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PostPosted: Aug 06, 2013 12:58 pm 
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Ideally I would like something closer to the Gentilly-1 Boiling Water CANDU than the AHWR which derives its concept more from the Advanced CANDU.

The neutron economy that gives the ability to burn unenriched fuel brings with it the capability to burn more enriched fuels for extremely long burnups, especially if we have some fancy fuels that are made up of LEU doped with thorium.

So if we can have a relatively large moderator tank at relatively low cost I think its a good idea.
Unfortunately this does probably require on load refueling which is a massive cost headache.

My target is ~$1000/kW which is probably impossible to hit but I think it can be gotten close for the nth unit.


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PostPosted: Aug 06, 2013 3:24 pm 
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Longer burnup means more R&D. One of the reasons CANDU fuel is so reliable is precisely because of the low burnup.

Plus you're optimizing the lowest cost part of the power economics - the fuel, and get more capital cost and complication in return, the biggest cost area. Makes no sense.

I think that once you look at the costs beyond the heavy water inventory, such as the extra tritium control, monitoring, heat exchangers, chem cleanup, a light water moderator will add up considerably on capital cost reduction.


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PostPosted: Aug 06, 2013 4:39 pm 
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Well CANDU type fuel bundles are now qualified to ~20GWd/t, which is high for ~1% enrichment or equivalent. You can leverage the technology from BWR fuel assemblies as well I imagine.

One thing that worries me about a light water moderator is that the pressure tubes would almost have to be touching, which could lead to them welding together which is probably a bad thing.
I am going to look more into this idea of a compact de-tritiation system and get some cost figures, because if you can continuously cycle it that would reduce the tritium leaks and reduce the need for insane leak-tightness.

If you bury the heavy water calandria in a light water shield tank with either a mild steel or aluminium shell you might be able to avoid a heavy water heat exchanger.


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PostPosted: Aug 06, 2013 5:25 pm 
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There is some potential for returns on the tritium problem..... (might pay for the cost of the de-tritiation skid ?)

Quote:
Direct harvesting of Helium-3 (3He) from heavy water nuclear reactors
G. Bentoumi*, R. Didsbury, G. Jonkmans, L. Rodrigo and B. Sur
Atomic Energy of Canada Limited, Chalk River Laboratories, Chalk River, Ontario, Canada,

The thermal neutron activation of deuterium inside a heavy-water-moderated or -cooled nuclear reactor produces a build-up of tritium in the heavy water. The in situ decay of tritium can, for certain reactor types
and operating conditions, produce potentially useable amounts of 3He, which can be directly extracted via the heavy-water cover gas without first separating, collecting and storing tritium outside the reactor. It is estimated that the amount of 3He available for recovery from the moderator cover gas of a 700 MWe class Pressurized Heavy Water Reactor (PHWR) ranges from 0.1 to 0.7 m3 (STP) per annum, varying with the tritium activity buildup in the moderator. The harvesting of 3He would generate approximately 12.7 m3 (STP) of 3He, worth more than $30M at current market rates, over a typical 25-year operating cycle of the PHWR. This paper discusses the production of 3He in the moderator of a PHWR and its extraction from the 4He moderator cover gas system using conventional methods.
http://www.aecl.ca/Assets/Nuclear_Review/ANR_2-1_ENG.pdf


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PostPosted: Aug 07, 2013 3:13 am 
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Quote:
If you bury the heavy water calandria in a light water shield tank with either a mild steel or aluminium shell you might be able to avoid a heavy water heat exchanger.


This is already done in CANDUs. But you want to control the void fraction, basically have some subcooling in the calandria water. That means an actively pumped heat exchanger in today's CANDUs. The heat load is quite large.

However, a passive two phase HX would work. For example, there could be a little condenser carrying heated heavy water up a bit through a riser, where it starts to flash to two phase flow, producing driving head to the condenser. The condenser could be fully passive, cooled by the light water vault where it is submerged in. Then you'd only need a (nonradioactive) light water vault heat exchanger. With an oversized light water vault, you could have a day or two of subcooling, then weeks of boiloff out of the containment (the light water is not toxic or radioactive). AECL has worked on something like this, but work apparently stopped afterwards.


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PostPosted: Aug 07, 2013 8:42 am 
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Quote:
One thing that worries me about a light water moderator is that the pressure tubes would almost have to be touching, which could lead to them welding together which is probably a bad thing.


If the channels are vertical, then there is no sagging problem. Then it is as easy as with tubular fuel elements: CANDU fuel for example is extremely tightly pitched, with perhaps only a few mm in between fuel rods at the closest point.

With a water environment, and a metal as tenaciously oxide forming as zirconium, welding together is not a big concern anyway, unless perhaps you have components that are heavily pressed against each other (like a normally closed valve that could pressure weld after years of being closed).


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PostPosted: Aug 07, 2013 2:37 pm 
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Does moderator volume scale linearly with reactor power/
Finally found some decent stats on the Gentilly-1 plant and it seems to have over a tonne of Heavy water per megawatt.
Which is an awful lot. (an Enhanced CANDU 6 only has 457t of moderator, 192t of which is in the heat transport system and is thus not in the core much)

Would imply a thousand tonnes of moderator in some sort of hypothetical Gigawatt range reactor?
I would have thought it would scale by power^(2/3) but I am not sure.

Also, if we had a continually running on-site detritiation plant that cycled the entire moderator mass on average every year, would that be sufficient to suppress tritium concentrations to negligible levels - allowing for relaxation of the insane control measures in CANDUs?

EDIT:
I think I have an answer, the boiling coolant volume in the reactor had to be reduced by any means possible which meant that the fuel rod diameters were increased, which placed a hard limit on the power output of the assemblies thanks to linear heating problems.
Surely a more effective means would be to simply introduce more pins like hte smaller ones from CANFLEX?


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PostPosted: Aug 08, 2013 12:46 pm 
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The double post is annoying but I had another thought:

Can you put steam separators in the top of the pressure tubes themselves?

This would require a lot of identical steam separators but they would be identical which would be amenable to mass production and it would remove the need for the steam drum entirely?


Also in a light water pressure tube reactor, the rather small moderator tank is at 80C or so isn't it?
How about drawing turbine plant feedwater directly from it?
It avoids at least one stage of feedwater heaters.


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PostPosted: Aug 08, 2013 2:24 pm 
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Interesting ideas. I suppose you could have longer pressure tubes with steam seperators and dryers in them. Possibly this idea can be extended, with a re-entrant tube type arrangement, where the seperated moisture goes back around the annulus of the pressure tube, producing a recirculation flow path. Then you have basically a BWR but as modular pressure channels, each with their own recirc flow, in stead of a giant pressure vessel. This should be very amenable to mass manufacturing.

Calandria light water with feedwater should work. Put the condensate from the condenser in it, then take the heated moderator water and pump it up a few feedwater heaters upstream. It would avoid large heat rejection equipment for the moderator cooler.


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PostPosted: Aug 08, 2013 3:16 pm 
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How would a reentrant flow work?
I can't see how you could go around the annulus of the pressure tube without causing all sorts of problems, since that water will be as hot as the steam outlet temperature.

What about just bleeding off the water from the pressure tube headers separately from the steam and then just mixing it with the feedwater using a central downcomer that runs around the reactor?
If the feedwater is mixed in at the top of the reactor the downcomer could be kept at the reactor inlet temperature and could probably be made of relatively cheap steel?
The cooling of the water would also make it more dense which would help reduce the pumping power required in the circuit.


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PostPosted: Aug 09, 2013 4:12 am 
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E Ireland wrote:
How would a reentrant flow work?
I can't see how you could go around the annulus of the pressure tube without causing all sorts of problems, since that water will be as hot as the steam outlet temperature.


Insulate the pressure tube with a ceramic such as YSZ. As proposed by AECL. Any temperature becomes possible.

The re-entrant flow would of course complicate things. But then again it avoids a pressure vessel, as the possibility emerges to use only manifolds rather than a big plenum vessel with massively thick tubesheets.

It makes refuelling more complicated though, so is probably a bad idea.

I think a better idea is to have a vertical arrangement, with boiling light water rising through the pressure tubes to an upper little pressure vessel plenum, containing a central steam seperator and -dryer. During refuelling the seperator and dryer are simply unbolted just like a standard BWR. This would be a BWR with a tiny and short pressure vessel rather than a massive and tall one.

Quote:
What about just bleeding off the water from the pressure tube headers separately from the steam and then just mixing it with the feedwater using a central downcomer that runs around the reactor?
If the feedwater is mixed in at the top of the reactor the downcomer could be kept at the reactor inlet temperature and could probably be made of relatively cheap steel?
The cooling of the water would also make it more dense which would help reduce the pumping power required in the circuit


There is a need for recirculation flow in BWRs. In fact most of the flow (probably 70-80%) is recirculated. So we can't send such a huge amount of hot water to feedwater. It has to circulate several passes through the core on average. I've never seen a plausible once through boiling water reactor proposal, though they exist for fossil plant boilers. The recirc flow is a safety feature. Without it you have much higher heat fluxes and faster degradation during transients. All that recirc water is a big heat sink and natural circulator during accidents. In fact if you have a once through boiler there are probably issues with excessive dryout on the hottest part of the fuel, unless you're using supercritical water.

The reactor inlet temperature for BWRs is quite high, as a relatively large amount of feedwater heating is used to improve the efficiency. Usually the feedwater is >210 degree Celsius, modern BWRs are trending higher. Carbon steel would be a bad idea. Go stainless.


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PostPosted: Aug 09, 2013 10:20 am 
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Oops, i am getting my terminology mixed up.

