Deuterium retention in SiC coated graphite by D2+ implantation

The doped graphite tiles bolted to the active cooling heat sink, made of GBST1308 (1% B₄C, 2.5% Si, 7.5% Ti) coated with SiC, are now being used as the only plasma facing material (PFM) for the EAST device since the campaign of 2008. From the plasma density and fueling point of view, it is important to study thoroughly the hydrogen isotope retention in this kind of SiC-coated doped graphite. D₂⁺ implantations into the SiC coated doped graphite were performed at Shizuoka University. The chemical states of Si and C were studied by means of X-ray photoelectron spectroscopy (XPS), and the thermal desorption behavior of deuterium was analyzed by thermal desorption spectroscopy (TDS). It was found that deuterium was trapped by both C and Si in the SiC coatings. In the previous studies, Oya et al. reported the deuterium retention behavior in polycrystalline β-SiC. In this paper, difference of retention behavior in β-SiC and SiC coating will be also discussed.     

Tritium uptake in graphite tiles exposed to EAST plasma and then tritium gas

Tritium exposure experiments were carried out for three kinds of EAST SiC coated doped-graphite (SiC/C) samples, one from the original graphite tiles without being irradiated, and the other two from erosion and deposition areas of first wall after the 2009 campaign in EAST. β-ray-induced X-ray spectrometry (BIXS) was used to characterize the exposed samples. It is showed that the significant amount of tritium was absorbed in the surface of deposition sample in comparison with that of original sample, which was also supported by the results of imaging plate (IP) measurements. In addition, it was found that drastic decrease in tritium retention appeared by lowering exposure temperature, and the trapped tritium was maintained stably with time. Computer simulation is used to analyze the details of depth profile of tritium in different kinds of samples.     

Deuterium retention in plasma spray tungsten coatings exposed to low-energy, high flux D plasma

Two types of porous plasma spray tungsten coatings deposited onto stainless steel and graphite substrates were exposed to low-energy (76 eV ), high-flux (10²² D/m² s) D plasma to ion fluences of (3-4) × 10²⁶ D/m² at various temperatures. Deuterium retention in the W coatings was examined by thermal desorption spectroscopy and the D(³He,p)⁴He nuclear reaction, allowing determination of the D concentration at depths up to 7 μm. The relatively high D concentration (above 0.1 at.%) at depths of several micrometers observed after D plasma exposure at 340-560 K can be related to accumulation of D₂ molecules in pores, while at temperatures above 600 K deuterium is accumulated mainly in the form of D atoms chemisorbed on the inner pore surfaces. At exposure temperatures above 500 K, the D retention in the plasma spray W coating on graphite substrate increases significantly due to trapping of diffusing D atoms at carbon dangling bonds located at the edge of a graphite crystallite.     

Deuterium retention in tungsten at fluences of up to 10 D/m using D ion beams

Deuterium retention in two types of polycrystalline tungsten (PCW) was studied as a function of incident ion fluence, ion energy, and specimen temperature. (i) D retention at 300 K, as a function of D⁺ fluence, demonstrated a trend to saturation in both the Rembar hot-rolled thin foil and Plansee tungsten plate. At 500 K, new D retention results for the Plansee PCW showed an increasing trend with increasing incident D⁺ fluence without any indication of saturation, in agreement with previous results for Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895]. Even when the incident D⁺ fluence was increased to 8 × 10²⁵ D⁺/m², which is in the fluence range of plasma devices, there was still no sign of saturation. (ii) The temperature dependence results for the Plansee PCW show a decreasing trend in D retention as the temperature is increased from 300 to 500 K. These results differ from previous studies of Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895], but are similar to those seen for single crystal tungsten [M. Poon, A.A. Haasz, J.W. Davis, R.G. Macaulay-Newcombe, J. Nucl. Mater. 313-316 (2003) 199]; an explanation for the different behaviour is suggested. (iii) Varying the D⁺ energy from 100 to 500 eV/D⁺ plays a minor role in the amount of D retained, suggesting that D retention in W depends more on the W structure, incident ion fluence and specimen temperature, rather than on the incident ion energy when the energy is below the threshold for damage formation (∼960 eV for D on W).     

Online time-differential perturbed angular correlation study with an O beam - Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe ¹⁹F, the β⁻ decay product of ¹⁹O (t_1/2 = 26.9 s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣V_zz∣ = 2.91(17) × 10²² V m⁻², suggests that the incident ¹⁹O atoms are stabilized at an interlayer position with point group C_3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive ¹⁹O beam.     

