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PostPosted: Aug 17, 2010 12:35 pm 
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NOTE ADDED a week later. Nuscale's core does indeed seem to be the same power density of regular PWRs. Now the big mystery is how they can possibly get natural circulation to run at full power with only about 1 psi of driving force.

I am currently at a conference and the talk just after mine was a great one from Nuscale with their small 45 MWe modular PWR design that uses natural circulation. I really like the design in general but have always had a big question that I can't seem to get an answer (or at least one I believe yet without proof).

The design uses standard PWR fuel elements but only 6 feet tall instead of 12. Anyhow, with a PWR you only have a tiny thermosyphoning effect of hotter water being a little less dense than the colder return water (unlike a BWR with its phase change). I am quite certain in a normal PWR it takes a majority of the massive pumping power to push water through the core (the rest is in the steam generators). Thus if they are using natural circulation with only a modest chimney height and the same PWR fuel then my gut feeling says they must be going to much lower power density than a PWR. If so, and its the same fuel (4.95% enrichment) then they must need a lot more fuel and thus a much larger startup fissile inventory. Yes, the core is only half the height but if they claim the same power density then it must be twice the cross sectional area (yes I know its only 45 MWe but I'm talking scaled to match).

Having a large fissile starting load is no big issue for them if they are just trying to build on a small scale but this would really limit a large expansion and if the price of Uranium goes back up, their start charges could start pushing a billion$ plus per GWe. Anyhow, I emailed them with questions a couple years ago and wasn't too convinced of the answers. After the speakers great talk I asked again what sort of starting load they'd need and again he said about the same as a PWR (scaled of course). I just don't think I can believe that yet but I could of course be wrong. Anyone else have thoughts or seen technical data? Can you help with some questions?

What is the total pumping power in a PWR (say 1000 MWe), I think I recall about 8 MWe? Does anyone know the fraction of this power needed for pressure drop across the core? Does anyone know what this relates to in driving head across the core (given as feet of head or pressure drop in psi). Anybody care to figure out the actual pumping power they'd get from thermosyphoning (you just need to find the density difference between the inlet and outlet water and work in the chimney height from their tech figures). You typically get only a tiny bit of pumping power unless you have a phase change like a BWR.

If my gut feeling is right I'd guess they actually need a massive starting fissile load to start these things (which are otherwise a great little design). Could they be as high as sodium fast breeders? I.e. upwards of 20 tonnes U235 per GWe as compared to 5 tonnes current PWRs or 3 tonnes for older PWRs?

There website is http://www.nuscalepower.com

David LeBlanc


Last edited by David on Aug 24, 2010 3:12 pm, edited 1 time in total.

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PostPosted: Aug 17, 2010 1:17 pm 
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From the diagrams the reactor pressure vessel is 45' tall and it looks like from the bottom of the fuel to the top of the chimney is around 35'.
The fuel bundle is 21cm, the pressure vessel is 9' in diameter so it could hold around 36 bundles.

Wiki says there are 121-191 fuel assemblies in an PWR. Since the 17x17 is the largest assembly I'll use 121. Typical power for an PWR is 1.3GWe (?). Scaling we get
(1300/45) * (36/121) * (1/2 since 6' rather than 12' tall) or around 4.3 times as much specific inventory as a PWR.


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PostPosted: Aug 17, 2010 2:22 pm 
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Pumping power as a simplification goes about v^3, so it seems possible. This is usually how it works with overcoming speed, its nature telling us to take it easy :lol:


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PostPosted: Aug 18, 2010 12:20 am 
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Lars wrote:
From the diagrams the reactor pressure vessel is 45' tall and it looks like from the bottom of the fuel to the top of the chimney is around 35'.
The fuel bundle is 21cm, the pressure vessel is 9' in diameter so it could hold around 36 bundles.

Wiki says there are 121-191 fuel assemblies in an PWR. Since the 17x17 is the largest assembly I'll use 121. Typical power for an PWR is 1.3GWe (?). Scaling we get
(1300/45) * (36/121) * (1/2 since 6' rather than 12' tall) or around 4.3 times as much specific inventory as a PWR.


Thanks for digging out the ruler and calculator Lars. If those estimates are correct it does mean a whopping 20 tonnes U235 or more per GWe. That is even more than a standard fast breeder (which also doesn't need it all on day one, just over the first 2 or 3 core reloads). At today's cheap uranium price of about 100$/kg, U235 would cost about 35$per gram or adding 700 million to the price tag (scaled to a GWe). That won't rule out their concept (they quote a capital cost of just under 4 billion per GWe) but it certainly means any price increase, like back to the 2007 peak would seriously harm the concept.

Again, nobody sue me for libel, we are only guessing in the dark here.

