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PostPosted: Apr 02, 2014 9:38 pm 
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I recently became aware of the dry storage facitlity at the decommisioned Connecticut Yankee nuclear power plant. I was not aware that we are actually doing this in the US. I have a couple of questions.

http://www.connyankee.com/html/fuel_storage.html

Does anyone know roughly how long a particular worker is allowed to be near these casks? Is there a time limit or a radiation limit, or is that not a concern?

Also, why can't this facility be considered an adequate long-term storage solution?


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PostPosted: Apr 03, 2014 8:27 am 
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Dry SNF storage casks are designed with enough shielding to keep radiation dose levels below worker allowable limits within the site boundary, and below allowable limits for the public at the fence line.
Since the radiation levels decline over time, that means that the shielding must be designed for the high levels in the first year of storage (after, say, 7 years of storage in the spent fuel storage pool).
Any time after that, the shielding is more than necessary. So it pays to delay the transfer to dry storage as long as possible.

Regarding adequacy as a "long-term storage solution", dry storage casks or vaults are typically designed for either 50 or 100 years endurance.
Of course the condition of the structures is monitored and any degradation over the years noted.
It is not unusual to find small cracks develop in the concrete after a decade or two.
Depending on design details, degradation may be more rapid in seaside locations, due to the salty air.


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PostPosted: Apr 03, 2014 10:55 am 
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I have seen values generated from existing projects that storage in 100 year life casks comes to roughly $80/kgU

Which means for the price of reprocessing you could get something like a millenia or more of dry storage.
At which point you can reprocess using glove boxes in all likelyhood.


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PostPosted: Apr 03, 2014 1:53 pm 
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E Ireland wrote:
I have seen values generated from existing projects that storage in 100 year life casks comes to roughly $80/kgU

Which means for the price of reprocessing you could get something like a millenia or more of dry storage.
At which point you can reprocess using glove boxes in all likelyhood.


Or you put some money in a savings account and claim you can keep buying a new cask every 50 years on the interest alone.


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PostPosted: Apr 03, 2014 1:58 pm 
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jaro wrote:
It is not unusual to find small cracks develop in the concrete after a decade or two.


It is not unusual to find things that are badly designed.

:lol: :|


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PostPosted: Apr 03, 2014 10:54 pm 
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jaro wrote:
Dry SNF storage casks are designed with enough shielding to keep radiation dose levels below worker allowable limits within the site boundary, and below allowable limits for the public at the fence line.
Since the radiation levels decline over time, that means that the shielding must be designed for the high levels in the first year of storage (after, say, 7 years of storage in the spent fuel storage pool).
Any time after that, the shielding is more than necessary. So it pays to delay the transfer to dry storage as long as possible.

Regarding adequacy as a "long-term storage solution", dry storage casks or vaults are typically designed for either 50 or 100 years endurance.
Of course the condition of the structures is monitored and any degradation over the years noted.
It is not unusual to find small cracks develop in the concrete after a decade or two.
Depending on design details, degradation may be more rapid in seaside locations, due to the salty air.


I was under the impression that the spent fuel rods stayed in the cooling ponds for two or three decades at least. Let's say they are put into dry casks after 30 years in a cooling pond. Is the concern over radiation completely over at that point? If not, at what point would it be? In other words, how long would the rods have to cool in the ponds so that a person could live next to the dry cask all day every day and not worry at all about radiation?

Also, why are the dry casks only rated for 100 years? So what if some cracks develop? How would the radioactive material escape? Would the rain leech it out? If so, can't someone just throw a tarp over the darn thing? In other words, is the 100 year rating just very conservative?


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PostPosted: Apr 04, 2014 11:33 am 
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Russ wrote:
I was under the impression that the spent fuel rods stayed in the cooling ponds for two or three decades at least.

Most plants do not have cooling ponds large enough to store SNF for decades.
Some plants have tried "re-racking" to make more room in the pools, by using denser packing of the older fuel in the pools.
But even so, most plants are going to dry storage.
7 to 8 years is about the practical minimum for transfer from pool to dry storage. Anything fresher is really too hot, both radiologically and thermally, for dry storage, in an economical way.

Russ wrote:
Let's say they are put into dry casks after 30 years in a cooling pond. Is the concern over radiation completely over at that point? If not, at what point would it be? In other words, how long would the rods have to cool in the ponds so that a person could live next to the dry cask all day every day and not worry at all about radiation?

No, the concern over radiation is not completely over after 30 years. But the required shielding would definitely be less, so the cost would be less.
As for "living next to the dry cask all day every day and not worry at all about radiation," that would depend on the amount of shielding used on the dry cask.
After about 355 years most of the gamma emitters (fission products) are decayed, with the rest being materials with non-penetrating radiation (alpha & beta). At that point, the external radiation around the unshielded SNF is similar to the same mass of uranium ore (in equilibrium with its decay daughters).

Russ wrote:
Also, why are the dry casks only rated for 100 years? So what if some cracks develop? How would the radioactive material escape? Would the rain leech it out? If so, can't someone just throw a tarp over the darn thing? In other words, is the 100 year rating just very conservative?
Casks rated for longer than 100 years could certainly be designed, but they would be more costly -- and actually proving to the licensing authority that they would in fact last that long would be difficult and costly as well. It's just not worth the trouble, particularly with the expectation that the SNF will either be reprocessed at some point, or sent to a deep geological repository.
Also, depending on the actual condition of the casks at the end of their license period, the owners may decide to seek re-licensing for a few more decades - perhaps after some minor repairs, like patching small cracks.


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PostPosted: Apr 04, 2014 11:56 am 
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A typical dose rate you'd get from standing very close to a freshly loaded cask is in the ballpark of 1 mSv/hour. This is a very safe level of radiation, however due to radiophobia and crazy regulations, even 1% of that already safe level is utterly unacceptable to the general public.

Fresh storage is quite economical, though. Steel is very cheap.

Of course there are always morons that want to save a penny by using the cheapest crappiest ordinary concrete. Then find that this is really a thermal application and they should have used either steel or steel plate concrete with a high temperature cement, and they have been penny wise pound foolish. A first year student in structural engineering could have spotted this rookie mistake.

Just like there have been morons that build a nuclear plant that is highly reliant on electricity for emergency cooling, with a 3 meter flooding design basis in a location that regularly gets 15-25 meter tsunamis onshore.


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PostPosted: Apr 04, 2014 3:09 pm 
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steel is cheap and perimeter dirt beams are even cheaper.


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PostPosted: Apr 04, 2014 4:52 pm 
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Cyril R wrote:
Of course there are always morons that want to save a penny by using the cheapest crappiest ordinary concrete. Then find that this is really a thermal application and they should have used either steel or steel plate concrete with a high temperature cement, and they have been penny wise pound foolish.

Excellent point.
Of course the current situation is due to a competitive market in SNF dry storage equipment.
Any vendor who offers equipment that costs 2 or 3 times as much as the competition will never sell anything and will go under in short order. Not a great business plan.
The obvious solution is mandating more durable and expensive stuff, by the regulatory authority.
The vendors would love it, because it would mean more business & profit for them.
Socialist governments would love it too, because it would mean increased regulation and increased costs of nuclear -- utilities are already struggling to stay afloat, with all the subsidies to alternative energy, so this might be a good way to finish them off (too late for some plants, like Kewaunee and Vermont Yankee, which have already shut down, but there are lots of others to take care of).


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PostPosted: Apr 04, 2014 5:30 pm 
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For me it is simple minimum standard, common sense engineering. Concrete running over 100C for extended period, means you're in thermal concrete design area. A well thinking regulator would not allow ordinary concretes to be used here. It is the regulators job to punch through this.

In stead, the regulator is too busy with all sorts of regulations. Does the concrete comply with ASTM this, ASME that, are all the quality assurance documents in order, bla blah.

It was the same with the Fukushima sea wall. It was well constructed, to all the required codes and standards and quality control. I'm sure the regulator was very happy about it. Of course, had they used common sense rather than being paperwork tigers, they would have realized that the seawall was simply not tall enough.

