R.J. Pawelko

Steam-chemical reactivity for irradiated beryllium

This paper reports experimental results concerning the influence of neutron irradiation effects and annealing on the chemical reactivity of beryllium exposed to steam. The work entailed: (1) measurements of swelling, porosity and specific surface area for irradiated Be annealed at temperatures ranging from 700°C to 1200°C and (2) measurements of hydrogen generation rates for unirradiated Be, irradiated Be and irradiated-annealed Be exposed to steam at elevated temperatures. For irradiated Be, volumetric swelling increased from 14% at a 700°C anneal to about 56% at a 1200°C anneal. Gas-release measurements during annealing indicated the development of a surface-connected porosity network. Specific surface areas for irradiated-annealed Be increased with the anneal temperature. Steam-chemical reactivity was similar for irradiated and unirradiated Be at temperatures between 450°C and 600°C. For irradiated Be exposed to steam at 700°C, the reactivity accelerated rapidly and the specimen experienced a temperature excursion. Irradiated-annealed Be showed enhanced chemical reactivity related to its higher specific surface area.

Hydrogen transport behavior of beryllium

Beryllium is being evaluated for use as a plasma-facing material in the International Thermonuclear Experimental Reactor (ITER). One concern in the evaluation is the retention and permeation of tritium implanted into the plasma-facing surface. We performed laboratory-scale studies to investigate mechanisms that influence hydrogen transport and retention in beryllium foil specimens of rolled powder metallurgy product and rolled ingot cast beryllium. Specimen characterization was accomplished using scanning electron microscopy, Auger electron spectroscopy, and Rutherford backscattering spectrometry (RBS) techniques. Hydrogen transport was investigated using ion-beam permeation experiments and nuclear reaction analysis (NRA). Results indicate that trapping plays a significant role in permeation, re-emission, and retention, and that surface processes at both upstream and downstream surfaces are also important.

Hydrogen permeation properties of plasma-sprayed tungsten☆

Abstract Tungsten has been proposed as a plasma-facing component material for advanced fusion facilities. This paper reports on laboratory-scale studies that were done to assess the hydrogen permeation properties of plasma-sprayed tungsten for such applications. The work entailed deuterium permeation measurements for plasma-sprayed (PS) tungsten coatings, sputter-deposited (SP) tungsten coatings, and steel substrate material using a mass-analyzed, 3 keV D3+ ion beam with fluxes of ∼6.5 × 1019 D/m2 s. Extensive characterization analyses for the plasma-sprayed tungsten coatings were made using Auger spectrometry and scanning electron microscopy (SEM). Observed permeation rates through composite PS-tungsten/steel specimens were several orders of magnitude below the permeation levels observed for SP-tungsten/steel composite specimens and pure steel specimens. Characterization analyses indicated that the plasma-sprayed tungsten coating had a nonhomogeneous microstructure that consisted of splats with columnar solidification, partially-melted particles with grain boundaries, and void regions. Reduced permeation levels can be attributed to the complex microstructure and a substantial surface-connected porosity.

Tritium saturation in plasma-facing materials surfaces

Plasma-facing components in the International Thermonuclear Experimental Reactor (ITER) will experience high heat loads and intense plasma fluxes of order 10 20 -10 23 particles/m 2 s. Experiments on Be and W, two of the materials considered for use in ITER, have revealed that a tritium saturation phenomenon can take place under these conditions in which damage to the surface results that enhances the return of implanted tritium to the plasma and inhibits uptake of tritium. This phenomenon is important because it implies that tritium inventories due to implantation in these plasma-facing materials will probably be lower than was previously estimated using classical recombination-limited release at the plasma surface. Similarly, permeation through these components to the coolant streams should be reduced. In this paper we discuss evidences for the existence of this phenomenon, describe techniques for modeling it, and present results of the application of such modeling to prior experiments.

Steam Chemical Reactivity of Be Pebbles and Be Powder

Abstract This paper reports the results of chemical reactivity experiments for Be pebbles (2-mm and 0.2-mm diameter) and Be powder (14–31 μm diameter) exposed to steam at elevated temperatures, 350 to 900°C for pebbles and 400 to 500°C for powders. We measured BET specific surface areas of 0.12 m2/g for 2-mm pebbles, 0.24 m2/g for 0.2-mm pebbles and 0.66 to 1.21 m2/g for Be powder samples. These experiments showed a complex reactivity behavior for the material, dependent primarily on the test temperature. Average H2 generation rates for powder samples, based on measured BET surface areas, were in good agreement with previous measurements for fully-dense CPM-Be. Rates for the Be pebbles, based on measured BET surface areas, were systematically lower than the CPM-Be rates, possibly because of different surface and bulk features for the pebbles, especially surface-layer impurities, that contribute to the measured BET surface area and influence the oxidation process at the material surface.

