Hydrodynamic phenomena in the downcomer during flow rate transients in the primary circuit of a PWR

Mixing phenomena observed when the flow rate in a single loop of the primary circuit is changed can influence the operation of pressurized water reactor (PWR) by inducing local gradients of boron concentration or coolant temperature. Analysis of one-dimensional Laser Doppler Anemometry (LDA) measurements during the start-up and shutdown of pump on a single loop of the ROCOM test facility has been performed. The effect of a step change and a ramped change in the flow rate on the axial and azimuthal velocities was examined. Numerical simulations were also performed for the step change in the flow rate that gave quantitative agreement with the axial velocities. Phenomenological agreement was made on the turbulent kinetic energy; however, observed values were a factor of 2.5 less than the turbulent kinetic energy derived from the measurements. © 2007.

Microlocal Approach to Tensor Tomography and Boundary and Lens Rigidity

2000 Mathematics Subject Classification: 53C24, 53C65, 53C21.

The nitrate to ammonia and ceramic (NAC) process for the denitration and immobilization of low-level radioactive liquid waste (LLW)

Hazardous radioactive liquid waste is the legacy of more than 50 years of plutonium production associated with the United States' nuclear weapons program. It is estimated that more than 245,000 tons of nitrate wastes are stored at facilities such as the single-shell tanks (SST) at the Hanford Site in the state of Washington, and the Melton Valley storage tanks at Oak Ridge National Laboratory (ORNL) in Tennessee. In order to develop an innovative, new technology for the destruction and immobilization of nitrate-based radioactive liquid waste, the United State Department of Energy (DOE) initiated the research project which resulted in the technology known as the Nitrate to Ammonia and Ceramic (NAC) process. However, inasmuch as the nitrate anion is highly mobile and difficult to immobilize, especially in relatively porous cement-based grout which has been used to date as a method for the immobilization of liquid waste, it presents a major obstacle to environmental clean-up initiatives. Thus, in an effort to contribute to the existing body of knowledge and enhance the efficacy of the NAC process, this research involved the experimental measurement of the rheological and heat transfer behaviors of the NAC product slurry and the determination of the optimal operating parameters for the continuous NAC chemical reaction process. Test results indicate that the NAC product slurry exhibits a typical non-Newtonian flow behavior. Correlation equations for the slurry's rheological properties and heat transfer rate in a pipe flow have been developed; these should prove valuable in the design of a full-scale NAC processing plant. The 20-percent slurry exhibited a typical dilatant (shear thickening) behavior and was in the turbulent flow regime due to its lower viscosity. The 40-percent slurry exhibited a typical pseudoplastic (shear thinning) behavior and remained in the laminar flow regime throughout its experimental range. The reactions were found to be more efficient in the lower temperature range investigated. With respect to leachability, the experimental final NAC ceramic waste form is comparable to the final product of vitrification, the technology chosen by DOE to treat these wastes. As the NAC process has the potential of reducing the volume of nitrate-based radioactive liquid waste by as much as 70 percent, it not only promises to enhance environmental remediation efforts but also effect substantial cost savings. ^

Centrifugal separation of molten steel and glass mixtures

A lot of mixed vitrified waste exists at DOE sites, which contain valuable metal having great potential for being reused in industry. Of these useful metals, steel constitutes more than 45% of the volume. Using the differential centrifugal separation technology, steel is separated by using remote melting of the mixed waste. The high costs involved are directly proportional to the time involved in separation of the steel from the mixed waste. This is determined by using similitude principles. Having obtained a solidified steel ingot by melting, it is essential to determine the decontaminated portions of the ingot that can be released to industry. Two parameters representing measures of separation are proposed—the Centrifugal Fluid Separation Number and the Thermal Separation Number. Regression correlations are determined to express the estimated time of separation. Experimental analysis of solidified ingots has shown that when the Thermal Separation Number is less than 1700 the steel contains little or no trace of glass. This result can be used to recycle steel back to industry. ^

Resolução em tomografia de tempo de trânsito poço-a-poço: a dependência da iluminação

