Why are Reators Still Built Above Ground?

[ElectricFetus]

In general the opposite is true.

and that presumptuous because convection covers for decreasing surface area to volume and then some. If any reactor could cool by conduction alone they would not need active cooling! Also your assuming the reactor core (the amount of fissionable material in use) scales with the reactor volume linearly.

 

[billvon]

and that presumptuous because convection covers for decreasing surface area to volume and then some.

Convection depends on:

Length of vertical extent of surface
Surface area
Temperature differential

Vertical extent is directly proportional to size (or the cube root of volume) surface area goes by the square of size (or the root of volume.) Temperature differential is set by thermal conductivity of the containment and max temperature of the core material.

You can play a lot of games with the above to make convection more possible - make it tall and skinny, put fins on it, put heat paths between the reactor and the containment wall. But with two reactors of the same basic design you're better off with several smaller reactors than one big one if you are going to rely on convection to cool.

If any reactor could cool by conduction alone they would not need active cooling!

Reactors CAN cool by conduction/convection alone, but they need to be designed to do that. Most smaller reactors made nowadays (VK-300, SMR-160, Westinghouse SMR, GT-MHR) will cool passively through convection. It is possible but difficult to get larger reactors to cool passively.

Also your assuming the reactor core (the amount of fissionable material in use) scales with the reactor volume linearly.

I am assuming that the volume of the core scales with the volume of the containment. If you have a smaller core and a larger containment then passive cooling becomes easier.

 

[leopold]

the thing to remember about japan is that she practically relies on nuclear power because she has next to no other resources available.
dams is japans only other means of producing electricity.
wind energy cannot be used because it requires land that japan simply doesn't have.

according to the documentary i posted japans nuclear agency said a tsunami is unlikely within 30 years.
japan is an island nation, you can't tell me that japan isn't worried about tsunamis, 30 years or otherwise.

i see 2 valuable lessons to be learned from this:
1. the generators were put in the most defenseless place imaginable.
2. japans nuclear agency doesn't have any balls. it should have forced fukishima to have hardened generators that could not only withstand 30 year tsumanis but ANY tsunami.

 

[billvon]

i see 2 valuable lessons to be learned from this:
1. the generators were put in the most defenseless place imaginable.

But in a very good place in terns of efficiency. (The sea is a great heat sink.)

[quote2. japans nuclear agency doesn't have any balls. it should have forced fukishima to have hardened generators that could not only withstand 30 year tsumanis but ANY tsunami.[/QUOTE]

No such thing. There is no design that can survive the worst possible event. All you can do is choose what level of protection you want, and hope you guessed right.

 

[leopold]

There is no design that can survive the worst possible event.

we aren't talking about "the worst possible event".
we are talking about 2 weeks of electrical power to shut the cores down safely.
the cores themselves survived both the quake and the tsunami, it's the emergency power that failed.

you are correct, we can't design against the unknown but we can learn lessons.

in my opinion most of the blame lies on the head on japans nuclear agency for allowing the design to proceed.

 

[Carcano]

The containment building of a reactor can also hold more than enough cooling water for a short time. But then all that water starts boiling. And what do you do then? Vent the boiling radioactive water? Drain it somewhere to hold it safely? That's hard to do if it's in a pit and you have no power. The one benefit of the pit is that you can seal it off and just let the core melt, then hope you don't get hydrogen explosions or tunnelling of the core material.

We are discussing a scenario in which the containment building has been shattered by an internal hydrogen explosion or an external impact....so there is no question of relying on it to hold water.

http://en.wikipedia.org/wiki/AP1000#Safety_concerns

"In 2009, the NRC made a safety change related to the events of September 11, ruling that all plants be designed to withstand the direct hit from a plane. To meet the new requirement, Westinghouse encased the AP1000 buildings concrete walls in steel plates. Last year, a member of the NRC since it was formed in 1974, filed the first "non-concurrence" dissent of his career after the NRC granted the design approval. He argues that some parts of the steel skin are so brittle that the "impact energy" from a plane strike or storm driven projectile could shatter the wall."

If the melting core is at the bottom of a pit only two hoses are required to prevent the water from boiling off as radioactive steam...and these hoses can be supplied by emergency trucks from off site.

One hose pumps cold water into the pit, and another hose pumps hot water out from the bottom of the pit.

Emergency workers are protected somewhat from Gamma radiation by the insulating properties of the earth itself surrounding the core.

 

[Tero]

Water is an otherwise useful solvent for reactors, it can't burn etc., but the boiling point is too low.

 

[KitemanSA]

i believe most of fukishima could have been prevented by an adequate emergency power supply.
that's the real mystery.
why were these generators located underground at a sea shore?
what's up with that?

You are correct about the power supply though their underground position would have been fine if the generators and their controls and distribution systems were in a waterproof (submergible) containment with snorkels for the diesels.

 

[billvon]

One hose pumps cold water into the pit, and another hose pumps hot water out from the bottom of the pit.

A pump that will pump boiling radioactive water, full of core fragments, somewhere else? I think there might be some problems there. (including the 'somewhere else' part of that plan.)

Besides, if as you say the pit has been shattered by an explosion, the water's not going to stay there anyway.