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PostPosted: Aug 24, 2013 8:55 am 
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Very interesting Iain.

Some things seem to make no sense. How can the peak temperature possibly go down with increasing borehole diameter? I suspect you may have set the linear heat rating as fixed regardless of bore diameter, but that seems incorrect. Larger bores will have more radioactivity per meter borelength. What is the point of larger bores if you don't fill them any wider than the smaller bores?

Also, getting back to a previous discussion we had, did you consider thermal diffusivity? Or just thermal conduction? Because, for very large bodies, thermal diffusivity is very important. It will form a very long and tiny temperature gradient that will over time act like a kind of insulation. It is a major reason why the earth is still so hot inside.


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PostPosted: Aug 24, 2013 9:58 am 
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Cyril R wrote:
Very interesting Iain.

Some things seem to make no sense. How can the peak temperature possibly go down with increasing borehole diameter? I suspect you may have set the linear heat rating as fixed regardless of bore diameter, but that seems incorrect. Larger bores will have more radioactivity per meter borelength. What is the point of larger bores if you don't fill them any wider than the smaller bores?


He set the heat load constant and was trading off how much difference it would make to change the amount of inert material (like 238U or 232Th) is mixed with the fission products. So, a larger bore would imply the same fission product load regardless of bore diameter.


Current plans are for 60 year above ground storage rather than 10. This would reduce the heat load 2x and hence likely cuts the costs by 2. I believe current plans limit ground heating to less that 100C - for some reason there is concern about boiling ground water. I don't understand this as it seems like a natural driving force to keep water away from the fission products would be a good thing.

For LWR wastes this seems like a reasonable way to go. Will there be a pipe penetrate the ground water and having radioactive material stored below the ground water supply. Won't water want to run along the outside of the pipe until it is driven away by the heat (hence it is a good idea to have the temperature of the pipe >100C?)


Applying this idea to a thermal LFTR system with central processing:
Almost all the power at 60 years is in the Cs and Sr. Much of the Cs is in the decay products of the off-gas system and naturally separated. The remaining Cs could be reasonably separated during vacuum distillation. This would cut the heat load by a factor of two. If we also separate the Sr (no good idea on how this is done so no comment on whether this is practical) then the heat load from fission products drops almost to zero by 60 years. However, in a thorium fed, thermal LFTR the 238Pu heat load is surprisingly high. Due to proliferation concerns I was thinking that the power producing sites would not have the capability to separate plutonium from fission products. Instead they would both be sent to a separate site after a cooling period. At the central site, the 238Pu would be separated either for use for remote power or to be fissioned in a faster reactor. If the Cs and Sr are separated for longer term above ground storage the remainder is at a very low power - so low that even with one fast reactor servicing 50 thermal ones the borehole storage for all the waste (except Cs, Sr, and Kr) is less than shown here. Long term storage of Kr is not a risk in my mind simply due to the fact that Kr is a noble gas. Even if you breathe it in you will simply breathe it right back out. Cs, and Sr would likely want to be stored in containers in the buffer salt pool and add slightly to the passive heat removal requirements at each reactor. So roughly Iain's diagram would apply for a single site processing all the wastes for the whole of US.


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PostPosted: Aug 24, 2013 10:35 am 
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Ok, so the U238 is fillup. That's a clever idea, especially if UO2 since it will provide a reducing environment that inhibits fission product mobility, and prevents any sort of criticality accident.

Water is not much of a concern. In fact even criticality isn't a concern. See Oklo nuclear reactors info links.

http://www.ans.org/pi/np/oklo/

Lots of water, plus critical reactors, yet the fission products didn't get out even in 1.5 billion years. No engineered barriers, no isolating layers, just porous sandstone. Shows you how paranoid we are about geo storage.


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PostPosted: Aug 24, 2013 11:29 am 
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I read that the finnish site has a final depth if 520 metres.

A borehole could put all the waste in a repository between 2,000m and 6,000m. Assuming no vulcanisation in the next 100,000 years (a very short time frame), it should be totally safe.

Boreholes and horizontal boreholes are known technology and would cost a fraction the cost of a normal repository.

Any drawbacks?


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PostPosted: Aug 24, 2013 11:58 am 
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The main drawback may be that the powerplant now also becomes a geo repository. It's hard to site powerplants in terms of public acceptance, even harder to site geo repositories, and siting both at the same place could be a nightmare.


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PostPosted: Aug 24, 2013 12:37 pm 
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alexterrell wrote:
I read that the finnish site has a final depth if 520 metres.

A borehole could put all the waste in a repository between 2,000m and 6,000m. Assuming no vulcanisation in the next 100,000 years (a very short time frame), it should be totally safe.

Boreholes and horizontal boreholes are known technology and would cost a fraction the cost of a normal repository.

Any drawbacks?


I bet there will be an argument about future volcanoes if the spent fuel contains TRUs.

Second, the boreholes likely will penetrate an aquifer so there will be concern about whether there is some mechanism to allow fission products (and TRUs) to migrate into the aquifer. Seems like a legitimate question to ask - don't know enough to know if it is a real concern. Certainly, this has been brought out as an argument against fraking for gas.


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PostPosted: Aug 24, 2013 12:55 pm 
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Cyril R wrote:
The main drawback may be that the powerplant now also becomes a geo repository. It's hard to site powerplants in terms of public acceptance, even harder to site geo repositories, and siting both at the same place could be a nightmare.


But then again, communities near nuclear power stations tend to understand the risks and benefits than those from afar, and tend to be more in favour.

If you really want to get some protest, just frack the boreholes for gas first. Then bsckfill with nuclear waste. :)


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PostPosted: Aug 24, 2013 1:52 pm 
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Why would there be an argument about volcanism? It is very well understood on the timescale of 1 million years. If you go down more than 2000 meters, it is all tens to hundreds of millions of year old rock. Even deep saline aquifers are really old. The shallow aquifers could be an argument. If the waste form were vitrified it would not be an issue.

Having water around does reduce the peak temperatures a lot. Soaked up soil is a good coolant. Dry rock is an insulator.


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PostPosted: Aug 24, 2013 2:22 pm 
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I would suggest depositing waste between ~2000m and ~6000m depth, so well below any water tables.

Then the only question is whether there's a risk during loading of contamination. How would the waste be placed in the borehole?

If there was a case for heat extraction, would this use the same borehole (should be safe for 50 years or so) or special boreholes between the waste bearing ones?


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PostPosted: Aug 24, 2013 2:44 pm 
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Cyril R wrote:
Some things seem to make no sense. How can the peak temperature possibly go down with increasing borehole diameter? I suspect you may have set the linear heat rating as fixed regardless of bore diameter, but that seems incorrect. Larger bores will have more radioactivity per meter borelength. What is the point of larger bores if you don't fill them any wider than the smaller bores?

Also, getting back to a previous discussion we had, did you consider thermal diffusivity? Or just thermal conduction? Because, for very large bodies, thermal diffusivity is very important. It will form a very long and tiny temperature gradient that will over time act like a kind of insulation. It is a major reason why the earth is still so hot inside.


Thanks for looking, Cyril.

I rewrote the intro to the section on bore diameter to make that more clear. Lars has it right, I kept power per bore length constant.

And I did consider both the conductivity and thermal capacity of granite. I tried to make that more clear as well, although I think I need an explanatory graph early on.


Last edited by iain on Aug 24, 2013 3:10 pm, edited 1 time in total.

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PostPosted: Aug 24, 2013 3:09 pm 
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Lars wrote:
Current plans are for 60 year above ground storage rather than 10. This would reduce the heat load 2x and hence likely cuts the costs by 2. I believe current plans limit ground heating to less that 100C - for some reason there is concern about boiling ground water. I don't understand this as it seems like a natural driving force to keep water away from the fission products would be a good thing.


