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 Post subject: Lifespan of a LFTR?
PostPosted: Feb 22, 2011 3:36 pm 
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Given the design and operating challenges of LFTR how long would a typical LFTR run for before it needed to be replaced or refurbished?


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 22, 2011 4:22 pm 
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A two fluid LFTR will need its first wall replaced or refurbished at about four years.

Turbines need regular service, but I don't know a reasonable schedule. 5 years? Aircraft turbines are inspected and rebuilt about every thousand hours, but I imagine stationary turbines are sturdier and have less foreign object damage.

All reactors need their steam generators refurbished or replaced after about 20 years. This is horrendously expensive, >$100x10^6. Even LFTRs with helium turbines will have heat exchangers withe some comparable issues. Hopefully it will corrode less on the He side, but there will be some cumulative damage or corrosion mechanism, probably on the salt side. There is no long-term operational experience with FLiBe heat exchangers, and that is probably the biggest remaining engineering challenge for LFTRs.

At 50..60 years, any pressure vessels will have to be replaced, or inspected and heat-treated in place to restore the necessary strength. I recall some eastern-european reactors exceeded their pressure-vessel life, and were inspected and heat-treated in place to save money, because they could not be replaced. The LFTR has a pressurized heat-engine, and that will have comparable issues. Interestingly, part of a powerplant where I live, Huntington Beach, was down for years because of a cracked main turbine housing. It was cast iron, and welding a large iron casting really doesn't work (heat stress causes the crack to propagate). SOemtimes it can be nickel breazed, but often even that is too risky. When the power price went up during California's rolling blackouts, they fixed it with a peculiar non-welding technology called "metal sewing."

All that stuff can be fixed, and kept in service, often far more economically than by replacing it.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 22, 2011 6:08 pm 
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Quote:
A two fluid LFTR will need its first wall replaced or refurbished at about four years.


In the breeder/burner concept this maintenance issue can be avoided.

This frequent maintenance hit is why I favor a deeply thermal single fluid burner design build from molybdenum(TZM (Mo (~99%), Ti (~0.5%), Zr (~0.08%) and some C).

In the deep thermal spectrum molybdenum hardly sees slow neutrons with a maximum cross section of just 2.8 barns. Since the displacement per atom (DPA) is directly proportional to the absorption cross section, the DPA per year will be proportionally small.

Unlike helium intrusion caused by thermal neutrons interacting with Ni58 and Ni59 within Hastelloy-N, molybdenum is not affected.

With proper redox control of Filbe, little salt based corrosion problems will occur. If the heat exchangers are neutron shielded from the reactor core, only delayed neutrons will produce DPA damage in the heat exchangers extending their service life.

On the other hand, the U232/U233 fusion breeder will require annual overhaul, since 14.2 MeV neutrons are very damaging. The design of this type of hybrid should be such that parts can be easily and quickly replaced. With a breeder, capacity factor is not an issue.

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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 1:46 am 
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rgvandewalker wrote:
All reactors need their steam generators refurbished or replaced after about 20 years. This is horrendously expensive, >$100x10^6. Even LFTRs with helium turbines will have heat exchangers withe some comparable issues. Hopefully it will corrode less on the He side, but there will be some cumulative damage or corrosion mechanism, probably on the salt side. There is no long-term operational experience with FLiBe heat exchangers, and that is probably the biggest remaining engineering challenge for LFTRs.


This is no need! This is just the result of a poor material choice when they were constructed. Newer steam generators normally shouldn't have this inconvenience. Furthermore, reactor operators perform an uprating whilst replacing the SGs. Therefore the cost is actually benign.

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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 3:57 am 
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In fact, the new steam generator itself is often part of the uprate. Newer designs have improved materials and economisers for lower temperature drop and drier steam production. Expect 5-10% more net electric output when refurbishing old steam generators and old steam turbines with modern ones.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 6:58 am 
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The life span of fuel rods in ordinary solid fuel reactors (LWRs) is 18 or 24 moths.
The fuel channels inside a Molten Salt Reactor are the closest equivalent to the LWR fuel rods.
If we can run MSR fuel channels for 3 to 6 years between replacements, we're already way ahead.
The rest of the reactor, if designed properly, should last at least as long as an LWR.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 8:03 am 
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Importantly, Jaro is referring to single fluid designs. These can use replaceable fuel channels (Jaro’s HW-MSR) or a larger low power density core to deal with the materials issues. For two fluid it is harder. Lars has some ideas on making the barrier last longer, e.g. nearzero structural load on the barrier, thorium in the core to limit neutron load on the barrier, similar fluids in core/blanket making barrier breach a smaller issue, etc.

