Thursday, February 28, 2008

Carbon-Carbon Composites in Molten Fluoride Salt Reactors

I have begun a review of material input into molten salt reactors. I began a review of metals, but quickly realized that carbon-carbon composites represented a viable and interesting alternative to the nickel based Hastelloy H usually assumed to be the best material for building MSRs. Carbon-Carbons are quite expensive, Hastelloy H is also expensive, but less so than Carbon-carbon composits. Hastelloy H is mainly nickel, and the nickel is one of those materials thatr has undergone rapid inflation during the last few years. Thus plans for liquid fluoride reactors must consider the future cost of Hastelloy H as well as the advantages of alternative materials.

Carbon Carbon composites have been identified as having high potential for use in reactors. L.M. Manocha, A. Warrier, S. Manocha and D. Sathiyamoorthy report that carbon-carbon composites possess high thermal conductivity and ability to retain mechanical properties even at extremely high temperatures carbon-carbon materials can survive neutron bombardment. Manocha, Warrier, Manocha and Sathiyamoorthy state that the effects of “neutron interaction and Wigner energy [on] their microstructure has to be properly controlled through proper choice of fibrous materials, matrix precursor and processing route.”

Charles W. Forsberg, and associates envisioned reactors operating at temperatures as hjgh as 2300 K. Such reactors must be “built entirely from carbon-based materials that use salts (liquid or gas) as the heat transfer medium between the reactor and power-generation equipment and/or heat rejection systems to create reactor systems with very high power-to-mass ratios.”

Forsberg, et al report, “Based on theoretical considerations and the developments in carbon-
carbon technologies over the last 20 years, such machines appear to be potentially viable.
However, significant research is required to demonstrate feasibility and a major long-term
development program would be required to build such machines. “

They further report:

"Only two classes of fluids are chemically compatible with carbon-based materials: inert gases
(e.g., helium-xenon mixtures) and fluoride-based salts. Liquid metals are incompatible with
carbon-based materials."

Furthermore, “Carbon-carbon composites1 can operate at higher temperatures than other materials (Fig. 1), retain their room-temperature strength at temperatures up to >2500K (>2225°C), and have much higher strength-to-weight ratios than other candidate materials. The theoretical peak temperatures of carbon-based materials are limited by the carbon sublimation temperature of ~3350°C, which is close to the 3400°C melting point of tungsten. Carbon-carbon composites have been demonstrated to (1) maintain reproducible strength at 1650°C, (2) withstand large thermal gradients, (3) have low coefficients of thermal expansion and thus the potential to minimize thermal stresses, (4) have tolerance to impact damage, and (6) be manufacturable.”

Additionally: “Carbon-carbon composites have been developed for fusion2 and fission3 applications with short-term operating temperatures up to 1600°C. Graphite and carbon-carbon composites are used in a variety of high-temperature nuclear reactors. The characteristics of these materials under neutron radiation have been extensively studied and are dependent upon the specific material, the type of neutron damage, and the temperatures. In most cases, radiation damage is reduced as temperatures increase.”

Also: "Carbon-carbon composites have been developed for fusion and fission applications with short- term operating temperatures up to 1600°C."

Forsberg, et al note, “Two major challenges exist for very high temperature reactor applications of carbon-based materials.

* Radiation damage. Carbon-carbon composites and graphites are used in many nuclear
reactors; however, these materials degrade when subjected to in-core radiation levels. At
the same time, it is known that most types of radiation damage are reversed by treatment
at temperatures near 2000°C. Theoretical considerations and indirect experimental
evidence suggest that at very high temperatures there may be sufficient self-healing of the
carbon materials to enable extreme fuel burnups and long-term operations at temperatures
between 1500 and 2000°C
* Permeability. Unlike metals, the permeability of composites is not necessarily zero.
Development of very low permeability carbon-carbon composites for operation at
extreme temperatures for very long time periods may be a major challenge. Current
successful techniques include infusion of carbon and sometimes other materials into the
matrix by a variety of techniques. However, none of these methods have been tested for
very high temperatures and very long time periods.”

L.M. Manocha, A. Warrier, S. Manocha and D. Sathiyamoorthy have report that the effects of “neutron interaction and Wigner energy [on] their [carbon-carbon] microstructure has to be properly controlled through proper choice of fibrous materials, matrix precursor and processing route.”

