Energy From Thorium Discussion Forum

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PostPosted: Mar 18, 2017 9:11 am 
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E Ireland wrote:
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.


Well, even a quite large inventory of heavy water, say 1000 m3/GWe, is still only $200-300 million/GWe or so. So still a fraction of the all in prices we're seeing with any new build these days. I suspect it isn't that important. Also there are improved technologies to make D2O that are cheaper, and existing ones have been incrementally improved too.

I wonder if a D2O - ESBWR makes any sense. If you strip the tritium continuously the rad field is going to be less than a normal BWR. Might make an interesting "fast" reactor, or a thermal one if the water space between the FAs is increased. How much water is in the condenser hotwell? Might be a little too much (I guess turbine suppliers will freak out too. "heavy water are you kidding").

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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.


You're going to need flux flattening to get the most power out of your reactor. A totally flat profile gets you the most power output for a given fuel thermal limit. A totally unflattened profile gets lower neutron leakage, but if it costs you many MWe it isn't going to be a good deal economically.


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PostPosted: Mar 19, 2017 5:09 pm 
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Cyril R wrote:
You're going to need flux flattening to get the most power out of your reactor. A totally flat profile gets you the most power output for a given fuel thermal limit. A totally unflattened profile gets lower neutron leakage, but if it costs you many MWe it isn't going to be a good deal economically.


There would be some degree of core flattening in all practical designs thanks to the presence of a radial reflector around the core - in an unflattened core the outermost channels produce negligible power any way so there is little disadvantage to simply not having them and replacing them with more moderator water.

Bidirectional fueling gets a certain amount of axial power flattening (axial is along the channels right?) since the fuel at the insertion end will tend to be more active thhe fuel at the discharge end. [Atleast with an 8 bundle shift]

The question is whether you are willing to accept a core with only that level of flattening.
It appears that overburning the central fuel channels as is conducted at Bruce A is more efficient than the 15 milli-k reactivity loss associated with adjuster assemblies, but that it is still a significant reactivity loss as the Bruce A advantage over Bruce B is only about 1GWd/t - which is about 8 milli-k.

In the extreme entirely-unflattened case the average channel power would be ~27.5% the maximum channel power, which is obviously impractical, but this is improved with the reflectors. Apparently the cores at Pickering (with adjusters) manage something like 60% of maximum channel power on average. Bruce A appears to do better and obtain ~81% - probably thanks to the fine control possible with fuel burn induced reactivity control.
Perhaps a handful of thorium bundles in the centre positions of some of the highest power channels? That would reduce channel power and reactivity but not be truly parasitic absorption.


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PostPosted: Mar 22, 2017 6:54 pm 
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Estimates from a CANTeach graph suggests that bidirectional fueling and a neutron reflector gets you up to 35.5% of peak channel power on average.
With a licenced peak channel power of on order 7300kW that takes us to ~1MWe per channel on average.
Which means a 1500MWe reactor ends up with over 1500 channels, which would be a truly enormous core, containing huge amounts of heavy water. This might be improved upon however if the outmost channels use 28 element Pickering style bundles thanks to their very low core powers, which would give the core significantly improved reactivities in those regions. But would necessitate two fuel production lines, although with a fleet of these large reactors and with budnles as simple as CANDU ones that might not be much of an issue.

So some sort of power flattening is required - it would be interesting to see the effect of thorium bundles versus the effect of Bruce A style overburning, as the former will produce 233U instead of captures in fission products, but will also produce waste fissiles in the spent fuel that are effectively impossible to reuse.

I think Bruce A style overburning is probably the economic optimum.
Although if none of the auxiliary equipment gets bigger, is doubling the number of the channels in the core going to be a very large increase in capital costs?


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PostPosted: Mar 23, 2017 2:41 am 
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If you are working with enriched fuel instead of natural uranium, you could go for higher capacity channels. They could be less neutron efficiencient but still acceptable. Concentrate on using the strong points and overcome the weaknesses.


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PostPosted: Mar 23, 2017 9:38 am 
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Increasing channel power is probably one of the most effective methods of improving the economics of this type of reactor.

If you go for a SiC fuel cladding, almost certainly you can up the power per channel... indeed, SiC rather prefers running hot. It's stronger and more radiation resistant at elevated temperatures, remarkably.

You can also consider smaller dia fuel pins and have more of them.

Pump power will start to become quite huge, though, at some point.


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PostPosted: Mar 23, 2017 7:00 pm 
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jagdish wrote:
If you are working with enriched fuel instead of natural uranium, you could go for higher capacity channels.

Whilst enriched fuels does indeed allow for a flatter core, this risks obviating one of the major benefits of a NU CANDU fuel cycle - the shear simplicity of the entire thing.
Light water storage of the spent or fresh fuel has no criticality concerns, there is no need for conversion/deconversion and enrichment, and the fuel is relatively low heat producing.
Indeed with an average burnup of ~13MWd/t (which seems achievable here) we are near the optimum burnup in terms of disposal costs.
And we have already obtained much of the burnup gain from a conventional CANDU in terms of fuel burnup.
Indeed enrichments of higher than 1% would probably run us out of the region where burnup data is easily available in CANDU fuel.
jagdish wrote:
They could be less neutron efficiencient but still acceptable. Concentrate on using the strong points and overcome the weaknesses.

Improved neutron effficiency is higher burnup.
This equals less cost and less spent fuel lying around being embarassing.


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PostPosted: Mar 24, 2017 4:08 pm 
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It also occurs to me that if you were to provide two sets of circulating pumps (the tubing costs are negligible since our heat exchangers are modular regardless and there is no real pressure differential across the tubing) the outer portion of the core could be provided with a smaller pressure drop circulating loop than the centre of the core.

As the flow rate per channel is lower in the outer part of hte core the pump needs to have a smaller head to produce the same output header pressure.
This would reduce the need for deliberate pressure drops to properly divide the coolant flow - and thus reduce pumping power significantly.
Reduced pumping power equates to improved net efficiency - and potentially to improved natural circulation in a shutdown situation.
This is especially true in a less flattened core as the average pressure drop across the core will be much smaller.


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