Apparatus for in situ creep testing during irradiation with high-energy neutrons from a planar neutron source

An apparatus is described for investigating the in situ creep behavior of materials during irradiation with neutrons having a spectrum similar to that of a fusion power reactor. Techniques are discussed for remote, high-vacuum creep testing during irradiation with high-energy neutrons from a planar Be(d,n) source used on the 1.93-m Isochronous Cyclotron at the University of California at Davis. Significant effects of radiation damage on the creep of niobium at a flux of 2 x 10(16) n.m(-2) s(-1) are reported.     

Radiation Effects on the Components of Diagnostic Systems for a Thermonuclear Reactor: A Review

The review is devoted to describing how irradiation with neutrons, fast particles, and γ rays affects components of the diagnostic equipment for a thermonuclear reactor. It is shown that the radiation environment inside a reactor and in the space around it has a significant (and sometimes crucial) effect on the diagnostic instruments. Unfortunately, the available data on radiation effects do not allow us to reliably estimate the resistance of a particular diagnostic component to radiation damage from neutrons, γ rays, and other particles. Specifically, the shortage of data on the effect of 14-MeV thermonuclear neutrons is a major problem. Therefore, the radiation testing of both the instruments’ individual components of the instruments and diagnostic systems as a whole is in particular of paramount importance.__________Translated from Pribory i Tekhnika Eksperimenta, No. 4, 2005, pp. 5–14.Original Russian Text Copyright © 2005 by Gott.     

Application of excilamps in a flow reactor for recovery of stable toxic compounds

The preliminary results of the development and tests of a prototype of a small flow reactor for utilizing stable compounds are presented. A specific feature of the reactor is the use of two excilamps with different radiation wavelengths (λ ～ 222 nm, λ ～308 nm) and a polypropylene microfiber material. Then, design of the test bench allows studying the degradation of organic compounds before their mineralization. In accordance with the described technique, the experimental dependences of the decrease in organic compounds on the irradiation time are measured. The energy consumption is within 100 W. The reactor has been developed for training purposes, but it can be used when developing industrial reactors. The proposed design of the reactor equipped with a unit for photocatalytic degradation allows its application for recovery of a wide class of compounds.     

Radiation damage relative to transmission electron microscopy of biological specimens at low temperature: a review.

When biological specimens are irradiated by the electron beam in the electron microscope, the specimen structure is damaged as a result of molecular excitation, ionization, and subsequent chemical reactions. The radiation damage that occurs in the normal process of electron microscopy is known to present severe limitations for imaging high resolution detail in biological specimens. The question of radiation damage at low temperatures has therefore been investigated with the view in mind of reducing somewhat the rate at which damage occurs. The radiation damage protection found for small molecule (anhydrous) organic compounds is generally rather limited or even non-existent. However, large molecular, hydrated materials show as much as a 10-fold reduction at low temperature in the rate at which radiation damage occurs, relative to the damage rate at room temperature. In the case of hydrated specimens, therefore, low temperature electron microscopy offers an important advantage as part of the overall effort required in obtaining high resolution images of complex biological structures.     

Radiation damage in dry and frozen hydrated organic material

Electron-induced radiation damage can cause errors in interpreting electron micrographs. Radiation damage is distinguished from contamination (polymerization of hydrocarbons) and etching (radiolysis in the presence of water), both of which can be controlled by a proper specimen environment in the microscope. While temperature has little effect on the primary interactions of fast electrons with matter, most secondary radiation-damage processes are temperature dependent. Because damage mechanisms differ so greatly among materials, there is no simple factor by which specimen stability is improved as a function of temperature (some cases improve fivefold, others improve 100-fold). While some specimens are stable to almost arbitrarily high doses, some tests reveal damage at 1 e/nm². This paper surveys damage rates and temperature dependencies of various materials as a guide for future electron microscopic studies of organic specimens.     

A simple method for the direct visualization of cell surface-associated lectins and antibodies in freeze fracture electron microscopy

At very low object temperatures organic specimens suffer not only radiation damage but, to a higher extent, charging and heating during electron irradiation due to very low electrical and thermal conductivities of organic materials. The fading of electron diffraction patterns is seriously influenced by the superposition of both effects. For this reason an exact determination of the progress in radiation damage is nearly impossible by direct observations of the diffraction patterns, particularly for obtaining the cryoprotection factors of organics. Charging and subsequently heating cause intrinsic motions of the irradiated areas which also seem to lead to a destruction of the crystallinity. With a suitable preparation of organic specimens, charging and heating effects at very low temperatures could be avoided, and it could be demonstrated that the true cryoprotection was relatively good.     