I mean mixed in with the feedwater to make the recirculation flow back into the reactor.
I was assuming that with a pressure tube design you could consider the mixing of the recirculation flow with the feedwater as the final stage of feedwater preheating rather than just a recirculation flow.

As to the material for the downcomer, why are we exposing the pressure boundary to the water conditions there?
Would it not be a reasonable idea to use the ceramic insulation concept that is used in the core itself?
That way the structural elements barely change temperature between cold shut down and rated steam power. Without the neutronics issues we could use whatever ceramic was cheap and use the cheapest material for the structural strength we need at the moderator temperature (~80C).

Although a stainless steel downcomer might just be cheaper.


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PostPosted: Aug 09, 2013 11:49 am 
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Quote:
I mean mixed in with the feedwater to make the recirculation flow back into the reactor.


Ok, yes that's how BWRs add feedwater. It just gets mixed up with the recirc water coming out of the top of the core and from the drains of the seperators. The boiling pressure channels would do the same, except you also use pressure channels as downcomers rather than vessel annuli. Otherwise it is the same as BWR.

Quote:
As to the material for the downcomer, why are we exposing the pressure boundary to the water conditions there?
Would it not be a reasonable idea to use the ceramic insulation concept that is used in the core itself?
That way the structural elements barely change temperature between cold shut down and rated steam power. Without the neutronics issues we could use whatever ceramic was cheap and use the cheapest material for the structural strength we need at the moderator temperature (~80C).


Absolutely, I've previously suggested this for supercritical water: all of the SC water piping and vessels could be internally insulated with ceramic. And if you extend the idea by submerging everything in a big vault of water, then you have the entire primary loop at low temperatures, no more thermal transients on your pressure boundary, no more thermal creep, use of low cost high strength carbon steel, it's wonderful. It's also a passive core catcher that makes primary loop failure implausible.

It's not so important with BWR conditions (steam below 300C). Carbon steel is almost as strong at 300C as it is at 80C. A liner is needed anyway to protect the insulation, if internal, so you're not saving a lot of money here.

I'm very happy that you have similar enthusiasm about internal insulation. I've also suggested it for fluoride environments: some nifty carbon based internal insulation materials are commercially available.


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PostPosted: Aug 09, 2013 1:28 pm 
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I do like the idea of burying the entire thing in a vault full of water.
Internal insulation might not be the best idea for the down-comers then... but this general idea is amazing.
It makes me wonder why people haven't used it before.

I assume BWR recirculation pumps are generally comparable to PWR ones?
I wish we could up with some mechanism to eliminate the need for huge pump casings which are pretty much the last large forging left in the design.


Also a neutronics thing: I assume that although the pressure tubes in the core are very close together, there is no reason that the moderator tank cannot be effectively infinitely sized without affecting the handling the core?
Having the moderator tank be the buffer tank prevents having to have a buffer tank wall that would be exposed to neutron flux as the water will shield it.

But that would mean the buffer tank would have to be at ~80C which might make engineering more difficult.
If the design was simple enough though we could potentially make the buffer tank large enough to swallow all the steam and hot water in the primary circuit without boiling off explosively, then we can dispose of the pressure hardened containment.
Allow the buffer fluid to boil off slowly thanks to decay heat.


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PostPosted: Aug 09, 2013 2:24 pm 
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E Ireland wrote:
I assume BWR recirculation pumps are generally comparable to PWR ones?
I wish we could up with some mechanism to eliminate the need for huge pump casings which are pretty much the last large forging left in the design.


They are very similar, except the BWR pumps are much lighter. BWRs have less total pump power and use more pumps. So the pumps are not as big as PWR pumps. That means quite small casings. Definately less than 1 meter diameter. Maybe half a meter.

Quote:
Also a neutronics thing: I assume that although the pressure tubes in the core are very close together, there is no reason that the moderator tank cannot be effectively infinitely sized without affecting the handling the core?
Having the moderator tank be the buffer tank prevents having to have a buffer tank wall that would be exposed to neutron flux as the water will shield it.


Yes, this is the idea I had some time ago. Tight pressure tube pitch but a huge pool of water around it. Heat simply conducts through the tubes in an emergency.

Quote:
But that would mean the buffer tank would have to be at ~80C which might make engineering more difficult.
If the design was simple enough though we could potentially make the buffer tank large enough to swallow all the steam and hot water in the primary circuit without boiling off explosively, then we can dispose of the pressure hardened containment.
Allow the buffer fluid to boil off slowly thanks to decay heat.


80C is not a problem, even ordinary concrete can take this. Though it is also possible to operate below 40 degree C, at a small cost in thermodynamic efficiency (as you can't use the water for the power cycle preheating anymore).

One advantage with a fully submerged primary loop is inherent pressure suppression: there is no longer a need for pressurized containment. Not even a drywell/wetwell structure, so is simpler than BWR containments.


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