Numerical Simulation on the Shielding Efficiency of Magnetic Shielding Enclosures in the ITER Applications

Magnetic shielding needs to be employed to ensure proper operation of some electronic equipment which are sensitive to external magnetic interference, such as cryogenic valves located inside the ITER feeder cubicles. This paper is concerned with the shielding efficiency of the magnetic shielding enclosures. A 3-D theoretical model for Fe-Ni alloy magnetic shielding enclosures based on finite element method (FEM) is obtained with the nonlinear law of magnetization. The influence of shielding materials, enclosure configurations, single or multi- layer designs, and apertures on the shielding efficiency is investigated. It is shown that the proposed model can predict the shielding efficiency of shielding devices well with nonlinear magnetize relation, and some recommendations for an optimized magnetic shield design in the ITER feeder cubicles are given.     

Dose build up correction for radiation monitors in high-energy bremsstrahlung photon radiation fields

Conventional radiation monitors have been found to underestimate the personal dose equivalent in the high-energy bremsstrahlung photon radiation fields encountered near electron storage rings. Depth-dose measurements in a water phantom were carried out with a radiation survey meter in the bremsstrahlung photon radiation fields from a 450 MeV electron storage ring to find out the magnitude of the underestimation. Dose equivalent indicated by the survey meter was found to build up with increase in thickness of water placed in front of the meter up to certain depth and then reduce with further increase in thickness. A dose equivalent build up factor was estimated from the measurements. An absorbed dose build up factor in a water phantom was also estimated from calculations performed using the Monte Carlo codes, EGS-4 and EGSnrc. The calculations are found to be in very good agreement with the measurements. The studies indicate inadequacy of commercially available radiation monitors for radiation monitoring within shielded enclosures and in streaming high-energy photon radiation fields from electron storage rings, and the need for proper correction for use in such radiation fields.     

Shielding design for the front end of the CERN SPL

CERN is designing a 2.2-GeV Superconducting Proton Linac (SPL) with a beam power of 4 MW, to be used for the production of a neutrino superbeam. The SPL front end will initially accelerate 2 x 10(14) negative hydrogen ions per second up to an energy of 120 MeV. The FLUKA Monte Carlo code was employed for shielding design. The proposed shielding is a combined iron-concrete structure, which also takes into consideration the required RF wave-guide ducts and access labyrinths to the machine. Two beam-loss scenarios were investigated: (1) constant beam loss of 1 Wm(-1) over the whole accelerator length and (2) full beam loss occurring at various locations. A comparison with results based on simplified approaches is also presented.     

中性束注入器远程数据采集系统设计

设计了中性束注入器低温冷凝泵内多点温度及中性化室真空度的远程数据采集方案,采用现场数据采集模块进行信号转换.通过数字信号传输到上位计算机进行远程数据采集,达到了对系统中的液氮温区、液氦温区和真空度等工作状态的实时监测,为分析低温冷凝泵以及中性化室的工作特性提供了依据.     

Shielding design calculations for beam dump facility of KOMAC

A project to construct the Korea Multi-purpose Accelerator Complex (KOMAC) is currently underway targeting a high-intensity proton beam with an average current of 4.8 mA. As for the first stage of construction, a 20 MeV linac is planned to be built by 2007 and additional DTL sections will be added to increase the proton energy to 100 MeV by 2012. In this paper, preliminary shielding estimates, such as the evaluation of the gamma ray and neutron dose rate around the beam dump, have been carried out with the three-dimensional (3-D) Monte Carlo transport code MCNPX in order to determine the shielding requirements. The tentative flux calculations using the 3-D deterministic code KATRIN, which can handle a coupled charged-neutral particle transport, were also performed and their results were compared with the MCNPX calculations.     

Detection of bremsstrahlung radiation of 90Sr-90Y for emergency lung counting

This study explores the possibility of developing a field-deployable (90)Sr detector for rapid lung counting in emergency situations. The detection of beta-emitters (90)Sr and its daughter (90)Y inside the human lung via bremsstrahlung radiation was performed using a 3″ × 3″ NaI(Tl) crystal detector and a polyethylene-encapsulated source to emulate human lung tissue. The simulation results show that this method is a viable technique for detecting (90)Sr with a minimum detectable activity (MDA) of 1.07 × 10(4) Bq, using a realistic dual-shielded detector system in a 0.25-μGy h(-1) background field for a 100-s scan. The MDA is sufficiently sensitive to meet the requirement for emergency lung counting of Type S (90)Sr intake. The experimental data were verified using Monte Carlo calculations, including an estimate for internal bremsstrahlung, and an optimisation of the detector geometry was performed. Optimisations in background reduction techniques and in the electronic acquisition systems are suggested.     

