Evaluation of fracture mechanics parameters for a general corner using a weight function method

Summary The weight function method (WFM) has been used recently as a reliable tool for evaluation of fracture mechanics parameters, where cracks are represented by zero opening traction free surfaces. The purpose of this paper is to extend this technique to general opening corner problem. The two dimensional singular fields for displacements and stresses are introduced in terms of generalized Bueckner's strength. By means of eigenvalue analysis the stress intensity factors (SIF) are then formulated after appropriate splitting the regular stress and displacement fields into symmetric and antisymmetric modes. Using Betti's reciprocal theorem, a new expression in a more general closed form is derived for Bueckner's strength consisten with the given nonzero opening case. The potentiality of the method is demonstrated by a numerical example for =/2 corner problem. The stress intensity factor for the symmetric mode is evaluated by WFM and by a simple collocation procedure using both boundary element (BE) and finite element (FE) discretization.     

Development of a neutron tomography system using a low flux reactor

A neutron tomography instrument was designed and developed at the Royal Military College (RMC) of Canada with Queen's University to enhance these institutions' non-destructive evaluation capabilities. The neutron imaging system was built around a Safe Low-Power C(K)ritical Experiment (SLOWPOKE-2) nuclear research reactor. The low power and physical geometry of the reactor required that a novel design be developed to facilitate tomography. A unique rotisserie style rotary stage and clamping apparatus was developed. Furthermore, the low flux at the image plane (3×10⁴ n cm⁻² s⁻¹), necessitated that the image acquisition and reconstruction processes be optimized. Tomographs of numerous samples were obtained using the new tomography instrument at RMC.     

Normalization of a collimated 14.7 MeV neutron source in a neutron spectrometry system for benchmark experiments

In the present study a method used to normalize a collimated 14.7 MeV neutron beam is introduced. It combined a measurement of the fast neutron scalar flux passing through the collimator, using a copper foil activation, with a neutron transport calculation of the foil activation per unit source neutron, carried out by the discrete-ordinates transport code DOT 4.2. The geometry of the collimated neutron beam is composed of a D-T neutron source positioned 30 cm in front of a 6 cm diameter collimator, through a 120 cm thick paraffin wall. The neutron flux emitted from the D-T source was counted by an NE-213 scintillator, simultaneously with the irradiation of the copper foil. Thus, the determination of the normalization factor of the D-T source is used for an absolute flux calibration of the NE-213 scintillator.The major contributions to the uncertainty in the determination of the normalization factor, and their origins, are discussed.     

Sensitivity of peak positions to flight-path parameters in a deep-inelastic scattering neutron TOF spectrometer

The effects of small changes in flight-path parameters (primary and secondary flight paths, detector angles), and of displacement of the sample along the beam axis away from its ideal position, are examined for an inelastic time-of-flight (TOF) neutron spectrometer, emphasising the deep-inelastic regime. The aim was to develop a rational basis for deciding what measured shifts in the positions of spectral peaks could be regarded as reliable in the light of the uncertainties in the calibrated flight-path parameters. Uncertainty in the length of the primary or secondary flight path has the least effect on the positions of the peaks of H, D and He, which are dominated by the accuracy of the calibration of the detector angles. This aspect of the calibration of a TOF spectrometer therefore demands close attention to achieve reliable outcomes where the position of the peaks is of significant scientific interest and is discussed in detail. The corresponding sensitivities of the position of peak of the Compton profile, J(y), to flight-path parameters and sample position are also examined, focusing on the comparability across experiments of results for H, D and He. We show that positioning the sample to within a few mm of the ideal position is required to ensure good comparability between experiments if data from detectors at high forward angles are to be reliably interpreted.     

Fast response neutron emission monitor for fusion reactor using stilbene scintillator and Flash-ADC

The stilbene neutron detector which has been used for neutron emission profile monitoring in JT-60U has been improved, to respond to the requirement to observe the high-frequency phenomena in megahertz region such as toroidicity-induced Alfvén Eigen mode in burning plasma as well as the spatial profile and the energy spectrum. This high-frequency phenomenon is of great interest and one of the key issues in plasma physics in recent years. To achieve a fast response in the stilbene detector, a Flash-ADC is applied and the wave form of the anode signal stored directly, and neutron/gamma discrimination was carried out via software with a new scheme for data acquisition mode to extend the count rate limit to MHz region from 1.3x10(5) neutron/s in the past, and confirmed the adequacy of the method.     

