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PostPosted: Feb 02, 2014 12:59 pm 
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Thanks Jaro. So would you say that a 60 year calandria heavy water lifetime is acceptable? If, say, the PHT water is light water, or if it is heavy water replaced on a 25 year schedule?

The CANDU 6 has 120 tonnes of primary heavy water inventory. Needing another 120 tonnes batch for swapping means an additional 5 tonnes per year, or $1.5 million/year, or $0.3/MWh (0.03 cents/kWh). That is indeed hardly anything.

Running the numbers does bear out the lifecycle advantage of heavy water.


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PostPosted: Feb 02, 2014 1:32 pm 
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Cyril R wrote:
Thanks Jaro. So would you say that a 60 year calandria heavy water lifetime is acceptable? If, say, the PHT water is light water, or if it is heavy water replaced on a 25 year schedule?
Especially if the moderator cooling system is changed to a passively-cooled one, without pumps & valves that can have leaky seals and that need maintenance (although for maintenance the system should be drained, to reduce tritium vapors when equipment is opened....).
Passive cooling works well at a slightly higher temperature than current Candu units (80C) - still at low or negative pressure - using a condenser HX.


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PostPosted: Feb 02, 2014 2:42 pm 
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jaro wrote:
Cyril R wrote:
Thanks Jaro. So would you say that a 60 year calandria heavy water lifetime is acceptable? If, say, the PHT water is light water, or if it is heavy water replaced on a 25 year schedule?
Especially if the moderator cooling system is changed to a passively-cooled one, without pumps & valves that can have leaky seals and that need maintenance (although for maintenance the system should be drained, to reduce tritium vapors when equipment is opened....).
Passive cooling works well at a slightly higher temperature than current Candu units (80C) - still at low or negative pressure - using a condenser HX.


Since the amount of heating of the moderator is fairly large, easily over 150 MWt for a large reactor, it would be attractive to use that as feedwater preheating in normal operation, with the passive condenser in standby for emergencies and shutdown cooling. This also reduces the rating for the passive condenser making it easier to design. It does require pumps and HXs but if you use canned rotor pumps and PCHEs it is an all welded construction, no seals. There wouldn't be any valves other than isolation valves, the isolation valves could be canned as well (electromagnet requiring power for keeping the valve open).

The passive condensers could be mounted in the upper part of the giant pool of water (like an oversized reactor water vault). They would then use the same light water pumps and HXs to dump the heat.


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PostPosted: Feb 03, 2014 5:26 am 
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Unfortunately it appears the SGHWR at Winfrith required ~2.1% average enrichment to reach 20.7GWd/t.

Which is worse than a BWR.
I will see if they had actually burned the fuel as hard as they could, since it appears they used enrichment to make void coefficients negative and shrink the heavy water inventory.
Interestingly it seems they used Aluminium Calandria tubes instead of Zircalloy ones (not the pressure tube as I first thought).
Unfortunately it seems likely that a HEC tube design is impractical with the light water coolant since it will further reduce fuel economy by holding more light water in the core.

EDIT:

On the plus side, while it does not have on-load refueling it projects a refueling outage of 'over a weekend', with the test reactor using a plan of 1/9th of the core every 6 months.
During one winter when it was decided to suspend experimental work and generate as much power as possible availability rose to 97%.

Here is a handy document.


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PostPosted: Feb 03, 2014 12:01 pm 
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If we assume a tails enrichment of 0.2% and a feed enrichment of 0.7% that means that to produce 1 tonne of 4.2% enriched uranium will require 8 tonnes of natural uranium.

This will produce 50GWd of thermal power.

To do the same with 20.7GWd/t would require roughly 2.4 tonnes of fuel.
At 2.1% enrichment each tonne of fuel will require 3.8 tonnes of natural uranium

That means that 50GWd would require 9.12t of natural uranium.

So the SGHWR requires 15% more fuel but benefits from not requiring any bottleneck parts like pressure vessels.
If wea re in a "build nuclear as fast as possible" scenario this may be a major advantage.

The limiting factor on SGHWR rollout is likely access to sufficient nuclear grade zirconium or more likely to large quantities of heavy water.
We can probably add capacity faster.


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PostPosted: Feb 03, 2014 12:17 pm 
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The SGHWR you are referring to is a tiny one so it doesn't do well in fuel efficiency compared to a massive 4500 MWt BWR. For a given size it should do much better than the BWR even with a poorer burnup from lower enrichment.

British have their own technology, such as giant CO2 cooled reactor with 20 meter (!) diameter spherical pressure vessels, magnesium-aluminium alloy cladding, etc. It was mostly politics. The native deviant design was a lot more politically enticing than imported LWRs, but it was technically inferior (thermal efficiency, burnup, capital cost, all poorer). SGHWR is no exception, it is different than other HWRs of the time.


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PostPosted: Feb 03, 2014 12:20 pm 
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There are projections for fuel enrichment for proposed ~600MWe commercial units, they show a similar ~2.1% enrichment and ~20.7GWd/t burnup.

Remember that Magnox was primarily for the production of plutonium for weapons, only in later years did the programme morph into a purely civilian one. This dictated the use of natural uranium and thus heavily restricts the choices for reactor geometry.
Aluminium-Magnesium alloy fuel cladding was also used by the French for their own carbon dioxide cooled graphite moderated reactors, before they abandoned them in favour of dedicated weapons production reactors and LWRs.

