Fine-tuning Hastelloy-N in MSRs: Better, Costlier Alloys?

Molten salt has heat transfer properties similar to those of water. To verify the cooling effects of salt, just place some epsom salts in a glass of hot water and recheck the temperature.

Although the melting point of many salts hovers between 800 and 900 degrees Celsius, salts can be mixed so as to solidify at relatively low temperatures (400 to 500 degrees Celsius), making them suitable for use as coolants in nuclear reactors operating above these temperatures. (It may even be possible to use low-melting point salts to transfer solar heat.)

More Efficiency, Less Pressure

According to Encyclopedia of Earth, the MSR is "not a fast reactor, but with some moderation by the graphite," its neutrons interact at intermediate speeds. Higher temperature reactors mean more efficient burning of fuel and nuclear waste, but salt is so efficient at transferring heat that using it in a reactor yields, according to Sabharwall et. al., an increase in reactor efficiency equivalent to upping reactor temperature by almost 200 degrees Celsius. Molten salt also puts little pressure on pipes. (2010; Idaho National Laboratory)

Lowered Risk for Proliferation?

Molten salt fuel is, initially, thorium, relatively abundant when compared with Uranium-235 used in Generation III reactors. Thorium "breeds" another kind of uranium fuel, Uranium-233.

Some proponents of MSRs believe that issues incurred in making weapons from Uranium-233, plus the fact that all fuel is dissolved in the salt in a closed cycle reduce security issues with the reactors, as do the MSR's more complete burning of waste. The relatively stable (half-life of more than 0.33 million years; not fissile by slow neutrons) Plutonium-242 is the main plutonium isotope in the MSR. Plutonium-242 is not used in proliferation.

Issues

Alas, molten salt reactors still need some tweaks. These may delay the adoption of the technology in the U.S.

Issues include:

  1. Excessive time required for thorium-based fuel in the molten salt reactor to "become critical" (produce enough fuel to be useful; this may be solved with the addition of Uranium-235 to the fuel salt).
  2. Concerns about fuel salt leakage into "blanket" (cooling salt; an issue physicist LeBlanc proposes to solve by having fuel salt without blanket salt in the core).
  3. Problems with the graphite core (with possible "work arounds") and disposal of graphite.
  4. Storage of excess Uranium-233.
  5. Poison fission products (such as proactium), which either must be filtered from the salt, posing cost and security issues, or left in.

Corrosion

Another problem with salt reactors is corrosion, which worsens as temperatures rise. Corrosion affects nickel alloys that coat the pipes and outside of the reactor core, although at present operating temperatures (700 degrees C), with today's Hastelloy-N alloy, corrosion is relatively low.

Perhaps because of this the U.S. has to date preferred sodium-cooled reactors for generation IV, in spite of molten salt's "passive cooling," plus built-in safety valve which works without electricity (a salt plug melts if things get too hot, letting the radioactive salt into storage tanks; but for this pipes have to work), and in spite of relative stability of salt over sodium. Also, because of radiation product deposits, the alloy forms cracks, particularly at higher temperatures.

Iconel: the Original Alloy

Iconel, a nickel-based alloy containing chromium and iron, was the first alloy developed for molten salt reactors. Nickel corrodes little if at all in fluoride salts. Not so for this nickel alloy: corrosion rates for it were "excessive" for long-term use at above 700 degrees Celsius.

INOR-8/Hastelloy-N

When Iconel proved unsatisfactory, INOR-8, available under the trade name Hastelloy-N, a nickel-based alloy with 16% niobium, 7% molybdenum, 5% chromium, and 0.05% iron, was developed at Oak Ridge National Laboratory for use in molten salt reactors with fluoride salts, say McCoy and McNabb (1972). Hastelloy-N, "was found to afford the best combination of strength and corrosion resistance among the alloy compositions tested," says DeVan (1969).

Chromium and Molybdenum in Hastelloy-N

The amount of chromium in the alloy had to be just right: in the presence of oxygen, the chromium in the alloy helps reduce corrosion, but excessive chromium dissolves into the flouride salts, leaving "perforations" in the alloy, weakening it. Molybdenum, which may keep iron from dissolving out of the alloy, was also included. Molybdenum gives strength at high temperatures (Delpech, et. al.)

Higher Temperatures?

Typical molten salt reactors, as noted, have medium neutron speeds, operating at around 700 degrees Celsius, where Hastelloy-N works fairly well. However, French researchers at the Centre Nationale de la Recherche Scientifique (CNRS) have proposed a high-speed neutron (fast) molten salt reactor, with operating temperatures reaching 850 degrees Celsius.

Hastelloy-N Problems: Chromium, Electrical Acceleration of Corrosion, Cracking

In nickel-molybdenum-chromium or nickel-tungsten-chromium alloys, most "corrosion is due to the dissolution of chromium," say the French. The higher the temperature, the more chromium dissolves in fluoride salts. Also graphite in the core (graphite modulates the reactor core) can act as a "cathode" with the piping acting as an "anode," further increasing corrosion. (Note that graphite itself does not corrode; graphite's also resistant to neutron decay.) And with corrosion, tritium (radioactive hydrogen) may leak out.

Another problem is post-irradiation brittleness. At above 750 degrees Celsius, under radiation, tellurium fission products leave precipitates in alloy grains, causing brittleness and "intergranular cracking." Such cracking has sometimes been controlled by controlling fuel chemistry, according to MacPherson's "The Molten Salt Reactor Adventure."