Radiation damage in soft X-ray microscopy of live mammalian cells

When specimens are observed by soft X-ray microscopy, they always absorb many photons, causing radiation damage at the imaged site. The problems of radiation damage were studied in view of the principle of image formation; absorption contrast, scattering (holography), or phase contrast. In all cases, photons with a wavelength of 1–10 nm interact with the specimen mainly through the photoelectric effect followed by the transfer of energy to the imaging site either directly (absorption imaging) or indirectly (holography or phase contrast). This absorbed energy will cause structural changes to the imaging site. From a review of the literature the absorbed dose is estimated to be as high as 10⁷ Gy when the expected resolution of the specimen (1–10 thick) is 10 nm. This dose is far in excess of the amount required for cells to be able to survive when live mammalian cells are exposed. The levels of radiation effects were extrapolated to the estimated absorbed dose from the reported values for cell survival, chromosome aberrations, and DNA strand breaks with respect to observations on mammalian chromosomes. The extrapolated results show that some damage will occur in every 10 times 10-nm (expected resolution) size unit. Although these studies focused only on the effects on mammalian chromosomes, the present results are more or less common phenomena in the observation of biological specimens. Hence, the results suggest that dynamic observations will be difficult. On the other hand, a time-scale study of the effects of radiation on structural integrity suggests that single-shot imaging with short-pulsed (probably shorter than a few milliseconds) X-rays may be appropriate for the observation of intact live biological specimens in the hydrated condition, before they have deteriorated.     

Effect of neutron irradiation on the electrodes of a vacuum photoemission detector

The effect of neutron irradiation on Ta-Be photocathodes of a vacuum photoemission detector (VPD) has been investigated. The VPD has been designed for tomography of plasmas using thermal X-ray radiation on the ITER international tokamak reactor. The exposure of VPD electrode samples to neutrons with a fluence of 1.6 × 10¹⁹ neutrons/cm² (E > 0.1 MeV) is shown to change the relative concentration of Ta and Be in the surface layer. As a result, the detector sensitivity in the DT operating mode of the ITER is inevitably altered. Some recommendations concerning selection of materials for the VPD electrodes are presented. The conclusion is drawn that sputtering by thermonuclear neutrons in the ITER may lead to deposition of a conductive film on the insulator surface and that a special shape of insulators must be therefore used to avoid formation of such a coating.     

The effect of gas on the irradiation of aluminium inside a high voltage electron microscope

An environmental cell was used in the high voltage electron microscope to observe the formation of radiation damage in aluminium and dilute aluminium alloys in the presence of gas atmospheres of hydrogen, helium and air. The types of defect produced under such conditions are classified and it is shown that the characteristic of electron irradiation in the presence of a gas is the promotion of void nucleation. The implication of this observation with regard to the design of reactor materials is discussed.     

Radiation Hardness of a Fast Neutron Scintillation Spectrometer with a Stilbene Crystal

Neutron detectors with organic oscillators such as stilbene crystals are suggested for use in measuring the characteristics of neutron radiation in the International Thermonuclear Experimental Reactor (ITER). These detectors will be used as neutron-flux monitors in a multichannel neutron collimator for obtaining the spatial characteristics of a plasma source of thermonuclear neutrons with a 1-ms temporal resolution. In addition, operation in the spectrometric mode will ensure the measurement of the thermonuclear-neutron energy spectrum, which yields data on the fusion-plasma temperature and the ratio of the deuterium and tritium concentrations. During the operation of the facility, the detectors will be exposed to irradiation with a large fluence of fast neutrons. The first results obtained in studies of the characteristics of a stilbene crystal irradiated by fast neutrons with a fluence of up to 10¹⁴ neutrons/cm² are presented.     

