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PostPosted: Jul 24, 2015 10:11 am 
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Well the EU-APWR claims a steam cycle efficiency of 39% - allowing it to make 1700MWe net out of a reactor with comparable thermal output to an ESBWR or similar ~1500MWe class unit.

It seems to do this in two ways:
1 - it has larger steam generators, presumably reducing the delta-T between the primary and secondary circuits which will marginally increase steam conditions
2 - It has 6 foot long low pressure turbine blades, which apparently allows the turbine to be run to a lower outlet pressure, allowing it to capture more energy from the steam.

I am more interested in point 2 as this seems to be generalisable to a wide range of reactors, for example the aforementioned ESBWR.
Does anyone (I am thinking Lindsay more than anything) have any information on this and if it is indeed generalisable?

Additionally, why has noone apparently ever tried a double reheat machine in an LWR?


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PostPosted: Jul 24, 2015 3:45 pm 
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As ancient as they are steam turbines keep getting better and better.

Improvements come from many areas:
- more efficient blade designs that literally extract more energy from the steam, rejecting less energy to the condenser;
- longer more efficient blades in the last two stages, more efficient transitions from the last stage to the condenser;
- more of and more efficient feedwater heaters;
- lower pressure drops in all parts of the system including moisture separators;
- improved moisture drainage from the STG;
- better feed pump efficiencies. etc

Most of these techniques are directly transferable to fossil or high temperature units, but depending on the improvement will make a bigger or smaller difference between those two major turbine types. For example any improvements in the back and of the LPT and condenser have a much bigger impact on a wet nuclear turbine than they do on a fossil unit, but both benefit.

Most of these techniques are so much easier now with the advanced finite element flow simulation software that is really starting to have some major impact on the overall performance of these machines, where they can now iteratively optimise every single stage of the machine to work at peak efficiency with ever other stage. Think powerful software and scalable computing environments.

>Double reheat on a LWR
I don't know about that, it might be that the gains in efficiency and power are insufficient to cover the costs of increasing the peak pressure in the system, that would be my guess. I can tell you that DR technique on fossil units creates only a very small improvement in cycle efficiency for a significant increase in complexity and cost.


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PostPosted: Jul 24, 2015 6:04 pm 
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I don't suppose you would happen to be have handy tables or some such which would enable you to estimate the efficiency available using current condenser vacuums and steam turbines, if you had 325 Celsius ~125atm steam?


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PostPosted: Jul 25, 2015 1:13 pm 
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Its interesting to track the development of these reactors through the years, ESBWR early documents talked about 33-34% efficiency, then they uprated the reactor and changed the turbine blading (presumably the long turbine blade stuff), and they got 35%.

The Areva designs also modernized the steam systems, with optimal economizer and hotter steam, and a big turbine, getting 37%. Areva claims the somewhat smaller Kerena reactor also gets the 37% turbine efficiency. Seems like APWR is going a few steps further. I imagine you could push 40% efficiency with a nice cold heat sink, in the colder places of this world. I sometimes wonder if it would make sense (energy wise) to lay an enormous, insulated pipe from a coastal nuclear plant, over the seabed, to get the coldest water around. The condenser then heating that water up to normal surface temperatures and rejecting water at the surface. No more thermal pollution. Pumping might be the issue here.


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PostPosted: Jul 25, 2015 1:48 pm 
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E Ireland wrote:
I don't suppose you would happen to be have handy tables or some such which would enable you to estimate the efficiency available using current condenser vacuums and steam turbines, if you had 325 Celsius ~125atm steam?

To get useful numbers, you have to build a complete model of the system including all the feedwater heaters which are a key element in designing a regenerative cycle. More regeneration => more complexity and capital cost, but greater efficiency. So no, there aren't a lot of useful tables that you can just look up, but I do have something that might help; I will look for it.

Note: Regeneration is where you partly or fully expand your working fluid, take the residual energy in that fluid and use it to heat the incoming cold/cool fluid. Rankine cycles do this with feedwater heaters, Brayton cycles with recuperators.


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PostPosted: Jul 25, 2015 4:02 pm 
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Cyril R wrote:
Its interesting to track the development of these reactors through the years, ESBWR early documents talked about 33-34% efficiency, then they uprated the reactor and changed the turbine blading (presumably the long turbine blade stuff), and they got 35%.

