CENTRAL RESEARCH LIBRARY DOCUMENT COLLECTION 2 .‘."\'”\ll' i | | R 0515 ORNL-4345 UC-70 — Waste Disposal and Processing TEMPERATURE PROFILES WITHIN CYLINDERS CONTAINING INTERNAL HEAT SOURCES AND MATERIALS OF TEMPERATURE-DEPENDENT THERMAL CONDUCTIVITIES. DESCRIPTION OF FAST COMPUTER PROGRAMS AS APPLIED TO SOLIDIFIED RADIOACTIVE WASTES OAK RIDGE NATIONAL LABORATORY operated by UNION CARBIDE CORPORATION for the U.S. ATOMIC ENERGY COMMISSION Printed in the United Stotes of America. Available from Clearinghouse for Federal Scientific and Technical Information, National Bureau of Standards, inia 22151 Printed Copy $3.00; Microfiche $0.65 U.S. Deportment of Commerce, Spri Price: LEGAL NOTICE This report was prepared o5 an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Mokes any warranty or representation, expressed or implied, with respect o the accuracy, completeness, or usefulness of the information contained in this report, or that the use of formation, apparatus, method, or process disclosed in report may not infri privately owned rights; or B. Assumes any liabi any = with respect fo the use of, or for damages resulting from the use of formation, apparatus, method, or process disclosed in this report. As used in the above, “‘person octing on behalf of the Commission' includes any employee or contractor of the Commission, or employee of such contractor, fo the extent that such employes or contractor of the Commission, or employee of such contractor prepares, disseminate provides access to, any information pursuant to his employment or contract with the Commi or his employment with such contractor. ORNL-4345 Contract No. W-7405-eng-26 - CHEMICAL TECHNOLOGY DIVISION Chemical Development Section B TEMPERATURE PROFILES WITHIN CYLINDERS CONTAINING INTERNAL HEAT SOURCES AND MATERIALS OF TEMPERATURE-DEPENDENT THERMAL CONDUCTIVITIES. DESCRIPTION OF FAST COMPUTER PROGRAMS AS APPLIED TO SOLIDIFIED RADIOACTIVE WASTES W. Davis, Jr. JANUARY 1969 OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee operated by UNION CARBIDE CORPORATION for the U. S. ATOMIC ENERGY COMMISSION i W e iii CONTENTS Abstract . . . . . . L e e e 1. Introduction . . A 2. Methods of Solution . . . . .. .. .. L 3. Input Statements. . .. . ... .. .. 4. Execution Timesand Output ... ... ... ... ... .. ........ D, References . . . . v i i s e e e e e e e e e e e e e e e e e e e e e e e e TEMPERATURE PROFILES WITHIN CYLINDERS CONTAINING INTERNAL HEAT SOURCES AND MATERIALS OF TEMPERATURE-DEPENDENT THERMAL CONDUCTIVITIES. DESCRIPTION OF FAST COMPUTER PROGRAMS AS APPLIED TO SOLIDIFIED RADIOACTIVE WASTES W. Davis, Jr. ABSTRACT The safety and economic aspects of producing and storing radio- active wastes as solids, usually in vessels having a cylindrical geometry, require that we know the maximum internal temperatures to be expected as a result of decay heat. Such information is mandatory for materials that have variable thermal conductivities under different storage con- ditions. ' This report presents a computer program (STORE), which was written to permit more rapid calculation of temperature profites within cylinders containing homogeneously distributed heat sources and materials whose thermal conductivities can be expressed as a tabular function of tem- perature. A simplified version of this program was prepared for the cases in which the thermal conductivity is constant or is a linear function of temperature. Both of these programs have short execution times, typi- cally from a few to 20 seconds on the IBM/360-75 as compared with the five or more minutes required for the more accurate finite-difference method. They are based on the assumption that the material density and, therefore, the power density of the heat source are independent of tem- perature; this assumption is, of course, contrary to physical reality. However, in a test example involving a hypothetical vessel of glass con- taining a large internal (fission product) heat source with a specific power density of 0.2 cal sec™! cm™3 (i.e., 80,910 Btu hr~! £+73), the temperature difference between the wall and the center of a 6-in.