Graphite surface erosion by 100 keV helium and hydrogen bombardment

Surface erosion in pyrolytic graphite by 100 keV ⁴He⁺ and 200 keV H⁺₂ ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10¹⁷ to 5 × 10¹⁸ particles/cm². Although the surface is eroded at 1 × 10¹⁷ particles/cm² in helium bombardment, it is not eroded so heavily even at 5 × 10¹⁷ particles/cm² in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10¹⁸ particles/cm², and a cone structure appears at 5 × 10¹⁸ particles/cm², which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10¹⁸ particles/cm², many circular blisters are formed which are sputtered off at 5 × 10¹⁸ particles/cm². The surface roughness of the target also affects the erosion.     

Comparison Of Results From Two Spent Fuel Sabotage Source Term Experiments

Abstract

Estimation of the radiological impact of a sabotage attack on a spent fuel cask requires values for the respirable spent fuel aerosol that might be produced by the attack. Two potential sources of relevant source term data are reviewed and shown to provide consistent information which leads to values of 6 × 10^?5 to 8 × 10^?4 g of respirable surrogate spent fuel aerosol released from the cask per gram of surrogate fuel matrix disrupted by a sabotage attack using a high energy density device (HEDD). Using a respirable spent fuel to respirable surrogate conversion factor of 3 enables estimation of the spent fuel respirable source term from the experiments reviewed.     

Chemical Dosimeters for Mixed Radiations in a Nuclear Reactor

The dose rates due to mixed reactor radiations were measured by five gaseous chemical dosimeters – nitrous oxide (natural), ¹⁵N-enriched nitrous oxide, ethylene, ethane and carbon dioxide. The observed dose rates for these gases at the same irradiation position in a nuclear reactor were, 1.8×10⁸, 1.5×l0⁸, 1.9×10⁸, 2.5×l0⁸ and 1.0×10⁸ rad/hr, respectively. These values were compared with those calculated from the mass stopping power of the gases for secondary electrons produced by γ-rays and those from thermal and fast neutron fluxes. No contradiction was found among them. A method of analysis of the reactor radiation dose rates into γ, thermal and fast neutron components is proposed, which is based solely on chemical dosimetry.     

A Powerful Remote Source of O Atoms for the Removal of Hydrogenated Carbon Deposits

We describe a system to deliver a large flux of O atoms for the removal of hydrogenated carbon films from surfaces in remote areas of tokamaks with carbon divertors. The oxygen plasma is generated via electrode-less radiofrequency discharge in a discharge chamber connected to a remote chamber by a 2 m long complex-shaped glass tube 4 cm in diameter. The density of O atoms in the remote chamber was measured with a nickel catalytic probe and its variation with discharge power obtained. The density was close to the detection limit of the probe (around 1 × 10¹⁹ m^?3) as long as the vacuum system was pumped with a rotary pump at a nominal pumping speed of 80 m³ h^?1. The density increased well over 10²⁰ m^?3 when a Roots pump was added. The effective pumping speed at the current setup was up to 200 m³ h^?1. At such conditions, the maximal O-atom density at 2 m from the source was up to 3 × 10²⁰ m^?3. The density depended on the pressure as well as the discharge power. The behavior of O-atom density far away from the source was explained by gas phase and surface phenomena. The effective pumping speed was found to be of crucial importance. The setup was used for removal of model hydrogenated carbon films. Experiments were performed at sample temperatures up to 600 K and etching rates up to 50 nm/s were obtained. We found that the experimental setup is suitable for removal of hydrogenated atoms on a large scale.     

Swift heavy ion induced modifications of optical and microstructural properties of silver–fullerene C60 nanocomposite

In this paper, we study the optical and microstructural properties of silver–fullerene C₆₀ nanocomposite and their modifications induced by swift heavy ion irradiation. Silver nanoparticles embedded in fullerene C₆₀ matrix were synthesized by co-deposition of silver and fullerene C₆₀ by thermal evaporation. The nanocomposite thin films were irradiated by 120 MeV Ag ions at different fluences ranging from 1 × 10¹² to 3 × 10¹³ ions/cm². Optical absorption studies revealed that the surface plasmon resonance of Ag nanoparticles showed a blue shift of ～49 nm with increasing ion fluence up to 3 × 10¹³ ions/cm². Transmission electron microscopy and Rutherford backscattering spectroscopy were used to quantify particle size and metal atomic fraction in the nanocomposite film. Growth of Ag nanoparticles was observed with increasing ion fluence. Raman spectroscopy was used to understand the effect of heavy ion irradiation on fullerene matrix. The blue shift in plasmonic wavelength is explained by the transformation of fullerene C₆₀ matrix into amorphous carbon.     