R&D and design for the cryogenic and mechanical vacuum pumping systems of ITER

The next generation thermonuclear fusion machine ITER comprises large cryogenic pumping systems to process very high gas throughputs. The paper starts from a summary of the requirements for the torus exhaust pumping system and the neutral beam injection system for plasma heating and discusses the design solutions derived to match them. All ITER force-cooled cryopumps are based on a modular set-up and incorporate similar design of charcoal-coated cryopanels cooled to 4.5 K with supercritical helium. A systematic approach for how to design a tailor-made cryopump is presented. It is based on Monte Carlo simulation to calculate the overall capture coefficient being the primary design parameter. Experimental results measured with a model pump of representative scale are used to validate the design procedure. In the last part, the paper addresses the design issues to be solved for tritium-compatible forepumps with high pumping speeds.     

Microstructural design for thermal creep and radiation damage resistance of titanium aluminide alloys for high-temperature nuclear structural applications

Microstructure plays an important role in strengthening of metallic materials. Various microstructures can be developed in titanium aluminide (TiAl) alloys, which can enable different combinations of properties for various extreme environments in advanced nuclear systems. In the present paper the mechanisms for microstructural strengthening and the effects of various microstructural features on thermal creep and radiation damage resistance of TiAl alloys are reviewed and compared. On the basis of the results, the evidence-based optimum microstructure for the best combination of thermal creep and radiation damage resistance of TiAl alloys is proposed. The heat treatment processes for manufacturing the optimal microstructure are also discussed.     

Shielding design for the PANDA spectrometer at the Munich high-flux reactor FRM-II

The start-up of the Munich high-flux reactor FRM-II is in progress on. At the beam tube SR-2 the spectrometer PANDA has been installed. It is at three-axis neutron spectrometer looking onto a slightly under-moderated cold neutron source. For polarisation analysis, PANDA is equipped with a vertical cryomagnet producing fields up to 14.5 T for the sample. To get an appropriate shielding of the high-intensity instrument, one has to take into account the large cross section of the primary beam, several restrictions using magnetic materials, limitations in loading the site and finally, has to keep the lateral extent of the shielding small to allow for high-scattering angles. The shielding has been designed on the basis of the results, which were achieved by the combined use of both the Monte Carlo code MCNP-4B2 and an analytical method based on one-dimensional dose transmission functions.     

Coherent bremsstrahlung and parametric X-ray radiation from nonrelativistic electrons in a crystal

X-ray radiation generated by nonrelativistic electrons interacting with a crystal target exhibits several distinctive features in comparison to the relativistic case. The difference is related to the interference of the parametric X-ray radiation and coherent bremsstrahlung, which takes place for the nonrelativistic electrons. The characteristics of this radiation have been studied in the Bragg and Laue geometries in an electron microscope using a beam of electrons with energies in the 50–100 keV range. The necessary requirements on the target parameters, the measuring instrumentation, and the experimental geometry are established. Variation of the X-ray radiation frequency depending on the angle of electron beam incidence on the target in the region of non-relativistic electron energies has been observed for the first time. The X-ray radiation frequency has been also studied as a function of the primary electron beam energy. Tunable soft X-ray radiation with quantum energy in the range below 1 keV is obtained. The radiation quantum yield per electron within a unit solid angle amounts to ～10^?8.     

电磁屏蔽理论及屏蔽材料的制备

介绍了电磁屏蔽材料的屏蔽原理及其发展现状,复合导电纤维和金属化织物具有高的电导率、良好的电磁屏蔽效果好,是极具发展前景的一类包装材料.目前我国在电磁屏蔽材料领域同国际水平差距较大,应当加强电磁屏蔽材料的研究与开发,不断提高产品的竞争能力.     

NBI漂移管道内低温和真空性能实验

在T okam ak实验装置中,为了改善中性束注入器和HT-7装置内的真空度不匹配的状况,在漂移管道内放置了一台低温冷凝泵。本文对漂移管道内的真空条件进行了性能测试实验并将实验数据通过O rig in软件进行了处理和分析。结果表明:在LH e充注30 m in后,低温泵冷凝板的温度稳定在5 K,漂移管道的极限压强达到4.8×10-6Pa,完全满足HT-7装置对NB I系统的要求。     

Estimation of photoneutron yield from beryllium target irradiated by variable energy microtron-based bremsstrahlung radiation

The possibility of setting up microtron-based photoneutron source by utilizing bremsstrahlung radiation interaction with beryllium targets is critically examined. The bremsstrahlung yield for tantalum (Ta) target is obtained by EGS4 simulation. The neutron yield is estimated theoretically by MCNP simulation. The yield was measured experimentally by neutron irradiation of calibrated SSNTD CR-39 films. The total neutron yield is found to be of the order 10¹⁰ n/s for 250 Hz PRR and 10⁹ n/s for 50 Hz PRR. A detailed comparison shows good agreement between theoretical and experimentally measured yields.     

