Supercritical nuclear power plants 4

[1capybara]

Coal plants have used supercritical heaters for along time, and now even "ultra-supercritical" coal power plants are in operation.
Nuclear power plants are stuck at the theoretical stage for supercritical power? Whats the bottleneck?

 

[racookpe1978]

Temperature.
Conservative thought about steam heat transfer processes at the most limiting point.
Length of piping (number of pipes and pipe penetrations going in and out of the primary systems and through the primary pressure boundaries.

Coal and gas-fired boilers blow extremely hot air and combustion gases past the saturated steam to gain their superheat, right?
You take the saturated steam from the boiler piping at the end of the conventional runs, run it back to the front of the boiler in new rows of pipe being hit directly by the hottest of the boiler gases as they leave the burning area.
So, in a nuke, where are you getting the the "extra" heat? The core, right?
So, you need to run hundreds of pipe (or regroup all of your existing pipes into a single one again) back into the the hottest part of the core (where you are trying to control the nuclear reaction by modulating the water fraction, fuel fractions as they change, and control rod fractions, poison fractions as the fuel burns up, and the "contamination" of the various components as fuel becomes replaced by nuclear fission products. Now into that, you are introducing a varying steam+water mix of pipes and pipe materials - none of which are fuel - and all of which vary in density and nuclear cross-section with temperature.

It makes your nuclear control more difficult, your metalurgy more difficult, your primary containment system more difficult. A few plants - I don't recall which, DID try to even use conventional burners + superheaters to reheat the saturated water post-steam generator, but that didn't last long. Too much cost for too little benefit.

All of these problems could be solved, have been looked at before in various ways, but the simple, solid design of "leave it as is" weighs out. You have to be able to sell your design across the public desks of local and state politicians, to the local public.

 

[1capybara]

iirc, the plan was single pass through the reactor, not multiple passes as you described. but the problem maybe the single-pass idea didnt work in practice?

i must respectfully disagree with this:
"Now into that, you are introducing a varying steam+water mix of pipes and pipe materials - none of which are fuel - and all of which vary in density and nuclear cross-section with temperature."
a supercritical fluid isnt a mix of stteam and water, its a homogenous supercritical fluid, that is the definition of supercritical.

but to cut to the chase, i agree with this:
"It makes your nuclear control more difficult, your metalurgy more difficult, your primary containment system more difficult. ......Too much cost for too little benefit. All of these problems could be solved, have been looked at before in various ways, but the simple, solid design of "leave it as is" weighs out. You have to be able to sell your design across the public desks of local and state politicians, to the local public. "

and that is the answer. thank you racookpe1978

 

[davefitz]

The supercritical nuclear reactor was studied in the 1950's and calculations at that time demonstrated that the single pass thermal hydraulic sensitivity S was too high ( highly negative), meaning that the hottest channel would draw far less water flow than the "average" channel, leading to unacceptable overheat of the hottest channel. ( ref: Hyman et al) . Multi pass circuits are required, with fully mixed intervening headers ( equalizing pressure and enthalpy) between passes.

Modern coal fired USC ultrasupercritical units also use multi pass designs, basically :
economizer==>lower evapaporator==> upper evaporator==>primary superheater==>secondary superheater(s), with full mix headers between all circuits, except the 2 furnace evaporator circuits have a partial mix header.


Latest Gen IV USC designs also have a less-than-credible design configuration, based on the false assumption of equal characteristics of each channel . The truth is each channel has widely varying charcteristics ( heat generation, fluid friction , inlet and outlet header flow unbalance)- I would think twice before buying a house next to one of those demonstration plants.

"Whom the gods would destroy, they first make mad "

 

[davefitz]

The first paper on the above issue was publiched in 1958, see AICHE journal, V4, issue 1, pp 33-36 mar 58 by S.C. Hyman, A.R. Gruber and L. Joseph, and their followup papers.

"Whom the gods would destroy, they first make mad "

 

[Duwe6]

If they ever work out the construction/operational problems with using liquid metal* for a heat transfer medium, then superheated steam can easily be generated. That system should have a high enough temperature to reheat that steam into the supercritical range. But as davefitz has proven, using water/steam as the primary coolant [transfering the heat from the reactor core] just doesn't work.

*USA and USSR have tried and abandoned liquid sodium. Soviet Alpha Fast Attack subs with a sodium-cooled reactor were so fast that the US & UK had to develop faster torpedos to catch them. But the reactor suite problems outweighed even this enormous speed/power advantage.

 

[racookpe1978]

The USS Seawolf started with a liquid metal reactor, but it "quickly" ( less than 2 years of attempted operations) and replaced with a modified light water reactor similar to USS Nautilus.

One of Rickover's few technical failures. He had many other problems, but was usually very good on applying technology safely and reliably when facing thousands of new problems with little experience.

 

[davefitz]

Other options exist for improving the efficiency of the nuclear cycle.

One proven method is to use a separately fired superheater/reheater , and today's low cost of natural gas would trend in the direction of such a hybrid cycle

Anther method tied to the LMFBR is to use supercritical CO2 as the secondary working fluid. The CO2 does not have the same reaction hazards as would H20 during a HX leak, and the critical temperature of CO2 is so low that metalurgical overheat issues associated with traversing the psuedo-critical point are not nearly as bad as with H2O.



"Whom the gods would destroy, they first make mad "

 

[racookpe1978]

I have not worked with "liquid metal" except in very small amounts (10's of pounds) of molten aluminum while manually pouring castings in sand molds.

That said, please educate me.

