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PostPosted: Aug 02, 2013 1:06 pm 
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Well I have been thinking about reducing the capital cost of power plants alot, and it occurs to me that excluding the price of heavy water CANDU is drastically cheaper than conventional reactors, what with the lack of huge forgings.

What would happen if you filled the Calandria of the CANDU reactor with light water?
Would the enrichment required to get reasonable burn-ups be much worse than in a conventional LWR?

I imagine you would have to shrink the Calandria and make the spectrum relatively hard to get around the higher capture cross sections of the light water.

You could further simplify the design by using boiling coolant.


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PostPosted: Aug 03, 2013 12:39 am 
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Light water would necessarily require higher enrichment.
While designing a new reactor, you could have thorium-RG Pu fuel. It will give you a higher burn up. Will suite UK just fine.


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PostPosted: Aug 03, 2013 7:44 am 
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The fuel choice is irrelevant at this point.

Merely interested in a way of pushing the capital cost of a reactor as low as possible.


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PostPosted: Aug 04, 2013 3:20 am 
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I was also interested in this topic, and we discussed this on the forum about a year ago:

viewtopic.php?f=55&t=3689

It appears that Canada went for heavy water because of the overwhelming concerns about getting enrichment. Whereas the US went for monolithic vessels because this is how the LWR was developed, as a compact vessel PWR for a submarine. Nuclear anything was difficult to develop in later years, so we ended up with an entrenched "path of least resistance" where almost all reactors are PWR, BWR, or CANDU.

So politics and history explain why there are no light water moderated, light water cooled pressure tube reactors.

Technically, it is a lot more interesting. It would be a lot more compact than heavy water calandria. In fact you could ditch the calandria altogether and just use an oversized shield tank, filled with pure light water. This is the moderator and thermal and radiation shield, just like a pool type research reactor.

The most difficult part is probably the engineering of a very tight lattice for the pressure tubes. Light water means much less moderator will be optimal, this means a more compact reactor but difficult to do with manifolds and such.

Unsurprisingly, the recent work on tighter lattice configurations is trending towards a shell-and-tubes arrangement, where a tubesheet and plenum replace the manifolds and headers. This is entirely reasonable: modern PWR steam generators are all built this way.

A final note, supercritical water works well with this arrangement. The small tube diameter means the tube thickness is low, despite the supercritical pressures.


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PostPosted: Aug 04, 2013 3:52 am 
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Cyril R wrote:
Technically, it is a lot more interesting. It would be a lot more compact than heavy water calandria. In fact you could ditch the calandria altogether and just use an oversized shield tank, filled with pure light water. This is the moderator and thermal and radiation shield, just like a pool type research reactor.


CANDU designers also didn't have the option to go for a single pressurized vessel. The diffusion length of heavy water is too large and to have a good reflector one needs a large radius of heavy water. The pressurized tubes, atmospheric vessel was thus a compromising result.

If you would like to do the same for light water, you could in fact heavily reduce the size of the calandria due to the low diffusion length of water. Yet, from a first qualitative analysis, the pressure tubes also have to be positioned much closer next to each other. Otherwise, one would lose to many neutrons and as such an even higher enrichment would be necessary.

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Liking All Nuclear Systems, But Looking At Them Through Dark And Critical Glasses.


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PostPosted: Aug 04, 2013 8:57 am 
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Quote:
CANDU designers also didn't have the option to go for a single pressurized vessel. The diffusion length of heavy water is too large and to have a good reflector one needs a large radius of heavy water. The pressurized tubes, atmospheric vessel was thus a compromising result.


True, though this problem could also be solved with enrichment - it would be a more epithermal spectrum, with a higher fissile loading, but it would work. Of course there's the enrichment need again, which seemed always paramount to Canada in the early days of its nuclear programme. Today's offerings of CANDU type reactors include light water coolant with a tigher lattice pitch, such as ACR1000, as enrichment isn't such a big deal anymore with a high capacity of efficient centrifuge enrichment available on the market. In fact, from a fuel efficiency viewpoint.

I wouldn't call pressure tubes a compromising result. They have many economics, engineering and safety advantages over monolithic vessels. As just one example, there is an inherent and small limit to the break size with pressure tubes. Another advantage is that tubes are more amenable to mass manufacturing, avoiding the bottleneck of large forgings capacity. Quality control and assurance is also much easier, with thinwalled components. In fact I imagine the entire pressure tube arrangement could be completely welded in place by one or two robots.

Coal plants are also pressure tube type powerplants, in a sense. Large pressure vessels are only used where really necessary - such as the turbines which have a certain, relatively large diameter. Where that requirement does not exist, what we see in industry is that long slender pressure tubes or vessels are preferred.

