Small Amounts Of Thorium Research Articles

MODELING STUDIES AND CODE-TO-CODE COMPARISONS FOR PRESSURE TUBE HEAVY WATER REACTOR CORES

Preliminary, conceptual studies have been performed previously using deterministic lattice physics (WIMS-AECL) and core physics codes (RFSP) to estimate performance and safety characteristics of various thorium-based fuels and uranium-based fuels augmented by small amounts of thorium for use in pressure tube heavy-water reactors (PT-HWRs). To confirm the validity of the results, the WIMS-AECL/RFSP results are compared against predictions made with the stochastic neutron transport code MCNP. This paper describes the development of a method for setting up an MCNP core model of at PT-HWR for comparison with WIMS-AECL/RFSP results, using a core with 37-element natural uranium fuel bundles as a test case for sensitivity studies. These studies included evaluating the sensitivity of the bias of the effective neutron multiplication factor (keff), a source convergence study, uncertainties correction with multiple independent simulations, the impact of irradiation map binning methods, and the impact of reflector models. A Python-based software scripting tool was developed to automate the creation, execution, and post-processing of reactor physics data from the MCNP models. The software tool and algorithm for creating an MCNP core model using data from the WIMS-AECL and RFSP models are described in this paper. Based on the preliminary evaluations of the simulation parameters with the base model, reactor physics analyses were performed for PT-HWR cores with thorium-based fuels in a 35-element bundle type. Code-to-code results demonstrate good agreement between MCNP and RFSP, giving confidence in the method developed and its applicability to other fuels and core types.

Simulations of Pressure-Tube–Heavy-Water Reactor Cores Fueled with Thorium-Based Mixed-Oxide Fuels

Thorium, a fertile nuclear fuel which is nearly three times as abundant as uranium, represents a long-term energy resource that could complement uranium by making the current nuclear fuel cycle more sustainable. With the expected refurbishment and new construction of pressure-tube–heavy-water reactors (PT-HWRs) within the international community, there is a good opportunity to gain experience with thorium-based fuels and to start the transition phase toward use of thorium as part of the nuclear fuel cycle. Previous studies have shown that in the near term (10 to 15 years), small amounts of thorium could be introduced into conventional 37-element fuel bundles made with natural uranium (NU) or slightly enriched uranium for use in PT-HWRs, which could help improve performance and safety characteristics relative to the use of NU alone. This work is an evaluation of various types of thorium-based oxide fuels in 35-element bundles that could potentially be implemented in current PT-HWRs to enable the transition toward the thorium fuel cycle to harness the energy potential from thorium and to conserve uranium resources. Such fuel concepts could have additional benefits, including enhanced reactor performance and safety characteristics, a reduction in the production of fissile plutonium, and the ability to consume stockpiles of plutonium. Thorium (in the form of ThO2) is mixed with a fissile driver of either reactor-grade plutonium (67 wt% Pu-fissile/Pu, 3.5 to 4.5 wt% PuO2), low-enriched uranium (LEU) (5 wt% 235U/U, 40 to 50 wt% LEUO2) or 233U (1.8 wt% 233UO2). The 35-element fuel bundle has a central graphite displacer rod to help maintain a low coolant void reactivity (CVR), while also flattening the radial power distribution in the fuel bundle due to supplementary neutron moderation. Analysis suggests that these thorium-based fuel types have lower fuel temperature coefficients (FTCs) and CVR values and higher fissile utilization (FU) compared to conventional 37-element natural uranium oxide (NUO2) fuel used currently in PT-HWRs. Lower values of CVR and FTCs will lead to lower and more negative values of the power coefficient of reactivity (PCR), which will help increase the safety margin for PT-HWRs further. Achieving higher FU will help better conserve nuclear fuel resources (both uranium and thorium), and should lead to lower fuel costs in a once-through fuel cycle. The use of a higher–fissile content fuel (in the form Pu, LEU, or 233U mixed with thorium) enables both higher fuel burnups and FU relative to NU, while also breeding and burning 233U. The low-burnup (LEU,Th)O2 core configuration shows the most promise as a feasible and practical means of producing energy from thorium (up to 33% of total energy produced) while also generating stockpiles of 233U. PT-HWR cores using (Pu,Th)O2 fuels may have some challenges in regard to the power balance in the bundle and reactor operation. Without reactivity control devices, some (Pu,Th)O2 cores may require power reductions to avoid exceeding bundle power limits, if a maximum linear element rating of 54 kW/m (which applies to uranium-based fuels) is assumed. PT-HWR cores fueled with (Pu,Th)O2 also demonstrate significant benefits as a means of consuming stockpiles of fissile plutonium, reducing inventories by 84% to 90% in a single pass through a PT-HWR core, while also producing substantial quantities of 233U (up to 314 kg/year/reactor) in the spent fuel for potential reprocessing and recycling in a closed fuel cycle.

