James S. Tulenko

Aqueous Dissolution of Urania-Thoria Nuclear Fuel

Abstract The aqueous dissolution of irradiated and unirradiated uranium-thorium dioxide, (U,Th)O2, fuel pellets in Yucca Mountain well water has been investigated. Whole and crushed pellets were reacted at 25 and 90°C for periods of up to 195 days. The fuel dissolution was measured by analyzing the concentrations of soluble uranium, thorium, and important fission products (137Cs, 99Tc, 237Np, 239Pu, 240Pu, and 241Am) in the well water. The surface-area-normalized fractional uranium release rates for unirradiated crushed uranium dioxide (UO2) pellets were 10 to 40 times higher than the values for (U,Th)O2 fuel. Similarly, the dissolution rates of irradiated (U,Th)O2 pellets with compositions ranging from 2.0 to 5.2% UO2 were at least two orders of magnitude lower than reported literature values for pure UO2. These results demonstrate an advantage of (U,Th)O2 over UO2 in terms of matrix dissolution in groundwater and suggest that (U,Th)O2 fuel is a more stable long-term waste form than conventional UO2 fuel.

Enhanced Thermal Conductivity for LWR Fuel

Abstract The thermal conductivity of the fuel in today’s light water reactors, uranium dioxide (UO2), can be improved by incorporating a uniformly distributed heat-conducting network of a higher-conductivity material: silicon carbide (SiC). The higher thermal conductivity of SiC along with its other prominent reactor-grade properties makes it a potential material to address some of the related issues when used in UO2 (97% theoretical density). This ongoing research, in collaboration with the University of Florida, aims to investigate the feasibility and development of a formal methodology for producing the resultant composite oxide fuel. Calculations of the effective thermal conductivity (ETC) of the new fuel as a function of percent SiC for certain percentages and as a function of temperature are presented as a preliminary approach. The ETCs are obtained at different temperatures from 600 to 1600 K. The corresponding polynomial equations for the temperature-dependent thermal conductivities are given based on the simulation results. The heat transfer mechanism in this fuel is explained using a finite volume approach and validated against existing empirical models. FLUENT 6.1.22 was used for the thermal conductivity calculations and to estimate the reduction in centerline temperatures achievable within such a fuel rod. Later, the computer codes COMBINE-PC and VENTURE-PC were employed to estimate the fuel enrichment required to maintain the same burnup levels corresponding to a volume percent addition of SiC.

Influence of Carbon Nanotube Dispersion in UO 2 –Carbon Nanotube Ceramic Matrix Composites Utilizing Spark Plasma Sintering

Abstract A novel uranium dioxide (UO2)–carbon nanotube (CNT) ceramic matrix composite fuel concept has been proposed for a nuclear fuel with increased thermal conductivity. Investigations were performed to analyze the dispersion of CNTs in a UO2 matrix utilizing homogenization and sonication techniques. Ethanol and ortho-dichlorobenzene (ODCB) were utilized as solvents during the mixing process. Distributions of both multiwalled carbon nanotubes and single-walled carbon nanotubes (SWNTs) were analyzed. It has been determined that CNTs can be homogeneously distributed into a UO2 matrix using mechanical processes, sonication, and homogenization in the organic solvent ODCB. The powder mixture of UO2 and CNTs was sintered at 1300°C with a hold time of 5 min and 40-MPa pressure in a spark plasma sintering furnace, and the resulting grain size distribution was analyzed. It was observed that where the distribution of CNTs was not well controlled, significant grain growth of UO2 occurred. However, where the CNT distribution is well controlled, the grain growth is limited by the pinning effect of the CNTs along the grain boundaries. The resulting pellet thermal conductivity was improved by 29.7% with the addition of 5 vol % SWNT, relative to pure UO2 values. Raman spectroscopy in conjunction with scanning electron microscopy shows that most CNTs survive both the mixing and sintering processes.

Virtual radiation fields : A virtual environment tool for radiological analysis and simulation

A virtual reality system was developed for computational and graphical modeling and simulation of radiation environments. This system, called Virtual Radiation Fields (VRF), demonstrates the usefulness of radiological analysis in simulation-based design for predicting radiation doses for robotic equipment and personnel working in a radiation environment. The system was developed for use in determining the radiation doses for robotic equipment to be used in tank-waste retrieval operations at the Hanford National Laboratory. As a reference case, specific application is made to simulate cleanup operations for Hanford tank C-106. A three-dimensional model representation of the tank and its predicted radiation levels are presented and analyzed. Tank cleanup operations were simulated to understand how radiation levels change during the cleanup phase and to predict cumulative radiation doses to robotic equipment to aid in the development of maintenance and replacement schedules.

Toward an Atomistically Informed Fuel Performance Code: Thermal Properties Using FRAPCON and Molecular Dynamics Simulation

Abstract A proof-of-principle study is presented in which the results of atomic-level simulations of the thermal expansion and thermal conductivity of UO2 are integrated into the fuel performance code FRAPCON. The beginning-of-life (BOL) thermal conductivity profile of a fuel pellet and the evolution of the pellet expansion over its lifetime are determined. It is found that (a) modifying FRAPCON to accept input from atomistic simulations (or elsewhere, such as new experiments or other calculations) is relatively straightforward, at least for these two properties, and (b) the properties determined from atomistic simulations yield predictions in FRAPCON that are in good agreement for the BOL thermal conductivity, but much less satisfactory for the pellet expansion.

