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PostPosted: Feb 27, 2017 11:33 pm 
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Just decided to calculate what would happen with a conventional CANDU with a moderator tank at roughly 180C, which is roughly 100C greater than normal.

If we assume such a lattice is still overmoderated (or is close to the optimum) then we could expect a reactivity change ranging from -10mk with fresh fuel to +4mk with equilibrium fuel.
Since this reactor is on load fueled equilibrium ist he important one, and that could add nearly 500MWd/t to the burnup of the core.
Since the equilibrium xenon poisoning of such a reactor is at least 30mk it should not lead to any problems with being unable to start the reactor if the core is cold, although that might lead to problems with poison override if we want that.

Ofcourse we could always just fit an electrode boiler in the moderator tank if worst comes to worst. Although if we just ensure the moderator stays hot for the first 30 minutes after a shutdown to enable a hot restart before the poison-out occurs. Although I am not sure how valuable poison ovveride actually is, and it cost ssomething like 500MWd/t in burnup.


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PostPosted: Mar 05, 2017 2:14 pm 
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Hi Ed,

You might want to take a look at this study:

http://web.ornl.gov/info/reports/1963/3445605494453.pdf

Some interesting concepts on a prestressed RPV structure in there. The one in fig. 3 seems interesting, as there's no tendons in the bottom and top head as they would be concave so inherently loaded in compression. Then your vessel head and bottom might be as simple as steel concave boxes filled with concrete.


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PostPosted: Mar 05, 2017 10:50 pm 
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The idea of a concave end vessel is one I thought about for a while - and it is certainly interesting as we could probably make a large enough casting of either ductile iron or mild steel, however if the vessel head has to be highly concave, it might cause issues for the 700 fuel channels, which the section of the pressure vessel that seperates the fueling machine cavities from the reactor will have to have penetrating it.

I suppose you could use it as the reactor shield - at the centre of the vessel end-face the shielding is outside the pressure vessel but at the edges that outer shielding is thinner and there is an inside-the-vessel shield as compensation?


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PostPosted: Mar 06, 2017 8:15 am 
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Far more benefit at lower development cost could be achieved by changing only the fuel cycle. Some work has been done on thorium fuel cycle by Indians for the AHWR.
You could continue with horizontal tubes and
Have Th-Pu bundles but have only half meter lengths. You could also continue with existing loading machines.
Have two end bundles in core tubes as axial blanket of thorium bundles.
Have two outer rings as radial blanket of thorium bundles.
Burn the axial blanket for as long as rest of the tube. U-233 created will add to fissile fuel towards end of the fuel life. It could be 2000 to 2500 days.
Use radial blanket for an optimum period for creation of U-233. It could be 400-500 days of irradiation.
You could have a thermal breeder or at the very least a good production of U-233. U-233 is the best fissile fuel for any type of reactor. A fast reactor on Th-U233 cycle with blanket can be a breeder with solid or liquid fuel.


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PostPosted: Mar 06, 2017 8:19 am 
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Yes, you can operate a CANDU or AHWR (which is a CANDU-BLW with the serial numbers filed off).
But the superior neutron economy of this concept would allow higher burnups in an equilibrium cycle, if that is how you wanted to operate teh reactor.
As the cost of fuel recycling dominates all other costs in an SSET cycle, this drastically reduces the cost of the power.


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PostPosted: Mar 06, 2017 8:33 am 
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E Ireland wrote:
The idea of a concave end vessel is one I thought about for a while - and it is certainly interesting as we could probably make a large enough casting of either ductile iron or mild steel, however if the vessel head has to be highly concave, it might cause issues for the 700 fuel channels, which the section of the pressure vessel that seperates the fueling machine cavities from the reactor will have to have penetrating it.

I suppose you could use it as the reactor shield - at the centre of the vessel end-face the shielding is outside the pressure vessel but at the edges that outer shielding is thinner and there is an inside-the-vessel shield as compensation?