Evaluation of the neutron spectrum and dose assessment around the venus reactor

An assessment of the neutron field near the VENUS reactor is made in order to evaluate the neutron dose to the operators, particularly in an area near the reactor shielding and in the control room. Therefore, a full MCNPX model of the shielding geometry was developed. The source term used in the simulation is derived from a criticality calculation done beforehand. Calculations are compared to routine neutron dose rate measurements and show good agreement. The MCNPX model developed easily allows core adaptations in order to evaluate the effect of future core configuration on the neutron dose to the operators.     

Three-dimensional shielding calculations for the IFMIF neutron source using a coupled Monte Carlo/Deterministic computational scheme

Shielding calculations for the International Fusion Materials Irradiation Facility (IFMIF) are complicated due to the geometrical complexity of the target system and the large-scale bulk shields around the source target. Three-dimensional shielding calculations were performed by using a newly developed Monte Carlo/Deterministic computational scheme. The neutron-photon fluxes and dose rate distributions in the back wall of the Test Cell and the access/maintenance room are presented and compared with previous shielding calculations. The results demonstrate that this coupled scheme is an useful computational tool for three-dimensional shielding analyses of complex and large nuclear facilities.     

GEANT4 used for neutron beam design of a neutron imaging facility at TRIGA reactor in Morocco

Neutron imaging has a broad scope of applications and has played a pivotal role in visualizing and quantifying hydrogenous masses in metallic matrices. The field continues to expand into new applications with the installation of new neutron imaging facilities.In this scope, a neutron imaging facility for computed tomography and real-time neutron radiography is currently being developed around 2.0MW TRIGA MARK-II reactor at Maamora Nuclear Research Center in Morocco (Reuscher et al., 1990 [1]; de Menezes et al., 2003 [2]; Deinert et al., 2005 [3]).The neutron imaging facility consists of neutron collimator, real-time neutron imaging system and imaging process systems. In order to reduce the gamma-ray content in the neutron beam, the tangential channel was selected. For power of 250 kW, the corresponding thermal neutron flux measured at the inlet of the tangential channel is around 3×10¹¹ ncm²/s.This facility will be based on a conical neutron collimator with two circular diaphragms with diameters of 4 and 2 cm corresponding to L/D-ratio of 165 and 325, respectively. These diaphragms' sizes allow reaching a compromise between good flux and efficient L/D-ratio. Convergent-divergent collimator geometry has been adopted.The beam line consists of a gamma filter, fast neutrons filter, neutron moderator, neutron and gamma shutters, biological shielding around the collimator and several stages of neutron collimator. Monte Carlo calculations by a fully 3D numerical code GEANT4 were used to design the neutron beam line (http://www.info.cern.ch/asd/geant4/geant4.html[4]).To enhance the neutron thermal beam in terms of quality, several materials, mainly bismuth (Bi) and sapphire (Al₂O₃) were examined as gamma and neutron filters respectively. The GEANT4 simulations showed that the gamma and epithermal and fast neutron could be filtered using the bismuth (Bi) and sapphire (Al₂O₃) filters, respectively.To get a good cadmium ratio, GEANT 4 simulations were used to define the design of the moderator in the inlet of the radiation channel. A graphite block of 22 cm thickness seems to be the optimal neutron moderator.The results showed that the combination of 5 cm of bismuth with 5 cm of sapphire permits the filtration of gamma-rays, epithermal neutrons as well as fast neutrons in a considerable way without affecting the neutron thermal flux.     

Performance analysis of a monolithic integrated transversal filter using mixed-mode scattering parameters

A broadband transversal filter based on a new cell design that enables positive and negative tap gain weight control is proposed and its behaviour is analysed in the frequency domain. The non-ideal behaviour of distributed circuits and associated effects on the functional characteristics of an integrated transversal filter design are examined through consideration of the principles of generic differential circuits. It is shown that the transversal filter can be modelled as a coupled-pair drain line structure with the ability to propagate both common and differential-mode signals. Coupled wave interactions on artificial transmission lines are characterised using a set of derived mixed-mode scattering parameters and voltage and current phasors at input/output ports of a monolithic filter design. Analytical and simulation results confirm the suitability of the approach for broadband filter implementations, in applications requiring predefined time-domain responses as in high-speed correlators and encoders as used in optical code division multiple access (CDMA) systems.     

