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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 2:22 pm 
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Yes Lars, well summarized.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 7:15 pm 
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Lars wrote:
the context of this thread was specifically the risk that the spent fuel rod holding tanks in Fukushima present to the whole world. In this light every fuel rod is at least 2 1/2 years since removal from the reactor and presumably most are considerably older.
The fuel from Unit 4 was offloaded into the storage pool 3.5 months before the Tohoku earthquake/tsunami.
That's why the pie chart was taken at 2.8 years.
There is other SNF in the U4 pool, but it's much older.
BWR fuel assemblies have about 180 kgs of U in them. About 500 of them would have been offloaded from the reactor.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 7:52 pm 
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The problem with SFP #4 is that if another severe earthquake hits and brings the building down as has been suggested, the jumble of fuel assemblies will not necessarily have "windows and doors"to be opened but may be covered with rubble sufficient to suppress the convective loss capability. The 588C assumes free convection from the outer surface, no?

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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 11:10 pm 
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KitemanSA wrote:
The problem with SFP #4 is that if another severe earthquake hits and brings the building down as has been suggested, the jumble of fuel assemblies will not necessarily have "windows and doors"to be opened but may be covered with rubble sufficient to suppress the convective loss capability. The 588C assumes free convection from the outer surface, no?

I suppose - and I agree it would have been smarter not to put them high up in the building. But as far as I understand the conclusion is that the pool area suffered no structural damage. It survived a magnitude 9 earthquake so just how strong an earthquake are you positing?
If this is a serious problem then it seems reasonable to build a new spent fuel pool at a higher elevation and move the spent fuel from SFP #4. I understood this has been considered but deemed unnecessary when they found no structural damage.

Still it would probably be good PR (something TEPCO could really use) to move the older stuff to dry cask storage and the fresh stuff to a pool at ground level but at a high enough elevation to avoid any concerns about another tsunami.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 11:20 pm 
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The only way Tepco might get good PR in the current climate would be if their directors ordered all their plants bulldozed immediately, and then committed ritual seppuku live on television.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 1:42 am 
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KitemanSA wrote:
The problem with SFP #4 is that if another severe earthquake hits and brings the building down as has been suggested, the jumble of fuel assemblies will not necessarily have "windows and doors"to be opened but may be covered with rubble sufficient to suppress the convective loss capability. The 588C assumes free convection from the outer surface, no?


Yes, it assumes free convection from the outside surface.

Though such a catastrophic failure mode, an entire building collapse, is not plausible with this kind of heavily reinforced concrete building. Did you check how thick the walls and bottom of the spent fuel pool are? Did you know the building has a reinforced concrete foundation of more than 8 meters thick? It has survived the initial earthquake plus massive hydrogen explosion and after that, many powerful aftershocks. We don't even have an indication of any piece of equipment failing because of the seismic loading, so it is more than a giant stretch of imagination that the entire building would collapse. The US NRC has done a lot of analysis on so called alpha mode failure, where there are multiple catastrophic building and vessel failures, and it was considered completely physically unreasonable, even with eartquakes that have peak ground accelleration 3x higher than the design basis.

The NRC has also done a lot of analysis on dry cask storage. They looked at plugging of the dry cask vents by debris froms storms or aircraft crash, and found it still wouldn't cause fuel damage.

The built up energy has been relieved by all these eartquakes. There's not enough built up spring energy left right now for a powerful earthquake. Takes many years to build that up again.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 1:56 am 
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Lars wrote:
Still it would probably be good PR (something TEPCO could really use) to move the older stuff to dry cask storage and the fresh stuff to a pool at ground level but at a high enough elevation to avoid any concerns about another tsunami.


Good PR, maybe, but technically pointless. Tsunamis are no longer a concern. It would take more than 2 months to boil off all the water in unit 4 spent fuel pool, and after that it still wouldn't sustain fuel damage.

Like Kiteman says, the residual risk is a massive eartquake collapsing the building around the spent fuel pool into it. It is completely physically unreasonable, for a large number of reasons, and it still wouldn't drain the water in the pool so you still get a 2 month response period. Which is huge, by the way; TEPCO built a whole new makeshift cooling system in about half a month, despite disaster conditions.

Nuclear concrete and rebar isn't your ordinary home building stuff. The rebar is as thick as a man's wrist. The concrete thickness of the pools is set by radiation shielding requirements, which makes it much thicker than needed for seismic reasons. This stuff doesn't collapse catastrophically in an eartquake, not before everything else in a 30 mile radius has collapsed and there's no one left alive to care about the nuclear plant.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 6:50 am 
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Cyril R wrote:
Nuclear concrete and rebar isn't your ordinary home building stuff. The rebar is as thick as a man's wrist. The concrete thickness of the pools is set by radiation shielding requirements, which makes it much thicker than needed for seismic reasons. This stuff doesn't collapse catastrophically in an eartquake, not before everything else in a 30 mile radius has collapsed and there's no one left alive to care about the nuclear plant.


