POTENTIAL FOR NANOSTRUCTURED COATINGS FOR MAGNETICALLY CONFINED PLASMA FUSION REACTOR - FIRST WALL APPLICATION

In the field of nuclear fusion, like many other industrial fields, materials survivability in harsh operating environments is of major obstacle towards full commercialization.  One of the most challenging applications from a materials point of view involves protection for the plasma-facing first wall ^{[[i], [ii], [iii]]} of a magnetically confined plasma fusion reactor.  The first wall must possess the following qualities:

• Resistant to electric arcs
• Resistant to high temperatures
• Resistant to chemical attacks (i.e., chemical sputtering)
• Resistant to high temperatures
• High hardness
• Low specific weight
• High thermal conductivity
• High cross section for neutron absorption
• Resistant to physical wear
• Resistant to irradiation

Identifying a single material that can meet all these requirements has been elusive.  Some significant gains in attaining these requirements have been realized with the introduction of coatings.  Coatings such as thermal spray B4C and W have proven to enhance the life of first walls.

B4C brings with it high hardness, low specific weight, high cross section for neutron absorption, as well as, resistance to heat, wear and corrosion.  Although B4C has a relatively low thermal conductivity, this can be compensated by applying the coating onto a conductive substrate such as graphite.  Should a nanostructured coating of B4C be successfully developed and applied, one may see further enhancements in wear resistance and hardness.  A couple of additional possible enhancements that are specific to this application include increases in both electric arc resistance and neutron absorption cross section.  A few research studies on arc erosion/work function of nanostructured metal and metal-matrix composite materials in bulk and coating forms have shown that a finer grain structure leads to reduced work function and, hence, increased arc erosion resistance ^{[[iv], [v], [vi]]}.  Artem’ev ^[[vii]] states that since the structural units of a dispersed nanoparticles is comparable to the wavelength of a slow neutron, neutron diffusion coefficient may decrease, resulting in an increase in neutron absorption compared to its conventional counterpart powder.  To transfer this characteristic to coatings, the key would be to try to attain a structure with uniformly dispersed nanopores to simulate dispersed nanoparticles.  It is important to note that depositing B4C is quite challenging due to issues of decomposition during spraying.

Tungsten and its alloys’ high thermal conductivity, resistance to sputter erosion and high melting temperatures have led to their use as a coating in first wall application ^{[[viii], [ix]]}.  To date, most of the thermal spray deposition techniques used for W have been via the plasma spray processes, APS or VPS, due to its high melting temperature.  Because of its high melting temperature, attaining a uniform W coating with limited porosity and unmelts is difficult to achieve.  In addition, tungsten’s low coefficient of thermal expansion (CTE) relative to many metals often creates high residual stress at the interface that can lead to reduced bond strength and cracking.  Should a dense and flawless nanostructured W coating be successfully deposited for first wall application, one will likely see improvements in resistance to arc erosion, wear, and corrosion.  To retain the nanostructure at elevated temperatures, one may have to incorporate an immiscible send phase material to help pin the grain boundaries and mitigate grain growth.

There are potential benefits to introducing nanostructured thermal spray coatings for first wall application.  These benefits may contribute towards the safe introduction of magnetically confined plasma fusion reactors for commercial use.

REFERENCES

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[i] H.R. Baharvandi, N. Talebzadeh, N. Ehsani, and F. Aghand Synthesis of B4C-Nano TiB2 Composite Powder by Sol-Gel Method, Journal of Materials Engineering and Performance Volume 18(3) April 2009, 273-277

[ii] L. B. Begrambekov, O. I. Buzhinskij, Features and advantages of boron carbide as a protective coating of the tokamak first wall, Plasma Devices and Operations Vol. 15, No. 3, September 2007, 193–199

[iii] D. M. MATTOX, A. W. MULLENDORE, H. 0. PIERSON and D. J. SHARP, LOW Z COATINGS FOR FUSION REACTOR APPLICATIONS, Journal of Nuclear Materials 85 & 86 (1979)1127-1131

[iv] C. ZHANG, Z. YANG, and B. DING, LOW ELECTRODE EROSION RATE OF NANOCRYSTALLINE CuCr-50 ALLOY IN VACUUM, Modern Physics Letters B, Vol. 20, No. 21 (2006) 1329{1334

[v] L. Rao, R.J. Munz  and J.-L. Meunier, Vacuum arc velocity and erosion rate measurements on nanostructured plasma and HVOF spray coatings, J. Phys. D: Appl. Phys. 40 (2007) 4192–4201

[vi] J.-S. Kim, Y.-S. Kwon, D. V. Dudina , O. I. Lomovsky, M. A. Korchagin and V. I. Mali, Nanocomposites TiB2-Cu: Consolidation and erosion Behavior, Journal of Materials Science, Volume 40, No. 13, July, 2005, pp. 3491-3495

[vii] V. A. Artem’ev, ESTIMATE OF NEUTRON ATTENTUATION AND MODERATION BY NANOSTRUCTURAL MATERIALS, Atomic Energy, Vol. 94, No. 4, 2003

[viii] F Koch and H Bolt, Self passivatingW-based alloys as plasma facing material for nuclear fusion, Phys. Scr. T128 (2007) 100–105

[ix] R. Montanari, B. Riccardi, R. Volterri, L. Bertamini, Characterisation of plasma sprayed W coatings on a CuCrZr alloy for nuclear fusion reactor applications, Materials Letters 52(2002) 100–105

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca