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PostPosted: Feb 19, 2012 11:41 am 
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There is a new design proposed by Sandia National Laboratory researchers: the Supercritical CO2 Direct-cycle Gas Fast Reactor. This seems interesting because GCFR's are usually cooled by helium. It is also interesting because of the super-efficient supercritical CO2 power conversion system it uses.

The document in which the concept is described, can be found at:

http://prod.sandia.gov/techlib/access-c ... 112525.pdf


Last edited by camiel on Feb 26, 2012 4:07 pm, edited 1 time in total.

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PostPosted: Feb 20, 2012 6:14 am 
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Nice, thanks. Kind of surprising that this concept is "new". Probably due to the fact that the S-CO2 power cycle itself is new.

They can probably use the newly developed triplex SiC cladding from MIT.

The higher density and density changes of CO2 allow excellent natural circulation. However, the primary loop is still high pressure and LOCA/dryout after accidents remains a serious issue for this type of reactor. I still prefer a lead or fluoride coolant. But any work that helps to develop an S-CO2 power cycle is most welcome.


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PostPosted: Feb 22, 2012 8:04 am 
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Thanks for the link, Sandia are doing some very interesting work on this. The small scale (and so power density) I still find astonishing
Sandia wrote:
...{100 Mw(e)} power generating turbine unit operating at 60 Hz, the turbine wheel is estimated to be 1.05 m (41.3 inches) in diameter with a blade height of about 5 cm (2 inches).....
and if they allowed a gearbox, as is used on gas turbines to better match the natural turbine rpm to the grid, even smaller diameters would be possible. However, it would look less 'new' if their references list had included Dostal et al, which covers much of the same ground on the power-cycle side, thought without looking at the neutronics. They do reference several other MIT reports, some with the same co-authors, but Dostal's thesis is the only one that isn't paywalled.


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PostPosted: Feb 22, 2012 10:42 am 
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Small yes - too small I'm afraid. Small fast reactors leak neutrons like a sieve, increasing the startup fissile load. This reactor needs about 24 tonnes fissile to get started. It would cost $1500/kWe just for the first core. Not prohibitive but hefty.

Looks like they have a small positive void coefficient as well. Perhaps using neutronic protection, behind the nickel reflector, resolves the issue (while protecting the reactor wall as a bonus).


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PostPosted: Feb 22, 2012 4:55 pm 
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From an efficiency viewpoint, a direct cycle, combining a supercritical CO2 cooled reactor with a supercritical CO2 turbine seems very attractive. This technology is still in its infancy though: Sandia is working with its partner, turbomachinery manufacturer Barber-Nichols Inc., on a prototype S-CO2 turbine and its power output is still very low: 240 KWe, although there are plans to develop a 10 MWe version.


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PostPosted: May 20, 2013 6:21 am 
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I have been re-reading the SC-GFR paper (see attachment) and the following statement in the paper (page 10) caught my eye: "One key advantage of the S-CO2 direct cycle over a helium Brayton cycle is the capability to develop natural convection flow through the reactor and power conversion flow loop."

Why is a natural convection flow not possible in a helium Brayton cycle ? It is an important safety feature with regard to decay heat removal.


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112525.pdf [1.74 MiB]
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PostPosted: May 20, 2013 8:01 am 
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Check out the density differences around the SC-CO2 cycle.

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PostPosted: Nov 30, 2013 12:43 am 
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The supercritical CO2 direct cycle fast reactor is attractively simple and compact, without some of the issues of sodium cooling, but would have problems with decay heat removal after a loss of coolant accident. Would it be possible to surround the core with an annulus full of molten salt, which passively flooded the fuel rods if pressure was lost? Instead of having a low neutron cross section, you would choose a salt with high absorption, to simultaneously cool the fuel and quench fission. The only other requirements would be low corrosion and the right liquid temperature range. It would never need to be pumped or go through the heat exchangers. For normal shut down, once the core was cool enough, the salt could be drained out the top of the pressure vessel, and never even touch the fuel rods except in an emergency.


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PostPosted: Nov 30, 2013 5:01 am 
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jon wrote:
The supercritical CO2 direct cycle fast reactor is attractively simple and compact, without some of the issues of sodium cooling, but would have problems with decay heat removal after a loss of coolant accident. Would it be possible to surround the core with an annulus full of molten salt, which passively flooded the fuel rods if pressure was lost? Instead of having a low neutron cross section, you would choose a salt with high absorption, to simultaneously cool the fuel and quench fission. The only other requirements would be low corrosion and the right liquid temperature range. It would never need to be pumped or go through the heat exchangers. For normal shut down, once the core was cool enough, the salt could be drained out the top of the pressure vessel, and never even touch the fuel rods except in an emergency.