Interaction between B4C, 304 stainless steel,and Zircaloy-4 in H2O/H2 atmosphere at 1473 K

The control blade and the fuel rod installed in channel box melt in steam during severe accident of a boiling water reactor. In order to clarify the melting phenomena and relocation of the structural material in the core of the reactor, interaction and melting behavior among B₄C, 304 grade of stainless steel (SST), and Zircaloy-4 in atmosphere containing H₂/H₂O at 1473 K are investigated. The results showed that the reaction at the interface between B₄C and SST under H₂O atmosphere was slow and an oxidation was observed after 3600 s. Under H₂O/H₂ atmosphere, the concentration of B and C in the SST increased and the SST melted. Despite the atmosphere, an oxide layer formed on the surface of Zircaloy-4, and thus the reactions proceeded slowly when the Zircaloy-4 was contacted with B₄C and SST. Under H₂O atmosphere, continuous oxidation happened to SST, and SST was partially melted. Under H₂ atmosphere, the SST was also melted due to the diffusion of B and C from the B₄C. In addition, the oxidation of B₄C affected the oxidation behavior of SST and Zircaloy, and thus the oxidation and the hydroxylation of B₄C in a severe accident was discussed thermodynamically.     

Saturation in deuterium retention of CFC graphite targets exposed to beryllium-seeded plasmas on PISCES-B

ITER strike-plates are foreseen to be of carbon-fiber-composite (CFC). In this study the CFC bulk deuterium retention in ITER-relevant conditions is investigated. DMS 701 (Dunlop) CFC targets were exposed to plasma in PISCES-B divertor plasma simulator. Samples were exposed to both pure deuterium plasma and beryllium-seeded plasma at high fluences (up to ) and high surface temperature (1070 K). The deuterium contents of the exposed samples have been measured using both thermal-desorption-spectrometry (TDS) during baking at 1400 K and ion beam nuclear reaction analysis (NRA). The total deuterium inventory has been obtained from TDS while NRA measured the deuterium depth distribution. In the analysed fluence range at target temperature of 1070 K, no fluence dependence was observed. The measured released deuterium is . In the case of target exposure with beryllium-seeded plasma no change in the released amount of deuterium was found. The deuterium concentration inside the samples is almost constant until the probed depth of ?m, except in the first 1 μm surface layer, where it is 5 times higher than in the bulk. No C erosion/redeposition was observed in the Be-seeded plasma cases. The measured retention, applied to 50 m² of ITER CFC surface, would imply a tritium saturated value of 0.3 gT, much lower than the ITER safety limit of 350 g.     

Localization by TEM and EELS of deuterium trapping sites in CFC exposed to plasma irradiation in Tore Supra

Carbon fiber composite (CFC) Sepcarb^® N11 is used in the tokamak Tore Supra as plasma-facing components. To investigate the fuel retention capability of this material, a mobile sample holder was used to expose CFC N11 samples to direct irradiation by the scrape-off layer plasma of Tore Supra at fluences up to 1 × 10²⁵ m⁻². Deuterium (D) elemental mapping using nuclear reaction analysis for the most-exposed CFC sample showed that D retention occurs at depths greater than 8 μm due to the presence of deep (>3.5 μm) local retention sites. In this work, combining transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS), we describe at a high spatial resolution where and how D atoms are trapped in these sites. TEM experiments performed on thin cross-sections of the plasma-modified surface show evidence of the presence of a 3.5 μm-thick deuterated amorphous carbon layer deposited on the CFC surface. We show that specific localized retention sites correspond to the filling of relatively large (∼3 μm.) and deep (at least 3 μm below the initial CFC surface) cracks between fibres and matrix by the deuterated amorphous carbon layer.     

Measurement of neutron emission from Ti and Pd in pressurized D2 gas and D2O electrolysis cells

Experiments using high-efficiency neutron detectors have detected neutron emission from various forms of Pd and Ti metal in pressurized D₂ gas cells and D₂O electrolysis cells. Four independent neutron detectors based on³He gas tubes were used. Both random neutrons (0.05–0.2 n/s) and time-correlated neutron bursts (10–280 n) of 100-s duration were measured using time-correlation counting techniques. The majority of the neutron burst events occurred at –30°C as the samples were warming up from the liquid nitrogen temperature.     

Studies of 2D reflector effects in cross section preparation for deep burn VHTRs

The prismatic VHTR neutronic simulation presents challenges due to thermalization of the neutrons in the graphite reflector which leads to a spectral change in the peripheral fuel blocks. Two calculation schemes were tested on a simple 2D core calculation: a single block path wherein a classical single block lattice calculation provides the homogenized cross-sections, and a supercell path where the homogenized cross-sections are generated using a lattice of a fuel block surrounded by some of its surroundings. In both paths, several group condensations were performed to assess the effect of increasing the number of groups in the core calculation from 2 to 295. Core and lattice calculations were validated with respect to MCNP. The study revealed that the supercells lead to improvement in the calculation of power shape over the single-block path. This improvement is rather pronounced with small numbers of energy groups. For larger numbers of energy groups, however, both solution methods appear to yield adequate accuracy and the improvement gained through supercells in these cases may not warrant the computational cost. Lattice depletion calculations also show that the presence of the reflector creates strong heterogeneities on isotopic densities after 1000 days of burning.     