1. How do the liquid metal specialists (iron foundries, iron and magnesium and titanium or aluminum casting sites where air (oxygen) contamination is dangerous or destructively reactive, etc.) manage shutdowns and startups? How do they handle catastrophic (sudden or unexpected) rapid shutdowns or "black island" loss of power incidents without losing their facilities?

2. When we were pouring melted aluminum, if the heat were lost, the single "pot" would cool and solidify of course, but could be remelted with only lost time or that one bad mold as a penalty. But how do you "remelt" a complex piping arrangement with pumps, sensors, backflow spots and multiple flowpaths inside and around the core of a very radioactive source?

3. In a liquid metal reactor, do they anticipate electrically "re-melting" the core and pipes and pumps and heat exchangers first after every shutdown? How do they propose maintenance in piping or heat exchangers or valves when the valve interior is going to be coated with "solid" debris and sludge after shutdown even if "drained" first?

 

[1capybara]

Duwe6 - according to

http://en.wikipedia.org/wiki/LMFBR#Fast_breeder_reactor

it says, "As of 2006, all large-scale fast breeder reactor (FBR) power stations have been liquid metal fast breeder reactors (LMFBR) cooled by liquid sodium." so liquid sodium cooling is state of the art - in practice not theory. but if this is wrong pls lmk. ok?

 

[Duwe6]

Sodium - in actual practice - has proven to be impractical [i.e. it is an Operational and Maintenance nightmare]. Fast Breeders are really only used for plutonium production - thus kept under Top Secret veils. How much the few sodium-cooled ones have been used, how many sodium-water explosions, how many sodium fires, how many operators/maintenance personnel maimed, etc. are all well hidden.

Basically, it is like 100-mile/gallon carbureators. Everybody wants to get one to work, but nobody has accomplished it yet. It might be do-able, but it appears to need either an engineering or a materials-of-construction breakthrough.

 

[Drexl]

Have not read anything about supercritical reactor designs. But if you would start from a BWR-type design but make it a once-through with pressure of 240ish bar what would the biggest problems be? The first thing that comes to my mind is that you can't control output using core circulation pumps anymore. The higher pressure/temperature would of course require different materials and physical design, but I don't see why this would be so problematic. Actually sounds like a good idea, have to read up on what actual problems they have encountered.

Why would you use burners for reheating when you can reheat using live steam? Superheating is of course a different chapter.

 

[racookpe1978]

Where are you going to get the "hotter temperature" live steam?

Mentioned above: If you take "live steam" off of the top of a BWR, then you need to go back into the core (through the core) to heat it higher. that requires hundreds of (potentially leaking!) penetration - each very expensively and very slowly welded individually - to get the steam back through the core and into the pressure vessel and into the reactor containment.

Can't afford it; Too risky; no effective payback for little gain.

 

[Drexl]

I'm perhaps reading you wrong. Reheating is when you superheat the steam AGAIN after the high-pressure turbine. Then you heat the low-pressure steam up to near live steam temperture again (thus REheat). If you mean something else, then i'm not familiar with that terminology, but please enlighten me.

If you are talking about superheating the live steam then for sure this seem like a VERY challenging task in a NPP.

 

[racookpe1978]

Thank you. Reheat (look up MSR's please) is already done as standard in all the nukes I've worked on or worked configuration management programs on. Usual practice is 2x MSR's on the turbine deck per LP turbine.

 

[davefitz]

Nearly all water cooled reactors generate saturated steam out of the steam /water separators. Entering these separators is a 2-phase mixture of steam + excess circulating water- the excess circulating water is supplied by the recirculating water pumps to ensure that the film conditions at the outlet of the "worst channel" has sufficient liquid that it avoids "dryout" or "DNB"- 2 forms of CHF that would lead to overheat of the heat-generating fuel elements. A "circulation ratio" of perhaps 4:1 is maintained to ensure sufficinet excessliquid is provided to avoid CHF.

This has been proven to be a safe design, but it also limits the steam cycle to one based on the saturated steam conditions in the primary circuit, roughly 690 F.

To achieve better steam cycle heat rate ( or efficiency)one needs to raise the final steam temperature entering the steam turbine inlet far above 690 F. Currently installed commercial fossil fired steam cycles are operating now at 1100 F, with proposals to as high as 1300 F. Most are single reheat cycles, but further efficiency improvements are available at double reheat using the newly Elsom-patented " master cycle" ( where the HP feedwater extractions are not sourced from the power turbine) .

Higher steam temperatures can be had by use of a separately fired superheater ( as at indian point #1). If one substituted a supercritical water flow for the lower pressure boiling water flow used at conventional reactors, the "circulation ratio" would need to be reduced to 1:1( ie it beconmes a "once thru unit"), which means it becomes extraordicanlrily important to ensure the "worst channel" does not become oveheated - thus the need to calacualte the thranl hydraulic sensistivity characteristic S of the circuit- and a single pass circuit is not feasible.



"Whom the gods would destroy, they first make mad "

 

[EnergyMix]

This is an interesting theoretical discussion, but I believe in the current world climate (i.e., post Fukashima), we're not quite ready for a super-critical reactor, if for no other reason than the current political climate. Additionally, because of the destructive potential, it's probably not one that can be discussed in any technical detail on a world-wide forum. This would be a good work or university discussion.

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[rodofgod]

Interesting reading,I would tend to agree with EnergyMix, not sure the world is ready for super-critical reactors just yet. On a technical point, the USSR's Alfa class subs ran lead-bismuth cooled reactors, which required them to be kept running at all times or for a supply of superheated steam to heat the reactor vessel and keep the coolant in its liquid form.