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PostPosted: Feb 22, 2017 7:57 pm 
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Just dug up my old notes on this topic.

A key downside with LWRs and CANDUs is the low efficiency and the use of big, costly, wet steam turbine cycles. No one outside the nuclear (and possibly geothermal power) industry use these cycles, they are big and inefficient and special equipment such as moisture separator-reheaters are needed to deal with the relative wetness of the steam. It also limits the productivity of the reactor: for a given reactor power, less energy is generated, hence increasing reactor cost per unit electrical power.

Any means to increase power cycle efficiency is going to reap huge economic benefits. Molten salt reactors do it by having a high reactor temperature.

I wondered though why fossil fired superheat was never more popular with nuclear plants. Especially today, with the wealth of expertise and experience in combined cycle gas turbines, that need very large combustion gas heated steam generators and - heaters.

The idea is pretty simply - add a burner to the saturated steam coming out of LWRs or CANDUs.

Just take one example, the AP1000. Based on various sources here is some data:

Electrical power: 1150 MWe
Reactor power: 3400 MWth (bit higher when adding primary pump heat too, 3415 MW?)
Steam flow: 1889 kg/s
St. temp: 272.8 C
St. press: 5.76 MPa
Fw temp: 226.7 C
St. Enth.: 2787 kJ/kg
Efficiency 34%

Superheating with a combustor to a modern steam outlet temperature of, say 620C and with some pressure drop going down to 5.5 MPa would result in a steam enthalpy of 3710 kJ/kg.

Thus an enthalpy gain of 923 kJ/kg. (with my steam tables and data. Someone better check).

923 * 1889 kg/s = 1744 MJ/s = 1744 MWth

3415 + 1744 = 5159 MWth

So that burner has just added ~50% heat load!

Next the question is the efficiency. At a 620 C live steam this is going to be good, only downside is the pressure is on the lower end of superheated cycles so that will hurt a bit. But then this is a huge thermal output so the largest available most efficient machines can be used. If 45% efficiency is possible with some tricks and optimizations, basically the electrical output becomes 2322 MWe which is the output of TWO AP1000s.

One way to think of that is two AP1000s for the cost of one plus a burner. Oversimplifying here but you get the point.

With nuclear plants expensive and gas cheap, and tons of experience with CCGTs, GTs, etc. these days you'd think this idea would be more popular.

For the purists - yes, it's natural gas, but it's an efficient way of using it. When optimized it should be more efficient, in terms of gas to power, than state of the art CCGTs. From a nuclear viewpoint it may help deployment of more nuclear plants which will bring down nuclear costs in the longer term and gear up towards more nuclear powered grids.

Lots to like I would think... what am I missing?

(can someone check what the actual achievable efficiency is? I'm sure 45% is overly ambitious for such a low steam pressure. Also I'd like to know if reheat makes sense considering the pressure is already pretty low not sure if it can even be engineered...)


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PostPosted: Feb 23, 2017 6:59 pm 
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How would this compare to NACC-FIRES?

Using the reactor effectively as a preheater for working fluid is functionally the same for both.


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PostPosted: Feb 23, 2017 7:16 pm 
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Asteroza wrote:
How would this compare to NACC-FIRES?

Using the reactor effectively as a preheater for working fluid is functionally the same for both.


FIRES is a thermal energy storage system. And a really inefficient one. Using electricity to make heat to make electricity... fighting up to thermodynamics here. Never a good place to do battle.

This superheater concept is about making more power and improving the thermal efficiency of nuclear plant. Not doing the opposite by wasting it in heating up a pile of bricks.


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PostPosted: Feb 24, 2017 2:19 am 
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Cyril R wrote:
Asteroza wrote:
How would this compare to NACC-FIRES?

Using the reactor effectively as a preheater for working fluid is functionally the same for both.


FIRES is a thermal energy storage system. And a really inefficient one. Using electricity to make heat to make electricity... fighting up to thermodynamics here. Never a good place to do battle.

This superheater concept is about making more power and improving the thermal efficiency of nuclear plant. Not doing the opposite by wasting it in heating up a pile of bricks.


The criticism of FIRES standalone is not without merit, but my point here is that the CCGT in NACC-FIRES is using reactor heat (and FIRES heat when available) to upgrade/warm air prior to combustion (if any), spinning the gas turbine, and eventual exhaust use in a conventional CCGT steam recovery generator. The primary working fluid here is air.

Your superheat concept is effectively using combustion heat to upgrade a working fluid (nuclear steam). I guess one could say you are flipping the NACC the other way around (steam focused, using combustion to upgrade reactor heat, rather than reactor heat to upgrade combustion).

One could easily imagine your superheater being a HX block past a gas turbine combustor but before the turbine. You could run the combustor hotter as you are less worried about exceeding turbine inlet temperature limits as your HX/superheater block mellows out the air temps. You could push even farther by replacing the conventional combustor with a high temperature fuel cell. If one were really pushing beyond conventional gas turbines, there's that Allam cycle work with oxyfuel/CO2 turbine setups and an ASU that can run on waste heat.

Since the aim is to upgrade the steam, what will be done with the combustor exhaust after the gas turbine (where a conventional CCGT's HRSG is located)? Use for steam reheat or feedwater heating?

I suppose it's a bit unfair to compare against NACC-FIRES since that assumes a higher temperature gen4 reactor, when you are proposing something that is much more accessible now and simpler to implement that works with existing designs.

