J.B. Whitley

Bulk-boronized graphites for plasma-facing components in ITER

Abstract Newly developed bulk-boronized graphites and boronized C-C composites with a total boron concentration ranging from 1 wt% to 30 wt% have been evaluated as plasma-facing component materials for the International Thermonuclear Experimental Reactor (ITER). Bulk-boronized graphites have been bombarded with high-flux deuterium plasmas at temperatures between 200 and 1600 °C. Plasma interaction induced erosion of bulk-boronized graphites is observed to be a factor of 2–3 smaller than that of pyrolytic graphite, in regimes of physical sputtering, chemical sputtering and radiation enhanced sublimation. Postbombardment thermal desorption spectroscopy indicates that bulk-boronized graphites enhance recombinative desorption of deuterium, which leads to a suppression of the formation of deuterocarbon due to chemical sputtering. The tritium inventory in graphite has been found to decrease by an order of magnitude due to 10 wt% bulk-boronization at temperatures above 1000 °C. The critical heat flux to induce cracking for bulk-boronized graphites has been found to be essentially the same as that for non-boronized graphites. Also, 10 wt% bulk-boronization of graphite hinders air oxidation nearly completely at 800° C and reduces the steam oxidation rate by a factor of 2–3 at around 1100 and 1350 °C.

Beryllium—a better tokamak plasma‐facing material?

The plasma–material interaction and high heat flux properties of beryllium are reviewed to determine its suitability as a plasma‐facing component in magnetic fusion energy reactors. Consideration is given to beryllium’s outgassing, erosion, and hydrogen retention characteristics. Its responses to normal and off‐normal high heat fluxes are compared to graphite in both the as‐received and the neutron‐irradiated states. Beryllium’s performance in present‐day devices is assessed, and its expected behavior in future reactors is summarized. It is concluded that beryllium is potentially a better plasma‐facing material than graphite and that more development and testing is warranted.

The development and laboratory testing of low Z refractory coatings for fusion reactor limiters

Abstract A materials development program for low Z ( Z ) coated tokamak limiters is described. A variety of refractory coatings applied by chemical vapor deposition, plasma spraying, and chemical conversion to graphite and copper substrates have been evaluated. The results of laboratory testing for low energy hydrogen ion erosion, arc erosion, and thermal fatigue by pulsed electron beam heating are reported. The 250-V hydrogen ion erosion data, analysed in terms of the effect on plasma radiation losses from resultant contaminants, indicates near equivalence of the very low Z (B and B4C, for example) and the moderate Z (e.g. TiB2) coatings. Thermal fatigue testing has identified several candidate materials which can survive 1000, 3 kJ/cm2 pulses of electron beam heating. The development of prototype limiters of TiB2, TiC, and B coated graphite for testing in the ORNL ISX-B tokamak is described.

Thermal-shock- and fatigue-resistant coatings for magnetically confined fusion environments☆

Abstract Candidate coating systems of materials of low atomic number Z under development for fusion reactor applications were exposed to a maximum of 1000 cycles of electron beam pulse heating at 2 kW cm-2 for 1.5 s. The coating/substrate systems that we investigated include chemically vapor-deposited TiB2/graphite, chemically vapor-deposited TiC/graphite, chemically vapor-deposited B/graphite, plasma-sprayed TiB2/Cu, clad V/Cu, clad Ti/Cu and clad plus borided VB2/V/Cu. All the thin coatings obtained by chemical vapor deposition (CVD) survived the testing as did the plasma-sprayed TiB2/Cu. Coatings of CVD TiB2 and CVD TiC on Poco graphite showed fatigue cracking which may have propagated into the graphite substrates. Some of the clad materials showed failure due to coating melt and deformation and separation due to thermal ratcheting.

Preliminary design analysis of the ALT-II limiter for TEXTOR

Abstract Installation of a large toroidal belt pump limiter, Advanced Limiter Test II (ALT-II), on the TEXTOR tokamak at Julich, FRG is anticipated for early 1986. This paper discusses the preliminary mechanical design and materials considerations undertaken as part of the feasibility study phase for ALT-II. Since the actively cooled limiter blade is the component in direct contact with the plasma edge, and thus subject to the severe plasma environment, most preliminary design efforts have concentrated on analysis of the blade. The screening process which led to the recommended preliminary design consisting of a dispersion strengthened copper or OFHC copper cover plate over an austenitic stainless steel base plate is discussed. A 1 to 3 mm thick low atomic number coating consisting of a graded plasma-sprayed Silicon Carbide-Aluminum composite is recommended subject to further experiment and evaluation. Thermal-hydraulic and stress analyses of the limiter blade are also discussed.