Photoelectronic properties of HgI2 crystals for nuclear radiation detection

Photoelectronic properties of red mercuric iodide single crystals, grown from its saturated solution in tetrahydrofuran, have been studied for the wavelength range 450–700 nm at temperatures 80,110, 175, 235 and 300 K. Various aspects of the optical generation of charge carriers have been discussed. The computer simulation of the room temperature photoconductivity has generated the optimized values of the mobility-lifetime products μ_eτ_e = 5.67 × 10⁻⁵ cm²/V, μ_hτ_h = 0.18 × 10⁻⁵ cm²/V), and surface recombination velocities (S_e = 3.2 × 10⁵ cm/s, S_h = 4.5 × 10⁵ cm/s) of the charge carriers in these crystals. The estimated values of the electron and hole drift lengths for typical electric fields suggest that, under the negative electrode illumination, THF α-HgI₂ crystals have high potential as regards to their use as photodetectors in most of the scintillation spectrometers.     

Monitoring bremsstrahlung from 4 MeV electrons by nuclear excitation and Al2O3 thermoluminescence dosemeters

Isomer excitation by gamma,gamma' reactions and aluminium oxide thermoluminescence dosemeters (TLDs) have been used to monitor bremsstrahlung from the 4 MeV electron beam of a linear accelerator type LPR4 produced on a 0.9 mm Pt converter foil. Natural indium and osmium as well as TLDs have been irradiated at different distances (2-11 cm) and angles (0 degrees-90 degrees). Dose rates measured by TLDs were 5-110 kGy x h(-1). Isomer excitation of 115In (half-life 4.5 h) was used for monitoring bremsstrahlung of energies above 1 MeV, while that of 189Os (half-life 5.8 h) extended the available range down to 200 keV. Isomer production yields measured by gamma spectrometry and found to be about 10(-19)-10(-18) Bq per nucleus were calibrated against dose rate. A graphical method based on a semiempirical formula was used to evaluate the bremsstrahlung flux as well as the dose rate from the activity of isomeric monitors with uncertainties below 20%. The method is simple, of linear response in a large scale, independent of temperature, and able to monitor extremely high gamma intensities.     

Optical fiber design and the trapping of Cerenkov radiation

Cerenkov radiation is generated in optical fibers immersed in radiation fields and can interfere with signal transmission. We develop a theory for predicting the intensity of Cerenkov radiation generated within the core of a multimode optical fiber by using a ray optic approach and use it to make predictions of the intensity of radiation transmitted down the fiber in propagating modes. The intensity transmitted down the fiber is found to be dominated by bound rays with a contribution from tunneling rays. It is confirmed that for relativistic particles the intensity of the radiation that is transmitted along the fiber is a function of the angle between the particle beam and the fiber axis. The angle of peak intensity is found to be a function of the fiber refractive index difference as well as the core refractive index, with larger refractive index differences shifting the peak significantly toward lower angles. The angular range of the distribution is also significantly increased in both directions by increasing the fiber refractive index difference. The intensity of the radiation is found to be proportional to the cube of the fiber core radius in addition to its dependence on refractive index difference. As the particle energy is reduced into the nonrelativistic range the entire distribution is shifted toward lower angles. Recommendations on minimizing the quantity of Cerenkov light transmitted in the fiber optic system in a radiation field are given.     

Concept and design procedure for cylindrical element monochromators for synchrotron radiation

The design of a new type of soft X-ray grating monochromator which utilizes cylindrical optical elements is proposed. This design is the outcome of a return to simplicity. If not only offers high resolving power and transmission, but also has other advantages over the conventional designs. A simple analytical design procedure (which has been raytracing justified) for designing such a monochromator is described. The raytracing results also confirm that replacement of the cylindrical figure by a spherical one will not degrade the overall monochromator performance.     

Sensitivity analysis for reliable design verification of nuclear turbosets

In this paper, we present an application of sensitivity analysis for design verification of nuclear turbosets. Before the acquisition of a turbogenerator, energy power operators perform independent design assessment in order to assure safe operating conditions of the new machine in its environment. Variables of interest are related to the vibration behaviour of the machine: its eigenfrequencies and dynamic sensitivity to unbalance. In the framework of design verification, epistemic uncertainties are preponderant. This lack of knowledge is due to inexistent or imprecise information about the design as well as to interaction of the rotating machinery with supporting and sub-structures. Sensitivity analysis enables the analyst to rank sources of uncertainty with respect to their importance and, possibly, to screen out insignificant sources of uncertainty. Further studies, if necessary, can then focus on predominant parameters. In particular, the constructor can be asked for detailed information only about the most significant parameters.