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Tue Oct 1, 2019, 09:59 PM

Corrosion Implications for Silicon Carbide in FLINAK in Contact with Hastelloy N

The paper I'll discuss in this post is this one: Corrosion of SiC induced by Hastelloy N alloy and its corrosion products in LiFNaFKF molten salt (Xia et al,Corrosion Science 109 (2016) 6267)

I stumbled across this paper while looking into an anomalous result in polymer chemistry, of all things, as I had a sort of long shot idea that proved to be silly. Being wrong can be good for you, particularly if you go looking for literature evidence for a ridiculous idea only to stumble upon some truth you might have otherwise missed.

Among the enlightened who have understood the importance and environmental superiority of nuclear energy - which whether we like or not is the only practical approach to addressing climate change - the idea of the molten salt reactor, fueled by thorium, has generated a lot of attention in the last 10 or 15 years, especially in popular imagination. The general idea has been based on the commercial development MSRE (Molten Salt Reactor Experiment) conducted at Oak Ridge National Laboratory - a truly fascinating place as I learned this summer when my kid interned there - in the 1960's. Upon capturing a neutron, thorium-232, the prevalent isotope on Earth, decays to uranium-233, uranium-233 being essentially the only actinide isotope that can act as a breeder fuel on thermal conditions, thermal conditions being those under which the average speed of neutrons is taken to be roughly equivalent to the average speed of molecules in common gases, such as those in air at room temperature, generally taken to be 0.253 eV. A breeding fuel is, of course, a fissionable nuclide that can release enough neutrons to convert a fertile nuclide into a fissionable one.

I spent a lot of time looking into this type of reactor before concluding I liked other reactor concepts better; I'm a fast neutron kind of guy, because fast neutrons can breed, um, faster, than thermal (slow) neutrons, a happy parallelism in language for alternate meanings for homonyms. I think it important to phase out the expense, environmentally dubious work, and time for isotopic enrichment.

There are, however, proposals for fast molten salt reactors. Thermal molten salt reactors use a salt called "FLiBe" using the chemical symbols for fluorine, lithium and beryllium - light atoms that have the tendency to thermalize neutrons, although as a practical matter MSR's are moderated also with graphite - in a composition appropriate to produce a eutectic mixture. An alternative eutectic salt with less moderating ability is FLiNaK, composed of the symbols for fluorine, lithium, sodium and potassium. (A mixture of sodium and potassium metals has been widely used in breeder reactors running on plutonium; this metal coolant has been problematic, although no one ever seems to give up on it entirely.)

One proposal for a FLINAK cooled reactor is a sort of hybrid reactor, built around the concept of the helium cooled pebble bed reactor, which used something called TRISO fuel beads, immersed in a FLINAK molten salt. This reactor concept has been widely discussed by Charles Forsberg at MIT and Per Peterson at UC Berkeley.

TRISO (Tri-Structural Isotropic) fuel is a composite material which in part, consists of the refractory material silicon carbide, carbon and actinides in spherical layers.

Hastelloy is a "superalloy" which shows resistance to attack by fluoride (and inevitably fluorine generated in a radiation field) by virtue of being Nickel based. Superalloys also feature very high melting temperatures, in the case of Hastelloy N, this property is obtained by alloying nickel with molybdenum and chromium.

I'm not really a molten salt kind of guy anymore, considering that these salts have some major drawbacks. The first of these in both cases concerns lithium. Unless its two isomers are separated, 6 and 7, lithium under neutron bombardment tends to generate a lot of tritium. This would not be a problem in the case that someone someday builds a workable fusion reactor, something that's been 20 years off for well over half a century, and it might even be advisable as a source of light helium when the world supply of helium-4 runs out (it's definitively not "renewable" since tritium decays to the interesting isotope helium-3. Still it strikes me as a pain in the ass. FLIBE also contains beryllium, a toxic metal, which under neutron bombardment over long periods accumulates the long lived Beryllium-10 isotope which decays into the neutron poison boron-10.

This said, the most problematic nuclear reactor, even including the RBMK Chernobyl type reactor, is superior to the best dangerous fossil fuel plant in terms of risk, since dangerous fossil fuel plants kill people whenever they operate normally.

The FLiNaK fuel will tend to increase the concentration of the radioactive K-40 isotope, not terribly problematic inasmuch is this isotope occurs naturally, meaning human beings would die without ingesting a certain amount of radioactivity, but still is less than completely desirable to make potassium similar to that of a few billion years ago.

Both molten salts produce oxygen-18 as a result of certain nuclear reactions - 19F[n,2n]O18 - which might be of limited scientific utility for the study of chemical reaction mechanisms.

I don't think that any of these concerns are show stoppers, but for me, well, metallic fuels have the highest breeding ratios, and if I were to build a hybrid, it would involve liquid metals and certain types of interesting ceramics, perhaps with salts as coolants and heat transfer agents (which they are effectively in the TRISO/FLINAK hybrid) although I might choose different salts for different reasons.

Anyway, there are a number of ways in which silicon carbide can be utilized in nuclear technology besides TRISO. All this brings me to the paper referenced at the outset which discusses another application for SiC.

