T. R. Govindan Kutty, Joydipta Banerjee and Arun Kumar

Abstract The behavior of nuclear fuel during irradiation is largely dependent on its thermophysical properties and their change with temperature and burnup. Experi­mental data on out-of-pile properties such as melting point, density, thermal con­ductivity, and thermal expansion are required for fuel design, performance modeling, and safety analysis. The variables that influence the out-of-pile properties are fuel composition, temperature, porosity, microstructure, and burnup. Among the above-mentioned properties, thermal conductivity of nuclear fuel is the most important property which influences almost all the processes such as swelling, grain growth, and fission gas release, and limits the linear power. The changes in thermal conductivity occur during irradiation by the formation of fission gas bubbles, porosities, build-up of fission products, and by the change of fuel stoichiometry. Melting point plays a crucial role in determining the power to melt the fuel and decides the operating linear heat rating. The coefficient of thermal expansion (CTE) is needed to calculate stresses occurring in the fuel and cladding on change in temperature. In safety analysis, the values of thermal expansion data are required in determining the gap conductance and the stored energy.

1 Introduction

The behavior of nuclear fuel during irradiation is largely dependent on its phys­icochemical properties and their change with temperature and burnup [1]. Thermal conductivity is an important parameter to understand the performance of the fuel

T. R. Govindan Kutty (H)

Formerly at Radiometallurgy Division, Bhabha Atomic Research Centre,

Mumbai 400085, India

e-mail: trgovindankutty@gmail. com

J. Banerjee • Arun Kumar

Radiometallurgy Division, Nuclear Fuels Group, Bhabha Atomic Research Centre, Mumbai 400085, India

D. Das and S. R. Bharadwaj (eds.), Thoria-based Nuclear Fuels, Green Energy and Technology, DOI: 10.1007/978-1-4471-5589-8_2, © Springer-Verlag London 2013

pins under irradiation [2]. It is highly dependent on physical structure, state, chemical composition, and is one of the most important properties for predicting fuel and material performance [3]. If the thermal conductivity is low, the tem­perature gradient in the radial direction of the fuel pellet is large which results in high temperature at the central part of the fuel pin [2, 3]. The thermal conductivity of nuclear fuel influences almost all important processes such as fission gas release, swelling, grain growth etc. and limits the linear power [4, 5]. The changes in thermal conductivity occur during irradiation by the formation of fission gas bubbles, build-up of fission products, and by the change of oxygen-to-metal ratio (O/M) [6]. Hence, the knowledge of thermal conductivity is needed to evaluate the performance of nuclear fuels. The coefficient of thermal expansion (CTE) values is needed to calculate stresses occurring in the fuel and cladding. If the thermal expansion varies considerably between the fuel and cladding, then stresses will be accumulated during the thermal cycling [7]. This can lead to deformation of the cladding and eventually may result in the breakage of the cladding. Hence, precise evaluation of CTE data of the fuel is needed.

Other important thermophysical properties to be considered are melting point and density. Thorium and uranium oxide fuels used in nuclear reactors have very high melting points, but low density and they suffer from poor thermal conduc­tivity, because in these insulating oxides only phonons (lattice vibrations) conduct heat. Understanding the physics underlying transport phenomena due to electrons and lattice vibrations in actinide systems is an important step toward the design of better fuels [8].

Thermophysical properties of materials depend on various factors, such as microstructure, porosity and its distribution, thermal treatment employed, pro­duction technology used, radiation exposure undergone, and other unidentified factors leaving aside the temperature effect [1]. Improving the technology for nuclear reactors through better computer codes and more accurate data of materials property, which can contribute to improved performance as well as economics of future plants by getting rid of currently used large design margins. Accurate representations of thermophysical properties under relevant temperature and neu­tron fluence conditions are therefore, necessary for evaluating reactor performance under normal operation and accidental conditions [2].

Prior to deploying new fuels and structural materials in a nuclear reactor, its thermophysical properties must be known. The fuel temperature is determined by the thermal conductivity. Such properties of the fuel are not constant during the irradiation period in the reactor, but change with the burnup. Therefore, the evaluation of the thermophysical properties of fuel, including a reliable uncertainty assessment, is required by the nuclear reactor design [3]. A high confidence level on the fuel performance can only be reached from a good interpretation of the irradiation data followed by post-irradiation examinations. A prerequisite for this is to have data on out-of-pile properties such as thermal conductivity or thermal diffusion that allows to understand the influence of parameters such as tempera­ture, temperature gradient, stress, stress gradient, fission rate, and impurities that are effective during the operation. Safety analyses are required by regulatory authorities to prove that the fuel can be burned safely in the reactors. These safety analyses require calculations with safety codes that need the appropriate thermo­physical properties of the fuel. These important informations are used by ther­mohydraulic codes to define operational aspects and to assure the safety, when analyzing various potential accidental scenarios. For each property, the variables that influence the property are to be described, followed by a review of the available data and correlations. Variables considered are temperature, composi­tion, porosity (p), burnup (B), and oxygen-to-metal (O/M) ratio [9, 10].

Due to its higher thermal conductivity, during normal operation, ThO2-based fuel will operate with somewhat lower fuel temperatures and release less fission gas than UO2 fuel at corresponding powers and burnups. This will allow for higher prepressurization and thereby minimizes cladding creep down and fuel cladding mechanical interactions at high burnup and thereby possibly allow for higher burnup use of this material. During an accident such as a large loss-of-coolant accident (LOCA), ThO2-UO2 fuel will have less stored energy but a slightly higher internal heat generation rate than UO2 fuel at similar power levels [11]. As a result, certain parameters for accident evaluation such as the maximum cladding temperature and the timing of fuel rod rupture are expected to be slightly different [8]. These expected differences in behavior between ThO2-UO2 fuel rods and UO2 fuel rods need to be quantified for an objective evaluation of the performance of ThO2-UO2 fuel. The mixed ThO2-UO2 fuel reduces the amount of total plutonium production by a factor of 3.2 and the 239Pu production by a factor of about 4, when compared with conventional UO2 fuel irradiated to 45 GWD/t [9]. The plutonium that is produced in the mixed ThO2-UO2 fuel is high in 238Pu, producing copious amounts of decay heat and spontaneous neutrons making it proliferation resistant. A mixture of ThO2 and UO2 is much more resistant to long-term corrosion in air or oxygenated water than UO2. Thus, ThO2-UO2 is a superior waste form if the spent fuel is slated for direct disposal rather than reprocessing.