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PostPosted: Jan 26, 2014 11:38 pm 
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Well, as we know the capital cost of a nuclear power plant is the killer, so any way we can reduce the construction cost of a reactor is a boon.

It seems to me that pressure tube reactors are onto something interesting, but that the only two practical moderators available (heavy water and graphite) both have significant issues. Heavy water is ridiculously expensive and produces huge tritium containment problems, and Graphite is a refractory solid that must be run at more than roughly 500K, preventing thermal quenching and generating a positive void coefficient - this being the primary design flaw of the RBMK.

So I ask, is there any way to get a primarily graphite moderated pressure tube reactor core that does not have a positive void coefficient?

This could drastically reduce the cost of constructing such reactors.


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PostPosted: Jan 27, 2014 1:35 am 
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If the coolant is less absorptive than the graphite and the lattice is tight enough then it might just work.

Supercritical CO2 perhaps, or Pb-208.


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PostPosted: Jan 27, 2014 2:17 am 
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Such coolants are likely to hugely escalate the capital cost of the reactor though, for reasons that have previously been stated.

How about boiling heavy water?

2H's absorption cross section in the thermal spectrum is apparently 650x lower than 1H, which would presumably make oxygen a significant absorber of neutrons in the coolant, but its cross section is tiny.
Would that not reduce the void coefficient?

Although this would require a direct cycle turbine running on heavy water, could we not attach a tritium/deuterium trap to the condensor vacuum system and recapture them that way?
Do CANDUs reactor produce most of their tritium in the heat transfer system or the calandria system?

EDIT:

It appears there is no direct cycle boiling heavy water reactor in service anywhere, which leads me to believe that a boiling heavy water coolant would require a steam generator which might be expensive. PCSG might be the most cost effective in that scenario though, and it would eliminate the RBMKs instabilities due to hot water being returned from the steam seperators at low output.


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PostPosted: Jan 27, 2014 4:11 am 
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If you want to use light water then the easiest way is to go for tighter fuel channel pitch (undermoderated) and slightly enriched fuel.

Boiling may not be attractive as it reduces the power output per channel (because of dryout/boiling stability issues that run into thermal limits of the cladding).

Supercritical water is likely attractive. Could be used a direct cycle or indirect cycle with PCHE SGs.


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PostPosted: Jan 27, 2014 4:27 am 
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I am not sure limiting channel power is actually much of a problem.
The size of the core is almost not a factor as even the boiling water cooled RBMK-1500 only has roughly 1700t of core graphite.
While that sounds like a lot Nuclear grade Graphite is only ~$17000/t which means that the entire inventory is only ~$28.9m. As Graphite is moulded during production and is entirely nonradioactive and nontoxic the cost of the completed graphite mass will not be much more than that.

That amounts to only ~$20/kW which is nothing.

While the zirconium pressure tubes are considerably more expensive (I would imagine I cannot imagine them being incredibly expensive), and we must weigh the disadvantages of a pressurised primary circuit in terms of drastically increased pump power with specialised v. high pressure pumps.

If we were to adopt a boiling HW coolant we could dispense with the steam drums found on designs like the RBMK and feed the steam-water mixture from the tubes directly into a PCSG. (Well I think we could anyway).

That should keep the heavy water inventory in such a design under control.


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PostPosted: Jan 27, 2014 5:10 am 
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Its not so much about cost as it is about revenue. Boiling water is about half the power per channel as pressurized water flow. Double the power per channel results in double the revenue for a given physical size reactor. That's a big economic factor. Similarly a 50% increase in thermal to electric efficiency gets you a 50% higher revenue. Pressure tubes are well suited for high pressure supercritical fluids, because of the small thickness needed to contain high pressure, and because the tight fuel element spacing results in high pressure drop that is minimized with supercritical water (very low massflow).

Some previous work on supercritical CANDU has shown that quite high power can be generated with natural circulation supercritical water in indirect cycle.


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PostPosted: Jan 27, 2014 5:21 am 
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If our core is relatively cheap to build, the physical size of the reactor becomes unimportant compared to its cost.
(It doesn't matter if the core is twice the size it could potentially be if it doesn't cost more to build)

SCWR opens up a whole range of potentially very challenging materials challenges with regards to the fuel cladding and plant (like the pumps and the steam generators if it is not direct cycle).

Can we even build a PCSG that can handle supercritical water on potentially both sides of it?
How much would it cost.