A Preliminary Economic Assessment of Thorium-Based Fuels in a Pressure Tube Heavy Water Reactor

The fueling unit energy cost of various thorium-based fuels and uranium-based fuels augmented by small amounts of thorium in a pressure tube heavy water reactor (PT-HWR) are assessed building upon previous studies. The results suggests that fuel concepts using slightly enriched uranium augmented by thorium and thorium mixed with low-enriched uranium can be cost competitive and have the greatest potential for near-term implementation in PT-HWRs. Thorium-based fuels that contain small amounts of recycled plutonium or 233U will require more effort to identify and develop technologies to reduce the costs of reprocessing and fabrication for these fuels. However, as an alternative nuclear fuel for reliable and sustainable low-carbon electrical energy generation, all thorium-based fuels are competitive for both short-term and long-term implementation.

A scenario analysis of once-through thorium fuel cycles with pressure tube HWRs in Canada

Thorium-based fuel cycles offer many potential benefits, including greater long-term energy sustainability and improved waste management, relative to uranium-based fuels. The purpose of this study was to analyze the potential impacts associated with deploying thorium-based fuels in Pressure Tube Heavy Water Reactors (PT-HWRs) in a once-through fuel cycle in Canada, and to compare them with the use of conventional Natural Uranium (NU) fuel. This study analyzed a medium-burnup (∼19.1MWd/kg) Slightly Enriched Uranium-based fuel augmented by small amounts of thorium (SEU+Th) and a high-burnup (∼40.6MWd/kg) fuel made with Low Enriched Uranium mixed with thorium (LEU/Th).The deployment of the medium-burnup SEU+Th in Canada reduced resource consumption by 23% relative to the low burnup NU fuel. The medium-burnup fuel required 3% to 60% fewer Deep Geological Repository (DGR) Used Fuel Containers (UFCs) relative to the low burnup fuel, depending on the decay time (10–70years) of the Used Nuclear Fuel (UNF). Extending the decay duration of UNF decreases its decay power per unit mass, and hence the required number of DGR UFCs per mass of UNF, at the expense of requiring more above-ground dry storage capacity.

Evaluation of Uranium-Based Fuels Augmented by Low Levels of Thorium for Near-Term Implementation in Pressure Tube Heavy Water Reactors

Thorium, a fertile nuclear fuel that is nearly three times as abundant as uranium, represents a long-term energy source that could complement uranium and eventually replace it. To facilitate the gradual transition from uranium-based fuels to thorium-based fuels, it may be advantageous in the near term to introduce small amounts of thorium (˂7% of the total fuel mass) into uranium-based fuels in pressure tube heavy water reactors (PT-HWRs). Downblending natural or slightly enriched uranium dioxide with thorium dioxide for fuel pellets placed at the ends of the fuel stack of a conventional 37-element fuel bundle could help reduce axial power peaking for fresh fuel, while incorporating thorium dioxide into the central element of the fuel bundle could reduce coolant void reactivity (CVR).A series of two-dimensional lattice physics simulations was carried out as part of conceptual scoping studies to evaluate the potential performance and safety characteristics of uranium-based fuel bundles with small amounts of thorium fuel added. The simulation results were complemented by an approximate model for evaluating the potential economic characteristics. The cases studied involve modifications to fuel composition, central element materials, and the addition of thorium dioxide to the fuel stack. In addition, a set of preliminary three-dimensional MCNP simulations was performed where fuel bundles were modeled to assess the effect of thorium end pellets and graded axial enrichment on end power peaking.Results suggest it should be possible to incorporate thorium into the fuel cycle using existing 37-element fuel bundle geometry. Advantages to incorporating thorium include a reduction in the CVR through a thorium central element, breeding of small amounts of 233U, maintaining front-end fuel costs at or below the price of natural uranium (NU) fuel, and maintaining maximum linear element ratings within 6% of those achieved using NU 37-element fuel.

