Energy From Thorium Discussion Forum

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PostPosted: Nov 24, 2012 6:08 am 
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iain wrote:
Some of the afterheat emitted comes in the form of gammas. I'm not sure how much, but surely it's less than half.


Don't be too sure of it. The ANL document I linked to suggests its around half, maybe even 2/3 for some actinides but it doesn't give U233, only U235, U238 and Pu239. For Pu239 it's about half, for U235 it's clearly more than half gamma energy.

Quote:
Of that, at most half will head away from the reactor, and the other half will head into the reactor.


If gamma ray direction is random, it will be more than half because line of sight away from the reactor is bigger than the line of sight into it (it is a 3d object not 2d so edge effects become important).

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Finally, only some fraction of the noncritical volume will be within 5 cm of the reactor walls, close enough for the gamma to leak out. Total gamma leakage from the reactor will be less than 0.2% of full power.


Certainly in energy terms we can just ignore everything inside the reflector. And it definately looks to be less than 0.2% of fullpower. But that's maybe 2 MW for a large reactor, a nontrivial amount of heat when it comes to moving the decay heat through the vessel.

For the vessel it will likely be much less than 1 MW for a large reactor. Only 1 or 2% or so of the fuel salt would be close enough to the vessel and there will likely be neutron protection between the vessel and that salt annulus. That neutron protection plus the vessel (heavy metal so likely has a half thickness of 1 cm or so) will easily absorb 90% of the 0.5 MeV gammas.

However I think we will have difficulty pinning down an exact figure with so much geometrical complexity. For example we haven't talked about the heat exchanger yet. Lots of salt in little thin tubes, this would allow most of the gamma radiation to move out of the tube. If the heat exchanger is inside the buffer salt, it's shell will be a significant heat sink for thermal and gamma radiation even if the secondary coolant is drained.

The energy level of the gamma radiation is also crucial, very nonlinear with penetrating power. That's why I think the prompt gamma's will be a meaningful contribution to buffer salt heat load. The gamma heat load from prompt gamma's is bigger than the decay gamma heat load instantly after shutdown, and the rays are much more energetic. This will also be important in the HX design, where perhaps 1/3 or 1/2 of that gamma radiation will be absorbed by the secondary coolant + shell.


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PostPosted: Nov 24, 2012 7:58 am 
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Ok, a sanity check: shield cooling for CANDUs:

http://www.nuceng.ca/canteachmirror/lib ... 053817.pdf

The heat removed from the shield around the calandria is under 0.1% of reactor full power during operation. End shields are higher but only because of the conduction of heat rather than the gamma's. Also, considerable neutron flux is in these shields, which we should not have lest we risk activating the buffer salt, and a molten salt reactor has graphite that is heavier than D2O.

This gives us an indication that the gamma's escaping the molten salt reactor vessel, into the buffer salt, will be under 0.1% of reactor fullpower. Decay gamma's will evidently be lower than that and drop off fast after shutdown.

This will obviously not be the case for the heat exchanger.


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PostPosted: Dec 10, 2012 2:27 pm 
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Hey Cyril...

Will this suffice for your pool pit? Looks like they can go to 30' (9000 mm :shock: ) diameter and more than enough depth.

Vertical Shaft Boring Machine

German company, with an office in WA state.

Herrenknecht Tunnelling
Systems USA, Inc.
1613 132nd Ave E, Suite 200
Sumner, WA 98390
Phone +1 253 447 2300
Fax +1 253 863 9376
marketing@herrenknecht.com
www.herrenknecht.com


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PostPosted: Dec 10, 2012 4:07 pm 
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Looks promising, MrGadget. Any idea of the cost of excavating hard rock like this?


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PostPosted: Dec 10, 2012 4:55 pm 
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Not a clue...I just found it and it's the first time the method has been used here, and it was successful, so maybe not well known in the States.

I'd seen other discussions on long cylinder reactor designs and thought it might be a good way to get a deep straight hole done and big enough diameter for the tank and lining layers you need.

Would probably solve the urban containment question if the shaft goes deep enough. At end of life, fill it with concrete and call it done.


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PostPosted: Dec 11, 2012 12:45 am 
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I wonder if the reactor buliding could be built over the location where the shaft is to be bored, with an overhead crane that could be used for the shaft bore, and then kept for the reactor assembly if the assembly was designed as a cylinder stack of bottom tank, reactor, and primary HX, all down the shaft and submerged up to the HX, with plumbing out the top to the FP separators, and then to 2nd HX and turbine hall adjacent. Could the primary HX be a cylinder tank design with spiral pipe inside? Then the crane could be used for lifting the whole assembly out for robotic service, maybe? I guess you need a fertile supply tank as well...not sure where that should be fit in.

I have the primary HX and FP separators in the "hot zone" in my head...that's correct?


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PostPosted: Dec 11, 2012 6:53 am 
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Not sure if I understand what you mean, but an integral design - HX in the reactor vessel - could certainly have benefits. Though a conventional arrangement would also have benefits. Integral design is mostly useful for high pressure coolants because it virtually eliminates the rapid loss of coolant accident. But for us I doubt this is any concern at all, with the piping at low pressure and everything in contact with cooler buffer salt (~500C) that also relieves deadweight, there would be low stresses and no possibility for overheating. Pipes rupturing off the vessel seems implausible for such a configuration.

