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PostPosted: Dec 21, 2011 8:01 am 
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I've been reading up on lead cooled fast reactors such as BREST and ELSY and I'm starting to like this concept. Unlike sodium, lead does not burn in air or explode in water. Boiling point is much higher also (>1700 degrees Celsius), so voiding and pressurizing the reactor/containment isn't plausible. So like the LFTR there is no plausible driving force to push out radionuclides.

Strategically, the lead cooled fast reactor is also attractive, because it is so different from the LFTR. LFTR is a thermal/epithermal/biomodal spectrum liquid salt fuel reactor, lead fast reactors are very fast spectrum solid fuelled liquid metal cooled reactors. Technological problems in one reactor system likely won’t be present in the other, and vice versa.

I’d like to know peoples opinion on lead cooled reactors.


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PostPosted: Dec 21, 2011 11:42 am 
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Lead-cooled reactors are excellent paper reactors.

I think they should have a gas-cooled startup and shutdown mode,
and switch to lead cooling when the whole circuit the lead needs
to circulate in has been warmed up.


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PostPosted: Dec 21, 2011 11:46 am 
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Keep it simple. Electrical resistance heaters for the hot chamber - not multiple modes for the reactor.


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PostPosted: Dec 21, 2011 1:04 pm 
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They are not paper reactors, the Russians had a very succesful submarine class, Alpha class submarine, which used lead-bismuth coolant. They were the fastest in the world. Lead-bismuth is much more corrosive (actually, dissolving) than pure lead and you produce thousands of times more polonium which is an annoying operational and exposure issue. The alloys and equipment used for lead-bismuth can be used for lead cooled reactors (they only operate slightly hotter, that's all).

It's very similar to the molten salt reactor position: we once had succesful reactors of molten salt and of lead-bismuth cooling, but have largely abandoned these. In stead we put all the money in a sodium cooled reactor which no one will accept in their backyards.

The reactor will have a helium reactor cover gas. That can be used in low pressure gas pumping for preheat. But like Lars said, induction or wire heaters are much simpler.


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PostPosted: Dec 21, 2011 1:55 pm 
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Mh two ships of the Alfa-class needed new reactors because the lead-bismuth coolant freezed and destroyed the reactor. Does a lead cooled reactor have the same problem ?


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PostPosted: Dec 21, 2011 2:17 pm 
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Quote:
the lead-bismuth coolant freezed and destroyed the reactor. Does a lead cooled reactor have the same problem ?


Without a gas-circulating mode for low-power initial thawing-out, or as people have been saying, resistance heaters, it has it much worse, because lead freezes at 327°C versus lead-bismuth eutectic freezing, IIRC, near 125°C.


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PostPosted: Dec 21, 2011 2:29 pm 
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Submarine reactors need a compact loop type reactor. This, combined with the small power core, and really cold water all around you (Barentz Sea, Kara Sea, Laptev Sea, brrrr!), makes freezing a big issue. Power reactors can be bigger, and pool type. This helps with prevention against freezing. All the smaller piping can be submerged in the lead pool. Huge pool of lead, takes months to freeze after shutdown.

Besides freezing, the problems that occured with the Alpha submarines were related to polonium and bismuth dissolution. Orders of magnitude smaller with pure lead as coolant. There were also problems with refuelling freezing. No such problem with a pool type reactor since you have an open core with metal extensions on the fuel rods (ELSY design), allowing fuel manipulation in the cover gas space at lower temperature. Manipulating in molten lead is awkward, so the ELSY design is pretty clever.

At any rate, molten salt fuel freezes between 450 and 600 degrees Celsius or beyond, depending on fuel and carrier salt composition. This is much worse than lead (melting point 328 degrees Celsius).


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PostPosted: Dec 21, 2011 2:35 pm 
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Cyril R wrote:
At any rate, molten salt fuel freezes between 450 and 600 degrees Celsius or beyond, depending on fuel and carrier salt composition. This is much worse than lead (melting point 328 degrees Celsius).


