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Languages : en
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Multicomponent time-dependent concentration diffusion and radioactive decay of isotopic species is an important aspect of fission product migration and release from fuel particles and fuel elements in a High Temperature Gas-Cooled Reactor (HTGR). After fission products escape from a fuel particle in an HTGR, it is still necessary for them to diffuse across the graphite web of a fuel block to a coolant hole before they can be entrained in the primary coolant. The time required for a given fission product species to diffuse across the graphite web has a direct influence on the time-dependent release associated with a significant increase in the power/flow ratio. The main purpose of the paper is to present the results of a study of the holdup time in graphite of Sr as a function of the diffusion constants. The study employs a newly-developed multicomponent time-dependent diffusion and decay code called DASH. Analysis methods for solving the type of problem discussed are well known, and some applications to fission product decay and diffusion in HTGRs have appeared in the literature. However, the methods employed are often subject to time step limitations, and the effects of decay are not adequately handled. The DASH code uses a one dimensional spatial discretization for the diffusion operator and an analytic matrix operator method to remove the time dependence. Comparisons of the solutions given by DASH with a number of analytic solutions have been made, and in all instances considered the agreement with analytical solutions is excellent and limited only by the inaccuracy inherent in the spatial discretization.
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Languages : en
Pages :
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Book Description
Multicomponent time-dependent concentration diffusion and radioactive decay of isotopic species is an important aspect of fission product migration and release from fuel particles and fuel elements in a High Temperature Gas-Cooled Reactor (HTGR). After fission products escape from a fuel particle in an HTGR, it is still necessary for them to diffuse across the graphite web of a fuel block to a coolant hole before they can be entrained in the primary coolant. The time required for a given fission product species to diffuse across the graphite web has a direct influence on the time-dependent release associated with a significant increase in the power/flow ratio. The main purpose of the paper is to present the results of a study of the holdup time in graphite of Sr as a function of the diffusion constants. The study employs a newly-developed multicomponent time-dependent diffusion and decay code called DASH. Analysis methods for solving the type of problem discussed are well known, and some applications to fission product decay and diffusion in HTGRs have appeared in the literature. However, the methods employed are often subject to time step limitations, and the effects of decay are not adequately handled. The DASH code uses a one dimensional spatial discretization for the diffusion operator and an analytic matrix operator method to remove the time dependence. Comparisons of the solutions given by DASH with a number of analytic solutions have been made, and in all instances considered the agreement with analytical solutions is excellent and limited only by the inaccuracy inherent in the spatial discretization.
Author: Lukas M. Carter
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Category :
Languages : en
Pages : 190
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Release of radioactive fission products from nuclear fuel during normal reactor operation or in accident scenarios is a fundamental safety concern. Of paramount importance are the understanding and elucidation of mechanisms of chemical interaction, nuclear interaction, and transport phenomena involving fission products. Worldwide efforts to reduce fossil fuel dependence coupled with an increasing overall energy demand have generated renewed enthusiasm toward nuclear power technologies, and as such, these mechanisms continue to be the subjects of vigorous research. High-Temperature Gas-Cooled Reactors (HTGRs or VHTRs) remain one of the most promising candidates for the next generation of nuclear power reactors. An extant knowledge gap specific to HTGR technology derives from an incomplete understanding of fission product transport in major core materials under HTGR operational conditions. Our specific interest in the current work is diffusion in reactor graphite. Development of methods for analysis of diffusion of multiple fission products is key to providing accurate models for fission product release from HTGR core components and the reactor as a whole. In the present work, a specialized diffusion cell has been developed and constructed to facilitate real-time diffusion measurements via ICP-MS. The cell utilizes a helium gas-jet system which transports diffusing fission products to the mass spectrometer using carbon nanoparticles. The setup was designed to replicate conditions present in a functioning HTGR, and can be configured for real-time release or permeation measurements of single or multiple fission products from graphite or other core materials. In the present work, we have analyzed release rates of cesium in graphite grades IG-110, NBG-18, and a commercial grade of graphite, as well as release of iodine in IG-110. Additionally we have investigated infusion of graphite samples with Cs, I, Sr, Ag, and other surrogate fission products for use in release or profile measurements of diffusion coefficients.