Quote:
If you would like to do the same for light water, you could in fact heavily reduce the size of the calandria due to the low diffusion length of water. Yet, from a first qualitative analysis, the pressure tubes also have to be positioned much closer next to each other. Otherwise, one would lose to many neutrons and as such an even higher enrichment would be necessary.


That's what I said. A tubesheet approach avoids the engineering (fitting) problems of the manifolds/headers, while retaining the advantages of pressure tubes, except for a smaller "pressure vessel" plenum. Of course, it likely means no online refuelling, not a big deal really (PWRs do just as well in availability as CANDUs) and it simplifies the refuelling systems a lot.


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PostPosted: Aug 04, 2013 9:59 am 
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I found a proposal to use a water-logged yttrium stabilised zirconia insulator to protect the pressure tube of a supercritical CANDU from the coolant and allow the tube to work at the temperature of the moderator which would be only ~80C.
That allowed supercritical channels with wall thicknesses of 6.5mm using Zirconium alloys that would never be feasible if directly exposed to said coolant. (It appears you talk about this in the early thread... sorry).

Either way, what is the spacing between fuel elements in a BWR core? I assume it is much smaller than in a CANDU reactor?

If you dial the pressure down to BWR levels then you can reduce the thickness still further.
At present prices of reactor grade zirconium it might be worthwhile.

Also, has anyone got a recent figure on the price of heavy water? I find estimates of ~$300/kg for reactor grade product but that sounds a little low since the moderator tank in an ACR-1000 would only cost ~$57.5m, which is only ~$55/kW.


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PostPosted: Aug 05, 2013 2:32 am 
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Yes, the option to operate a cold pressure boundary is a major advantage.

BWR fuel assemblies are usually on a tight pitch. Maybe only 15-20 mm in between assemblies.

http://www.freepatentsonline.com/6925138-0-large.jpg

So a pressure tube reactor with light water as moderator would use a roughly similar pitch.

Re the cost of heavy water, with a big industrial demand from CANDUs the cost has been reduced a lot, with earlier estimates of $1000-2000/kg, now usually around $300/kg. But keep in mind there are lots of other costs, such as tritium control, including much more expensive HVAC, detritiation, etc. The capital and operating costs of these systems could easily exceed the cost of the initial heavy water inventory. Tritium in CANDU amounts is also a big PR issue.

Without a heavy water inventory cost and tritium issue, you can simplify a lot of things, with the entire reactor just being in a huge below grade water silo. Good for boiloff and blowdown without a pressurized containment (inherent pressure suppression from submerged steamlines).


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PostPosted: Aug 05, 2013 3:06 pm 
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I thought the primary reason the tritium was causing such a problem in CANDUs is because they are constantly cycling heavy water through the reactor and then through steam generators that might have numerous holes.
If we just kept the heavy water in a giant calandria and it never goes anywhere that might reduce the problems surely, keeping it under an oxygenated atmosphere should prevent free tritium so can't it just keep it in there the entire reactor life?

Either way, I am going to study the Steam Generating Heavy Water Reactor and the Gentilly-I concept to see if I can determine how they managed on load refuelling.

Also, would it be possible to rig it so if the reactor suffers a LOCA but for some reason SCRAM fails, htat the insulation would degrade to an extent that the full reactor power would disperse through the pressure tube before the fuel element suffers serious damage?

Then we could rig the moderator to boil in such a scenario (albeit still at room pressure) which should prevent a runaway meltdown.
Also we could remove heat through the top of the Calandria using some light water filled tubes.


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PostPosted: Aug 05, 2013 3:36 pm 
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Yes, just heavy water in the calandria and light water in the coolant circuit helps a lot. It's clearly a good compromise - ACR1000 and the AHWR are going for this. AHWR is most similar to the idea that you're proposing, heavy water calandria, boiling light water coolant, and passive calandria water cooling system.

With enrichment, plus boiling or supercritical water, online refuelling isn't so attractive anymore. Needs a lot of R&D and money to test rigs. Offline refuelling can be done quickly these days, and with enrichment you can vary the enrichment (zoning the core) plus have more burnable poison for longer operating times between refuellings. So I think an ideal arrangement is a vertical, offline refuelling, with poisoned enriched fuel, heavy water calandria is probably still attractive for neutron economy. There's no doubt though that light water calandria would be much cheaper, you could even avoid the calandria, shields, cooling systems altogether (looks somewhat expensive) and have just a pool of light water with one cooling system. So if you're really focused on capital cost reduction and simplification, light water moderator is still attractive.