Neutron Resonance Parameters of Silver-107 and Silver-109

Neutron transmission measurements were carried out on the separated isotopes of silver using the time-of-flight facility at the Japan Atomic Energy Research Institute electron linear accelerator. Neutrons were detected with the ⁶Li-glass detectors at 56 and 191 m. The samples used were metallic powder enriched to 98.2% for ¹⁰⁷Ag and 99.3% for ¹⁰⁹Ag. Transmission data were analyzed with the multi-level Breit-Wigner formula incorporated in a least squares fitting program. Resonance energies and neutron widths were determined for the large number of resolved resonances in the neutron energy region of 400 eV~7 keV. The s-wave strength functions and average level spacings were obtained to be; S₀= (0.43±0.05) × 10^?4, D₀ = 20±2 eV for ¹⁰⁷Ag and S₀= (0.45 ± 0.05) × 10^?4, D₀ = 20 ± 2eV for ¹⁰⁹Ag.     

Research of Radioisotope Production with Fast Neutrons, (V)

The preparation of carrier-free ⁶⁵Ni was studied using the reaction ⁶⁵CU(n,/p)⁶⁵.Ni in a nuclear reactor. High purity copper metal was irradiated for 20 min in the JRR-2 reactor. Nickel-65 formed in the copper target was separated by anion exchange method. About 1 μ ⁶⁵Ni per gram of copper was obtained by irradiation with a fast neutron flux of 9×10¹¹ n/cm=²/sec and thermal neutron flux of 3.6 ×10¹³ n/cm²/sec. The specific activity was greater than 3 mc/mg Ni which was over 600 times more than that produced by n, γ) reaction on nickel irradiated at the same time. However, the reaction yield of ⁶⁵Ni per gram of target by the (n, p) reaction was only 1/5,000 of that by the (n, γ) reaction, so that it proved that the method would be unsuitable for practical application.

The cross section for the reaction ⁶⁵Cu(n/p)⁶⁵Ni has been estimated semi-theoretically to be 0.6 mb, an experimental value of 0.2 mb was calculated from the results of the present work.     

Fabrication of the First-Loading Fuel of the High Temperature Engineering Test Reactor

The High Temperature Engineering Test Reactor (HTTR). which is the first high temperature gas-cooled reactor (HTGR) in Japan, attained its first criticality in November 1998. The fabrication of the first-loading fuel started June 1995 and in December 1997, 150 fuel assemblies were completely formed. A total of 66,780 fuel compacts, corresponding to 4,770 fuel rods, were successfully produced through the fuel kernel, coated fuel particle and fuel compact processes. Fabrication technology for the fuel was established through a lot of research and development activities and fabrication experiences of irradiation samples. As-fabricated fuel compacts contained almost no through-coatings failed particles and few SiC-defective particles. Average through-coatings and SiC defective fractions were as low as 2 × 10^–6 and 8 × 10^–6, respectively. This paper describes (1) characteristics of as-fabricated fuel, (2) the experiences obtained from the first mass-production and (3) prediction of irradiation performance of the fuel in the HTTR.     

Particle Release from Vanadium Metal Surface by Neutrons

Sputtering yields from vanadium metal surface due to neutron irradiation were studied. A carefully prepared Pyrex glass tube, containing a vanadium foil as target and a polyethylene film pasted on a nickel plate as catcher, was sealed after evacuation, irradiated in a reactor, disassembled to take up the film, and the ⁵²V activity on it was counted for estimating the thermal neutron sputtering yield due to the recoil by(n, γ) reaction. The reactivation of the film gave the fast neutron sputtering yield. These values were found to be 2.3×10^?9 and 2.1×10^?1 respectively.     

The effect of low-fluence neutron irradiation on silver-electroded lead-zirconate-titanate piezoelectric ceramics

The properties of several different versions of near equi-molar proportioned lead-zirconate-titanate ceramic piezoelectric plates were measured after irradiation for up to 48 h in an MTR hollow fuel element. The irradiation temperature was 180 ± 50°C and the maximum fluences 3.5 × 10¹⁹ thermal and 1.4 × 10¹⁹ fission neutrons/cm². The irradiation decreased the capacitance, increased the thickness-mode resonant frequencies and decreased the elevated temperature electromechanical coupling in all of the samples tested. The effects are considered to be due to a change in the electrode bonding and a reduction in the polarisation of the ceramic.     

