THERMAL DIFFUSIVITY AND THERMAL CONDUCTIVITY OF (Th, U)O> FUELS by A. K. Sengupta, T. Jarvis, M. R. Nair, R. Ramachandran, S. Mujumdar and D. S. C. Purushotham Radiometallurgy Division 2000 32/ 05 PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK BARC/2OOO/E/O15 -, GOVERNMENT OF INDIA < ATOMIC ENERGY COMMISSION THERMAL DIFFUSIVITY AND THERMAL CONDUCTIVITY OF (Th, U)O2 FUELS by A. K. Sengupta, TJarvis, M.R. Nair, R. Ramachandran, S. Majumdar and D.S.C. Purushotham Radiometallurgy Division BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2000 BARC/2000/E/015 BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT (as per IS : 9400 - 1980) 01 Security classification: Unclassified 02 Distribution : External 03 Report status: New 04 Series : BARC External 05 Report type: Technical Report 06 Report No. : BARC/2000/E/015 07 Part No. or Volume No. : 08 Contract No. : 10 Title and subtitle : Thermal diffusivity and thermal conductivity of (Th,U)O2 fuels 11 Collation : 25 p., 4 figs., 10 tabs. 13 Project No. : 20 Personal autnorts) : A. K. Sengupta; T. Jarvis; M.R. Nair; R. Ramachandran; S. Majumdar; D.S.C. Purushotham 21 Affiliation of authors): 1) Radiometallurgy Division, Bhabha Atomic Research Centre, Mumbai 22 Corporate authors): Bhabha Atomic Research Centre, Mumbai - 400 085 23 Originating unit: Radiometallurgy Division, BARC, Mumbai 24 Sponsors) Name: Department of Atomic Energy Type: Government Contd... (ii) -i- BARC/2000/E/015 30 Date of submission: April 2000 31 Publication/Issue date: May 2000 40 Publisher/Distributor: Head, Library and Information Services Division, Bhabha Atomic Research Centre, Mumbai 42 Form of distribution : Hard copy 50 Language of text: English 51 Language of summary: English 52 No. of references: lOrefs. 53 Gives data on : Abstract: India has vast reserves of thorium (> 460, 000 tons) and sustained work on all aspects of thorium utilization has been initiated. In this context work on fabrication of sintered thoria and mixed (Th,U)O2 pellets and evaluation of their thermophysical properties have been taken up in Radiometallurgy Division. Thermal conductivity, being the most important thermal properties, has been calculated using the experimentally measured thermal diffusivity, density and literature values of specific heats for ThO2 and thoria containing 2,4,6,10 and 20% UO2 Thermal diffusivity was measured experimentally by the laser flash method from 600 to 1600°C in vacuum.It was observed that thermal conductivity of ThO2and mixed (Th,U)O2 decrease with increase in temperature. It was also observed that the conductivity decreases with increase in UO2 content, the decrease being more at lower temperature than that at higher temperatures. Emperical relations correlating thermal conductivity to temperatures have been generated by the least square fit method and reported. 70 Keywords/Descriptors : THORIUM OXIDES; URANIUM OXIDES; MIXED OXIDE FUELS; FUEL PELLETS; THERMAL CONDUCTIVITY; THERMAL DIFFUSIVITY; SINTERING; EXPERIMENTAL DATA; TEMPERATURE DEPENDENCE; VERY HIGH PRESSURE; DENSITY 71 IMS Subject Category: S36 99 Supplementary elements: -n- ABSTRACT India has vast reserves of thorium (> 460, 000 tons) and sustained work on all aspects of thorium utilization has been initiated. In this context work on fabrication of sintered Thoria and mixed (Th,U)O2 pellets and evaluation of their thermophysical properties have been taken up in Radiometallurgy Division. Thermal conductivity, being the most important thermal properties, has been calculated using the experimentally measured thermal diffusivity, density and literature values of specific heats for ThO2 and thoria containing 2, 4, 6, 10 and 20% UO2. Thermal diffusivity was measured experimentally by the laser flash method from 600 to 1600°C in vacuum. It was observed that thermal conductivity of TI1O2 and mixed (Th,U)O2 decrease with increase in temperature. It was also observed that the conductivity decreases with increase in UO2 content, the decrease being more at lower temperature than that at higher temperatures. Empericai relations correlating thermal conductivity to temperatures have been generated by the least square fit method and reported. Thermal diffusivity and thermal conductivity of (Th,U)O2 fuels A.K. Sengupta, T. Jarvis, M.R. Nair, R. Ramachandran, S. Majumdar and O.S.C. Purushotham Radiometallurgy Division Bhabha Atomic Research Centre Trombay, Mumbai - 400085, INDIA. 