Nano-scale modification of 2D surface structures exposed to 6 keV carbon ions: Experiment and modeling

In this work, a Si pitch grating with typical lateral dimensions of 200-250 nm was exposed to 6 keV C⁺ ions at normal incidence and at an angle of 42° both parallel and perpendicular to the grating structure. In contrast to volatile and recycling ions (like Ar⁺ or H⁺), non-recycling ions are able to modify the surface not only due to sputtering, but also due to implantation of incident ions and the re-deposition of projectile atoms following sputtering or reflection. The target-projectile combination used in this work is an example of such a system forming a mixed Si-C surface. The interaction between the ion beam and the surface has been studied both experimentally and numerically with the focus on validation of the numerical model of the newly developed SDTrimSP-2D code. SDTrimSP-2D is capable of following the evolution of the Si-C system including ion-surface interactions with 2D micro- and nano-structured surfaces. The SDTrimSP-2D code takes the interdependency of surface morphology, sputtering and implantation into account. The simulated surface morphology has been compared to cross-sections of bombarded Si pitch grating obtained by SEM, revealing good agreement between experiment and simulation. The calculations also provide improved insight into the mechanisms of surface modification by sputtering, implantation and material transport by redeposition.     

Synergism in sputtering of copper nanoclusters on graphite substrate at low energy Cu2 bombardment

Molecular dynamics simulation of Cu cluster sputtering by 50-200 eV/atom Cu₂ dimers and Cu single atoms has been performed. The clusters were located on a (0 0 0 1) graphite surface and consisted of 13-195 atoms. Synergy features were identified in the sputtering yield and energy distributions of sputtered particles calculated for the cases of cluster bombardment with Cu dimers and monomers at the same velocity. The reason for the nonlinear effects in surface cluster sputtering is the overlapping of collision cascades generated by each of the dimer atoms.     

Formation of water in lithium ceramics bed at hydrogen addition to purge gas

It has been proposed to make use of the isotope exchange reaction for enhancement of the tritium release rate from the blanket material by adding hydrogen isotopes to the purge gas. However, it is found by the present authors that formation of water in the lithium oxide bed occurs when hydrogen isotopes are introduced to the He sweep gas. The amount of water generated in various lithium ceramics beds at hydrogen addition to purge gas is discussed in this paper. The reaction rate of the water formation reaction is also discussed. The water formation reaction in each lithium ceramics bed is considered to be second order reaction which is affected by the concentration of active point as the oxygen supplier on the lithium ceramics and the concentration of hydrogen in the purge gas.     

Temperature dependence of surface morphology and deuterium retention in polycrystalline ITER-grade tungsten exposed to low-energy,high-flux D plasma

Surface topography and deuterium retention in polycrystalline ITER-grade tungsten have been examined after exposure to a low-energy (38 eV/D), high-flux (10²² D/m² s) deuterium plasma with ion fluences of 10²⁶ and 10²⁷ D/m² at various temperatures. The methods used were scanning electron microscopy equipped with focused ion beam, thermal desorption spectroscopy, and the D(³He,p) ⁴He nuclear reaction at ³He energies varied from 0.69 to 4.0 MeV. During exposure to the D plasma at temperatures in the range from 320 to 815 K, small blisters of size in the range from 0.2 to 5 μm, depending on the exposure temperature and ion fluence, are formed on the W surface. At an ion fluence of 10²⁷ D/m², the deuterium retention increases with the exposure temperature, reaching its maximum value of about 10²² D/m² at 500 K, and then decreases below 10¹⁹ D/m² at 800 K.     

Structural stability of Cu nanocrystals in SiO2 exposed to high-energy ion irradiation

Cu nanocrystals (NCs) were synthesized in SiO₂ by ion implantation and thermal annealing. Annealing at two different temperatures of 950 °C and 650 °C yielded two different nanocrystal size distributions with an average diameter of 8.1 and 2.5 nm, respectively. Subsequently the NCs were exposed to 5.0 MeV Sn³⁺ ion irradiation simultaneously with a thin Cu film as a bulk reference. The short-range atomic structure and average NC diameter was measured by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. Consistent with the high regeneration rate of bulk elemental metals, no irradiation induced defects were observed for the reference, whereas the small NCs (2.5 nm) were dissolved as Cu monomers in the matrix. The latter was attributed to irradiation-induced mixing of Cu, Si and O based on dynamic binary collision simulations. For the large NCs (8.1 nm) only minor structural changes were observed upon irradiation, consistent with a more bulk-like pre-irradiation structure.     

Decrease of Cesium Release from Irradiated UO2 Fuel in Helium Atmosphere under Elevated Pressure of 1.0 MPa at Temperature up to 2,773K

In the case of severe accidents, the radionuclides release from fuel could mostly occur at high temperature under elevated pressure. The effect of temperature on the release has been clarified in many previous studies while the pressure influence has been scarcely investigated so far due to difficulty in the experimental operation. To investigate the effect of pressure on the release, two tests under the same conditions except for the system pressure were performed in the VEGA program at JAERI by heating up the irradiated UO₂fuels up to 2,773 K in inert helium. The test results uniquely showed that the release rate of cesium for the temperatures below 2,773 K at 1.0 MPa could be suppressed by about 30% compared with that at 0.1 MPa. This article describes the outlines of the two tests and the observed effects of system pressure on cesium release as well as the results of various post-irradiation examinations. Moreover, the mechanisms and models that explain the pressure effect are discussed.