The key aspect limiting resolution in crosswell traveltime tomography is illumination, a well known result but not as well exemplified. Resolution in the 2D case is revisited using a simple geometric approach based on the angular aperture distribution and the Radon Transform properties. Analitically it is shown that if an interface has dips contained in the angular aperture limits in all points, it is correctly imaged in the tomogram. By inversion of synthetic data this result is confirmed and it is also evidenced that isolated artifacts might be present when the dip is near the illumination limit. In the inverse sense, however, if an interface is interpretable from a tomogram, even an aproximately horizontal interface, there is no guarantee that it corresponds to a true interface. Similarly, if a body is present in the interwell region it is diffusely imaged in the tomogram, but its interfaces - particularly vertical edges - can not be resolved and additional artifacts might be present. Again, in the inverse sense, there is no guarantee that an isolated anomaly corresponds to a true anomalous body because this anomaly can also be an artifact. Jointly, these results state the dilemma of ill-posed inverse problems: absence of guarantee of correspondence to the true distribution. The limitations due to illumination may not be solved by the use of mathematical constraints. It is shown that crosswell tomograms derived by the use of sparsity constraints, using both Discrete Cosine Transform and Daubechies bases, basically reproduces the same features seen in tomograms obtained with the classic smoothness constraint. Interpretation must be done always taking in consideration the a priori information and the particular limitations due to illumination. An example of interpreting a real data survey in this context is also presented.

Resolução em tomografia de tempo de trânsito poço-a-poço: a dependência da iluminação

The key aspect limiting resolution in crosswell traveltime tomography is illumination, a well known result but not as well exemplified. Resolution in the 2D case is revisited using a simple geometric approach based on the angular aperture distribution and the Radon Transform properties. Analitically it is shown that if an interface has dips contained in the angular aperture limits in all points, it is correctly imaged in the tomogram. By inversion of synthetic data this result is confirmed and it is also evidenced that isolated artifacts might be present when the dip is near the illumination limit. In the inverse sense, however, if an interface is interpretable from a tomogram, even an aproximately horizontal interface, there is no guarantee that it corresponds to a true interface. Similarly, if a body is present in the interwell region it is diffusely imaged in the tomogram, but its interfaces - particularly vertical edges - can not be resolved and additional artifacts might be present. Again, in the inverse sense, there is no guarantee that an isolated anomaly corresponds to a true anomalous body because this anomaly can also be an artifact. Jointly, these results state the dilemma of ill-posed inverse problems: absence of guarantee of correspondence to the true distribution. The limitations due to illumination may not be solved by the use of mathematical constraints. It is shown that crosswell tomograms derived by the use of sparsity constraints, using both Discrete Cosine Transform and Daubechies bases, basically reproduces the same features seen in tomograms obtained with the classic smoothness constraint. Interpretation must be done always taking in consideration the a priori information and the particular limitations due to illumination. An example of interpreting a real data survey in this context is also presented.

Codomain Rigidity of the Dirichlet to Neumann Operator for the Riemannian Wave Equation

We study the Dirichlet to Neumann operator for the Riemannian wave equation on a compact Riemannian manifold. If the Riemannian manifold is modelled as an elastic medium, this operator represents the data available to an observer on the boundary of the manifold when the manifold is set into motion through boundary vibrations. We study the Dirichlet to Neumann operator when vibrations are imposed and data recorded on disjoint sets, a useful setting for applications. We prove that this operator determines the Dirichlet to Neumann operator where sources and observations are on the same set, provided a spectral condition on the Laplace-Beltrami operator for the manifold is satisfied. We prove this by providing an implementable procedure for determining a portion of the Riemannian manifold near the area where sources are applied. Drawing on established results, an immediate corollary is that a compact Riemannian manifold can be reconstructed from the Dirichlet to Neumann operator where sources and observations are on disjoint sets.