If the reactor ends up at $5000/kW we have gained nothing.


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PostPosted: Jan 27, 2014 6:51 am 
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The cost of bigger reactors extend to more than just the primary coolant loop cost. The building, foundation ,containment cost is usually significant. Bigger primary loop inventory means bigger containment. Coal fired powerplants give a clue as to the competitiveness of higher steam conditions. No new coal build uses saturated steam despite higher materials cost/issues.

Annealed inconel 718 does very well in PCHE in supercritical water environment. Almost no corrosion at all. For cladding it is the biggest issue, but not with SiC cladding. The MIT tubular triplex SiC and the Russian SiC TRISO type fuel (without graphite matrix) are very promising. PWRs are likely to go into SiC cladding in the long run anyway so a zircaloy new reactor concept would not be enticing for the public. Funding it would be hard to sell politically.


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PostPosted: Jan 28, 2014 1:30 am 
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Nuclear fission started with graphite but it is going out of fashion. French have already given up. British Magnox is past and AGR is on its way. Russian RBMK got a bad name at Chernobyl and is also obsolescent. Indians considered carbon as moderator for AHWR but gave it up in favor of heavy water moderator and light water coolant.
The MSR's could best be used in fast spectrum without graphite or with BeO moderator.


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PostPosted: Jan 28, 2014 3:42 am 
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If we had a SiC clad fuel system available then it would be feasible I suppose.

There is already dozens of reactor years worth of experience with stainless steel clad in the AGR, albeit not with water.
Even so I imagine there is far more information about the irradiation behavior of stainless steel cladding than SiC cladding (which apparently has some issues regarding thermal conductivity under neutron flux to sort out).

Stainless steel clad oxide fuel would probably be best in the short term.

Do we have any figures on Supercritical Water - Supercritical Water PCSGs?
Or would we run with a high temperature subcritical turbine?


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PostPosted: Jan 28, 2014 4:03 am 
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The conductivity of SiC does degrade with fluence, but it isn't a concern with thin cladding, especially not with MFE fuel.

The performance of Inconel 718 in supercritical water is well known. Even SS316 will do at the cost of a slightly bigger HX.

Direct supercritical versus indirect is a tough tradeoff. The simplicity of a direct once through cycle is very attractive. Then again the indirect cycle has an easier job on the turbine (restricting radiation to a tiny PCHE in stead) and you get a passive natural circulation heat removal path, possibly even at full power (huge density difference without obstructing two phase flow losses). The direct cycle would need an isolation condenser or similar device to get that going.


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PostPosted: Jan 28, 2014 7:33 am 
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Also we can use heavy water relatively cheaply to produce a reactor with a near zero (comparable to CANDU probably) void coefficient.

There is one problem with a high power density core: Graphite lifetime.
We want to avoid the problems that many of the MSR concepts on this board have with regards to limited graphite lifetime, since attempting graphite replacement on the core is going to be impossible, since you would have to pull everything apart to get to it in the first place.

Although the power generation capacity of a large unit could be titanic.
Comparing the CANDU to the RBMK is difficult but it appears that thermal power can be drastically increased, and the increase in efficiency will raise thermal power still further.

What is the largest available supercritical steam turbine set?


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PostPosted: Jan 28, 2014 8:19 am 
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One of the advantages of a solid moderator like graphite is that you can omit the calandria vessel. So you can have just logs of graphite that can be pulled out for replacement if they swell too much. Probably the refuelling machine like a CANDU type reactor can push out moderator logs just as well as it can push out spent fuel bundles. The graphite is not in a pressurized circuit and not very radioactive (compared to spent fuel assemblies) so this should be easy.

max sc turbine size should be near 1000 MWe currently, but industry seems confident in 1400-1500 MWe sizes (based on SCWR work).


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PostPosted: Jan 28, 2014 12:28 pm 
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jagdish wrote:
... or with BeO moderator.
How about a BeF2 moderator? Could we effectively eliminate the graphite swelling issue by replacing the solid C log with a solid BeF2 core + thin graphite or CCComposite skin?
At that point, you have a phase change heat dump inside the core in the form of meltable BeF2.
Hmmm.

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PostPosted: Jan 28, 2014 12:35 pm 
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And just how expensive is Beryllium fluoride and its associated containment structures going to be?
You have to assemble the core in hazmat suits if you use such materials, which will put your capital cost through the roof.


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