Full-Core Evaluation of Uranium-Based Fuels Augmented with Small Amounts of Thorium in Pressure Tube Heavy Water Reactors

Thorium, a fertile nuclear fuel that is nearly three times as abundant as uranium, represents a long-term energy source that could complement uranium and eventually replace it. With the expected refurbishment and new construction of pressure tube heavy water reactors (PT-HWRs) within the international community, there is an opportunity to gain experience with thorium-based fuels and to start the transition toward the use of thorium as part of the nuclear fuel cycle.This paper presents an evaluation of fuel types that could be implemented in the near-term to transition into thorium-based fuels in current PT-HWRs. The near-term fuel consists of small amounts of thorium (in a traditional 37-element fuel bundle that is mostly filled with natural uranium or slightly enriched uranium). In addition, a modified 37-element fuel bundle type comprised of slightly enriched uranium fuel (1.2 wt% 235U/U or less), a thorium central element, and the mass equivalent of 1-cm thorium end pellets was studied. Both lattice physics depletion simulations and full-core time-averaged neutron diffusion simulations were carried out to evaluate the performance and safety characteristics of the different studied full-core configurations.The results demonstrate that adding small amounts of thorium into the fuel of a 37-element bundle is feasible, through enrichment, without reducing power in the reactor or incurring a severe burnup penalty. The most viable core configuration is a core filled with modified 37-element fuel containing slightly enriched uranium dioxide with 1.2 wt% 235U/U. Even with the addition of 1.2 kg of thorium metal to the bundle, significant gains are achieved, including an increased margin to maximum bundle power limit of 40 kW and a 50% increase in fissile utilization.

Theoretical studies of Ir5Th and Ir5Ce nanoscale precipitates in Ir

Experimentally, it is known that very small amounts of thorium and/or cerium added to iridium metal form a precipitate, Ir5Th/Ir5Ce, which improves the high-temperature mechanical properties of the resulting alloys. We demonstrate that there are low-energy configurations for nanoscale precipitates of these phases in Ir, and that these coherent arrangements may assist in producing improved mechanical properties. One precipitate/matrix orientation gives a particularly low interfacial energy, and a low lattice misfit. Nanolayer precipitates with this orientation are found to be likely to form with little driving force to coarsen. The predicted morphology of the precipitates and their orientation with the matrix phase provide a potential experiment that could be used to test these predictions.

Phagocytic potential of pulmonary alveolar epithelium with particular reference to surfactant metabolism.

Finely divided thorium and carbon were injected into the rat lung via the trachea and their uptake was studied electron microscopically. Small amounts of thorium were found in the lamellar vacuoles of the granular pneumocytes. Their presence was maximal at 24 hours and they later disappeared from these cells. This phenomenon is thought to represent a passive inflow as the vacuoles open on to the alveolar surface to discharge their content of surfactant, rather than evidence that the granular pneumocyte is concerned in surfactant disposal. An explanation reconciling the secretory nature of the vacuoles with their previously demonstrated content of lysosomal enzymes is advanced. Small quantities of thorium dioxide were ingested by the squamous pneumocytes and a little transferred to interstitial macrophages, but this is a minor route of disposal, the bulk of the material being ingested by alveolar macrophages. The limited phagocytic potential of the alveolar epithelium contrasts with that of the alveolar macrophages and is not taken to support the concept of an epithelial derivation of alveolar macrophages.

Examination of the hexamethylenetetramine procedure for separation of thorium from rare earths and for determination of thorium

An examination has been made of the efficiency of the gravimetric reagent hexamethylenetetramine in the separation of thorium from rare earths and in the determination of thorium. Losses to the filtrate, beaker and filter paper are evaluated as well as the extent of rare earth and other contamination of the thorium hydroxide precipitate. When hydroxylamine is used as the reducing agent to keep cerium in the tervalent state, excellent separations are obtained. The efficacy of the reagent is offset by the loss of small amounts of thorium to the precipitation vessel, the scavenging properties of the precipitate for silica and the difficulty in evaluating a true reagent blank.

Evaporation and Diffusion Phenomena in Thoria Coated Cathodes†

ABSTRACT Evaporation from a thoria coated tungsten filament for the temperature range 1900-2300°K was studied by measuring the thermionic emission of a nearby tungsten receiver. Below 2100°k the evaporated product was mainly thoria with a small amount of thorium. Above 2100°K there was an increased proportion of thorium. The thermionic emission of thoria coated iridium was studied after evaporating thoria from a source on to the clean metal. After only a few monolayers had been deposited on the iridium the thermionic emission decreased during operation below 2200°k due to diffusion of thorium into the metal. Later when more thoria had been deposited a relatively high and stable thermionic emission was obtained following operation of the iridium at 2400°K. This is explained by assuming that at 2400°K considerable crystal growth occurred and that the thorium was stored in the grain boundaries.