David


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PostPosted: Aug 18, 2010 12:25 am 
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Cyril R wrote:
Pumping power as a simplification goes about v^3, so it seems possible. This is usually how it works with overcoming speed, its nature telling us to take it easy :lol:


Yes, I have no doubt they could get natural circulation to work, just I doubted the claim they can do so and still keep roughly the same power density as a PWR. Interesting to see how this plays out. I know companies have to be on the secretive side but I'd love to see a lot more published technical support of the concept.

David L.


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PostPosted: Aug 18, 2010 12:56 am 
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Maybe I'm dense but isn't the question of natural circulation set by the height of the chimney? Does it have anything to do with the size of the core?
The core looks to be in the first 6' of the chimney, the next 30' look pretty empty.
The HX is at the top of the return.
It seems like all the water above the heat source will be lighter than all the water below the HX.
Doesn't that difference dictate the force for circulation?


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PostPosted: Aug 18, 2010 2:24 am 
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Smaller reactor has without doubt poor neutron economy and poor fissile economy. Thorium use can mitigate it to a large extent. World's smallest fuctioning reactor is experimental KAMINI working on Th-U233 cycle.
Indian DAE put on display a brochure at IAEA for AHWR300-LEU with the declared intention of providing a proliferation-proof design. It's real merit lies in 40% energy from thorium and lower U235 consumption. Such fuel is the way to fissile fuel economy for all sizes including small ones. The brochure was for new entrants in nuclear energy but is useful for all cases where smaller reactors are required.


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PostPosted: Aug 18, 2010 7:41 am 
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Lars wrote:
Maybe I'm dense but isn't the question of natural circulation set by the height of the chimney? Does it have anything to do with the size of the core?
The core looks to be in the first 6' of the chimney, the next 30' look pretty empty.
The HX is at the top of the return.
It seems like all the water above the heat source will be lighter than all the water below the HX.
Doesn't that difference dictate the force for circulation?


Yes height of chimney along with density difference gives you the driving force. However, without a phase change (BWR) the effect is typically tiny. At standard power densities, PWR cores take a large amount of driving force, I'm guessing at least 50 psi pump pressure. Kirk and others ran into this issue with their grad design project a couple years back on a natural circulation MSR. Even with a big chimney height and a big temperature difference they were only ending up with a fraction of a psi driving force (if memory serves). They ended up putting a couple IHXs in parallel but to be frank I doubt those numbers worked either, it typically takes a hundred times more pump pressure to push liquid through a core and heat exchanger. I wonder why they didn't just make the concept a BWR in the first place? That can easily work on natural circulation as GE has already shown (in design, not production though).

ORNL looked briefly at a natural circulation design but if I recall that was a simple tank of salt core (almost no frictional drag) a big chimney and then super oversized low drag heat exchangers so they ended up with a majority of the salt out of core (bad for control).

Jagdish is also right that smaller cores also have worse fuel economy since they tend to lose a lot more neutrons to leakage. My concern though was more in the starting load and my guess that it might be huge in this design.

David L.


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PostPosted: Aug 18, 2010 8:26 am 
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I also don't think molten salt natural convection will work out for molten salts. The figures of merit are terrible compared to water. Water is much better for natural circulation regimes, whether tuburlent or laminar, compared to any molten salt.


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PostPosted: Aug 18, 2010 9:51 pm 
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Wow, I think it is even worse than Lars' calculation indicated. A PWR has a power density of about 100 MW/m3. This relates to about a core volume of 28.6 m3 per GWe. Older PWRs with lower enrichment needed about 3 to 3.5 tonnes per GWe and newer ones with upwards of 5% enrichment is now about 5 t/GWe. Anyhow, the Nuscale core certainly seems to be 6 feet high and about 9 feet wide for 45 MWe. Those dimensions give about 10.7 m3 of volume which scales to 238 m3/GWe which is over 8 times that of a PWR! Since Nuscale uses higher enrichment like new reactors that means they could be using 8x5t/GWe or 40 tonnes of U235 per GWe! Even at cheap uranium of 100$/kg and 35$/g U235 this would add 1.4 billion$ per GWe (they quote a capital of 3.9 billion/GWe which never includes first core costs). At 300$ uranium U235 is about 80$per gram or a first core cost of over 3 billion! The other bizarre thing is if they do have 1/8th the power density to get their natural circulation that means the fuel will burn up 8 times slower. How long are current fuel elements left, 3 years? 5 years? In their core this would mean 24 to 40 years! Sure the burnup is the same but can you be sure cladding will last that long in the core environment. Again these are just guesses in the dark (so no law suits Nuscale) but when the CEO tells me directly that they have about the same starting fissile load as a PWR this just does not make sense.