Mechanical properties of chemical vapor deposited coatings for fusion reactor application

Chemical vapor deposited coatings of TiB2, TiC and boron on graphite substrates are being developed for application as limiter materials in magnetic confinement fusion reactors. In this application severe thermal shock conditions exist and to do effective thermo‐mechanical modelling of the material response it is necessary to acquire elastic moduli, fracture strength, and strain to fracture data for the coatings. Four point flexure tests have been conducted from room temperature to 2000 °C on TiB2 and boron coated graphite with coatings in tension and compression and the mechanical properties extracted from the load‐deflection data. In addition, stress relaxation tests from 500 ° to 1150 °C were performed on TiB2 and TiC coated graphite beams to assess the low levels of plastic deformation which occur in these coatings. Significant differences have been observed between the effective mechanical properties of the coatings and literature values of the bulk properties.

Thermal-fatigue testing of coatings for fusion-reactor applications

Abstract Thermal fatigue testing was performed on eight coated or clad materials which have potential application as limiters in pulsed tokamak fusion devices. They are (1) chemically vapor-deposited coatings of TiC, TiB 2 and boron on graphite, (2) plasma-sprayed TiB 2 on copper, (3) a chemical conversion coating of VB 2 on vanadium-clad copper, (4) titanium-clad copper and (5) vanadium-clad copper. Testing consisted of up to 1000 cycles of electron beam heating for 1.5 s at beam power densities of 1 and 2 kW cm −2 . Three materials, chemically vapor-deposited TiC and TiB 2 on graphite, and plasma-sprayed TiB 2 on copper, survived the 1000 cycle 2 kW cm −2 test with slight but acceptable damage. The most notable test failure was VB 2 on vanadium-clad copper which deformed severely by a thermal ratcheting mechanism and displayed subsurface melting.

Correlation of experimental and theoretical results for vaporization by simulated disruption

Abstract Samples of Al, 1100 Al, SS 304, Ni, Mo, and TZM were subjected to a simulated plasma disruption produced by a 25 kV electron beam for times of 100 to 500 ms with an energy flux of 0.5 to 7.0 kJ/cm2. The net vaporized thickness was measured as a function of absorbed energy density for experimental results and predicted by theoretical models. Agreement was found for the threshold energy density for vaporization and the functional dependence of vaporization on energy density over a wide range of parameters. The usefulness of electron beam experiments to verify and guide the development of vaporization models for high heat flux component design was shown.

Particle removal with pump limiters in ISX-B

Abstract The first pump limiter experiments were performed on ISX-B. Two pump limiter modules were installed in the top and bottom of one toroidal sector of the tokamak. The modules consist of inertia cooled, TiC-coated graphite heads and ZrAl getter pumps each with a pumping speed of 1000–2000 l/s. The objective of the initial experiments was the demonstration of plasma particle control with pump limiters. The first set of experiments were performed in ohmic discharges (OH) in which the effect of the pump limiters on the plasma density was clearly demonstrated. In discharges characterized by Ip = 110 kA, B T = 15 kG , n e = 1−5 × 10 13 cm −3 and t = 0.3 s, the pressure rise in the pump limiters was typically 2 mTorr with the pumps off and 0.7 mTorr after activating the pumps. When the pumps were activated, the line-average plasma density decreased by up to a factor 2 at identical gas flow rates. The second set of measurements were performed in neutral beam heated discharges (NBI) with injected powers between 0.6 MW and 1.0 MW. Due to a cooling problem on one of the ZrAl pumps, the NBI experiments were carried out with one limiter only. The maximum pressure observed in NBI-discharges was 5 mTorr without activating the pumps, i.e., approximately twice as high as in OH-discharges. The exhaust efficiency, which is defined as the removed particle flux divided by the total particle flux in the scrape-off layer, is estimated to be 5%.

Depth dependent void swelling rates in self-ion irradiated nickel

Abstract Specimens of high purity nickel were irradiated with high energy heavy ions and the resultant microstructure examined along the ion path using a cross sectioning sample preparation technique. Void nucleation was found to be very sensitive to hydrogen introduced into the samples by electropolishing prior to irradiation. Samples containing hydrogen had void densities of about two orders of magnitude greater than outgassed samples. A series of hydrogen doped samples were irradiated with 14 MeV nickel ions to fluences from 2 × 1015ions/cm2to 1.4 × 1017ions/cm2 (peak damage from 2 to 150 dpa). The increase in the collision cross section as the incident ion slows down causes an increase in displacement rate with depth. The variation in the observed void density, void size and void swelling rates with depth indicate the importance of displacement rate on void simulation studies.