A did a quick calc, and water reaches it's critical pressure at 833 m underground. Below that it's supercritical and can't be boiled, in the sense of a phase change that would drive vigorous convection.

The proposed holes would have a steel lined cemented to the surrounding rock, as per standard oil well drilling practice. I should probably write this up.

Quote:
Applying this idea to a thermal LFTR system with central processing:
Almost all the power at 60 years is in the Cs and Sr. Much of the Cs is in the decay products of the off-gas system and naturally separated. The remaining Cs could be reasonably separated during vacuum distillation. This would cut the heat load by a factor of two. If we also separate the Sr (no good idea on how this is done so no comment on whether this is practical) then the heat load from fission products drops almost to zero by 60 years.


If the point of the sequestration system is to relieve our descendents of the obligation to take care of our wastes, then holding 30 year half-life stuff at the surface isn't really doing that. I'd prefer to just bury it and be done -- it's a simpler story.

We agree that strictly economically you could just put the stuff in casks on the surface for at least hundreds of years. It appears Rancho Seco is on that plan. But I think the customer wants a story that says all the unnaturally radioactive stuff is gone forever. Nevermind that by boring through granite we're going to make a pile of radioactive granite cuttings.

I think your central fast reactor story is too complex. I prefer, and I suspect others would as well, a story which has a well-defined site boundary. Uranium and thorium ores go in, and nothing comes out. The reactors are air-cooled: so no contaminated or heated water leaves. Spent fuel processing and sequestration are on-site: no spent fuel leaves.

I think there might even be an argument for building something like this site near Searles Valley in California. I've not looked at the underlying geology closely yet, but I think that might be endorheic (no outlet but evaporation)... maybe not fault-free though. Anyway, the idea of putting it in California is that the site might also be the long-term disposal for spent fuel from Diablo Canyon, San Onofre, and Rancho Seco. This fixes this state's problem with waste, without opening the interstate transfer can of worms.

Making the disposal site a power plant as well also solves the problem of making the disposal site a (big enough) profit center. If the site takes in PWR SNF, and processes it into sequestered fission products and MSR fuel for use on-site only, then there is a local economic benefit to taking the SNF from other places.


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PostPosted: Aug 24, 2013 3:24 pm 
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alexterrell wrote:
Then the only question is whether there's a risk during loading of contamination. How would the waste be placed in the borehole?

If there was a case for heat extraction, would this use the same borehole (should be safe for 50 years or so) or special boreholes between the waste bearing ones?


The bore is drilled and lined with steel pipe cemented to the surrounding rock. The cement prevents vertical flow of water. The upper service portions of the bore are larger diameter than the lower portions, and have a production casing inside the cemented casing. As is noted below, this production casing can be removed if necessary.

The waste is formed into... let's call them lozenges. These are formed into a string, perhaps hundreds of meters long, and pushed down the cased well by a more usual pipe string. During this time there is mud circulating down the center of the pipe string and also through the lozenge string. The mud cools and lubricates the lozenges, and balances the pressure in the well.

When a lozenge string is in place at the end of the bore, cement is pumped down the string, displacing the mud. The pipe string decouples from the lozenges and is pulled out so that the next string of lozenges can be inserted.

If the lozenge string is jammed and can't be dislodged, then if it's deep enough it can be cemented into place in the service bore, and a new service bore drilled. This may happen a few times. If it's not deep enough, the production casing that it's jammed into can be pulled out of the well.


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PostPosted: Aug 25, 2013 2:47 am 
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What are these lozenges? Strings of fuel assemblies as-is, or vitrified waste glass?


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PostPosted: Aug 25, 2013 9:13 am 
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This topic is about lots of interesting design work for something that need not, and SHOULD not happen. Don't waste it, USE it!

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PostPosted: Aug 25, 2013 4:57 pm 
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KitemanSA wrote:
This topic is about lots of interesting design work for something that need not, and SHOULD not happen. Don't waste it, USE it!


Lots of people say "I'm against nuclear power until there's a solution for the waste problem".

Two solutions are better than one.


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