It is not clear what the best approach is but all of the above appear workable. All MSRs require hot cells as they cannot deluge-shield the core with nonradioactive liquids such as water, upon maintenance shutdown. This means fully automated robotics is required one way or the other. So it is an advantage when parts can be replaced in a simple and effective manner. It would be nice if the parts last more than just a few years for reliability, uptime and equipment cost reasons, but is not strictly necessary.

There is very little commercial experience with what is required. Commercial experience is with water, medium temperature, very high pressure. What is needed is fluoride fuel and fission product environment, low pressure, high temperature. Work by ORNL reveiled no red flags, and problems were quickly solved. Now we need large scale component design and testing. It will take a lot of money and time.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 3:24 pm 
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The Japanese are working on single fluid designs, which should last a very long time and are simple to build. Improvements in fuel processing can be on-going as the core technology of a single vessel design remains constant. I would bet the Japanese are right to stick with the original concept of the LFTR. The vessel walls can be made as thick as you want, and the corrosion is so slow that they could last indefinitely. The fuel processing equipment can be changed out at lower cost as improvements and natural wear mandate.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 4:31 pm 
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In the original design you need to plan on replacing the graphite at regular intervals. You can replace every four years with the turbine maintenance or put in 7 times as much graphite originally and replace every 30 years. But if you have a graphite based design plan on replacing it.

The neutron load of the outer (and only in a single fluid design) reactor wall can be made very small by providing a lining of boron&carbon in front of the wall to absorb the neutrons so the lifetime of the reactor wall will be very long.

I would plan on reworking/replacing the HX several times during the life of the reactor. It has to deal with two kinds of salt - which means higher corrosion wear rates, possible noble metal deposition, wants to be as thin as possible for best heat transfer, and has lots and lots of thin pipes.

The other areas to watch for would be the fluorinator and vacuum distiller. These likely will take a few iterations to come up with a long lasting solution.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 23, 2011 4:37 pm 
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guys, guys, we're not selling LFTRs yet, are we? So what are the problems with:

TZM (any long term reactor experience? Salt interactions are small, but zero? Fission product interactions? Plate-out of the noble-metal fission products? Cracking from differential thermal expansion during start-up (Mo is brittle at room temperature) Steam loop creep damage? Blade fatigue in the He turbines?)

Slow thermal reactors (Any machine is likely to have problems after 50 years... but it sounds good.)

Breeder issues, since the slow thermal reactor fleet needs breeders to feed it. First wall issues? What about graphite replacement?

Many LFTR designs may need graphite replacement or refurbishment after 10 to 15 years. The MSRE, for example, would probably need that. There are potential mitigations: Sealing the graphite to keep out fission products, annealing the graphite. Still, they can change shape appreciably, and will need refurbishment.

An epithermal LFTR can avoid graphite, but if it's a two-fluid reactor, it has the first-wall replacement, instead. In particular, the elegant LeBlanc tube-in-tube 2-fluid designs still need first-wall replacement, although they avoid a graphite moderator and are self-breeding.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 24, 2011 8:16 pm 
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Quote:
TZM (any long term reactor experience? Salt interactions are small, but zero? Fission product interactions? Plate-out of the noble-metal fission products? Cracking from differential thermal expansion during start-up (Mo is brittle at room temperature) Steam loop creep damage? Blade fatigue in the He turbines?)



See

Development of Mo base TZM
(Mo-0.5Ti-0.1Zr-0.02C) alloy and its shapes
Sanjib Majumdar and I. G. Sharma
Materials Processing Division

In this recent news letter

http://www.google.com/url?sa=t&source=w ... B5vfYqrYEg

Quote:
Refractory metals and their alloys possess high temperature strength, creep resistance, low coefficient of thermal expansion, high thermal conductivity and favorable nuclear properties, which enable them to withstand prolonged exposure to aggressive environments of radiation, temperature, corrosion (gaseous and liquid metal) and stress.

These materials have an edge over conventional superalloys as high temperature structural materials and are, therefore, being considered for new generation reactors such as, accelerator driven systems, advanced high temperature reactor, fusion devices and reusable launch vehicles.