From the introductory text:

Molten salt Reactor (MSR) is one of six reactors proposed by the Generation IV International Forum [1]. There are graphite, molten fluoride salt, and Nickel-based alloy, such as Hastelloy N alloy, in MSR [2]. The Molten-Salt Reactor Experiment (MSRE) per-formed in the 1960s revealed that Hastelloy N alloy and graphite are compatible with molten fluoride salt [2]. However, the permeation of molten fluoride salt into porous graphite is a critical issue, which can create local hot spots where the damage rate would be increased [2]. Silicon carbide (SiC) coating can be used to protect graphite moderator against the permeation of molten fluoride salt [3]. SiC ceramic and SiC fiber reinforced SiC ceramic composite (SiCf/SiC) are potential materials for MSRs due to their superior high-temperature properties, corrosion resistance, and irradiation resistance. SiCf/SiC composite is also one of candidate materials for shut down rod channel liners, core support plates, ex-core control blade guides and wetted refueling mechanism in MSRs [4,5].Experiments performed at Oak Ridge National Laboratory revealed that SiC is not obviously corroded by molten FLiNaK (46.5 mol% LiF,11.5 mol% NaF, and 42 mol% KF) salt [6]. However, impurities may cause the corrosion of SiC in molten fluoride salt. There are intrinsic impurities from molten fluoride salt and impurities from the corrosion of Hastelloy N alloy. Oxygen is common impurity in molten fluoride salt. SiC may react with oxygen in salt to form oxide that can be corroded by molten fluoride salt [7,8]. Fluoride anions can cause the breakage of Si O Si bonds and the formation of Si F bonds [7]. Oxides of SiC may react with molten FLiNaK salt to formSiF4, K2SiF6, Na2SiF6, [SiO4]4−, [Si2O5]2−, [SiO3F]3−, [SiO2F2]2−, andSi4O7F2[79]. The corrosion of SiC may be aggravated because of an increase in the concentration of metal corrosion products in salts induced by the corrosion of Hastelloy N alloy. Hastelloy N alloy is nickel-based alloy and is composed of 1517 wt% Mo, 68 wt% Cr, 46 wt% Fe...

...Molten FLiNaK salt is one of candidate coolants in MSR due to its chemical and radiolytic stability [28]. In the whole MSR system, the salt-containing piping and equipment are composed of Hastelloy N alloy. The corrosion of Hastelloy N alloy can increase the concentration of metal impurities in salt. The effect of corrosion products of Hastelloy N alloy on the corrosion of SiC should be investigated. Ni is the base element of Hastelloy N alloy. Cr in Hastelloy N alloy can be easily corroded. Therefore, we studied the effect of Hastelloy N alloy and its corresponding corrosion products, CrF2and NiF2...

The pictures should give a feel for the ways they investigated these effects.

Fig 1. Raman spectrum of CVD SiC before and after corrosion in FLiNaK salt at 700 C for 15 days.

Fig. 2. Cross-section EDS line-scan spectrum of Hastelloy N alloy after corrosion with SiC in FLiNaK salt at 700 C for 15 days.

Fig. 3. (a) Cross-section SEM image of Hastelloy N alloy after corrosion with SiC in FLiNaK salt at 700 C for 15 days, (b) GIXRD pattern of Hastelloy N alloy before and after corrosion with SiC in FLiNaK salt at 700 C for 15 days.

Fig. 4. Surface SEM image of SiCf/SiC composite with CVD SiC coating, (a) and (b) is before experiment, (c) and (d) is after corrosion in FLiNaK salt with NiF2 at 700 C for 45 days. The inset of (a) is GI-XRD pattern with a fixed grazing angle of incidence at 0.3.

Fig. 5. Raman spectrum of pure FLiNaK salt and FLiNaK salt mixed with NiF2 after corrosion of SiC at 700 C for 45 days.

Fig. 6. (a) Surface SEM image of CVD SiC after corrosion with Hastelloy N alloy in FLiNaK salt at 700 C for 15 days, (b) GIXRD pattern of CVD SiC before and after corrosion with Hastelloy N alloy in FLiNaK salt at 700 C for 15 days, (c) surface SEM image and (d) GIXRD pattern of CVD SiC after corrosion in FLiNaK salt mixed with CrF2 at 700 C for 15 days.

4. Conclusions

The effect of Hastelloy N alloy and its corresponding corrosion products on the corrosion of SiC in molten FLiNaK salt was investigated by SEM/EDS, GIXRD, and GDMS. Results reveal that Hastelloy N alloy and the corresponding corrosion products can induce the corrosion of SiC in FLiNaK salt. Three factors affect the corrosion of SiC.

(1) Ni in Hastelloy N alloy can trigger the corrosion of SiC. Ni can react with Si in salt to form NiSi and Ni31Si12on the surface of Hastelloy N alloy. Silicide deposited on the surface of alloy can drive the dissolution of Si from SiC into salt and alloy.

(2) The corrosion product, NiF2can drive the corrosion of SiC. NiF2can make SiC with a thickness of ∼50 _m almost disappear after experiment for 45 days. Thus, the Si content in salt increased up to 0.5 wt%. Raman spectrum indicates that Si in salt is in the form of [SiF6]2−.

(3) The corrosion product, CrF2, can induce the corrosion of SiC. CrF2 can react with SiC to form Cr7C3 and CrC on SiC, and then induce the dissolution of element Si from SiC into salt.

From my perspective, it might be useful to evaluate various types of nitride, as opposed to carbide, coatings, but that's just my opinion. I'm not really a FLiNaK kind of guy, although I have been interested in other, less studied, fluorides.

I wish you a pleasant Wednesday tomorrow.

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