The reactor could be pretty much guarenteed subcritical fairly easily, so there would be no chance of power excursion. (mod poison/drain, rods locked in core, etc)

Assuming the grid would be down, then the plants emergency power systems would be maintained by some form of generator which requires fuel, of which there is only a certain amount on site. Emergency power is required for certain loads to stay safely shutdown, to ensure long term cooling, removing the fuel decay heat.

I guess the question, which I don't know the answer to, is how long it would take an equilibreum bundle's decay heat to drop below the value which would cause the bundle sheath to fail with no cooling (about 1800 dec C). If Emergency power lasted longer than this, then there would be no fuel failures, and then no releases.

All in all, my bet is release could be completely avoided.

But really, if a tree falls in the forest, and nobody hears... just kidding.

Check out this link regarding fuel bay draining
http://www.fluent.com/solutions/examples/img/ex155.pdf

 

[mgb_phys]

Chernobyl-type failure after about 1 week, which resulted in a virtual "doomsday shroud" of radiation over large geographic areas.

In fact Chernobyl has been a great benefit to wildlife - after the inital accident the background radiation isn't that high and the lack of people have turned it into effectively a wildlife reserve. http://news.bbc.co.uk/2/hi/europe/4923342.stm

 

[joema]

...Assuming the grid would be down, then the plants emergency power systems would be maintained by some form of generator which requires fuel, of which there is only a certain amount on site. Emergency power is required for certain loads to stay safely shutdown, to ensure long term cooling, removing the fuel decay heat...

That was exact situation the show considered. Allegedly the grid would fail within a few hours without human intervention.

The spent fuel storage pools aren't passive tanks, but require waterflow to maintain cooling.

On-site emergency diesel generators would automatically activate and maintain the pumps while the diesel lasted, typically one week. Afterward the pumps stop, the decay heat boils the water away, the zircaloy-clad fuel rods would melt and catch fire, then the entire building.

However the TV show depicted it more as an explosion (very Chernobyl-like) than a fire. Also it depicted high-level contamination (dead forests, 50% dead fauna) over thousands of square miles for each plant. The source was the wet storage area, not the reactor itself.

Just curious how accurate that depiction is.

 

[mgb_phys]

Allegedly the grid would fail within a few hours without human intervention.

Then the reactor would shut down automatically - without a grid load there is nowhere for the power to go and the reactor has to shutdown or the lines would melt.
This is what happened in Florida a few weeks ago, the power grid failed and 3 reactors shutdown safely- although the TV news told it as 'nuclear plant failure puts state into darkness'.

However the TV show depicted it more as an explosion (very Chernobyl-like) than a fire. Also it depicted high-level contamination (dead forests, 50% dead fauna) over thousands of square miles for each plant. The source was the wet storage area, not the reactor itself.

A fire is going to put a lot of particles of radioactive heavy metals over a large area but this isn't going to devastate it - even a huge explosion like chernobyl didn't damage very much forest. Most of the animals didn't die, main victims were those very close to the explosion and embryos of pregnant animals, within a generation they are pretty much back to normal.
Uranium and even plutonium aren't too much of a danger in the enviroment, they form mostly insoluble compounds it's the highly active short lived compounds that would cause the fire.

 

[Homer Simpson]

However the TV show depicted it more as an explosion (very Chernobyl-like) than a fire. Also it depicted high-level contamination (dead forests, 50% dead fauna) over thousands of square miles for each plant. The source was the wet storage area, not the reactor itself.

Just curious how accurate that depiction is.

I haven't seen the show, but I'm going to say not accurate at all. Chernobyl was indeed an explosion like you say, which was caused by power in the reactor increasing to huge levels.

In the fuel bays, there will be no chain reaction of neutrons, all the heat is just decay heat, caused by beta radiation as the fission products decay. The only possibility that even comes to mind for an explosion of any sort is the Zirconium Steam reaction, where steam interacts with the Zr fuel sheath at temps above 1100 C and produces hydrogen, which would pose explosion hazard. Seems really unlikely, but I really don't know.

edit: after saying that, try googling Andreyeva Bay. Apparently the rooskies have some neglected spent fuel, and are at risk of their fuel breaking down due to salt water, and suspending the fuel in such a way that criticality is reached. That would lead to risk of explosion if conditions were just right.
re-edit: upon furthur review, this whole criticallity in the fuel bay thing seems to be only promoted by a greenpeaceish type group, and therefor is almost guaranteed to be b-s.

Oh my god, I am scared. OK, maybe not.

I'm used to thinking about natural uranium, where you can not arrange the fuel bundles or the fuel pellets in any way to make critical, without moderator.

 

[vanesch]

It wouldn't be difficult to design an "all-safe" storage pool, with a closed circuit with passive cooling (thanks to natural convection).

That said, I think a week is really short. The thermal power generated by fresh spend fuel is of the order of 2KW per ton of spend fuel, which isn't that much. It is comparable to body heat (if you take a person to produce 200W per 100 kg)!

EDIT: uh, that last number is wrong, it is after a cooling period of 4 years