In terms of pumping power, if they have the same inlet and outlet of a PWR with about 40 degrees temp difference and 30 feet of chimney this gives them only 0.9 psi of driving head. If they go much colder, say 100C temp difference they get 1.9 psi. 1/8th the power density and really slow moving water is probably needed if this is their pump power.

I can't seem to dig up the driving head of a standard PWR and how much is core and how much is in the steam generators but 150 psi rings a bell and more in the core than out (help please). The only number I could find in my data sheets is an old CANDU design that needed 188 psi of operating head.

Curiouser and curiouser.

David L.


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PostPosted: Aug 19, 2010 7:05 am 
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For standard PWRs this page gives the specs from a manutacturer of main coolant pumps. 60 - 135 m head (85 - 192 PSI), 6,000 - 24,000 m3/hr, 0.65 - 13 Mw input power


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PostPosted: Aug 23, 2010 9:16 am 
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Found an article with some relevant data - for French 3-loop PWR

Nuclear Engineering and Design, Volume 211, Issues 2-3, February 2002, Pages 189-228. Paywalled unfortunately, but got it at work via my employer's Science Direct subscription. The article is an analysis of a large break LOCA, but they give the pre-accident nominal conditions.

Code:
Zone              Pressure drop / MPa       Pressure drop / PSI
------------------------------------------------------------------
Hot Leg                 0.026                     3.8
Steam Generator         0.232                    33.6
U leg                   0.027                     3.9
Cold Leg                0.027                     3.9
Reactor inlet nozzle    0.049                     7.1
Downcomer               0.002                     0.3
Core support plate      0.04                      5.8
Core                    0.13                     18.8
Reactor outlet nozzle   0.012                     1.7
------------------------------------------------------------------
Total                   0.545                    78.9

So more in the steam generators than anywhere else, but I don't see how this is supposed to work without a pump. Decay heat, sure. Full power???


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PostPosted: Aug 23, 2010 10:57 pm 
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I just saw this thread, sorry I have been very busy. I can give you some representative numbers off the top of my head, more exact numbers when I get a chance to get to work and review some design documents.

David wrote:
... Anyone else have thoughts or seen technical data? Can you help with some questions?
What is the total pumping power in a PWR (say 1000 MWe), I think I recall about 8 MWe?

1000MWe is about a 3300MWth core and would need about 5-6MWe per RCP. A 4 loop plant would be 20-24MW of pumping power total.

David wrote:
Does anyone know the fraction of this power needed for pressure drop across the core? Does anyone know what this relates to in driving head across the core (given as feet of head or pressure drop in psi).

~60ft head moving 400E3 gpm @ 550F/2300psi. Temperature rise across the core is about 60F.

David wrote:
Anybody care to figure out the actual pumping power they'd get from thermosyphoning (you just need to find the density difference between the inlet and outlet water and work in the chimney height from their tech figures). You typically get only a tiny bit of pumping power unless you have a phase change like a BWR.

It's about 4-5% of the normal flow with the same 60F temperature rise.

Remember these numbers are from my sometimes fuzzy memory, I can look this up tomorrow and post the actual numbers from an operating PWR. Assuming I can find the time.

David you may also find this paper interesting: http://www-pub.iaea.org/MTCD/publicatio ... 74_web.pdf


Last edited by USPWR_SRO on Aug 25, 2010 12:15 am, edited 1 time in total.

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PostPosted: Aug 24, 2010 3:08 pm 
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Thanks so much USPWR_SRO and Luke. The link has a wealth of information. Nuscale appears to be based on MASLWR designed around 2002. Our guesses on the power density and core size are indeed all wrong. The active core is actually tiny in the MASLWR at least. The active core is only about a meter tall and meter wide which gives the same power density of around 100 kw/liter of regular PWRs. The original design called for 8% enrichment and a 5 year, once through cycle (I think they just looked to change out the whole core).

A new PWR core typically has a lot lower enrichment than for reloads so if their core was 8% and the same power density as a PWR then they might need triple the fissile load of a PWR (not 8 times like we were guessing). In the MASLWR design they only had a inlet to outlet core temp difference of about 40 C which seems to only give about 1 psi driving force. I know the core is shorter but I still can't see how this can possibly work on natural circulation. However, I guess I have to trust the guys that designed it since it was a large group from INL and Oregon State.

Strange...

David LeBlanc


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PostPosted: Sep 07, 2010 8:52 am 
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Pretty good approximations, Gentlemen.

There are a few things we have to keep as a trade secret, however.

I have two questions what would a Thorium Core look like. Triangular pitch?, p/d ratio, first core mixture of UO2 and ThO2.

Also if anyone is interested see our website for careers.

Thanks... Brent


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