Deuterium transport in Cu, CuCrZr, and Cu/Be

This paper presents the results of deuterium implantation/permeation experiments and TMAP4 simulations for a CuCrZr alloy, for OFHC-Cu and for a Cu/Be bi-layered structure at temperatures from 700 to 800 K. Experiments used a mass-analyzed, 3-keV D 3 + ion beam with particle flux densities of 5 x 10 19 to 7 x 10 19 D/m 2 S. Effective diffusivities and surface molecular recombination coefficients were derived giving Arrhenius pre-exponentials and activation energies for each material: CuCrZr alloy, (2.0 x 10 -2 m 2 /s, 1.2 eV) for diffusivity and (2.9 x 10 14 m 4 /s, 1.92 eV) for surface molecular recombination coefficients; OFHC Cu, (2.1 x 10 -6 m 2 /s, 0.52 eV) for diffusivity and (9.1 x 10 -18 m 4 /s, 0.99 eV) for surface molecular recombination coefficients. TMAP4 simulation of permeation data measured for a Cu/Be bilayer sample was achieved using a four-layer structure (Cu/BeO interface/Be/BeO back surface) and recommended values for diffusivity and solubility in Be, BeO and Cu.

Steam chemical reactivity of plasma-sprayed beryllium

Plasma-spraying with the potential for in-situ repair makes beryllium a primary candidate for plasma facing and structural components in experimental magnetic fusion machines. Deposits with good thermal conductivity and resistance to thermal cycling have been produced with low pressure plasma-spraying (LPPS). A concern during a potential accident with steam ingress is the amount of hydrogen produced by the reactions of steam with hot components. In this study the authors measure the reaction rates of various deposits produced by LPPS with steam from 350 C to above 1,000 C. They correlate these reaction rates with measurements of density, open porosity and BET surface areas. They find the reactivity to be largely dependent upon effective surface area. Promising results were obtained below 600 C from a 94% theoretical dense (TD) deposit with a BET specific surface area of 0.085 m{sup 2}/g. Although reaction rates were higher than those for dense consolidated beryllium they were substantially lower, i.e., about two orders of magnitude, than those obtained from previously tested lower density plasma-sprayed deposits.

Steam oxidation of PFC materials for advanced tokamaks

Steam chemical reactivity experiments were conducted for several ITER-like tokamak plasma-facing-component (PFC) materials: NB31 and NS31 carbon fiber composites (CFCs), W–1%La, DShG-200 Be, and Be (S65C and Kawecki PO Ductile Be) specimens irradiated to fast neutron fluences ranging from 5×1019 to 1×1021 n/cm2. Experiments were performed at 800–1100 °C for CFC, 550–1000 °C for W–1%La, 500–900 °C for unirradiated Be, and 600–800 °C for irradiated Be. Average hydrogen generation rates are presented for these new measurements as a function of temperature, and the results are compared to previous studies on similar materials. In general, the new results extend the temperature and material range from previous studies, are consistent with previous work, and provide Arrhenius-type hydrogen generation expressions useful for safety assessment calculations. Little irradiation influence on oxidation behavior was observed for Be irradiated to 1×1021 n/cm2, whereas significant influence was observed in previous tests on Be irradiated to 5×1022 n/cm2.

Steam chemical reactivity of Be pebbles and Be powder

Abstract This paper reports the results of chemical reactivity experiments for Be pebbles (2 and 0.2 mm diameter) and Be powder (14–31 μm diameter) exposed to steam at elevated temperatures, 350–900°C for pebbles and 400–500°C for powders. We measured BET specific surface areas of 0.12 m 2 /g for 2 mm pebbles, 0.24 m 2 /g for 0.2 mm pebbles and 0.66–1.21 m 2 /g for Be powder samples. These experiments showed a complex reactivity behavior for the material, dependent primarily on the test temperature. Average H 2 generation rates for powder samples, based on measured BET surface areas, were in good agreement with previous measurements for fully dense consolidated powder metallurgy (CPM)-Be. Rates for the Be pebbles, based on measured BET surface areas, were systematically lower than the CPM-Be rates, possibly because of different surface and bulk features for the pebbles, especially surface layer impurities, that contribute to the measured BET surface area and influence the oxidation process at the material surface.