Currently I'm thinking to have a metal, welded superstructure. Modules are connected to that structure. HXs, the vessel, the offgas system. All attached to a common superstructure. The superstructure can then be lifted out partially to do maintenance. Replacing core graphite and HX tubes, and pump impellers, is the only thing that is expected to need replacement. So these should be designed as accessible with a slight submergence. The rest would stay below the buffer salt at all times so there's no stresses on those components, and buffer salt radiation shielding to protect the robots. Anything that makes lots of heat (radiation) is put in a module submerged in the buffer salt. They could be visually inspected from above. Most of the things won't need maintenance. The dump tank or holdup tank, and the offgas decay tanks, for example would just be designed to last the life of the plant. So these can be put on the bottom of the pool.


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PostPosted: Dec 11, 2012 10:38 am 
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Cyril R wrote:
... Replacing core graphite and HX tubes, and pump impellers, is the only thing that is expected to need replacement.

I expect these will also need servicing:
1) bearings and seals for the pump
2) tritium collection (be it copper oxide pellets or titanium bricks)
3) Xe/Kr compressed gas cylinders
4) noble metal sponge
5) vacuum pumps for distilling
6) sensors

Also, I presume everything needs to be inspectable - even the sealed canisters at the bottom of the buffer salt.


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PostPosted: Dec 11, 2012 12:18 pm 
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Bearings and seals - I don't think we should service these seperately. When needed, just replace everything (including the impeller) in one operation. If Krytox is used for the coolant, that should last forever. Seals I think will not be in the buffer salt in the first place, they will extend on the shaft above the buffer salt area, along with the motor.

Tritium collection - looks like we can make the titanium bed big enough to last the life of the plant. One needs only about 10 grams of titanium for each gram of tritium. But I think it is more practical that this bed will not be in the buffer salt. The cooler it is the more efficient the tritium binding process. Titanium tridide doesn't need shielding, any flimsy container it is in will shield it. Not much cooling is needed either, just let it run warm. The Kr and Xe storage in gas cylinders can also be in a different area than the buffer salt.

Vacuum pumps will also be outside the buffer salt, as will be the helium compressor/fan for the offgas system. The piping then goes down the buffer salt.

Sensors will also be outside the buffer salt, except for a few gamma thermometers to measure core power level, and possibly a few laser optic fibers for level measurement in some of the equipment.

Everything in the buffer salt is visually inspectable without having to remove it. The buffer salt is transparent, clean, and nonradioactive. Some fiber optics mounted on the side of the pool can relay visuals to camera's outside the containment. This prevents needing actively cooled camera's or having to replace them inside the containment.


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PostPosted: Dec 11, 2012 7:54 pm 
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7-8 more photos of VSM drilling vertical shaft, in Magnolia neighborhood of Seattle.

http://www.kingcounty.gov/environment/w ... South.aspx


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PostPosted: Dec 12, 2012 4:44 am 
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This document talks about underground cavity construction for nuclear testing:

http://geology.er.usgs.gov/eespteam/pdf/USGSOFR0128.pdf

It has cost estimates of up to $300/m3 for hard rock excavation. With an 8000m3 cavity (for a large reactor) this is only $2.4 million. Not much for a billion dollar powerplant!

Excavation starting from the surface, with a simple vertical cylinder excavation, should be cheaper than this.

Things are looking good.


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PostPosted: Dec 12, 2012 2:20 pm 
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@ Cyril: Glad it looks positive.

Modules assembled into a superstructure to be raiesed for service and lowered to submerged for operation is what I had in mind too, just poorly described on my part.

Do you have a link to the fibrescopes you've made reference to in the past?

Where in the world are you...US or ?


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PostPosted: Dec 16, 2012 8:05 am 
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MrGadget wrote:
I wonder if the reactor buliding could be built over the location where the shaft is to be bored, with an overhead crane that could be used for the shaft bore, and then kept for the reactor assembly if the assembly was designed as a cylinder stack of bottom tank, reactor, and primary HX, all down the shaft and submerged up to the HX, with plumbing out the top to the FP separators, and then to 2nd HX and turbine hall adjacent. Could the primary HX be a cylinder tank design with spiral pipe inside? Then the crane could be used for lifting the whole assembly out for robotic service, maybe? I guess you need a fertile supply tank as well...not sure where that should be fit in.

I have the primary HX and FP separators in the "hot zone" in my head...that's correct?

In the river beds or under water in the sea, caissons are sunk as foundations. There should be no problem in building a structure on top if the casing is strong enough and designed for it. In India, they are known as well foundations.


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PostPosted: Dec 10, 2013 11:48 am 
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Interesting tunnel boring advance: Big Bertha, the largest in the world, is tunneling a 4 lane highway under 1st Avenue in Seattle. To an amateur like me, it looks like a cost-effective way to build underground reactor space.

http://www.seattlepi.com/local/transpor ... 048594.php

This horizontal tunnel could be met by a vertical shaft(s) using this new technology (below), as shown in photos of a Magnolia (Seattle) sewage project, posted earlier, but the link has changed.

website showing VSM boring machine: http://www.herrenknecht.com/en/products ... e-vsm.html

One could imagine a long "Big Bertha" tunnel with numerous vertical shafts for many SMRs (preferably MSRs) Of course, one's imagination might be more "free" if one was in China, where they are not aftraid to use basalt rebar; where they build skyscrapers like stacking Leggos; and where hopefully they will soon make a bid to dominate world nuclear construction and toss out the "gold standard" of the obstructionist NRC.

and many photos.

https://www.google.com/search?q=vsm+bor ... 2&dpr=1.25


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PostPosted: Dec 11, 2013 12:31 pm 
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Location: NoOPWA
With SM LFTRs, all you need is the vertical drill.

_________________
DRJ : Engineer - NAVSEA : (Retired)


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