Hydrogen damage in 34CrMo4 pressure vessel steel with high tensile strength

Various efforts have been made to improve the safety of high-pressure gas cylinders for hydrogen or natural gas with high strength steel liners. Metal liners with high tensile strength have a safety concern, particularly with hydrogen gas or hydrogen generating environments. The hydrogen can permeate into the liner material, and make the material brittle, causing hydrogen damage. This study investigated resistance to hydrogen damage for two kinds of 34CrMo4 steel with different strength levels. Hydrogen was charged with the electrochemical method, and the material strength was measured by the small punch testing technique. Hydrogen concentration of the specimen was also measured for every testing condition, with various charging periods. The specimens with high tensile strength absorbed more hydrogen than the regular tensile strength specimens. The absorbed hydrogen caused internal damage of intergranular cracking and blistering. Material ductility at failure decreased, as the hydrogen concentration of the specimen increased. But the hydrogen concentration had virtually no effect on the strength of the materials with hydrogen. These results confirm that the susceptibility to hydrogen damage of the high tensile strength materials is much higher than that of the materials with regular strength. If the metal liner of a hoop-wrapped cylinder vessel of type II has high tensile strength, general corrosion at the liner surface can cause a hydrogen rich environment, and the cylinder can suffer hydrogen damage and embrittlement. Therefore, controlling the strength level under an optimal level is critical for the safety of a cylinder made with 34CrMo4 steel.     

Radiation damage in coronene, rubrene and p-terphenyl, measured for incident electrons of kinetic energy between 100 and 200 kev

We have measured the sensitivity of three highly conjugated organic compounds to electron irradiation. Using a 200 keV TEM, loss of crystallinity was determined from quantitative electron-diffraction measurements. Degradation of the molecular ring structure was monitored from fading of the 6 eV pi-excitation peak in the energy-loss spectrum. Measurements at incident energies between 30 keV and 100 eV were made using a scanning electron microscope (SEM), by recording gradual decay of the cathodoluminescence (CL) signal. Expressed in Grays, the energy dose required for CL decay in coronene is a factor of 30 lower than for destruction of crystallinity and a factor of 300 lower than for destruction of the molecular structure. Below 1 keV, the CL-decay cross section shows no evidence of a threshold effect, indicating that the damage involved is caused by valence-electron (rather than K-shell) excitation. Therefore even relatively radiation-resistant organic materials may undergo some form of damage when examined in a low-energy electron microscope or a low-voltage SEM.     

Investigating the Thermal and Epithermal Neutron Spectra in the Moderators of Nuclear Reactors

A method for representing a neutron spectrum in the thermal range as a superposition of a Maxwellian equilibrium neutron spectrum with a temperature equal to that of the moderator and a Maxwellian quasi-thermal nonequilibrium neutron spectrum is proposed. A method for representing the reactor spectra in the energy range of epithermal neutrons as a superposition of five to seven partial Maxwellian equilibrium spectra of elastically scattered neutrons was studied.     

核聚变堆用钨表面超精密抛光的研究现状与趋势

钨作为未来核聚变堆中最有前景的面向等离子体材料,在反应堆工况下将承受高能粒子的辐照冲击。表面质量的好坏会直接影响材料的氢/氦滞留行为和辐照损伤程度,进而影响聚变堆的安全性和可靠性。现阶段,针对钨的抗辐照改性研究主要着眼于材料的成分、结构和组织设计,关于机械加工对材料表面抗辐照改性的研究甚少。文章聚焦前沿科学问题,从机械加工角度分析核材料领域科学问题,结合国内外相关研究成果及核聚变堆用钨(PFM-W)的机械加工现状,阐述了PFM-W表面超精密抛光的必要性。通过对比不同抛光方法,提出了磁流变抛光和力流变抛光是较为适合PFM-W表面超精密加工的观点,并对未来PFM-W表面超精密抛光研究趋势进行了分析,重点在抛光方法的探索以及抛光后材料表面质量对抗辐照性能影响的研究。     

A method for nondestructive assay of nuclear materials in facilities with a pulse neutron generator and a digital technology for discrimination between neutrons and photons

A method for determining the ²³⁵U concentration in fuel assemblies of a high-power channel-type PBMK reactor is described. The measure of ²³⁵U content of an analyzed sample is the number of neutrons from thermal-neutron fission of ²³⁵U, normalized to the number of γ quanta produced in thermal neutron capture by hydrogen nuclei in the scintillator or by ¹⁰B in the glass of a photomultiplier tube. A pulse neutron generator based on DT reaction is the neutron source, and an organic scintillator with the pulse shape discrimination between neutrons and γ rays with the aid of the digital technology is a detector. The scintillator is also used as a neutron moderator. Simulation of the method shows that the ²³⁵U content of the analyzed sample can be determined for 1 min with an accuracy of 1% or better. The efficiency of the method has been confirmed by experimental investigations on a model of the setup.     