It could certainly be done, just two questions right off the bat:

1. If you're going to bother with salt like this then why not use it as coolant in the first place? Certain materials and freezing problems occur with salt. If you can tackle those you can cool with salt directly (especially with SiC fuel rods).
2. The CO2 is under extreme pressure. The salt is at low pressure. How do you get the salt in the reactor quickly and efficiently? Even if you have a big pipe/vessel break, the CO2 pressure would remain somewhat high from decay heating and you'd only have an inefficient "chugging" type flow of bits of salt into the vessel. You'd need flooding devices like valves but that is not passive safety.


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PostPosted: Nov 30, 2013 6:30 am 
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S CO2 as coolant goes straight to the turbine, whereas with salt as the coolant you need, with your design, secondary and tertiary salts and then the working fluid for the turbine, along with a series of heat exchangers and pumps. A major pipe break, dropping from 20 Megapascals to atmospheric pressure, would dump the emergency coolant salt into the core straight away, but the pressure drop would also rapidly cool the core. If the pressure release was slower, decompression cooling would also be happening, along with natural circulation in the CO2. As long as the salt was released before there was any possibility of it freezing, the fuel should stay below damage temperatures. Hydrostatic pressure at the bottom of the salt would be low, but if you had something like an upside-down manhole cover there you could make it heavy enough to dump the salt at whatever pressure drop you chose.
(Haven't quite figured out how to get the salt up there in the first place though :roll:)


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PostPosted: Nov 30, 2013 8:22 am 
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I still have doubts about injecting the salt against the backpressure of CO2. If you have one outlet (the break) then you will not get a meaningful flow other than minor chugging of salt. You'll want a flooding mechanism and that means some kind of valve to open at the bottom and a tall SCO2 reactor vessel with some relief valve at the top that discharges CO2 into the containment.

I'd be more interested in a high pressure, tight fitting containment with this system. This way a pipe break keeps enough pressure in the reactor for natural circulation cooling.

If you have salt inside the vessel that means a total write-off. Very expensive decommissioning on your hands.

I'm not actually sure if natural circulation is that good with a direct cycle. It would need something like an ESBWR type vessel, meaning a huge vessel with mostly recirc flow (>70% of flow would be recirculated). At 20 MPa that's more costly than at the ESBWR's 7 MPa. Also pressurized CO2 has much poorer volumetric heat capacity than water (especially boiling water). Much larger delta T's would be needed, with the attendant thermal problems in a recirculating system.


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PostPosted: Nov 30, 2013 10:10 am 
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The British have been using advanced gas cooled reactors since 1976. They have not decided to continue with gas cooling.
http://en.wikipedia.org/wiki/Advanced_g ... ed_reactor
They have given up development of their own designs and are going for imports. A fast reactor would not have graphite but I guess that heat extraction would be a problem with gas cooling. They are going for PWR and considering sodium cooled PRISM fast reactors to use up their plutonium stocks. It gives a bad chit to CO2 cooling.
My wild idea about a fast reactor would be:-
A molten salt fuel. This can be periodically sparged for volatile fission product poisons but stay in the core till reprocessing.
A thorium container to also act as reflector/blanket. It should be changed and reprocessed before any significant fission.
An outer compartment with lead or a low cost salt with convectional flow for heat transfer.
A heat exchanger to extract heat from this secondary coolant. The last medium could change with turbine development to a gas if found feasible.


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PostPosted: Nov 30, 2013 11:54 am 
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The death of the gas cooled programme should not be taken as a comment on its viability. The reason AGR was not chosen for the Sizewell B was the large cost overruns caused by building every plant to a slightly different design, preventing any repeat construction savings from occuring.

Anything other than EPR never really had a chance when it was determined that the French state would be the body building the plants.


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PostPosted: Nov 30, 2013 11:57 am 
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The British CO2 reactors are very low power density cores with CO2 gas (not dense like near supercritical fluid). They have to run with giant steam generators heated by the gas, a poor coolant, hardly an optimal arrangement economically.

It's very different with supercritical CO2 and direct cycle. Everything is simplified and much more compact, and the thermal to electric efficiency increases greatly.

At the moment the SCO2 fast reactor is a paper reactor not very far in the design phase. No competition for EPR where that reactor is already being built and backed by multi billion dollar leader in the nuclear construction field.


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PostPosted: Dec 01, 2013 1:21 pm 
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Could the density on all that CO2 be tapped to moderate this concept into a thermal spectrum reactor? Then if there is a leak, the loss of moderator would shut it down quick.

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