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Languages : en
Pages : 24
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High and Very High Temperatures Gas Reactors (HTGRs/VHTRs) have five barriers to fission product (FP) release: the TRISO fuel coating, the fuel elements, the core graphite, the primary coolant system, and the reactor building. This project focused on measurements and computations of FP diffusion in graphite, FP adsorption on graphite and FP interactions with dust particles of arbitrary shape. Diffusion Coefficients of Cs and Iodine in two nuclear graphite were obtained by the release method and use of Inductively Coupled Plasma- Mass Spectroscopy (ICP-MS) and Instrumented Neutron Activation Analysis (INAA). A new mathematical model for fission gas release from nuclear fuel was also developed. Several techniques were explored to measure adsorption isotherms, notably a Knudsen Effusion Mass Spectrometer (KEMS) and Instrumented Neutron Activation Analysis (INAA). Some of these measurements are still in progress. The results will be reported in a supplemental report later. Studies ofFP interactions with dust and shape factors for both chain like particles and agglomerates over a wide size range were obtained through solutions of the diffusion and transport equations. The Green's Function Method for diffusion and Monte Carlo technique for transport were used, and it was found that the shape factors are sensitive to the particle arrangements, and that diffusion and transport of FPs can be hindered. Several journal articles relating to the above work have been published, and more are in submission and preparation.
Author: A. R. Saunders
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ISBN:
Category : Fission products
Languages : en
Pages : 42
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Category :
Languages : en
Pages : 63
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This Subprogram Plan describes cooperative work in the areas of HTR fuel and graphite development and fission product studies that is being carried out under US/FRG/Swiss Implementing Agreement for cooperation in Gas Cooled Reactor development. Only bilateral US/FRG cooperation is included, since it is the only active work in this subprogram area at this time. The cooperation has been in progress since February 1977. A number of Project Work Statements have been developed in each of the major areas of the subprogram, and work on many of them is in progress. The following specific areas are included in the scope of this plan: fuel development; graphite development; fission product release; and fission product behavior outside the fuel elements.
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Languages : en
Pages : 24
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High and Very High Temperatures Gas Reactors (HTGRs/VHTRs) have five barriers to fission product (FP) release: the TRISO fuel coating, the fuel elements, the core graphite, the primary coolant system, and the reactor building. This project focused on measurements and computations of FP diffusion in graphite, FP adsorption on graphite and FP interactions with dust particles of arbitrary shape. Diffusion Coefficients of Cs and Iodine in two nuclear graphite were obtained by the release method and use of Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) and Instrumented Neutron Activation Analysis (INAA). A new mathematical model for fission gas release from nuclear fuel was also developed. Several techniques were explored to measure adsorption isotherms, notably a Knudsen Effusion Mass Spectrometer (KEMS) and Instrumented Neutron Activation Analysis (INAA). Some of these measurements are still in progress. The results will be reported in a supplemental report later. Studies of FP interactions with dust and shape factors for both chain-like particles and agglomerates over a wide size range were obtained through solutions of the diffusion and transport equations. The Green's Function Method for diffusion and Monte Carlo technique for transport were used, and it was found that the shape factors are sensitive to the particle arrangements, and that diffusion and transport of FPs can be hindered. Several journal articles relating to the above work have been published, and more are in submission and preparation.
Author: Kevin Graydon
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ISBN:
Category :
Languages : en
Pages : 55
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The next generation of nuclear reactors (Generation IV) are specified to use graphite as the choice material for neutron moderation and structural components. For this reason, study of diffusion behavior of the fission product, ruthenium (Ru), in the candidate graphite grades, POCO AXF-5Q & ZXF-5Q, IG-110, NBG-18 and PCEA, is necessary. Diffusion data for Ru in these grades is absent due to the lack of prior studies and this work aims to fill this void and begin the study of this metallic fission product’s diffusion behavior. By utilizing physical vapor deposition (PVD), dynamic secondary ion mass spectroscopy (SIMS), and the thin film solution to the diffusion equation coupled with a short circuit diffusion correction term, the diffusivities and Arrhenius temperature dependence are determined in the range of 500-1000°C and reported for the first time. Diffusion of Ru in this range is slow, with both lower activation energies and diffusion coefficients than other metallic fission products. Simulations of Ru diffusion on a graphene plane give activation energies similar to those acquired in this study. This is consistent with Ru behaving as an intercalating species and using the region in between graphene-like basal planes of graphite to travel through the lattice.
Author: P.R. Rowland
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Category :
Languages : en
Pages : 27
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Category :
Languages : en
Pages :
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A proposal is given of an approach to a fission-product trapping system which appears feasible on the basis of thermodynamic and other data available. Reactor and trapping conditions are outlined. The half-lives, fission yields, and volatility of the fission products of interest are described. To provide the most effective retention at elevated temperatures, two types of reagents are required: a highly electropositive metal that will not melt or appreciably vaporize and which will form stable non-volatile compounds with non-metallic or near non-metallic fission products; and a reagent to provide a highly electronegative element to form stable, non-volatile compounds with metallic fission products. Thermodynamic properties are included for compounds formed by reactions between the fission products and the trapping reagents. (B.O.G.).
Author: John W. Prados
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Category : Fission gases
Languages : en
Pages : 22
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