-diam by 6.25-ft-long cylinder was overestimated by 36°C (an error of only about 10%, as compared with the "exact"” value that is obtained by solving finite-difference equations and compensating for the reduction in the heat-source strength as the density decreases with increasing temperature). Within the uncertainties inherent in thermal conductivity, density, and heat capacity measurements of systems of interest in the storage of solidified radioactive process wastes, the method and the program presented in this report offer adequate accuracy and a large time savings as compared with the more exact calculations. Also, they are applicable to cylinders with any specified length/diameter ratio. 1. INTRODUCTION On the basis of safety and economic studies, it appears rather probable that high-]-4 and ini‘ermedicfe-level5 radioactive wastes which accumulate from the processing of nuclear reactor fuels will be converted to solids for permanent storage at some time — from 30 days to 30 years — after removal of the fuel from the reactor. Essentially all of the beta energy, and more than half of the gamma energy, from the fission products will usually be absorbed by the solid within which they are contained; therefore, the temperature in the interior will be raised to a level higher than that of the surface. The extent of this temperature elevation depends on the thermal conduc- tivity of the solid, the diameter of the storage vessel (which is usually considered to be a right circular cylinder), and the specific fission product power density. In cal- culations it is assumed that the radioactive materials are distributed isotropically throughout the cylinder; then, atlsfecdy state, the temperature, T{r, z), at any point (r, z) is given by the equation " l_a_.!.Kré.I.‘+iiK-al +A=0, (n rarL er azL z where K = temperafure-dependent thermal conductivity, cal cm-] sec °C , T = temperature, °C, r = radial variable, cm, z = vertical variable, cm, A = sum of (fission product) power densities (i.e., absorbed power density), -1 -3 cal sec cm The quantity A is actually a function of temperature; that is, it decreases as the temperature of the solid increases because the material specific volume increases (the density decreases). However, the definition of A (above) points out that it is the absorbed energy that is important. The fraction of the total gamma-ray energy that is absorbed is a function of the dimensions of the cylinder. 3 Because of the temperature dependence of A, an "exact" solution of Eq. (1) can be obtained only by use of finite-difference methods. However, in practical cases the thermal conductivity is affected much more strongly by temperature than the density is. For example, the effect of temperature on the thermal conductivity 6 °C-], while the coef- of materials of interest is on the order of (50 to 200) x 10~ ficient of volume expansior. is in the range (1 to 5) x 10_6 °C-I. Thus, to a first approximation, the quantity A may be assumed to be independent of temperature. This report presents a computer program (STORE) which was written to provide approximate solutions of Eq. (1) in terms of temperature as a function of spatial . location within a cylinder of arbitrary length/diameter ratio. Such a program can be very useful for evaluating the advantages, with respect to safety and economics, of storirig radicactive waste materials because it can be executed much faster than a program based on the more exact finite-difference method. Additional reports, which are now being written, will illustrate the application of this program, and the simpler programs.derived from it, to the calculation of internal temperatures in cylinders containing intermediate-level waste solids that are dispersed in an organic mairix (such as asphalt, polyethylene, or other plastics) or high-level waste, existing as calcine or as solids that are dissolved, or dispersed, in an inorganic matrix (such as a glass or a microcrystalline solid). 2. METHODS OF SOLUTION It is convenient to express Eq. (1) in terms of a dimensionless temperature, v, defined as v = (T - TO)/TO 7 (2) where To is a convenient reference temperature. In this report, To was chosen as temperature of the surface of the cylinder. By combining Egs. (1) and (2), we obtain a3l af avi, AL }--a-r—‘:Krsl-_-j""g-z-[K'—z'}"‘T;—o, (3) with the boundary conditions (4) v=0atz=0hfor0=