AGR is another question but it mainly suffered because every single station was built to a different design, leaving us with a cluster of FOAK units that were never repeated.

As far as I know the only other people to do anything involving a boiling water cooled heavy water reactor were the Canadians who tried to do it with Natural Uranium at Gentilly and gave up after it failed to live up to expectations.
Criticising this as merely being contrary is like accusing the designers of BWRs of being so because they didn't adopt PWR technology.

EDIT:

It is interesting in that the west now finds itself in the position Canada and similar states were in in the previous century - we lack the heavy engineering capability to produce pressure vessel reactors in the number required domestically.
This I think rather improves the economics of these pressure tube reactors, especially in economic multipliers from domestic manufacture of parts. (I believe a 4RPV/yr facility in the US was priced at $2bn with 5-7 years to first product)


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PostPosted: Feb 03, 2014 8:51 pm 
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Here is a nice overview of early HWRs, including the British SGHWR, from a former AECL colleague.....

https://dl.dropboxusercontent.com/u/11686324/Blair_Bromley_20110204.pdf


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PostPosted: Feb 04, 2014 3:51 am 
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Oooh, this document seems to indicate SGHWR was able to push to 28GWd/t.
That is more like it, although with a 5-batch refueling schedule....

Information is rather scarce for the entire project so I can't even confirm a comparable level of enrichment.


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PostPosted: Feb 05, 2014 6:00 am 
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Carbon moderator has allotropy problems. How about using a carbide with a stable diamond like crystal form? Beryllium Carbide (Be2C) may fill the bill.
http://en.wikipedia.org/wiki/Beryllium_carbide
http://www.americanelements.com/bec.html
It could be clad in SiC to protect it from water.


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PostPosted: Feb 05, 2014 6:26 am 
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Beryllium carbide will produce tritium like you wouldn't believe.

It appears that Heavy Water may be the best choice after all, but there are serious production capacity limitations.

EC6 requires ~620t of heavy water per gigawatt-electric.
Even the SGHWR would require 224t.

This is the bottleneck to mass rollout of EC6s which could easily occur on the scale of multiple units per year otherwise.


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PostPosted: Feb 05, 2014 7:15 am 
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E Ireland wrote:
Beryllium carbide will produce tritium like you wouldn't believe.

It appears that Heavy Water may be the best choice after all, but there are serious production capacity limitations.

EC6 requires ~620t of heavy water per gigawatt-electric.
Even the SGHWR would require 224t.

This is the bottleneck to mass rollout of EC6s which could easily occur on the scale of multiple units per year otherwise.


Is this really such a bottleneck? Enriching heavy water is easier and simpler than enriching uranium. Uranium mass difference is tiny, the stuff is radioactive resulting in more bureacracy delays, and it can be misused to make simple gun type fission bombs. Despite its problems uranium enrichment hasn't turned out to be a major bottleneck. Heavy water production has none of the downsides, its just like any other chemical production facility.


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PostPosted: Feb 05, 2014 7:19 am 
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Cyril R wrote:
Is this really such a bottleneck? Enriching heavy water is easier and simpler than enriching uranium. Uranium mass difference is tiny, the stuff is radioactive resulting in more bureacracy delays, and it can be misused to make simple gun type fission bombs. Despite its problems uranium enrichment hasn't turned out to be a major bottleneck. Heavy water production has none of the downsides, its just like any other chemical production facility.


The problem is the shear quantity required.
Even at its peak Canadian annual production was only 2000t per year, and they have access to far greater quantities of fresh water to feed into the plants than most countries could dream of.
Improvements in recovery efficiency have changed things somewhat but the scaling up of heavy water production takes time, just as LWR RPV construction capacity takes time, with the big difference in that an LWR's RPV has to be available several years before the plant opens whereas you can deliver the heavy water to a CANDU the day before you pull the control rods the first time.


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PostPosted: Feb 05, 2014 7:53 am 
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If there is a bottleneck then the light water pressure tube, light water calandria becomes more attractive.

Heavy water production is no more complicated than many chemicals, though. As for fresh water input, the global water consumption is something like 5 trillion tonnes a year. Putting the most convenient (largest scale flows) 1/1000th of that amount in heavy water production means 5 billion tonnes of fresh water making some 1 million tonnes of heavy water. @ 500 tonnes/GWe this makes 2000 GWe/year, which is a ridiculous amount. At a more reasonable but still ambitious 200 GWe/year you'd have to process 1/10000 of global freshwater consumption.

Lack of tritium and easier direct cycle (supercitical heavy water turbines???) would be a better argument for light water I think. Light water cooled with heavy water moderator remains an attractive hybrid though.


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PostPosted: Feb 05, 2014 11:01 am 
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Attempting mass scale production of a light water moderated pressure tube might run headlong into a scale up bottleneck in uranium enrichment though. AS we have recently discovered, enrichment plants take quite a long time to build.
The advantages of HWRs involve less SWUs required per kWhe and in the case of CANDUs, reactor startup can continue independently of uranium enrichment upscaling, with Natural Uranium being used until sufficient SEU becomes available to switch to the optimal fuel cycle.

As to water shortage issues, perhaps it woudl be best to build systems near large lakes (such as Loch Ness in the UK or the Great Lakes) so they can access 'stored' heavy water, and increase production rates above what would normally be available at those sites.

There is moret han enough Deuterium in Loch Ness to supply my preliminary target of 100GWe for the first 'dash' phase of a UK rollout.


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