Energy From Thorium Discussion Forum

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PostPosted: Aug 08, 2015 11:30 am 
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Anyone know if 304 Stainless is compatible with molten fluorides. The attached link is to a paper about an improved Stainless steel fast reactor cladding material. Could it be useful in a MSR?
http://www.researchgate.net/publication/270965682_Superior_radiation-resistant_nanoengineered_austenitic_304L_stainless_steel_for_applications_in_extreme_radiation_environments

Mike


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PostPosted: Aug 08, 2015 11:32 am 
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DOI: 10.1038/srep07801

just in case the link doesn't work.
mike


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PostPosted: Aug 08, 2015 4:18 pm 
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Yes, all the 300 series austenitic stainless steels are good with fluorides. The lower chromium ones do better. 304 and 316 do well, proven by ORNL tests. The low carbon versions have limited creep strength, however, unless you get solid solution strengthening or other strengthening mechanism as substitute (like most superalloys do).


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PostPosted: Aug 09, 2015 4:43 pm 
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I looked at the paper closely.

It appears that this grain size engineering, while high tech, is a typical trade-off that James Watt also faced: more strength for less ductility. Its too bad the authors of the paper did not check fracture toughness and impact toughness. The stress-strain curve suggests much lower ductility than large grain boundary 304. Void swelling is one thing but with austentics, and pretty much any other material under irradiation, you always get embrittlement before void swelling, so there's not much point in not considering embrittlement first.

It also appears that the grain size increases a lot with exposure to >600 Celsius, which should lose the properties. See the hardness graph.

One also wonders how welding will affect the grain size. It would have to be welded very quickly with minimal heat input and perhaps using heat sinks. Otherwise the grains will undergo thermal induced growth.


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PostPosted: Aug 09, 2015 11:54 pm 
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My common-sense says that the surface should be protected from corrosion by Nickel and a Nickel Fluoride coating. The rest of material can be designed for stresses, heat and neutron flux. The stainless steels could well be fit for main structures.


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PostPosted: Aug 10, 2015 8:31 am 
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jagdish wrote:
My common-sense says that the surface should be protected from corrosion by Nickel and a Nickel Fluoride coating. The rest of material can be designed for stresses, heat and neutron flux. The stainless steels could well be fit for main structures.


Nickel may work, nickel fluoride won't work in salt wetted parts (only in gas areas).

I have mixed feelings about liners. On the one hand this is how LWRs are constructed, using a high strength steel for the vessel with corrosion resistant stainless liner. For LWRs there seems some sense to this approach.

But, if your vessel is already stainless steel (if for no other reason, high temperature means normal steels are unsuitable) then it is a more difficult tradeoff. The stainless won't corrode catastrophically, unlike the low alloy steel used in LWR RPVs. So the liner becomes more difficult to justify.

Another problem with a liner is that it is so thin you have to consider liner failure as an accident scenario (most likely). So then you still need to know corrosion rates. Yet another problem (maybe) is tellurium attack, I believe tellurium attacks nickel very rapidly. It attacks Hastelloy N, and that still has some chromium and other protective elements in it.

Yet another issue is this. Liners basically mean 2x the welds. Granted they are thin, but it looks expensive to me especially around the more complicated areas such as nozzles. A lot of nuclear grade welding is added.

Liners are rarely used in industry which suggests to me high cost.

300 series stainless corrodes at around 0.03 mm/year in fuel salt. Typically you get 0.1 mm quickly, then the surface chromium is depleted and corrosion rate trends down to 0.01 mm/year long term.

So if you are going to consider a (say) 6 mm nickel liner. Then why not just make the vessel 6 mm thicker stainless in stead? This should get you 600 years of corrosion allowance, plenty of design margin to convince regulators. And no tellurium attack, added welding, or dissimilar metal issues.

In actual fact based on ORNL data, the corrosion basically asymptotes out at about 0.5 mm, and even less at lower than 700C operating temperatures.


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PostPosted: Aug 11, 2015 12:36 am 
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At wear rates like .03mm per year, it is definitely preferable to provide for a 100 yrs margin and leave it at that.


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PostPosted: Aug 11, 2015 12:58 am 
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In case of molten salt fuel, it may be preferable to provide a metallic thorium wearing liner cum blanket in the main vessel. 233U created is more soluble in the fluoride salts used and could compensate for depletion of fissile fuel. Decades long intervals may be possible between fuel changes. It could effectively replace thorium salt in second fluid which could then be clean salt.


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