The Areva designs also modernized the steam systems, with optimal economizer and hotter steam, and a big turbine, getting 37%. Areva claims the somewhat smaller Kerena reactor also gets the 37% turbine efficiency. Seems like APWR is going a few steps further. I imagine you could push 40% efficiency with a nice cold heat sink, in the colder places of this world. I sometimes wonder if it would make sense (energy wise) to lay an enormous, insulated pipe from a coastal nuclear plant, over the seabed, to get the coldest water around. The condenser then heating that water up to normal surface temperatures and rejecting water at the surface. No more thermal pollution. Pumping might be the issue here.


I get this for a reactror rejecting 1GW:
Density at 4C 999.972 kg/m3
Density at 20C 998.2071 kg/m3
Difference 1.7649 kg/m3
Difference ratio 0.00176807 (for pure water - assume similar for salt water)

Heat rejection 1 GW
Delta T 16 C

Flow 14.88095238 tons per second
Flow difference 0.026310565 tons per second
Power per metre 0.257843538 KW

So, if your source was 200m deep, that's 50KW of extra pumping only. (Please check - I can't believe it's so low).

Inefficiencies and pipe friction though will dwarf that, but if you had a steep shore, and cold currents bringing cold water near the shore (as in North California?), it should be feasible. Around the UK, the coninental shelf goes quite far out so pumping cold water in is probably not feasible.

When thermal power stations discharge into the sea, do they use the drop (3-10m) for power? The logical way is to create a negative head so the falling, warm water sucks up the risng cold water.

The US Navy will know what the efficiency difference is for a submarine at 4C compared to 20C.


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PostPosted: Jul 25, 2015 6:43 pm 
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The density difference is quite small, like you say it will be the friction factor that dominates. Then again that just depends on the pipe diameter. [crunching numbers... wrrrr...click] Now that is interesting, a pipe of 5 meters dia (not difficult to make) and 10 km length has only about 100 kWe of pump power need from wall and fluid friction*.

Our civilisation pumps liquids over thousands of km, so the tech feasibility isn't in question. Just wondering if it makes sense from the energetic viewpoint... I guess it must if we're only talking hundreds of kW pump power for a GW plant. It would be quite easy to recover that in thermal efficiency, if ice cold water is available relatively close by. If its far out then a bigger pipe could be used; it might be a pipe cost issue then (400 km pipe of 10 m dia still gets only a few hundred kWe pump need).

*though it may end up being 200 kWe when mister and misses Barnacle, Spongebob and mister Crab take on permanent residency in our pipe.


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PostPosted: Jul 25, 2015 8:22 pm 
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Increasing condenser vacuum is possible but extraordinarily expensive.
The back end of the turbine blows up.
Worse, the condenser blows up.
The TG costs blow up.
And the condenser and the turbine last stage are already
the two biggest maintenance headaches in a steam plant.

Other than Lindsay's incremental improvements due to better modeling, better materials,
I'm guessing the only reason the latest PWR's have gone down this route
is that the steam generation side has become so expensive
these marginal improvements in efficiency at great cost make sense.
For a less costly, low pressure, high temperature nuclear technology
they almost certainly do not.


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PostPosted: Jul 26, 2015 3:13 am 
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Quote:
Increasing condenser vacuum is possible but extraordinarily expensive.


OK. 15 tons per second dropping 5-10m is a power source of 750-1500 KW, minus inefficiencies.. Pretty small for a 500MW-1GW plant. However, there are turbines specifically designed for this kind of low head, so 500-1000KW could be recovered. An extra 0.1%.


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PostPosted: Jul 26, 2015 3:26 am 
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Cyril R wrote:
The density difference is quite small, like you say it will be the friction factor that dominates. Then again that just depends on the pipe diameter. [crunching numbers... wrrrr...click] Now that is interesting, a pipe of 5 meters dia (not difficult to make) and 10 km length has only about 100 kWe of pump power need from wall and fluid friction*.

Our civilisation pumps liquids over thousands of km, so the tech feasibility isn't in question. Just wondering if it makes sense from the energetic viewpoint... I guess it must if we're only talking hundreds of kW pump power for a GW plant. It would be quite easy to recover that in thermal efficiency, if ice cold water is available relatively close by. If its far out then a bigger pipe could be used; it might be a pipe cost issue then (400 km pipe of 10 m dia still gets only a few hundred kWe pump need).

*though it may end up being 200 kWe when mister and misses Barnacle, Spongebob and mister Crab take on permanent residency in our pipe.


Bear in mind that the pipe may have to be double walled to stop it warming up as comes through warmer water. I think this would be worthwhile if deep water is nearby.