Is this a problem for LFTR ? Has anybody a short answer if not i will start a new topic about it.


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PostPosted: Dec 21, 2011 2:56 pm 
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Marcus wrote:
Cyril R wrote:
At any rate, molten salt fuel freezes between 450 and 600 degrees Celsius or beyond, depending on fuel and carrier salt composition. This is much worse than lead (melting point 328 degrees Celsius).


Is this a problem for LFTR ? Has anybody a short answer if not i will start a new topic about it.


ORNL had some issues especially with smaller pipes where the insulation deteriorated. The easiest way to solve the problem is to immerse the entire primarly loop in hot gas (the hot cell is an oven) or hot molten buffer salt (the hot cell is cooler but there is a pool of hot nonradioactive buffer salt in it).

It's also possible to operate in a colder hot cell but then you must use electrical heat tracing plus good insulation and you have the scenario of station blackout or insulation deteriorating to deal with. Better to operate the entire primary loop in a temperature that is higher than the melting point of the fuel salt.


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PostPosted: Dec 21, 2011 7:47 pm 
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A lead-tin eutectic would be lower freezing if you can manage corrosion/dissolving of metal by a suitable coating. Can you think of a suitable enamel?


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PostPosted: Dec 22, 2011 4:38 am 
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Tin is much worse than lead in its metal dissolving abilities. However most flat glass production today is with molten tin baths. The molten glass floats on the tin. Glass, of various types is therefore compatible with molten tin.

Molten tin is also compatible with niobium, molybdenum, tungsten, and very likely compatible with silicon carbide.

A big disadvantage with tin though is its higher neutron capture rate compared to lead. It would be interesting to see how bad it would be for a fast reactor. It is also more expensive.

Still the lower melting point of 183 degrees Celsius is a major advantage.

http://www.chemguide.co.uk/physical/phaseeqia/snpb.html


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PostPosted: Dec 22, 2011 8:28 am 
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Marcus wrote:
Cyril R wrote:
At any rate, molten salt fuel freezes between 450 and 600 degrees Celsius or beyond, depending on fuel and carrier salt composition. This is much worse than lead (melting point 328 degrees Celsius).


Is this a problem for LFTR ? Has anybody a short answer if not i will start a new topic about it.



A short answer is that the fuel salt of a MSR/LFTR is very hard to freeze or at least keep frozen because of all the decay heat generated within it. A secondary salt might freeze but really isn't a safety implication like it could be for a lead or sodium cooled reactor that needs to keep the coolant flowing.

Regarding Lead cooling in general, there are indeed pretty great advantages including even getting rid of all heat exchangers and just mixing lead and water to make steam (see this overview)

http://www.inl.gov/technicalpublications/Documents/3318134.pdf

However, if you want to hear a sodium proponent make a case for why lead is just no good.

Attachment:
leadvssodium.pdf [1.6 MiB]
Downloaded 232 times


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PostPosted: Dec 22, 2011 2:10 pm 
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One of the major selling points for lead coolant is that it is extremely nonvolatile. Mixing water in the molten lead to make steam, means you lose that substantial advantage. The reactor then becomes a huge pressure vessel with all the assorted problems and costs. This also limits the temperature achievable in the steam cycle, reducing plant efficiency. Then there's issues with lead particles entraining the steam turbine, which is a materials and operational nightmare.

Stick to heat exchangers. A heat exchanger is a simple thing. Some tubes, some baffles, a shell. Existing lead cooled reactors use one heat exchanger, but even that gives issues with steam tube leakage, letting water in the fast reactor core seems like a really bad idea. It could potentially pressurize the containment, and produce hydrogen. We've seen what kind of problems that can give, in Fukushima.