1. Introduction : India has reached a major milestone in her ambitious thorium resource utilisation programmes by using Zircaloy clad ThC>2 bundles in the initial core of all new PHWRs for flux flattening. Apart from this, several tons of stainless steel clad ThC>2 pins have been fabricated for use as blanket material for fast breeder test reactor(FBTR) at Kalpakkam. Attempts are also being made to develop Thoria based fuels for PHWR. An Advance Heavy Water Reactor(AHWR), is being designed specifically for utilization of Thorium and generation of electricity. The behaviour of nuclear fuels during irradiation is dependent on the physico chemical properties of the fuel and their variation with temperature and burn up. For example, the fuel temperature is determined, amongst others, by the thermal conductivity of the fuel material. Other properties of importance are the coefficient of thermal expansion, specific heat, melting point etc. In the present investigation thermal conductivity of (Th,U)O2 solid solutions with UO2 content varying from 2 to 20w% have been calculated from the experimentally measured thermal diffusivity by laser flash method, literature values of specific heats of the solid solutions of ThO2 and UO2 and density. 2. Experimental: 2.1 Sample Preparation : ThO2 is a highly stable stoichiometric compound with very high melting point (3573K). Hence to obtain high density pellet at lower sintering temperature, it is necessary to dope thoria powder with about 500 ppm MgO. For better homogeneity, this doping was done in the solution stage before precipitation of oxalate. The characteristics of ihoria and urania powders used are given in Table 1. ThC>2 powder has a platelet morphology and hence it was milled for 4 hrs. The premilled ThC>2 powder was comilled with UO2 powder in a planetary ball mill for 2 hrs. Progressive milling technique was used for obtaining good homogeneity. The milled powders were double pre-compacted at 105 MPa and 150Mpa before granulation. Double precompaction was necessary for obtaining defect free green pellets which were produced by compacting the granules at 300 MPa in a double acting hydraulic press using a 12 mm diameter die. Sintering of the green compacts was carried out in a Molybdenum resistance batch furnace at 1923 K for 4 hrs. in Nitrogen + 8% Hydrogen atmosphere. Addition of UO2 to ThC>2 brings down the sintered density progressively. Average Green and Sintered density values as well as chemical analysis results for Uranium and Thorium contents in the sintered peilets are given in Tables 2 and 3. The details of the fabrication procedures along with the flow sheet are given in Reference [1J. The sintered pellet samples of diameter 10 mm and average height of about 15 mm for all the compositions were sliced using a low speed cut-off wheel to a thickness ranging from 2 to 2.5 mm for measurement of thermal diffusivity. The density of these sliced samples was calculated from the volume and mass of the sample. These samples were then coated on both the circular sides with a thin layer of graphite, using graphite spray(emulsion) and then were dried. Graphite spray ensures complete and uniform absorption of the laser energy by the sample and on the measuring side of the sample it maintains a black body condition. 2.2 Measurement Procedure: Thermal diffusivity was measured by the transient Laser flash method where a ruby laser with a pulse (square) energy of 6J was used as heating source. The disc-shaped specimens were held under adiabatic condition in a tungsten-mesh high temperature(upto 2000°C), high vacuum (10"6 Torr) furnace. The details of the experimental set up has been given in Ref. 2. The specimen was positioned along the optical axis of the ruby laser. The measurement procedure consisted of irradiating one surface of the sample by the pulsed laser and measuring/recording the rise in temperature (AT) on the other surface of the sample by an In-Sb I.R. detector. The temperature rise vs. time data was stored in the transient memory module for further calculations; using an on-line computer. The time taken for the rear surface of the sample to reach 20, 30, 40, 50, 60, 70 and 80% of the maximum temperature rise was recorded. Subsequently thermal diffusivity was calculated following the radiation heat loss correction procedures proposed by Clerk and Taylor [3]. Measurements were carried out in the temperature range 600°C to 1600°C in vacuum and at each temperature, the sample was soaked for about 5 minutes before laser irradiation was made. Few measurements were also performed during cooling cycle of the sample in order to ensure that the characteristics of the sample has not changed due to heating. Before carrying out measurements on actual samples, a standardization run was taken using Graphite standard(NBS SRM 8425). The data obtained are given in Table 4. It is observed that the experimental data is within ±2% of the recommended value of NBS.USA. 3. Results and Discussion : Experimental data on thermal diffusivity of thoria containing 2-20% UO2 are given in Table 5-10. Thermal conductivity of these materials were calculated from the literature value of specific heat(Cp), measured density(p) and the experimentally measured thermal diffusivity(a) using the following relation : k = a x p x Cp The specific heat data of the mixed (Th.U) oxides which form complete solid solution in the whole range of composition were calculated by adding the weighted fraction of the specific heats of UO2 and ThO2 for each composition.