Development and Testing of a Nanotech Nuclear Battery for Powering MEMS Devices

Abstract The paper describes a micronuclear battery that utilizes the conversion of beta particles into photons and back into electrons through a photoelectric cell to potentially deliver a nuclear battery of higher efficiency than other nuclear battery concepts and with much greater energy per gram and lifetime than chemical batteries. The Monte Carlo nuclear code MCNP has been used to analyze the performance of the proposed battery, and the photoelectric stage has been shown to be insensitive to the expected radiation for at least 1 yr of performance.

Screening Experiments for Removal of Low-Level Tritiated Water

Screening experiments for low levels of tritiated water (HTO) remediation based upon selective adsorption/desorption mechanisms utilizing equilibrium isotope effects have been carried out. Several organic and inorganic high surface area materials were investigated to assess their ability to selectively adsorb low concentrations of HTO. Ion-exchange resins with cation functionalities, chitosan, sodium alginate, and several inorganic media modified with metal cations exhibited promising results. Biomaterials, for example, chitosan and modified alginate, demonstrated positive results. Based on the literature and our preliminary testing, we postulate four possible mechanisms for selected tritium adsorption: hydrogen ion exchange, HTO coordination with surface cation sites, hydrogen bonding to surface basic sites, and secondary hydrogen bonding (structural water) in fine pores.

An Innovative Ceramic Corrosion Protection System for Zircaloy Cladding

Light Water reactor (LWR) fuel performance is currently limited by thermal, chemical and mechanical constraints associated with the design, fabrication, and operation of the fuel in incore operation. Corrosion of the zirconium based (Zircaloy-4) alloy cladding of the fuel is a primary limiting factor. Recent success at the University of Florida in developing thin ceramic films with great adhesive properties for metal substrates offers an innovative breakthrough for eliminating a major weakness of the Zircaloy clad. ?The University of Florida proposes to coat the existing Zircaloy clad tubes with a ceramic coating for corrosion protection. An added bonus of this approach would be the implementation of a boron-containing burnable poison outer layer will also be demonstrated as part of the ceramic coating development. In this proposed effort, emphasis will be on the ceramic coating with only demonstration of feasibility on the burnable outer coating approach. This proposed program i s expected to give a step change (approximately a doubling) in clad lifetime before failure due to corrosion. In the development of ceramic coatings for Zircaloy-4 clad, silicon carbide and zirconium carbide coatings will first be applied to Zircaloy-4 coupons and cladding samples by thermal assisted chemical vapor deposition, plasma assisted chemical vapor deposition or by laser ablation deposition. All of these processes are in use at the University of Florida and have shown great potential. The questions of adhesion and thermal expansion mismatch of the ceramic coating to the Zircaloy substrate will be addressed. Several solutions to these conditions will be examined, if needed. These solutions include the use of a zirconium oxide compliant layer, employment of a laser roughened surface and the use of a gradient composition interlayer. These solutions have already been shown to be effective for other high modulus coatings on metal substrates. Mechanical properties and adhesion of the coatings will be monitored as a function of the coating process parameters. The corrosion protection of the various coatings will be evaluated by accelerated corrosion testing. Engineering requirements for coating a full size Zircaloy clad tube will be determined. It is expected that the coating process will add approximately 10 dollars or 10% to the price of a tube. In the second approach, the University of Florida will demonstrate the feasibility to add a boron carbide outer layer to functions as a burnable poison.(B204)

Verification and reconciliation of virtual world model for radioactive waste cleanup

The main task of sensing for robotics and automation is to provide 3D geometrical environment information to robot control and visualization systems, which is often referred as facility characterization or environment mapping. Particularly in radioactive waste cleanup operations, such as the Tank Waste Retrieval (TWR) task where the environment is hazardous, automated sensing techniques are necessary for accurate facility characterization that will be used by remote controlled robots for a safe and orderly cleanup process. This research proposes a facility characterization system which combines the strength of computer vision and computer graphics and which maximizes the use of a priori information. Using a novel image registration method, this system is able to detect the difference between the pre-modeled virtual world and sensed real world. Combined with 3D sensing data, the information can be used for verification and reconciliation of the virtual world database. In the proposed system, the environment is pre-modeled as the virtual world. This virtual world database provides the template for the virtual/real world registration. Once the virtual/real images are registered, the comparison can be accomplished by image subtraction. As the result of the comparison, any missing objects or unanticipated objects will be detected. Utilizing the 3D information, the surfaces of these objects can be reconstructed. This information in turn is used for geometric primitives detection and virtual world updating. The initial testing demonstrates that this system has potential to accomplish the TWR task.

Development of Innovative Accident Tolerant High Thermal Conductivity UO2-Diamond Composite Fuel Pellets

The University of Florida (UF) evaluated a composite fuel consisting of UO2 powder mixed with diamond micro particles as a candidate as an accident-tolerant fuel (ATF). The research group had previous extensive experience researching with diamond micro particles as an addition to reactor coolant for improved plant thermal performance. The purpose of this research work was to utilize diamond micro particles to develop UO2-Diamond composite fuel pellets with significantly enhanced thermal properties, beyond that already being measured in the previous UF research projects of UO2 – SiC and UO2 – Carbon Nanotube fuel pins. UF is proving with the current research results that the addition of diamond micro particles to UO2 may greatly enhanced the thermal conductivity of the UO2 pellets producing an accident-tolerant fuel. The Beginning of life benefits have been proven and fuel samples are being irradiated in the ATR reactor to confirm that the thermal conductivity improvements are still present under irradiation.