Amongst the high temperature alloys, the refractory metal alloys are the only materials for structural applications beyond 900°C. Molybdenum base alloys such as, TZM, are the most suitable ones amongst the refractory metal alloys for application temperatures up to 1500°C.

Amongst the different molybdenum base alloys, TZM (Mo-0.5Ti-0.1Zr- 0.02C)) is the most suitable one, in terms of yield strength to density ratio, at temperatures above 900°C. In this alloy, small amounts of titanium, zirconium and carbon are added, so as to obtain a coarse distribution of carbides with some titanium and zirconium remaining in solid solution. The high strength of this alloy at elevated temperatures and its excellent corrosion resistance against liquid metals makes it suitable for application in advanced high temperature nuclear reactors.

Other alloys of this class include: TZC (1.2% titanium, 0.3% zirconium, 0.1% carbon), MHC (1.2% hafnium, 0.05% carbon), and ZHM (1.2% hafnium, 0.4% zirconium, 0.12% carbon). The preparation and fabrication of TZM components is challenging due to its high melting temperature (~2600°C). Of the feasible methods of preparation and fabrication of TZM alloys, viz. (i) aluminothermic reduction of mixed oxides followed by arc or electron beam melting, (ii) component melting and (iii) powder processing, the last route has the advantage of ease of operation, consistency of alloy composition, homogeneity and phase distribution.

The present work was taken up with the aim of producing TZM and its components by the powder processing route consisting of the following steps: preparation of pure Mo powder by hydrogen of reduction of MoO3, preparation of TZM alloy powder from elemental powders by mechanical alloying, compaction of powders into green compacts and sintering for densification. Each of these steps are described in the following sections.

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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 24, 2011 8:41 pm 
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Thanks Axil, that's HUGE. I've looked at powder metallurgy techniques and I believe that for MSR/LFTR they applications could be many and extremely powerful. I did not know that TZM could formed through powder techniques and hot isostatic pressing (HIP). One can make all many of very useful geometries to fine tolerances and extremely high material properties, ie much stronger an denser than cast material and as good as wrought material, sometime slightly better.


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 24, 2011 9:17 pm 
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Breeder issues, since the slow thermal reactor fleet needs breeders to feed it. First wall issues? What about graphite replacement?


No grahite.

If the fusion core of a fusion breeder is small enough (one to two meters in diameter), the fusion core can be submerged into the center of a molten chloride salt thorium blanket. This core can be replaced yearly by lifting it out of the removable top of the hot cell. A few replacement fusion cores could be kept in reserve as replacements to keep the mean time to repair (MTTR) low.

After it cools off, the fusion core that has been removed for maintenance could be refurbished at a leisurely pace in a shirt sleeve environment in a remote maintenance area during the year.

When this rebuild is complete, the fusion core is returned to the inventory of spare fusion cores.

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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 24, 2011 9:28 pm 
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Supposing TZM is suitable for the first wall of LFTR but it is too hard/expensive to make everything out of it. Do you think we would have severe problems using a mix of Hastalloy for most things but TZM for the first wall?

We could make the first wall pretty simple (in the extreme a straight cylinder but definitely better if we could do something like:

| |
| |
/ \
/ \
| |
| |
\ /
\ /
| |
| |

We could have no hard connections to the Hastalloy and allow enough slack for the differences in TCE.

Would we have problems with:
1) dielectric effects due to the two metals being in contact with each other
2) fluoride making the metals so clean that they weld together and this creating problems when the temperature changes


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 Post subject: Re: Lifespan of a LFTR?
PostPosted: Feb 25, 2011 2:43 am 
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Hastalloy-N is 16% molybdenum. Doesn’t that mean that nickel and molybdenum are totally compatible? It seems to me that a molybdenum first wall could be coated with a layer of NI60 if required. I say Ni60 because it does not have the helium inclusion problems that Ni58 and Ni59 do. A coating of a specific nickel isotope will be relatively inexpensive since there is not much of it required. Molybdenum could supply the high temperature strength. Since nickel does not need to carry any load to speak of, the coating could get very hot (1450C) without ill effect. Another good thing, the nickel coating would keep the fluoride from getting to and eating away at the titanium and zirconium in TZM near the surface.

The only reason why the old timers never considered using molybdenum was that they couldn’t figure out how to fabricate reactor structure using it. They added lots of nickel to make it malleable.

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