Modeling and simulation of ammonia removal from purge gases of ammonia plants using a catalytic Pd-Ag membrane reactor

In this work, the removal of ammonia from synthesis purge gas of an ammonia plant has been investigated. Since the ammonia decomposition is thermodynamically limited, a membrane reactor is used for complete decomposition. A double pipe catalytic membrane reactor is used to remove ammonia from purge gas. The purge gas is flowing in the reaction side and is converted to hydrogen and nitrogen over nickel-alumina catalyst. The hydrogen is transferred through the Pd-Ag membrane of tube side to the shell side. A mathematical model including conservation of mass in the tube and shell side of reactor is proposed. The proposed model was solved numerically and the effects of different parameters on the rector performance were investigated. The effects of pressure, temperature, flow rate (sweep ratio), membrane thickness and reactor diameter have been investigated in the present study. Increasing ammonia conversion was observed by raising the temperature, sweep ratio and reducing membrane thickness. When the pressure increases, the decomposition is gone toward completion but, at low pressure the ammonia conversion in the outset of reactor is higher than other pressures, but complete destruction of the ammonia cannot be achieved. The proposed model can be used for design of an industrial catalytic membrane reactor for removal of ammonia from ammonia plant and reducing NO(x) emissions.     

Characterization of microcracks in YCrO3 using small-angle neutron scattering and elasticity measurements

The mean crack radius, crack opening displacement, number density, and volume fraction have been estimated for a population of microcracks in polyerystalline YCrO₃ using small-angle neutron scattering in tandem with elasticity measurements.     

Activation calculations for the target of a spallation ultra-cold neutron source at PSI

A spallation ultra-cold neutron source--UCN source--is scheduled to start operation at PSI in 2006 using up to 2 mA 590 MeV protons from the ring cyclotron. It will be operated in a pulsed mode with an average current of 20 microA. For safe maintenance, during operation as well as handling, transport and storage of the UCN target assembly after its lifespan, detailed knowledge about the activation induced by the impinging protons and secondary radiation fields is required. The Monte Carlo transport code MCNPX was coupled with the European Activation System--EASY--to calculate the residual nuclide production in the UCN target assembly. The nuclide inventory is finally used to design the shielded exchange flask that is needed to safely remove and transport the UCN target assembly after its lifespan to a hotcell for dismantling.     

The standardization of Nb for use as a reference material for reactor neutron dosimetry

A solution of ^93mNb in 1M HF + 1M HNO₃ was dispensed into teflon containers and distributed to several laboratories to allow a definitive measurement of the emission probability for the 17 keV K X-rays. The material was characterized for stability, activity concentration, and K X-ray emission rate. The activity concentration of the solution was measured by liquid-scintillation counting, with an estimated uncertainty (one standard deviation) of 0.76%. The K X-ray emission rate was measured in five laboratories using three different detector types: defined solid-angle photon detector, a 4π pressurized proportional counter, and calibrated semiconductor photon spectrometers. The K X-ray emission-rate value for the five laboratories had an estimated uncertainty (one standard deviation) of 1.9%. Combining the activity and emission rate values gives a X-ray emission probability of 0.1112±0.0022.     

In-pile collimator and shutter for neutron beam research at a light water moderated reactor

The construction of an in-pile collimator/shutter system for neutron beam research on a reactor is described. A series of vertically rotatable cylinders, fitted inside the existing beam tube, gives an optimum beam cross-section in the opened position and a very good shielding effect in the closed position.     

Measurement of parameters of simple lenses using digital holographic interferometry and a synthetic reference wave

The use of digital holographic intrerferometry in the testing of simple thin lenses is explored. Focal length, radius of curvature, and refractive index are the lens parameters that can be determined using this method. The digital holograms using the lens under test are recorded at various positions of the test lens using off-axis geometry. This is combined with a digitally computed plane wavefront to determine the curvature of the light beam emerging from the test lens. Focal length is the position of the test lens where a single fringe results. The radius of curvature of the test lens is also determined similarly using a long focal length lens to concentrate a collimated beam onto the test lens. The nonuniformities on the lens surface could also be found by using this method. The implementation of the method is shown by using computer simulations in the case of biconvex lenses. The method can be utilized to measure the parameters of plano-convex and concave lenses also.     

14 MeV neutron irradiation effects on cryostability of a composite superconductor for a fusion reactor

The neutron irradiation effects on cryostability of composite superconductors for fusion reactors are studied based on Maddock's condition. In particular, to estimate the effects of 14 MeV neutrons we assumed that the irradiation-induced degradation of critical temperature, critical current density and conductivity of stabilizer are determined by the damage energy depending on the neutron energy spectrum. The cryostability is found to decrease sensitively with increasing the fraction α of fusion neutrons with energy of 10 ～ 14 MeV to the total neutrons, ie, the Cu/superconductor ratio R_ns, to stabilize the conductor, must be increased remarkably with increasing α as well as the total dose of the neutron fluence. For the small R_ns (～4) the stabilized overall current density decreases by several ten percents even at the fluence when T_c and J_c change by only a few percent. This effect is dominated by the severe increase of ρ.