Theoretical and experimental investigations of neutron and γ-ray spectra and doses in the experimental room of the ГИР-2 reactor

A mathematical model of the ГИР-2 reactor core and its experimental room is described. Neutron spectra and absorbed doses at different points in the room, calculated using the Monte Carlo method, are presented. By comparing these results to experimental data, it is shown that the model proposed can be used to calculate the main characteristics of the reactor radiation field with an accuracy of no worse than 40%.     

A combined γ-neutron detector for fissile material monitoring using the pulse photonuclear technology

Prompt and delayed neutrons and secondary γ rays are the informational radiation in the photonuclear technology for recording fissile materials. Due to the high cost of detectors and difficulties in their placing around a container, it is desirable that the same detectors be used to record all kinds of informational radiation. The design of a multilayer γ-neutron detector is proposed, and the effect of the layer thickness on the recording efficiency is estimated. It is shown that background from prompt neutrons in recording of γrays can be reduced by proper selection of the time interval for γ-ray measurements.     

Selected abstracts from the thirty-sixth annual meeting of the Scandinavian society for electron microscopy

This article reviews work on the temperature dependence of electron radiation damage in organic materials using the electron diffraction method to monitor the destruction of crystalline order. It is clear from work on crystalline 1-valine, polyethylene and paraffin that radiation resistance improves by a factor of about four when the temperature is lowered from room temperature to liquid nitrogen temperature. At lower temperatures there is no apparent improvement.     

Testing the neutron attenuator based on <Superscript>6</Superscript>LiH for γ-ray diagnostics of plasmas in the JET tokamak

A ⁶LiH attenuator of a neutron flux incident on a detector is used to reduce the γ-ray background induced by neutrons in the detector material. This attenuator has been tested during experiments with deuterium (DD) plasmas on the JET tokamak. A specimen of the neutron attenuator with dimensions of ?30 × 300 mm has been developed by the Russian Academy of Sciences’ Ioffe Physico-Technical Institute and inserted into a vertical collimator used for γ spectrometry of plasmas. To compare γ-ray spectra recorded with and without the ⁶LiH attenuator being mounted, identical discharges with heating of the DD plasma by a neutral particle beam have been selected. For γ rays with energies of <3 MeV, which are induced by neutrons in the detector material, the suppression factor is found to be ～100. A low attenuation (～2) observed at energies of >3 MeV can be attributed to the transparency of the ⁶LiH attenuator for γ rays. This portion of the spectrum is due to γ radiation of the plasma and γ rays induced by neutrons in the constructional materials of the tokamak. To estimate the efficiency of the ⁶LiH attenuators as a mandatory component of the ITER γ-ray diagnostic system, it is necessary that measurements be taken in deuterium-tritium (DT) discharges.     

A digital method for pulse-shape discrimination between neutrons and γ rays at a high counting rate and a low energy of detected radiation

A digital method for pulse-shape discrimination between neutrons and γ rays was used in measurements at counting rates of up to ～10⁶ counts/s in the energy range of ～2–800 keV. Pulses produced by neutrons and γ rays in a stilbene-based scintillation detector were digitized by a digital oscilloscope and transmitted to a computer for carrying out particle identification. Identification was performed for radiation of radionuclide sources and a pulsed neutron generator operating in a repeated triggering mode. Amplitude spectra of pulses identified as neutrons and γ rays of radiation from the generator were measured. At a detector counting rate of ～8.5 × 10⁵ counts/s, ～90% of all recorded pulses were recognized as neutrons. In the energy range of ～30–800 keV, the γ-ray suppression factor was ～10⁴–10³ at counting rates of ～1.5×10⁵–5 × 10⁵ counts/s, while the efficiency of identifying neutrons was >0.9. The suppression factor for γ rays with an energy of ～10 keV was ～300, and the neutron identification efficiency was ～0.75.