One of the problems in Europe, is that even coastal sites can have heat rejection problems. This is why Dungeness was deemed not suitable for 3.5GW of WR, with a heat rejection of 7GW. There's a similar problem with the Bristol channel reactors, where the heat is basically taken away by tidal wash.

Which begs the question, can't you just pump the waste water through a 10km pipe, made of copper with fins to reject heat. Personally, I'd run the Dungeness pipe close to the beaches. They are seriously nice beaches but the water could do with s bit of heating :)

Of course, MSRs at 50% efficiency halve the problem.

If pumping water is so easy, an undersea pipeline from Washingon State to Southern California to pump fresh water might be helpful for California. It could even be made of flexible plastic.It just needs weighting down, to carry less dense fresh water.


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PostPosted: Jul 26, 2015 7:54 am 
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As I recall the reason that Dungeness was deemed unsuitable is the massive beach nourishment exercise currently conducted to stop the plant's position being eroded away.

(They dig up vast tonnages of gravel on the lee side of the headland and dump it on the upwind side).


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PostPosted: Jul 26, 2015 8:18 am 
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Quote:
Bear in mind that the pipe may have to be double walled to stop it warming up as comes through warmer water.


Yes, in fact it must be fairly well insulated pipe, that does increase the cost. Running kilometers of conductive metal pipe through a bath of coolant, definately insulate that. Pipe might be reinforced concrete though and it wouldn't be so bad.

Quote:
Which begs the question, can't you just pump the waste water through a 10km pipe, made of copper with fins to reject heat.


Sounds expensive. Just go with a dry cooler then. Point of the cold seawater sucking is to lower the cold sink temperature.

Quote:
Increasing condenser vacuum is possible but extraordinarily expensive.
The back end of the turbine blows up.
Worse, the condenser blows up.


Well, power is quite valuable too. Another 0.5% out of a 1000 MWe turbine is 5 MWe, worth $2-3 million/year.


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PostPosted: Jul 26, 2015 11:23 am 
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E Ireland wrote:
As I recall the reason that Dungeness was deemed unsuitable is the massive beach nourishment exercise currently conducted to stop the plant's position being eroded away.

(They dig up vast tonnages of gravel on the lee side of the headland and dump it on the upwind side).


I always sort of chuckle when I hear this class of argument. Half of my country of birth (Holland) is one big fight against the erosion, the "sea wolf" as it is known locally. We are pretty good at just throwing up more sand in strategic locations. It is tedious and constant, and costs money, but is perfectly doable. I always wonder - perhaps other people think Holland is an unsuitable country for living? After all, we tend to have similar prejudices against people living next to an active volcano. Gotta have something to worry about, right?


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PostPosted: Jul 26, 2015 12:09 pm 
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Flood defence is a dirty word now.

The only thing the Environment Agency is prepared to countenance is managed retreat.

EDIT:

Additionally I made up a spreadsheet that produces an optimum uranium enrichment scheme based on the price of a SWU/kg of Uranium.
I used the ESBWRs 4.2% 50GWd/t fuel producing 420MWh of electricity per kg (35% cycle efficiency) as the benchmark.

At $105/kgU and $70/SWU that gives us a fuel cycle cost of around $2k/kg enriched uranium.
If we take the seawater extraction guys at their word and set the uranium price to $660/kg then the optimum is to enrich with a tails fraction of 0.065% with a cost of $5875/kg of enriched uranium.

That comes to roughly $14/MWh, or an increase of $9.20/MWh (which is surprisingly small - it is likely small enough to crush any chance of breeders being economically superior).
The point of all this is that this means that increasing efficiency by ten percent (35% to 38.5% - roughly from the ESBWR to the EU-APWR) would save us $1.28/MWh - which for a 1500MW reactor is $15+m/yr in saved fuel.

That is probably enough to justify the steam cycle efficiency improvements.


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PostPosted: Jul 30, 2015 8:35 am 
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That's very interesting data, Ed. Looks about right too, though I imagine no one has ever gone down in tails fraction that much. (any technical issues there or just more SWU?). I guess low tails is going to be the future standard with modern efficient enrichment technology being now standard (no more diffusion plants).

Probably more important than the fuel saving will be the value of the electricity. For example, your 10% improvement in cycle efficiency by changing out the turbine or other improvements, means the ESBWR is upgraded by going from 1550 MWe to 1705 MWe. @8000h/year this is another 1240,000 MWh. If you sell for $50/MWh then that brings in another $62,000,000 per year. That is a lot of money to pay for your upgrades! And you haven't used more fuel or reduced safety because it is from efficiency improvements. Same reactor...


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