To check, is the superheater concept's primary assumption the various costs of adding a second reactor/unit versus using fairly conventional but high performance steam hardware and operational issues of needing natural gas fuel? How well does that assumption hold in the face of potential licensing allowing (SMR) multi-unit plants to be licensed/operated as one whole? It does seem somewhat predicated on the continuing production of large reactors (LWR, CANDU, other gen3+) which is not unreasonable, but will that hold?

There's the secondary issue of the natural gas itself. I suppose since many reactors are coastal, it's not unreasonable to colocate with a LNG terminal/transhipment point for the barges/tankers...


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PostPosted: Feb 24, 2017 12:41 pm 
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I think this concept is basically applicable to LWR SMRs.

Take NuScale as example. Assuming:

540 MWe
1920 MWth
71.3 kg/s steam flow/unit *12 =855.6 kg/s
St. Pr. 3.1 MPa

This gives me 2803 kJ/kg steam enthalpy.

Assuming 2.9 MPa after the superheater, 620C, gives 3729 kJ/kg.

3729-2803 = 926 kJ/kg added enthalpy by the superheater

926 * 71.3 * 12 = 792 MJ/s = 792 MW

792 + 1920 = 2712 MWth

Assume 40% efficiency. 2712 * 0.4 = 1085 MWe

More than a doubling of power output. So same story. One 24 pack nuscale output for the price of a 12 pack plus a burner. (ok I admit they need to redesign the steam turbine generator).

Interestingly it seems even better for NuScale because of the poor live steam conditions they get - very cold, low pressure steam resulting in only 30% efficiency.


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PostPosted: Feb 24, 2017 12:47 pm 
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It's also interesting to look at the natural gas to electric efficiency.

In the working example I gave above, 545 MWe added power from 792 MWth input. Assuming a net burner efficiency of 0.8, this would require 990 MWth of natural gas. 545/990 = 55%.

That's similar to an average modern CCGT and almost as good as the best available CCGTs.

I hope a better burner efficiency than 0.8 is achievable.

The other interesting option is to pair a gas turbine to create a NuScale - CCGT hybrid. This would have even higher natural gas efficiency, though at that point natural gas will dominate - it'll be a gas fired powerplant with a nuclear steam plant strapped on for added power in the steam turbine.


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PostPosted: Feb 24, 2017 3:56 pm 
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Burner efficiencies of up to 95% are achieavable in GTs.
If you use the waste heat from the burner exhaust to do feedwater heating instead of bleeding steam out of turbines you might be able to achieve that.
Which leaves you with 65% efficiency.


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PostPosted: Feb 24, 2017 7:59 pm 
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E Ireland wrote:
Burner efficiencies of up to 95% are achieavable in GTs.
If you use the waste heat from the burner exhaust to do feedwater heating instead of bleeding steam out of turbines you might be able to achieve that.
Which leaves you with 65% efficiency.


Is that true for such a high inlet and outlet product temp burner though? I know home furnace boilers with condensing get up to 99% efficiency or so, but that is with room temperature in, 30-40 C out.

I imagine the high inlet temperature will set the burner efficiency (ie the outlet flue gas temperature) I'm sure we can regenerate some in the feedwater, but we already have a regenerative cycle. Perhaps there will be less regeneration on steam bleed-off and more on natural gas "economizing" (not sure if that's the correct term here). With such dry steam there is much less need to bleed off... Hard to tell without having run the numbers.

Natural gas burns clean so a condensing flue gas & flue gas condensate HX is definately interesting, especially if you have a nice cold heat sink handy like a lake or sea.


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PostPosted: Mar 05, 2017 9:26 pm 
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Perhaps the best place to test this hybrid LWR-gas option is a nuclear plant that a utility is considering to shutter for "economic" reasons, and replace with a gas plant. Instead of closing the LWR, add a burner and replace the STG with a high capacity superheated reheated cycle.

Might make for a stunning re-powering project...


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PostPosted: Mar 05, 2017 10:43 pm 
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A suitable PWR-type unit would require no changes to the nuclear side, which would be quite useful in holding down the conversion cost.
But I don't think I can kickstarter enough money to buy a Gen II PWR. I believe the ones Entenergy put up for sale last year were still $50m or so.


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PostPosted: Mar 06, 2017 9:38 am 
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A varient of this was done at the Zimmer plant on the Ohio River.
This was a standard PWR plant which was almost ready to start up
when the anti-nukes found problems with the QA paperwork.
After a long fight, the utility gave up and converted Zimmer to a supercritical coal plant
by adding a boiler and an HP "topping turbine" upstream of the PWR TG,
resulting in a power upgrade.
AFAIK the conversion was successful and they ended up with near coal plant
thermal efficiency.

But I still think the idea of mixing nuclear and fossile fuel in the same cycle is nuts,
especially with MSR's.


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PostPosted: Mar 06, 2017 8:08 pm 
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Quote:
But I still think the idea of mixing nuclear and fossile fuel in the same cycle is nuts,
especially with MSR's.


I'll agree for MSRs, there is almost no gain in efficiency or in improvements in the STG.

Today's reactors seem a different thing altogether. Getting the output of two [insert favorite LWR or HWR here] for the price of one [insert favorite LWR or HWR here] and a burner has got to be a good deal in today's difficult nuclear economic climate, even when gas gets more expensive...


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