I hope one day to read truth like this in a newspaper.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 1:00 pm 
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Thank you Nicholas, you are most optimistic about the media.

Let us be pessimistic about the heat transfer.

I made another model, this time for 2 week old spent fuel, the maximum possible heat load (takes more than a week to boil off the water so less than 2 weeks seems impossible). It assumed 1600 fresh fuel rods (not possible of course but let's exaggerate to see max temps). To make things more realistic, I've reduced the heat flow from the sides, by adding insulation, 10 cm of 1 W.m/K material. This reflects the fact that conduction won't work as well horizontally as it works vertically, since there is no continuous zircalloy heat bridge horizontally.

Interestingly, the fuel rod surface area is about 1000x higher than the area of the sides of the assemblies if they are packed tightly. This means that even a choked laminar flow regime (reflecting debris and tight rod pitch) of just 0.5 W/m2/K would result in an effective thermal conductivity of about 500 W/m/K on the whole. This is 20x the thermal conductivity of zirconium!!!

Putting in 2 week old fuel making about 4x as much heat as the 3 year old fuel, we get the following:
Attachment:
dry spent fuel bay model fresh fuel with internal convection.png
dry spent fuel bay model fresh fuel with internal convection.png [ 129.88 KiB | Viewed 1005 times ]


The peak temperature hovers around 450 to 485 degrees Celsius (oddly enough the model doesn't reach equilibrium). This is even lower than the case without internal convection but with 4x lower decay heat load (current fuel).

This shows that even in a choked laminar flow regime, there won't be fuel failure. Suprising, not sure if I believe my own model here...


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 3:37 pm 
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GRLCowan wrote:
Lars wrote:
It is hard to imagine that these things could get hot enough to melt. But I'll grant you that they will get considerably hotter than one rod free standing in space.


There is a recent paper: "Predictions of Spent Fuel Heatup After a Complete Loss of Spent Fuel Pool Coolant". As I recall, it essentially agrees with you.

The sun goes at 0.000194 kW per tonne, and look how hot it gets.


See http://pbadupws.nrc.gov/docs/ML0037/ML003727905.pdf It talked about reaching temperatures of up to 800C. From the report: The clad oxidation reaction and radiative heat transfer are not included in this model. The effect of these omissions is considered small at temperatures below 600 0C.

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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 4:14 pm 
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Great find Paul, a very relevant reference. Looks in rough agreement to what I've modelled at first, especially the cases with comparable assumptions (35 month old fuel), though they are considering over 4000 spent fuel assemblies, many more than in the unit 4 spent fuel pool. More relevant to the central fuel storage pool, though dryout there is not a plausible scenario.

Assuming a fixed building ventilation and adiabatic walls is of course silly. If only I had adiabatic walls in my house, I wouldn't have a heating bill. Fixed building ventilation during a heatup is also silly, ventilation goes up a lot when you heat up a room that much. Still the sensitivity studies are useful here in the study.

Oxididation isn't significant in a dryout scenario. When the fuel cladding gets to >300C, all the water is gone, so no water reactions. Zr reaction with air is not significant below 1000 degrees Celsius, ZrO2 stays protective up to searing hot temperatures in dry environments.

The lower values for convective heat transfer through the spent fuel are a bit of a mystery to me. They seem to assume a porous medium, but that's not accurate due to favorable heterogeneity of the spent fuel rods for laminar air cooling flow, they appear to be greatly underestimating this airflow.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 19, 2013 8:07 pm 
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Lots of great information on this, it's really nice to have a place to go that debunks all the utter nonsense some people try and create to kill off further development of nuclear power.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 20, 2013 9:10 am 
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Here's a useful online calculator where we can check some of the heat transfer findings.

http://www.efunda.com/formulae/heat_tra ... e.cfm#calc

Here we can enter the fuel assembly data, eg making 900 Watts, rod surface area of around 90 m2, standard air properties, average fuel rod temperature of 300-400 C, etc.

Even if we assume an air flow rate of just 0.01 m/s through the assemblies, reflecting totally choked (covered by coarse debris) laminar flow, the heat transfer of internal convection alone is still several times the decay heat, and in fact is still bigger than the decay heat of fresh fuel assemblies. This agrees with my last model where even fresh spent fuel assemblies won't be damaged.

I now don't understand what anyone has been worrying about at all regarding the spent fuel, even in the beginning of the Fukushima saga.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 20, 2013 1:26 pm 
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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.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 20, 2013 2:01 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.


The water also provides a radiation shield for workers in the plant.
I also understand that we need to keep the spent fuel for at least 5 years in the water pool before moving to dry cask. I would guess this is because the dry cask storage would have the fuel assembles encased in a steel shell so we lose a lot of surface area compared to individual rods.


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