It would thus appear that Carbon-Carbon composites are extremely well suited for liquid fluoride salt cooled reactors. And would be ideally suited for heat exchange materials in a liquid fluoride salt to helium heat exchange. Such a reactor would have the potential of capable of operating at a temperature limit set by the boiling point of fluoride salts – a little over 1400 C. Thus would enable a liquid fluoride salt reactor to operate at a remarkably high level of thermal efficiency.

The principle disadvantages of carbon-carbon composites might be their cost, and the problem of Wigner energy, the deformation of graphite structures by exposure to neutron radiation. If Manocha, Warrier, S. Manocha and Sathiyamoorthy are correct, the problem of Wigner energy on carbon-carbons can be avoided by materials selection and adopting certain processing approaches. Cost penalties, if any for the use carbon-carbon materials can be recouped through the higher electrical output from the more efficient system. But the case has been made that Carbob-barbons will actually lower manufacturing costs. It should be noted that liquid metal reactors are incompatable with carbon-carbon composites, and thus unlike reactors that use fluoride salts coolants, liquid metal reactors cannot take advantage of CCC's high heat tolerence and the benefits of thermal efficiency that it brings.

Charles W. Forsberg, Per F. Peterson, and HaiHua Zhao note that "Liquid silicon infiltrated carbon-carbon compositesmposites provide a potentially very attractive construction material for high-temperature heat exchangers, piping, pumps, and vessels for MSRs, because of their ability to maintain nearly full mechanical strength at high temperatures (up to 1400°C), the simplicity of their fabrication, their low residual porosity, their capability of operating with
high-pressure helium and molten fluoride salts, and their low cost."

There you have it, you can build a fluoride salts reactor on the cheap with carbon-carbon technology. Industrial production would not be difficult, "Chopped carbon fiber can provide a particularly attractive material that can be readily formed by pressing with dies, machined using standard milling tools, and assembled into complex parts. "

There you go.

Forsberg, Peterson and Zhao conclude: "Three technological developments since then (Brayton power cycles, compact heat exchangers, and carbon-carbon composites) have (1) eliminated or significantly reduced several major technical issues associated with MSRs, (2) created the potential for major improvements in performance, 2400 MW(t) Power Conversion System Point and (3) significantly reduced costs. These major technological advances and changing goals for nuclear reactors strongly support a major investigation and assessment of MSRs as future GenIV reactors for deployment. "

We have then clear roadmap to the future of energy for the entire world. Carbon goes into the reactor as building material, not as fuel. This solution will lower reactor building costs, and produce a reactor that is a virtually a renewable source of electrical energy.

Clearly then the flawed and politically motivated judgement of WASH-1222 should be reversed. In comming post I intend to layout the numerous advantages of the Molten Salt Reactor concept. These include the superior safety - no China syndrome with an always molten core - elimination of the problem of nuclear waste, a reactor that can breed new nuclear fuel without the risk of nuclear proliferation, and ability to use plentiful, virtually renewable thorium far more efficiently than uranium is now used as a nuclear fuel. And, of course, carbon-carbon liquid fluoride salt reactors can be mass produced.

Not only do I think that this is what should happen, I think that this is what will happen.  


DV8 2XL said...

This is an interesting line of inquiry. If my experience in aviation is any guide, there will be about a twenty year lag between initial experiments and widespread use of carbon composites. The advantage that the nuclear industry has is that aviation has done most of the leg work on fabrication and process issues, which might shorten the lag time.

Fifi said...

How about so-called vitreous carbon ? It's used at very high temperatures with very good oxidative corrosion properties. It goes above 3000K in fine metallurgy and ceramic melting .

If the structure is truly amorphous, I would tend to think it would do rather well against swelling and fractures.

DV8 2XL said...

If I'm not mistaken there is some work with reticulated vitreous carbon for sodium purification system to capture cesium in fast reactors.

friend2all said...

Just a little suggestion -
Hastelloy-H mentioned in this post is probably Hastelloy-N which is an ASME Section III material approved for use in nuclear reactors. Hastelloy-N has been shown by ORNL to be resistant to attack by fluoride salts.

Jim Bowery said...

See the discussion of glassy carbon plumbing for LFTRs.


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