I'd prefer another heat exchanger, using nonradioactive lead as intermediate loop, and then to the steam cycle. There are then no means to pressurize the containment and you can have simple filtered confinement in stead of a pressure containment, a big cost and safety saving easily worth another heat exchanger. Looking at how much engineered stuff the ELSY has to put in to prevent steam leaks from entraining the core, that looks very complicated to me.

What do the sodium fast reactor advocates have to say about lead fast reactors? What does General Motors have to say about buying Toyota? Jaro pointed out that there are numerous problems with that presentation, for example lead's low lethargy allows wide coolant spacing which improves natural circulation, reducing pumping requirements. It is in fact the sodium fast reactor that has the bigger pumping requirement, due to the extremely high power density core (>400 kWt/l) and subsequent high flow speeds. High flow speeds also means increased issues with vibration such as grid fretting and even up to eigenvalue issues, plus any debris in the coolant will abrade the cladding.

But the real problem with sodium cooling is that people know what sodium does when in contact with water. They don't appreciate engineered mitigation, like double walled steam generators and intermediate nonradioactive loops. There was one sodium cooled fast reactor that was going to be built at Kalkar nearby. Public opposition, mostly due to the use of sodium, caused the project to be abandoned. It is now an entertainment park.


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PostPosted: Dec 22, 2011 2:38 pm 
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Continuing my critique of the sodium vs lead presentation.

- pCp (volumetric heat capacity) being similar: lead's pCp is 70% higher than sodium. That's not similar, that's much better.
- seismic challenge due to lead's density. There is no basis for this claim. Much heavier sky scrapers have 2D seismic isolation. Works fine, and they're typically 20-40x taller than the reactor vessel. Seismic isolation is not very expensive in $/kW, often cheaper than massive concrete basemats. In some ways having a dense coolant is attractive; think in terms of the fuel core and support structures dead weight, it is nearly eliminated because of the nullifying plumbostatic pressure.
- corrosion to structural materials. Wrong assumptions, lead is corrosive to iron and nickel, those are not the only structural materials. Vessels can be made of stainless steel with no nickel and some silicon added for passivation protection with some oxygen in the lead. Zirconium is also compatible for the cladding, this means proven cladding can be used. Sodium is not corrosive but its corrosion products are. If any air or moisture gets in you get big corrosion issues. Whereas with lead, some oxygen in the system is attractive for passivation. This, combined with lead's lower volatility and higher inertness than sodium, limits the containment requirements. Loss of cover gas is a design basis accident in the USA. I wonder how the sodium designs deal with this.
- Above 550 degrees C, oxide layer can become thick and unstable. Again a poor argument, there is no need to operate higher than 500 degrees Celsius. Also the oxide layer instability problem is there for lead-bismuth with the wrong structural materials, not with lead with the right structural materials.
- Non-homogeneous oxygen distribution results in non-uniform coatings. Crevice corrosion and dissolution of occlusions can occur. Again a poor argument, maintaining flow speed over 1 m/s is sufficient to form uniform coatings. It is interesting that they make this argument, because all of today's power reactors rely on passivation chemistry control. It is ignoring what is being done today as standard practise.
- Magnetite (Fe3O4) undergoes phase transformation at 570oC. This is not the dominant passivation layer, it is SiO2 (Si modified stainless steel) or Al2O3 (aluminized steels), ZrO2 (Zr modified steels) or spinel (mg-al oxide system).
- Oxide layer impacts heat transfer from cladding to coolant. As it does in all existing power reactors (!). Again a weak argument.

I'm going to stop now because the presentation is clearly underinformed on lead fast reactor technology and its ideosyncrasies.


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PostPosted: Dec 23, 2011 5:19 am 
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Ok, I found this nice presentation that has a technical overview of the ELSY lead fast reactor design.

http://www-pub.iaea.org/mtcd/meetings/P ... mberti.pdf

Looks like a very pragmatic design with a strong focus on early development (standard steam turbine, available fuel type, relatively low temperature etc). A strong focus on passive safety as well, with two diverse passive short term decay heat removal systems, and a RVACS for long term heat removal.


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