Design a 10 kJ IS Mather Type Plasma Focus for Solid Target Activation to Produce Short-Lived Radioisotopes 12C(d,n)13N

A 10 kJ (15 kV, 88 μF) IS (Iranian Sun) Mather type plasma focus device has been studied to determine the activity of a compound exogenous carbon solid target through ¹²C(d,n)¹³N nuclear reaction. The produced ¹³N is a short-lived radioisotope with a half-life of 9.97 min and threshold energy of 0.28 MeV. The results indicate that energetic deuterons impinging on the solid target can produce yield of $ \langle y\rangle $  = 6.7 × 10^?5 with an activity of A = 6.8 × 10⁴ Bq for one plasma focus shut and A _ν  = 4 × 10⁵ Bq for 6 shut per mint when the projectile maximum deuterons energy is E _max = 3 MeV.     

Production of high-purity medical radio isotope 64Cu with accelerator-based neutrons generated with 9 and 12 MeV deuterons

We conducted a feasibility study for producing a high-purity medical radioisotope ⁶⁴Cu from natural zinc with accelerator-based neutrons. ⁶⁴Cu isotopes were produced via the ⁶⁴Zn(n,p) reaction. The accelerator-based neutrons were generated via the C(d,n) reaction using low-energy deuterons of 9 and 12 MeV on a 1-mm-thick carbon target. First, the production purity was estimated using the evaluated nuclear data library JENDL-4.0 and our previously measured thick target neutron yield. We found that even when natural zinc was used as the starting material, significantly high-purity ⁶⁴Cu could be obtained. Next, irradiation experiments for producing ⁶⁴Cu using natural zinc were conducted at Kyushu University Tandem Laboratory, with the amounts of ⁶⁴Cu isotopes and other gamma-emission nuclides measured by a high-purity germanium detector. As a result, high-purity ⁶⁴Cu isotopes of 1.11(49) × 10⁰ and 3.70 (17) × 10⁰ Bq/g/μC were produced with incident deuteron energies of 9 and 12 MeV, respectively.     

Absolute Measurements of the Disintegration Rates of Americium-241 Sources by Using Semiconductor Detectors

Low geometry a counting and α-particle/γ-ray coincidence counting were made to determine the absolute disintegration rate of an ²⁴¹ Am source.

Alpha particles were counted by silicon junction detectors and γ-rays by an Nal(Tl) crystalphotomultiplier system. The half life of the 60 keV level of ²³⁷Np was measured to check the effect of the coincidence resolving time in the absolute measurement of the americium source by α-γ coincidence counting; the half life measured was 6.3 × 10^-s sec.

The mean value of the absolute disintegration rate of a typical ²⁴¹Am source measured by coincidence counting was 1.696 ×10⁴ dps and the 90% confidence interval for the mean value was [1.679 ×10⁴dps, 1.713×10⁴dps]. Accurate absolute measurements by low geometry a particle counting are very difficult especially for very small semiconductor detectors because of the difficulty of measuring the accurate geometries.     

Release Behavior of Fission Gas from Coated Fuel Particles under Irradiation

TRISO coated fuel particles for HTGR were irradiated by two sweep gas capsules in order to study the release behavior of the fission gas and try to predict the failure fraction of the particles on the basis of the measurement. For verification of the predicted failure fraction, post irradiation examination was conducted, and failure fraction in a visual inspection and acid leaching fraction were measured. Agreement between the predicted failure fraction and the acid leaching fraction was good for these samples except one. From the release behavior from the intact particles, in-pile diffusion coefficients of Kr in LTI-PyC were estimated and expressed as D=(2.9–6.0)×10⁴exp(-2.55×10°/RT) (cm²/s), where R ids the gas constant (=8.314 J/K) and T the absolute temperature. It was recognized that the release from failed particles was controlled by diffusion at 1,600°C and that from intact particles, predominantly by recoil at 1,400°C.     

Suppression of sputtering of nickel by coverage with self-sustaining thin segregated carbon layers

Sputtering of two-layered films composed of nickel (~5000 Å) and nickel carbide (~1500 Å) at 600° C by 5 keV Ar⁺ bombardment on the nickel side has been studied using Rutherford backscattering of 1.3 MeV H⁺ ions. It is found that the removal rate of nickel atoms from specimens is dependent on ion current density and that the removal rate of nickel atoms is very much smaller than that of carbon atoms when the ion current density is low. During ion bombardments at a low current density carbon segregation by a thickness of nearly two monolayers is observed at the nickel surface. Thus suppression of the removal rate of nickel atoms is ascribed to coverage of the nickel surface with segregated carbon atoms which are continuously supplied by diffusion through the nickel film from the carbide layer.     