For Corrosion: Simple Coating?

Corrosion may be reduced perhaps if the alloy is coated with chromium, or alternately nickel. Chromium, after all, quickly forms oxides, and a thin oxidized layer where the alloy interacts with the salt may delay further oxidation.

Inexpensive Aluminum in Alloys: Plus or Minus for Corrosion?

DeVan (1969) reported corrosion increased as aluminum was added into alloys containing chromium. DeVan also reported that aluminum was problematic in alloys containing titanium. However, in spite of aluminum's low melting point, close to that of the molten salt reactor's operating temperature, DeVan argued that, without chromium or titanium in the alloy, and at below 2% concentrations of aluminum, aluminum formed highly stable compounds.

Despite problems caused by combining chromium and aluminum in alloys, DeVan also reported that introducing chromium plus aluminum corrosion products together inhibited corrosion of iron and niobium in the alloys. DeVan suggests that adding more elements to an alloy might serve to inhibit corrosion of any one element.

Alloys: Corrosion-Resistance, Strength

Additional elements also mean atoms of various sizes combine so as to fill voids in molecular structure, increasing strength. Of course, elements in alloys can also behave as distinct elements, melting out at different temperatures, although in some cases one element may bind to another rather than separate and dissolve, according to Energy from Thorium's discussion.

The salt used also affects corrosion. Aluminum may be less corrosive with beryllium salts than with other salts. Beryllium however is highly toxic and must be kept out of the environment.

Aluminum for Intergranular Cracking?

Alexander Surenkov (2010; Russian Research Center) investigated "intergranular cracking" that forms from deposits of tellurium fission products on metal grains, at current operating temperatures (650 or 700 degrees Celsius; maximum temperature 750 degrees Celsius). Surenkov, who compared cracking for various alloys at 200 and 1200 hours, recommends adding aluminum, plus decreasing titanium in the alloy, rather than adding niobium with titanium (used in aircraft and rocket engine alloys). However the rocket metals might nevertheless reduce intergranular cracking some he reported.

Molybdenum?

At Energy from Thorium's forums, it's argued that, since molybdenum is very flexible in its pure form, pure molybdenum might do. After all, molybdenum is highly corrosion resistant normally. The only thing needed to completely eliminate corrosion with molybdenum is a thin "overlay" of an "electrolytic copper coating," says one poster.

TZM, an approximately 99% molybdenum alloy with some titanium, zirconium, and chromium, resisted corrosion almost completely in fluoride and beryllium salt for 1100 hours at temperatures above 1300 degrees C. Some chromium and titanium in the alloy however were leached by the salt, with more leaching at higher temperatures. At increasing temperatures, TZM did not afford the strength of an alloy but behaved more like pure molybdenum. Nevertheless, India currently uses TZM in a Compact High Temperature reactor.

Tungsten for Higher Temperatures?

French researchers describe a possibly more efficient "MSFR" (Molten Salt Fast Reactor), designed without graphite modulation in its core, so as to operate at higher temperatures than those studied by Surenkov, up to 850 degrees Celsius. The French also removed beryllium fluoride from the salt.

The French alloy, like most, is nickel-based (TZM is not). Tungsten, with a slightly higher melting temperature than either niobium or molybdenum (and much higher than nickel), replaces the molybdenum in it. According to Delpech, tungsten resists corrosion better and is more easily shaped than molybdenum.

Chromium Corrosion With Tungsten

Dissolution of chromium nevertheless remains a problem with tungsten in the alloy. This say Delpech et. al. is best controlled by "control of salt properties." Since corrosion gets worse as more Uranium-233 (the primary fuel, bred from Thorium) is produced, Delpech et al. recommend adding more metallic thorium to the fuel salt in fluoride salt reactors.

Tungsten Under Radiation

Tungsten is likewise more susceptible than molybdenum to radiation, says Energy from Thorium's discussion. Delpech's team noted that it cracked some under radiation. To reduce cracking, they added titanium and niobium carbides with a tiny bit of chromium, according to Delpech et. al., while Russian researcher Surenkov favored aluminum (with a lower melting point) with decreasing titanium for his molybdenum alloy, used at lower temperatures, with beryllium fluoride in the fuel salt. The titanium and niobium carbide interface with "the nickel matrix" in their alloy say the French researchers, in such a way as to trap helium, including radioactive helium atoms.

Other Salts?

Corrosion of alloys varies, as noted, with the salt. The above information is for fluoride salts. Chloride salts are another salt option.

Cost of Metals

According to Metal Prices.com, niobium ($20.00 U.S. plus per pound), relatively rare, is more expensive than tungsten or molybdenum ($13.00 to $19.00 U.S. per pound; molybdenum's price is more stable than tungsten's, making molybdenum slightly more attractive pricewise), which in turn are more expensive than nickel (around $8.00 or $9.00 per pound), which is more expensive than copper ($3.50 per pound), which is more expensive than aluminum (around a dollar a pound). Iron ore is still cheapest. And rustiest, when no other elements are added.

Surenkov's relatively low-cost alloy with aluminum may work in some salts for lower temperatures. TZM (mostly molybdenum) and the nickel alloy with tungsten were investigated for higher temperatures.

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