Energy From Thorium Discussion Forum

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PostPosted: Apr 05, 2014 11:03 am 
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Cyril R wrote:
a simple steel cask would be very attractive. With steel you'd need less shielding which means you can put in more fuel assemblies for a given cask outer diameter.

Steel rusts.
You can apply rust protection on all the surfaces, but the interior surface coating will degrade rapidly due to the radiation: In a few years, all that's left is a small pile of paint chips at the bottom of the cask.

Some countries use casks with a lot of machined and welded stainless steel parts -- these are super expensive and have tiny storage capacity.
Here's a nice video of how they're made:
http://youtu.be/CzU8M0nU1xc


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PostPosted: Apr 05, 2014 3:41 pm 
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The interior coating is not necessary because steel casks can be fully sealed. Steel conducts heat well enough to allow a simpler cask that does not use vents. This avoids a number of (albeit rare) vent blockage type accidents (debris from natural events, cask toppling over, terrorist attack).

However, even for an airflow vent steel cask, the steel is so thick - and corrosion is only a surface phenomenon - that this is a minor issue for a 50 year lifetime. Not sure why you'd want an airvent steel cask - only gain is slightly lower peak cladding temperature, which is easier to deal with in other ways such as backfill inside the cask with conductive aggregate - iron particles for example. Iron particles also reduce the required cask overpack/shielding thickness.


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PostPosted: Apr 05, 2014 6:02 pm 
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All-steel casks present manufacturing problems: Due to the required shielding thickness, it would probably have to be cast in a mold. There are not many foundry shops that can melt that much steel for casting.

Transfer flasks for shuttling SNF between the pool and dry storage are typically all-metal construction, but NOT all-steel.
The main cylinder body is composed of concentric steel shells, with lead poured in-between. Often the other large parts, like the flask loading gate, are also made in this steel-lead-steel sandwich.
There aren't many, but a few foundries will melt the required amount of lead.
Dimensional control is tricky, because the high temperatures tend to distort the steel mold.
On some parts an extra thickness of steel plate is specified, so that the part can be machined to final shape after lead casting.

Concrete is much simpler, but it has other issues.

As for corrosion, it's not that simple: Just sealing the interior space may not be enough and - in some cases - not feasible.
There will be radiolytic decomposition of the air inside, which can result in the formation of nitric acid.

It's important that the spent fuel assemblies be as dry as possible inside the storage casks.
Achieving that may not be obvious, because the small movable storage casks used at many LWR plants are loaded by putting them into the cooling pond and transferring the SNF assemblies into the cask under water (for shielding).
The casks are then simply drained as they are lifted out of the pond: They are neither completely dry nor sealed inside.

But they may do OK for 50 years.


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PostPosted: Apr 05, 2014 8:46 pm 
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Could inert the flasks with argon prior to sealing, its rather cheap for the amounts required really.


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PostPosted: Apr 06, 2014 2:04 am 
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Making steel thick is quite easy, especially in this application. Only the inner steel plate must be hermetic. The others can simply be welded on as overlapping plates. Heck, you could make the sandwich steel double shell and then put in shielding plates without even bothering with welding the shield plates.

There is no air because the casks are pre-evacuated and then backfilled with helium. This is convention. So no radiolysis issues.

Using a highly conductive backfill such as iron particles, not only improves heat transfer, but also acts like a getter for residual water and oxygen.

This isn't rocket science. Its one of the simplest technologies imaginable in the field of nuclear engineering. There is no excuse for companies messing this up.


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PostPosted: Apr 06, 2014 3:52 am 
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jaro wrote:
Depending on design details, degradation may be more rapid in seaside locations, due to the salty air.
Thy should be using DUCrete with basaltic rebar. JMHO.

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DRJ : Engineer - NAVSEA : (Retired)


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PostPosted: Apr 06, 2014 10:02 am 
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Cyril R wrote:
There is no air because the casks are pre-evacuated and then backfilled with helium. This is convention. So no radiolysis issues.

How does this work ?
You pre-evacuate..... put the cask in the SNF pool for loading.... load the fuel without breaking the vacuum.... lift cask out of pool.... backfill with helium ?
Not clear on that transfer part.
What if the surfaces of the 5-year old fuel assemblies have a thin layer of hydrated crud on them ?


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PostPosted: Apr 06, 2014 10:05 am 
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Cyril R wrote:
Making steel thick is quite easy, especially in this application. Only the inner steel plate must be hermetic. The others can simply be welded on as overlapping plates. Heck, you could make the sandwich steel double shell and then put in shielding plates without even bothering with welding the shield plates.

At a couple of feet of thickness, that sounds like a lot of steel plate rolling -- and expensive.


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PostPosted: Apr 06, 2014 10:16 am 
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The big Korean shipyards roll their own big diameter pipe.
For a VLCC, you arre talking around 1 m OD and 25 mm thick.
They figure the cost of roilling and welding the pipe adds
about 200 USD per ton of pipe steel.


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PostPosted: Apr 06, 2014 1:59 pm 
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jaro wrote:
Cyril R wrote:
There is no air because the casks are pre-evacuated and then backfilled with helium. This is convention. So no radiolysis issues.

How does this work ?
You pre-evacuate..... put the cask in the SNF pool for loading.... load the fuel without breaking the vacuum.... lift cask out of pool.... backfill with helium ?
Not clear on that transfer part.
What if the surfaces of the 5-year old fuel assemblies have a thin layer of hydrated crud on them ?


Not sure about the specifics, but likely the fuel assemblies are loaded in underwater, then the cask is lifted out, then the water is drained and the thing is bolted or welded tight, then evacuated, then backfilled with helium.

Some residual steam would not be of great concern, though very little should remain if they purge with vacuum.


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PostPosted: Apr 06, 2014 2:03 pm 
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jaro wrote:
Cyril R wrote:
Making steel thick is quite easy, especially in this application. Only the inner steel plate must be hermetic. The others can simply be welded on as overlapping plates. Heck, you could make the sandwich steel double shell and then put in shielding plates without even bothering with welding the shield plates.

At a couple of feet of thickness, that sounds like a lot of steel plate rolling -- and expensive.


1 foot should be plenty. Steel is a rather good gamma shield, compared to concrete.

If rolling is felt too costly, then you can use simple steel or iron inserts as shielding filler for a steel (preferably stainless steel) sandwich.


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PostPosted: Apr 06, 2014 4:34 pm 
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Could fill the gap with cast iron rings.
There would be no structural loading on them, its just mass


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PostPosted: Apr 07, 2014 3:29 am 
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When remote handling becomes routine, you could
Keep the used fuel in a boiler to produce steam for one year or till the space is required for the next batch.
For next 5 yrs or so, keep in the Lead-tin bath and produce thermo-electric power.
After 5 yrs, process chemically to partition it in
Fission products
uranium
TRU
The last two can be stored dry till required for waste burning reactors.


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PostPosted: Apr 07, 2014 5:07 am 
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I doubt putting it in a boiler that has to be cooled is going to be cheaper than simply putting it in a cooling pond and then a dry cask.
And reprocessing after only 5 years sounds like a nightmare to me.
You want to be waiting at least a hundred, probably more.


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PostPosted: Apr 07, 2014 5:58 am 
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The economics of a SNF boiler don't work out. The heat load is small compared to the reactor power and the heat is declining which is annoying if you want to operate a boiler for 20 years. Operating a boiler for 5 years increases the capital cost. Also the low power output means low efficiency in terms of heat engines.


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