The pressure tubes are small and with boiling water you'll have lower element ratings. This means you can just let things heat up a bit in an emergency, without large fuel failure, by cooling through to the calandria water.

The simplest system for emergency cooling is probably an oversized water pool vault where the calandria sits in, which could itself be a calandria with tubes extending light water into the calandria heavy water without ever mixing the two fluids. This also avoids a seperate heat exchanger for the calandria heavy water. Just need a normal light water heat exchanger for the light water vault, with heatup and boiloff of light water to atmosphere in a pinch.


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PostPosted: Aug 05, 2013 4:15 pm 
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Apparently many of these boiling light water cooled heavy water moderator reactors have problems with starting up if the channels are full of light water which isn't boiling because it drastically reduces reactivity.

Would it be possible to have a "start up condenser" which would be used to sink steam during the reactor heat up phase, allowing you to bubble helium through the pressure tubes to displace light water coolant as if it was boiling?
The helium could be recovered from the condenser off gas and reused.... or you could just use argon and vent it to atmosphere.


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PostPosted: Aug 05, 2013 4:19 pm 
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E Ireland wrote:
I thought the primary reason the tritium was causing such a problem in CANDUs is because they are constantly cycling heavy water through the reactor and then through steam generators that might have numerous holes.

No.
In general there is no transfer between the PHT and secondary (SG/turbine/condenser) systems: Very little tritium gets out of the containment building, by way of the steam lines to the turbine building.
Tritium leaks (DTO actually) generally occur through the myriad seals on pumps, instrument ports, sampling lines, etc., etc. (the Candu system is very complicated, having many more systems than an LWR).
Of course the worst part is when you have to open the system somewhere for maintenance (even with PHT pressure reduced to ambient, during a maintenance shutdown).

Keep in mind though, that tritium buildup to problematic levels takes a couple of decades of operation: The neutron absorption x-section of deuterium is very small....

Recently, small skid-mounted tritium extraction systems have been developed, which could take care of the problem on a continuous basis.

E Ireland wrote:
If we just kept the heavy water in a giant calandria and it never goes anywhere that might reduce the problems surely, keeping it under an oxygenated atmosphere should prevent free tritium so can't it just keep it in there the entire reactor life?

Radiation causes a fair bit of dissociation of water in the calandria: the cover gas system includes electric recombiners to convert free oxygen & hydrogen back to water.

Its not quite true that "it never goes anywhere" -- the moderator gets several megawatts of heating, which must be removed by heat transfer through heat exchangers, to keep it at a constant ~75C.
But it is a much simpler system than the PHT, and its at ~ambient pressure, so leaks are not nearly as much of a problem -- even though tritium levels tend to be higher in the moderator than PHT.


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PostPosted: Aug 06, 2013 2:23 am 
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Light water helps a lot with maintenance of a calandria or replacement of a pressure tube. Very little tritium.

It also simplifies the systems a lot, especially with offline refuelling. Eliminate the end shields, plus cooling systems and chem cleanup, refuelling machine cooling, plus all other auxilliary gear for that, etc.

What appears to make CANDUs more complicated is the need for many seperate closed loops: calandria, end shields, vault water, primary coolant loop, secondary loop (power cycle), even the pressure tubes have closed gas in them for leak detection. Each of these systems require their own cooling systems, instruments, tritium control, leak detection, removal of corrosion products... this gets very complicated.

With light water, potentially all you need is the light water vault. No more calandria, end shields, refuelling machine cooling systems, tritium control systems and instruments. Boiling or SC water eliminates the steam generator.


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PostPosted: Aug 06, 2013 5:14 am 
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http://dae.nic.in/writereaddata/.pdf_37
AHWR, basically designed to use thorium as fertile fue with long burn upl, also covers most of the points brought up in the discussion. It has
1. A heavy water tank with moderator cooled rather than a heavy pressure vessel.
2. Boiling light water as coolant to save part of heavy water and one heat exchange.
3. Fuel in shorter vertical pressure tubes.
4. An overhead tank for emergency cooling.
The development has proceeded to a critical assembly pending availability of sufficient fissile feed. An alternative design using 19.5%LEU as fissile feed has also been prepared.
http://dae.nic.in/writereaddata/.pdf_31


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PostPosted: Aug 06, 2013 8:20 am 
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Yes, AHWR is most similar to what was described here. It has a rather large containment though for such a small reactor, which should cost more money. There's also a lot of long piping and elevated water reservoirs that seem to have some potential for common mode seismic failure. It's not exactly a simple system either.


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