Accelerator mass spectrometry II: Background reduction with a Wien (velocity) filter; 10Be contents of Central American basalts

Our limit of detection for ¹⁰Be by accelerator mass spectrometry has been lowered to a few million atoms by the installation of a Wien filter which screens out co-transmitted ⁹Be. Analyses of six Central American basalts gave ¹⁰Be contents ranging from 1 × 10⁶ to 60 × 10⁶ atom/g. The sample richest in ¹⁰Be has been contaminated by sediments. The other samples have ¹⁰Be contents in the range reported in the literature by Brown and co-workers.     

Chemical State and Deposition Rate of Nickel Ion Deposit Produced on Heated Surface in High Pressure Boiling Water

Nickel ion deposit was produced on a heated rod surface in high pressure boiling water (150–285°C, 0.4–7.0 MPa). The deposit under the same temperature and pressure conditions as those for BWR reactor water (285°C, 7.0 MPa) was identified as NiO by spectrum profile analysis of the NiLα, NiLβ and 9th-order NiKα₁ lines. Deposition rate was obtained from in situ measurements of deposit thickness, by a photoacoustic method, and from chemical analysis of deposit amount. The deposition rate coefficients obtained in temperature and pressure ranges of 150–250°C and 0.4–4.0 MPa were 2 × 10^?3–5 × 10^?2, which were 0.15–0.45 times as large as those of iron crud. This was attributed to a dissolution effect of Ni ion from NiO. The deposition rate coefficient at 285°C, 7.0 MPa was estimated to be 4.4 × 10^?2–1.3 × 10^?1.     

The development of surface morphological features on argon-bombarded silver single crystals

The morphological evolution of cones on 30 keV ⁴⁰Ar⁺ bombarded Ag (111) and (100) surfaces, as a function of consecutive doses from 1 × 10¹⁹ to 1.54 × 10²⁰ ions/cm² has been studied. The main observations are: (a) The cone density is orientation dependent, (b) Cones are not an equilibrium structure consistent with the observations of Auciello et al. (c) Some pits on the (111) face are conical and strongly faceted with the same symmetry as that in the sputtered ejection pattern from the same face. The results are explained with reference to the current models of cone formation.     

Microstructural evolutions of three-dimensional carbon–carbon composite materials irradiated by carbon ions at elevated temperatures

Carbon–carbon composites are deemed as candidate materials for applications in very high temperature reactors (VHTRs). In a VHTR, carbon–carbon composite materials would experience severe environmental impacts of both a high radiation fluence (about 3 × 10¹⁶/cm² per year) and high temperatures (∼1200 °C), and radiation damage is accordingly expected. In this study, samples prepared from a three-dimensional pitch-based carbon–carbon composite material were irradiated with a 4 MeV C²⁺ ion beam emitted by an accelerator to deliberately induce various levels of radiation damage on the samples at 1000 °C. Transmission electron microscopy analyses were then conducted to examine the evolution of microstructures in the samples, and a scanning electron microscope was used to examine the changes in surface morphology. It was found that an accumulated fluence of 7.0 × 10²¹/m² at 1000 °C resulted in cracks between the matrix and the fiber in the sample. These interfacial cracks were all parallel to the fiber orientation. The fiber/matrix bonding strength might play an important role in the mechanical property of the carbon–carbon composite material. In the meantime, cracks were also found at the fiber side of the sample irradiated under the same condition.     

Soft X-ray fluorescence measurements of irradiated polymer films

Fluorescent soft X-ray carbon Kα emission spectra (XES) have been used to characterize the bonding of carbon atoms in polyimide (PI) and polycarbosilane (PCS) films. The PI films have been irradiated with 40 keV nitrogen or argon ions, at fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁶ cm⁻². The PCS films have been irradiated with 5 × 10¹⁵ carbon ions cm⁻² of 500 keV and/or annealed at 1000°C. We find that the fine structure of the carbon XES of the PI films changes with implanted ion fluence above 1 × 10¹⁴ cm⁻² which we believe is due to the degradation of the PI into amorphous C:N:O. The width of the forbidden band as determined from the high-energy cut-off of the C Kα X-ray excitation decreases with the ion fluence. The bonding configuration of free carbon precipitates embedded in amorphous SiC which are formed in PCS after irradiation with C ions or combined treatments (irradiation and subsequent annealing) is close to either to that in diamond-like films or in silicidated graphite, respectively.