Isn't it just a question of volume? Which is cheap for a prestressed RPV. If you're concerned about heavy water inventory you could probably have some sort of space eaters there, in the corners.


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PostPosted: Mar 06, 2017 9:27 am 
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Cyril R wrote:
Isn't it just a question of volume? Which is cheap for a prestressed RPV. If you're concerned about heavy water inventory you could probably have some sort of space eaters there, in the corners.


Would probably have to have the fueling tubes in the centre of the vessel stand out from the vessel face to ensure that the fueling machine only has to deal with a 2D plane of tube ends.
I assume this is not a castrophic issue as that part is not inside the reactor so the tubes can be made out of steel to resist the pressure and corrosion etc.

The Atucha design has space fillers made of cast iron in the reactor vessel as it is near spherical and that is not a good shape for a practical core.


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PostPosted: Mar 06, 2017 11:13 am 
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I wonder if it is possible to have a vessel shape where everything is concave so everything is in compression? Could you do this with a square box or will there be tensile forces at the corners?


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PostPosted: Mar 06, 2017 11:25 am 
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I think it has to have some tension somewhere in the structure unfortunately.
The corners will want to burst open I think.


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PostPosted: Mar 06, 2017 3:16 pm 
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E Ireland wrote:
I think it has to have some tension somewhere in the structure unfortunately.
The corners will want to burst open I think.


Yes you're probably right. Maybe if you have a concave cylinder with concave head and bottom, all you'd have to do is tension the corners. If you have a removable head on top it would probably have that ability anyway since the head needs to be sealed tight on it via bolts etc. But maybe this is no improvement over tensioning the walls and use straight walls.


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PostPosted: Mar 08, 2017 6:46 am 
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I think that calandria heavy eater reactor is a good design. It served Canadians and others who followed to produce energy from natural uranium.
If you are not constrained to natural uranium, you could play with fuels and coolants,and use blankets to get many other results including a thermal breeder. We should aim at using all the tricks to reduce energy cost.


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PostPosted: Mar 10, 2017 11:55 am 
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Additionally it appears that a CANDU fuel element would have trouble with silicon carbide cladding - as in a CANDU the cladding is allowed to collapse onto the fuel element at the start of the irradiation - it is not a rigid component like it is in an LWR.
This could cause some issues as SiC can fail by brittle fracture.


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PostPosted: Mar 10, 2017 5:55 pm 
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E Ireland wrote:
Additionally it appears that a CANDU fuel element would have trouble with silicon carbide cladding - as in a CANDU the cladding is allowed to collapse onto the fuel element at the start of the irradiation - it is not a rigid component like it is in an LWR.
This could cause some issues as SiC can fail by brittle fracture.


Don't see why this is a problem. Just a matter of sufficient thickness to prevent failure of the SiC. For CANDU fuel the SiC should always be in compression (or be easily designed to be in that state by the tolerances and dimensions). This has some advantages especially for SiC.


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PostPosted: Mar 13, 2017 4:41 am 
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CANDU PHWR has originally designed for natural uranium fuel. Keeping a heavy water moderator in the tank as in original, you could change the fuel, burn up and coolant. You should borrow ideas from shippingport reactor and change to Th-Pu MOX or metallic enriched uranium-Thorium fuel. You could alternately use part of fuel space as a thorium blanket. U-233 produced will lead to a thorium breeder.
Metallic U-233 could be used in different enrichments as heavy water reactor fuel or fast reactor fuel.


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PostPosted: Mar 16, 2017 8:08 am 
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I think the primary economic considering in the design of the core will still be the quantity of required heavy water rather than any considerations regarding the size of the pressure vessel, after all the materials required are available in huge quantities at low cost and there appear to be no or very few large forging requirements.

This does lead to a question about how much flux flattening is required in such a core, the less that is permissable the lower the average channel power and the higher the possible core reactivity/burnup.


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