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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 20, 2013 2:50 pm 
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E Ireland wrote:
If that were true.... couldn't we just have convectively air cooled spent fuel silos?

Why bother with active pumping of water with all its inherent equipment requirements.


Like Lars said, it provides shielding. Without the water you can actually kill yourself in a few minutes of looking at the spent fuel as a worker looking down into the empty pool. And having 100 degree C air blowing in workers faces isn't nice either. They'd have to wear fire fighter suits in their normal day job! Fuel assemlies can take the heat, we humans are sadly pathetic in our heat resistance. Water means everything is cold (<40C), so you can have normal cranes, manipulators, and everything.

Dry casks will work with BWR fuel for about half a year of cooling. The dry cask is a fully enclosed loop which adds thermal resistance. The cask metal is actually pretty thick stainless steel, compared to a thin zircalloy sheet, and the surface area of the cask sucks compared to the surface area of the zircalloy. But even then considerable safety margin is available even with half year old spent fuel in dry casks, because the analysis done to please the regulator is very conservative, with a lot of worst case heat transfer assumptions. Even so it works well with half a year of cooling for BWR fuel (slightly more for PWR fuel which gets around 120% more decay heat per unit volume fuel).


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 20, 2013 8:56 pm 
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Location: Montreal
Cyril R wrote:
The dry cask is a fully enclosed loop which adds thermal resistance. The cask metal is actually pretty thick stainless steel, compared to a thin zircalloy sheet, and the surface area of the cask sucks compared to the surface area of the zircalloy.
It's worse than that:
Dry storage typically requires "double containment" or something close to that.
That means the fuel is inside two sets of metal cans, with only the outer one getting direct cooling from outside air.
The thick radiation shielding (concrete or metal) is outside those cans, and includes labyrinths which allow air circulation but avoid a direct shine path from the inside.
Some storage casks work by conduction only, with no internal ventilation: That greatly limits SNF storage capacity per module.

Besides that, maximum SNF temperature during storage is designed to be low, relative to normal operating temperature inside the reactor, because the fuel was designed and licensed for a limited life at the conditions in a reactor, not another bunch of years at similar or higher temperatures.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 21, 2013 2:24 am 
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The decay heat of used nuclear fuel is considerable and a part of it could be used. The bundles could be placed as fuel in a boiler and the pressure maintained to keep the boiling at 200K. The steam could be used for power via turbine/reciprocating engine/Stirling engine/thermo-electric couples. When the fuel is not hot enough, it could be transferred to dry casks or reprocessed. The requirement of water could be reduced substantially and pumping or other safety use should have first call on power produced.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 21, 2013 4:12 am 
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jaro wrote:
Cyril R wrote:
The dry cask is a fully enclosed loop which adds thermal resistance. The cask metal is actually pretty thick stainless steel, compared to a thin zircalloy sheet, and the surface area of the cask sucks compared to the surface area of the zircalloy.
It's worse than that:
Dry storage typically requires "double containment" or something close to that.
That means the fuel is inside two sets of metal cans, with only the outer one getting direct cooling from outside air.
The thick radiation shielding (concrete or metal) is outside those cans, and includes labyrinths which allow air circulation but avoid a direct shine path from the inside.
Some storage casks work by conduction only, with no internal ventilation: That greatly limits SNF storage capacity per module.


Thanks Jaro, that makes it even more obvious. I just did a little calculation of a 25 fuel assembly dry storage cask. The fuel assemblies have an internal surface area of around 2400 m2. This compares to the outside wall of the metal inner cask of about 8 m2! 300x less surface area. Then add to that the cask metal being at least 20x thicker than the cladding, and then yet another such metal cask around it. Now it's not hard to see why fresh fuel can't be placed in such casks - they're insulating!

Better engineered airflow can make up for some of the reduced heat transfer, but certainly no factor of 300, it would probably not even make up for the thickness of the metal cask.

Quote:
Besides that, maximum SNF temperature during storage is designed to be low, relative to normal operating temperature inside the reactor, because the fuel was designed and licensed for a limited life at the conditions in a reactor, not another bunch of years at similar or higher temperatures.


This makes little sense to me. As long as you stay below the thermal creep regime, which is above normal operating temperature, time will not be an important factor. But then again nuclear waste facility design is all about being overconservative every step of the way.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 21, 2013 4:26 pm 
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Joined: Dec 16, 2011 7:27 am
Posts: 262
And the beat goes on.

http://www.opednews.com/articles/Humank ... 21-90.html

Quote:
Spent fuel must somehow be kept under water. It's clad in zirconium alloy which will spontaneously ignite when exposed to air. Long used in flash bulbs for cameras, zirconium burns with an extremely bright hot flame.


"OEN- Progressive - Tough - Liberal; News and Opinion"

My guess is it's mostly opinion.

Here's a video of zirconium being heated in air with a blowtorch...and not burning.

http://www.youtube.com/watch?v=x__2yWx9zGY


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 24, 2013 12:47 pm 
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Location: Los Altos, California
Jaro, in that pie chart you posted, why is the Pu-241 slice (4.32 watts) much bigger than the Pu-238 slice (117.41 watts)? Is it that the pie slices show the mass, and the labels show the power?


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 24, 2013 1:11 pm 
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Location: Montreal
iain wrote:
Jaro, in that pie chart you posted, why is the Pu-241 slice (4.32 watts) much bigger than the Pu-238 slice (117.41 watts)? Is it that the pie slices show the mass, and the labels show the power?
Sorry, I should have explained:

The slices are scaled to activity (136kCi for Pu241; 3.54kCi for Pu238), while the labels are in heat power as you can see.

The relatively short halflife and higher production rate of Pu241 (~12% vs. ~3%) accounts for the higher activity, while the beta decay mode accounts for the relatively low power.

(The viewer messes up the display when you set both to show power)


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 Post subject: Re: Fukushima fuel pools
PostPosted: Feb 25, 2014 10:40 am 
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Joined: Apr 19, 2008 1:06 am
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The panic over damage at Fukushima is finally getting over and good sense returning.
[url]Japanese Draft Energy Policy Effectively Overturns Reactor Phaseout[/url]
[url]Japan Pushes to Revive Moribund Nuclear Energy Sector[/url]
[url]Japan To Continue Its Nuclear Fuel Recycling Policy[/url]
Will it have any effect on Germans and other Europeans?


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