Codomain Rigidity of the Dirichlet to Neumann Operator for the Riemannian Wave Equation

We study the Dirichlet to Neumann operator for the Riemannian wave equation on a compact Riemannian manifold. If the Riemannian manifold is modelled as an elastic medium, this operator represents the data available to an observer on the boundary of the manifold when the manifold is set into motion through boundary vibrations. We study the Dirichlet to Neumann operator when vibrations are imposed and data recorded on disjoint sets, a useful setting for applications. We prove that this operator determines the Dirichlet to Neumann operator where sources and observations are on the same set, provided a spectral condition on the Laplace-Beltrami operator for the manifold is satisfied. We prove this by providing an implementable procedure for determining a portion of the Riemannian manifold near the area where sources are applied. Drawing on established results, an immediate corollary is that a compact Riemannian manifold can be reconstructed from the Dirichlet to Neumann operator where sources and observations are on disjoint sets.

Evaluation of Two Ultrasonic Systems for Analysis of Porosity in Ceramic

The Ultrasound Laboratory of the Nuclear Engineering Institute (LABUS / IEN) has developed an ultrasonic technique to measure porosity in nuclear fuel pellets (UO2). By difficulties related to the handling of UO2 pellets, Alumina (Al2O3) pellets have been used in preliminary tests, until a methodology for tests with pellets of UO2 could be defined. In a previous work, in which a contact ultrasonic technique was used, good results were obtained to measure the porosity of Alumina pellets. In the current studies, it was found that the frequency spectrum of an ultrasonic pulse is very sensitive to the porosity of the medium in which it propagates. In order to define the most appropriate experimental apparatus for using immersion technique in future tests, two ultrasonic systems, available in LABUS, which permit to work with the ultrasonic pulse in the frequency domain were evaluated . One system was the Explorer II (Matec INSTRUMENTS) and the other the ultrasonic pulse generator Epoch 4 Plus (Panametrics) coupled with an oscilloscope TDS 3032B (Tektronix). For this evaluation, several frequency spectra were obtained with the two equipment, by the passage of the ultrasonic wave in the same pellet of Alumina. This procedure was performed on four different days, on each day 12 ultrasonic signals were acquired, one signal every 10 minutes, with each apparatus. The results were compared and analyzed as regard the repeatability of the frequency spectra obtained.

CHARACTERIZATION OF RADIATION DAMAGE TO A NOVEL PHOTONIC CRYSTAL SENSOR

New methods of nuclear fuel and cladding characterization must be developed and implemented to enhance the safety and reliability of nuclear power plants. One class of such advanced methods is aimed at the characterization of fuel performance by performing minimally intrusive in-core, real time measurements on nuclear fuel on the nanometer scale. Nuclear power plants depend on instrumentation and control systems for monitoring, control and protection. Traditionally, methods for fuel characterization under irradiation are performed using a “cook and look” method. These methods are very expensive and labor-intensive since they require removal, inspection and return of irradiated samples for each measurement. Such fuel cladding inspection methods investigate oxide layer thickness, wear, dimensional changes, ovality, nuclear fuel growth and nuclear fuel defect identification. These methods are also not suitable for all commercial nuclear power applications as they are not always available to the operator when needed. Additionally, such techniques often provide limited data and may exacerbate the phenomena being investigated. This thesis investigates a novel, nanostructured sensor based on a photonic crystal design that is implemented in a nuclear reactor environment. The aim of this work is to produce an in-situ radiation-tolerant sensor capable of measuring the deformation of a nuclear material during nuclear reactor operations. The sensor was fabricated on the surface of nuclear reactor materials (specifically, steel and zirconium based alloys). Charged-particle and mixed-field irradiations were both performed on a newly-developed “pelletron” beamline at Idaho State University's Research and Innovation in Science and Engineering (RISE) complex and at the University of Maryland's 250 kW Training Reactor (MUTR). The sensors were irradiated to 6 different fluences (ranging from 1 to 100 dpa), followed by intensive characterization using focused ion beam (FIB), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to investigate the physical deformation and microstructural changes between different fluence levels, to provide high-resolution information regarding the material performance. Computer modeling (SRIM/TRIM) was employed to simulate damage to the sensor as well as to provide significant information concerning the penetration depth of the ions into the material.