Regulations and paperwork is getting in the way of common sense.


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PostPosted: Apr 04, 2014 7:26 pm 
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I would prefer if we could avoid the political discussion of 'socialist' governments and how they are 'inherently' anti nuclear - since many such governments (arguably in France) have historically been very much in favour of nuclear generation.

But either way - dry casks have pretty much killed the case for early (within 100 years of fuel discharge) reprocessing and deep geological storage.
By the time the deep geological store is cheaper than a succession of dry casks you can do reprocessing so cheaply that the cost of surface storage drops even further.
(After 300 years of surface storage you could probably seperate out the uranium and store the fission products and actinides as bulk powder without running into thermal problems).


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PostPosted: Apr 04, 2014 7:35 pm 
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Cyril R wrote:
For me it is simple minimum standard, common sense engineering. Concrete running over 100C for extended period, means you're in thermal concrete design area.
The other, cheaper common sense engineering is to make sure the concrete doesn't exceed 60C (customers and regulators don't like fancy "thermal concrete" because it is not a common construction material -- meaning that its endurance is not well known in the nuclear industry. Remember that this is an extremely conservative industry: Radical unproven solutions are frowned upon - in addition to being more expensive.
Moreover, as mentioned before, that fancy thermal design would only be useful for the first couple of years - after which the radioactive decay heat drops.

In the design modeling of dry storage casks or modules, the problem is always finding a way for the heat to get out when you put enough shielding around the fuel to cut the radiation to acceptable levels.

"Common sense engineering" would say to use ventilation of the interior space, rather than just conduction through the thick shielding wall.
But it turns out that this technique is not commonly used, because it is difficult to implement on small casks. It works well on larger modules or building-size vaults, where appropriate labyrinths can be incorporated to allow adequate air circulation, while at the same time preventing radiation shine from getting out.


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PostPosted: Apr 04, 2014 11:13 pm 
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jaro wrote:
In the design modeling of dry storage casks or modules, the problem is always finding a way for the heat to get out when you put enough shielding around the fuel to cut the radiation to acceptable levels.

"Common sense engineering" would say to use ventilation of the interior space, rather than just conduction through the thick shielding wall.
But it turns out that this technique is not commonly used, because it is difficult to implement on small casks. It works well on larger modules or building-size vaults, where appropriate labyrinths can be incorporated to allow adequate air circulation, while at the same time preventing radiation shine from getting out.


It seems to me that they need large enough casks to have ventilation initially, with provision for filling the holes in later with concrete as the energy dissipates. Then you get the best of both worlds.

As for the cost of the concrete, I have a hard time imagining that this should be a significant issue. Looking at the photo of that Conn-Yankee site, the amount of concrete I see appears to be much less than what would be used for a common highway overpass. Even if the most expensive concrete is used, I can't imagine that the cost would amount to more than a tiny fraction of the decomissioning cost of a nuclear plant.


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PostPosted: Apr 05, 2014 2:34 am 
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Quote:
The other, cheaper common sense engineering is to make sure the concrete doesn't exceed 60C (customers and regulators don't like fancy "thermal concrete" because it is not a common construction material -- meaning that its endurance is not well known in the nuclear industry. Remember that this is an extremely conservative industry: Radical unproven solutions are frowned upon - in addition to being more expensive.


Thermal concrete is about as fancy as MS DOS.

The effects of low LET radiation on concrete is insignificant, so basically all effects are thermal.

There is a difference between being careful on the use of "new" materials, and producing an industry that lives in its own little stone age in isolation, alienating it from the rest of the world (which is exactly what has happened to the nuclear industry - it is still in the 1960s at best). It is safe to properly test materials. It is not safe to keep using coal powered steam engines in the year 2014.

Also materials cost is a tiny fraction of costs. From what I've heard, almost all the cost are in labor, in particular quality control, certification and the like. In that case, and assuming a regulator doesn't want to consider decades old concrete technologies, 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.


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